Table 1 – Application of signals Clause number and measurement in this standard unless otherwise specified Sine wave Pink noise Simulated speech Reference speech Combi Other
Procedure for setting up and commissioning an audio-frequency induction
The flow chart in Figure 1 shows the sequence of operations detailed in this standard
Figure 1 – Flow chart for the operations in this standard
Suitability of the site for the installation of an audio-frequency induction-loop
The flow chart in Figure 1 shows the sequence of operations detailed in this standard
Figure 1 – Flow chart for the operations in this standard
4.2 Suitability of the site for the installation of an audio-frequency induction-loop system
When planning an induction-loop system, it is crucial to assess the proposed location to ensure acceptable conditions can be achieved, as not all sites may be suitable for installation.
• the magnetic noise level from electric installations, e.g heating systems in the floor or roof, the electrical control of lighting systems (especially in theatres), (see Clause 7);
• the influence of magnetizable and electrically-conducting materials in the structure in which the loop is intended to be installed;
• the presence of other induction-loop systems in the neighbourhood, the signals of which may interfere with that of the planned loop system
NOTE Techniques exist to reduce the magnetic field strength outside an induction loop, but previously-installed systems may not be so designed
If adjustment is made in 8.3, repeat both 8.2 and 8.3
Setting up – commissioning the system
Adjustments made on system including microphone etc
Redo if 0 dB is not achieved
Relation of the magnetic field strength level at the telecoil to the sound
An acoustic input sound pressure level of 70 dB, along with a long-term average magnetic field strength level of −12 dB ref 400 mA/m (equivalent to 100 mA/m) at the telecoil in a hearing aid, is assumed to produce an equivalent acoustic output level.
5 Using components of a sound system in an induction-loop system
General
It may seem economically attractive to derive signals for an induction-loop system from a sound system serving the same space, but it may not be technically straightforward.
Microphones
To achieve optimal sound quality, microphones in a sound system should be strategically positioned to minimize ambient noise and reverberation Listening to the microphone signals through high-quality headphones is crucial for evaluating their clarity and suitability This assessment should be conducted for all microphone signals across various modes and configurations of the sound system.
Mixer
The induction-loop system will derive its signal from the mixer at a location where the signal level is independently adjustable, separate from the sound system's loudspeaker signal chain.
Power amplifier
A suitable signal from a power amplifier can be effective when connected to an induction-loop amplifier that has the right sensitivity and impedance Additionally, it should feature automatic gain control capable of adjusting to variations in the sound system's signal level.
It is generally unwise to directly connect a signal from a sound system to an induction loop without proper design Each interconnection should be specifically tailored to match the electrical characteristics of both the sound system and the loop system.
Meters
Meters in general
There are two types of magnetic field strength meters in use due to historical reasons, and both types remain practical While measurements from these meters yield identical results only for sinusoidal signals, the differences in most cases are not significant enough to cause major issues This standard outlines the expected discrepancies in certain situations In cases of uncertainty, the measurements obtained with the meter specified in section 6.1.3 should be considered definitive.
Requirements common to both types
The meter must maintain a frequency response that is flat within ± 1 dB from 50 Hz to 10 kHz, with a decline of at least 6 dB/octave beyond this range Additionally, A-weighting is required, and the A-weighted frequency response must align with the specifications for a Class 2 meter as outlined in IEC 61672-1, specifically within the 100 Hz to 5 kHz frequency band Other weighting characteristics may also be included as additional features.
True-r.m.s meter
This sound level meter is based on the IEC 61672-1 standard, featuring a magnetic pick-up coil and an amplifier with frequency response correction instead of a traditional microphone It includes a true root mean square (r.m.s.) detector and operates with a 125 ms averaging time constant in 'F' mode.
A useful additional feature is a peak-hold indication.
Peak programme meter (PPM)
This meter is based on the PPM Type II defined in IEC 60268-10, enhanced by incorporating a magnetic pick-up coil and a modern display, ideally a 'bar' type, replacing the traditional moving-coil pointer instrument.
It shall have dynamic responses conforming to the relevant requirements of IEC 60268-10, i.e an attack time-constant of approximately 5 ms and a release time-constant of approximately 1,0 s.
Test signals in general
Various types of test signals can be utilized for setting up and measuring the mid-band frequency value, typically the average over the octave band centered at 1 kHz, as well as the frequency response of magnetic field strength However, the appropriateness of certain signals varies based on the amplifier's amplitude characteristics, as outlined in IEC 62489-1 It is essential to refer to Table 1 for the range of applications for the specified test signals The test signal recommended by the amplifier manufacturer should be employed unless there is a valid justification for using an alternative signal.
Clause number and measurement in this standard
Sine wave Pink noise Simulated speech Reference speech Combi Other
7.1 Magnetic noise level N N N N N Y (no signal)
8.3 Frequency response Y Y See Note to
10.1 Commissioning the system N N N Y N Y (real signals)
To accurately determine the reference magnetic field strength using a wideband signal and meter, a specific procedure is essential to avoid significant errors Initially, it is crucial to measure the magnetic background noise level to ensure an adequate signal-to-noise ratio Next, assess the frequency response of the desired magnetic field, making necessary adjustments to the amplifier controls for optimal flatness in response Once these steps are completed, the reference magnetic field strength can be accurately established.
To ensure accurate measurement of the reference magnetic field strength at 1 kHz, it is crucial to set the frequency-response controls for the flattest response possible In environments with metal reinforcement, failure to do so can lead to significant errors Additionally, an inadequate signal-to-noise ratio, especially in the presence of strong noise components, can compromise the accuracy of this method.
Speech signals
Live speech signals
Live speech serves primarily as a test signal for the final verification of an induction-loop system's operation Nonetheless, it plays a crucial role in the subjective evaluation of loop systems.
Recorded speech material
Speech that has been recorded under controlled conditions and evaluated both subjectively and objectively may be used for test purposes See also B.2.1.
Simulated speech material
Simulated or synthetic speech material contains the features of speech in terms of its amplitude, frequency components and temporal characteristics, but has no recognizable intelligibility
ITU-T P.50 [2] is accompanied by a CD containing a standardized form of synthetic speech See also B.2.2
The International Speech Test Signal (ISTS) is recommended for objective measurements and was developed by the European Hearing Instrument Manufacturers' Association (EHIMA) It consists of recordings from 21 female speakers across six languages: American English, Arabic, Chinese, French, German, and Spanish Although the signal is largely non-intelligible due to segmentation and remixing, it is based on natural recordings Extensive analysis compared the ISTS to the original recordings across various criteria, including time, frequency, and amplitude distributions, confirming its representativeness.
Pink noise signal
The signal must be bandwidth limited, exhibiting a peak-to-peak voltage to true r.m.s voltage ratio of at least 18 dB (equivalent to a crest factor of 4) Additionally, the third-octave-band spectrum should remain flat within ±1 dB across the frequency range of 100 Hz to 5 kHz.
Bandwidth limitation will be achieved using at least one third-order Butterworth high-pass and low-pass filters, which provide -3 dB responses at 75 Hz and 6.5 kHz.
NOTE 1 This specification is given to ensure that the test signal stimulates the system in a manner similar to normal speech
The ±1 dB tolerance is essential due to the theoretical responses of the specified 3rd order Butterworth filters, which are −0.8 dB at 100 Hz and −0.7 dB at 5 kHz Component tolerances influence these exact values, and this impact is considered in the frequency response measurement method that utilizes a pink noise signal.
Sinusoidal signal
The signal source must deliver three specific frequencies: 100 Hz, 1 kHz, and 5 kHz, either individually or simultaneously, while maintaining total harmonic distortion below 2% within a 20 kHz bandwidth It should also allow output voltage settings ranging from 0 mV to 10 mV and 0 V to 1 V, making it suitable for both microphone and line-level testing Additionally, the output source impedance should be 600 Ω or lower.
A sinusoidal signal is suitable for tests outlined in sections 8.2 and 8.3 if the amplifier's amplitude characteristic, as per IEC 62489-1, includes a range of input signal voltages where the output current is proportional to the input voltage Typically, amplifiers with automatic gain control demonstrate this range However, amplifiers featuring expansion or more complex signal processing are generally not measurable beyond their amplitude characteristic and the field strength at 1 kHz when using sinusoidal signals.
Combi signal
This signal comprises 1 kHz sine wave tone-bursts interleaved with pink noise, making it appropriate for any measurements outlined in this standard that require either the 1 kHz sine wave or pink noise.
The measurement of a 1 kHz sine wave level, combined with an amplifier using peak detecting automatic gain control (AGC), results in reduced heating during long test durations due to the lower root mean square (r.m.s) levels of pink noise segments This advantage enhances the efficiency and longevity of the amplifier.
The minimum duration for a 1 kHz sine wave is set at 1 second to ensure that the meter specified in section 5.1 achieves the appropriate measurement level and stabilizes for accurate readings prior to the transition.
To effectively reduce overall amplifier heating, the duration of the pink noise signal should be at least four times longer than that of the sine wave signal This ensures that there are sufficiently frequent bursts of the sine wave signal for accurate measurement.
The specification of the combi signal is summarized in Table 2
Table 2 – Specification of the combi signal
Characteristic Sinusoidal part Pink noise part Notes
3 dB bandwidth, Hz Not applicable See 6.4 and B.2.3
Rise and fall times, ms 5 Not applicable All transitions between signals shall be at zero-crossings
Relative (r.m.s.) levels, dB 0 –6 The peaks of the sine wave signal are
3 dB below the maximum peak of pink noise having a crest factor of 4
Duration, s ≥1 ≥4 These minimum values may be increased but not decreased, and the ratio of 4:1 shall not be reduced
7 Magnetic background noise level of the installation site
Method of measurement
Magnetic noise levels will be measured using an A-weighting network in the measuring instrument The magnetic field strength values, obtained with a specified meter and a vertical pick-up coil (unless stated otherwise), will be expressed in decibels (dB) relative to the reference magnetic field strength level.
The induction-loop system must be turned off if already installed, while all other site equipment should remain operational, with dimmable lighting adjusted to half-dimmed It is essential to measure the magnetic background noise level at various points within the useful magnetic field volume The selection of measurement points can be random but should consider the height range of users—typically 1.2 m for seated listeners and 1.7 m for standing listeners—along with specific seating arrangements, the physical layout of the venue, and potential interference from metal objects and other signals.
NOTE It is useful to listen to the noise, to form a subjective impression of its spectrum, and thus the likely disturbing effect on listeners.
Recommended maximum magnetic noise levels
The reference signal-to-noise ratio should exceed 47 dB, particularly in environments where speech quality is crucial, such as theaters This standard ensures that the reference magnetic field strength significantly surpasses the A-weighted magnetic background noise level However, it is important to note that such low levels of magnetic and acoustic noise may not always be achievable.
Hearing-aid users encounter both acoustic and magnetic noise in their environment Typically, it is unnecessary to maintain a magnetic noise level significantly lower than the acoustic noise level, considering that hearing loss affects the perception of acoustic noise However, this guideline changes when users are equipped with ear defenders.
In situations where communication is prioritized over aesthetics, a higher level of magnetic noise may be acceptable, but it can be exhausting and should only be tolerated for brief, essential communications Therefore, a minimum signal-to-noise ratio of 32 dB is recommended If the actual ratio falls below this threshold, it must be reported and discussed with the system operator to explore potential solutions However, there are instances where a 32 dB signal-to-noise ratio may not be sufficient.
If the magnetic noise lacks significant tonal quality and primarily occurs at low frequencies, a higher level of interfering signal may be acceptable For instance, a reference signal-to-noise ratio as low as 22 dB can be tolerable for brief durations It is essential to evaluate the actual audible impact of the interfering signal to determine if the overall advantages of the system for hearing-aid users outweigh the absence of a loop system or the need for alternative techniques that require special receivers, such as infra-red or radio.
A hearing aid-compatible system requires the use of specialized headphones or accessories, like wireless modules, to provide a direct electrical input to the hearing aid Using a neck loop may result in the hearing aid picking up unwanted magnetic background noise when set to 'T'.
8 Characteristics to be specified, methods of measurement and requirements
Magnetic field strength
Characteristic to be specified
The peak magnetic field strength, as measured by a specified meter and a vertical pick-up coil (unless stated otherwise), is produced by the system at a minimum of one point within the designated useful magnetic field volume.
The measurement methods outlined in sections 8.2.2 to 8.2.5 utilize an amplifier with a 'loop drive' gain control after an AGC stage, specifically to demonstrate the amplifier's ability to generate the necessary magnetic field strength If this control is absent, it is essential to adhere to the manufacturer's instructions To ensure the entire system can produce the required magnetic field strength from the microphone(s) and other signal source(s), the procedure detailed in Clause 10 must be followed.
Method of measurement with a simulated speech signal
Apply the simulated speech signal specified in 6.3.3 to the amplifier and adjust its controls in accordance with the manufacturer's instructions so that the requirements specified in 8.4 are satisfied.
Method of measurement with pink noise
Apply the pink noise signal specified in 6.4 to the amplifier and adjust its controls in accordance with the manufacturer's instructions until the requirements specified in 8.4 are satisfied.
Method of measurement with a sinusoidal signal
Apply a 1 kHz sinusoidal signal to the amplifier and adjust its controls in accordance with the manufacturer's instructions until the requirements specified in 8.4.3 are satisfied
The manufacturer can determine a maximum test duration that allows for accurate measurements while preventing excessive temperature increases in the amplifier.
Method of measurement with a combi signal
To achieve the requirements outlined in section 8.4, apply the combi signal from section 6.6 to the amplifier and adjust its controls as per the manufacturer's guidelines For instruments with a 0.125 s averaging time, utilizing a peak hold feature is especially beneficial for this measurement.
Method of measurement – Other
Frequency response of the magnetic field
Characteristic to be specified
The frequency response of the magnetic field strength, measured with a pick-up coil whose magnetic axis is vertical (unless otherwise specified, see 8.1).
Method of measurement with a simulated speech signal
To measure the frequency response of the amplifier, first apply the specified simulated speech signal and adjust the controls according to the manufacturer's guidelines (refer to IEC 62489-1) Next, measure the one-third-octave band spectrum of the signal source, followed by measuring the one-third-octave band spectrum of the magnetic field at multiple points within the usable magnetic field volume Finally, subtract the spectrum results of the signal source from the magnetic field measurements to ensure the final results are independent of the source spectrum.
Using simulated speech or similar complex signals for measuring frequency response is challenging to execute accurately This method is more appropriate for research and development purposes rather than for commissioning tasks.
Method of measurement with pink noise
To measure the frequency response of the amplifier, first apply the specified pink noise signal and adjust the controls according to the manufacturer's guidelines Next, measure the one-third-octave spectrum of the signal source, followed by measuring the one-third-octave spectrum of the magnetic field at multiple points within the usable volume Finally, subtract the spectrum of the signal source from the magnetic field spectrum to ensure the results are independent of the source's spectrum.
Measurements should be conducted in one-third-octave bands centered at 100 Hz, 1 kHz, and 5 kHz, covering multiple points within the useful magnetic field volume It is advisable to analyze the frequency response variation within this volume, with a focus on the one-third-octave band centered at 5 kHz for initial testing This approach helps identify any spurious losses caused by conductive metal structures.
Method of measurement with a sinusoidal signal
To measure the frequency response of the amplifier, apply the specified sinusoidal signal from section 6.5 and adjust the controls according to the manufacturer's guidelines Ensure that the amplifier operates below the AGC threshold, as indicated by the manufacturer or as determined by IEC 62489-1.
NOTE 1 The manufacturer is free to specify the magnetic field strength at 1 kHz at which the measurement is made
NOTE 2 This method is not suitable for use with amplifiers that do not have a linear relationship between output current and input voltage at any input signal level See Clause C.4 b) Measure the magnetic field strength produced c) Measurements shall be made at least at 100 Hz, 1 kHz and 5 kHz, at a sufficient number of points within the useful magnetic field volume (see 8.4) Preferably, an analysis should be made of the variation of frequency response within the volume, and a frequency of
5 kHz is recommended as a primary test frequency This is to ensure that any spurious losses due to conductive metal structures are identified.
Method of measurement with combi signal
To measure the frequency response of the amplifier, apply the combi signal as outlined in section 6.6 and adjust the controls according to the manufacturer's guidelines Next, utilize the specified meter from section B.3.3 to measure the one-third-octave spectrum of the signal source and magnetic field, as detailed in section 8.3.3 b) to d), while disregarding the results from the sine wave burst portion of the combi signal.
Method of measurement – Other
Requirements
The frequency response shall be within the range ±3 dB with reference to the response at
1 kHz, from 100 Hz to 5 000 Hz.
Useful magnetic field volume
Characteristic to be specified
The volume within which the requirements recommended or specified in Clause 7, 8.2.7, 8.3.7 and 10.2.7 are met.
Methods of measurement
Measurements must be conducted at multiple points within the designated magnetic field volume, taking into account user locations, height variations, specific seating needs, the physical arrangement of the area, and potential interference from metal and other signals Typically, measurements should be taken at heights of 1.2 m for seated listeners and 1.7 m for standing listeners.
Requirements
Magnetic field strength measurements at designated points must remain within ± 3dB of the specified level as outlined in 8.2.7, excluding small volume systems which allow for a broader range Refer to Clause 9 and Annex A for additional details For other characteristics, adhere to the recommendations in Clause 7 and the requirements specified in 8.3.7 and 10.2.7.
Inapplicability of the 'useful magnetic field volume' concept
In certain systems, it is essential to define the locations of measurement points directly in the standard, eliminating the need for the 'useful magnetic field volume' concept However, for other systems where this is not feasible, the 'useful magnetic field volume' approach outlined in Clause 8 remains suitable.
Disabled refuge and similar call-points
Measurements should be taken at the six designated points shown in Figure 2, unless contractual requirements dictate otherwise The reference point is defined as the face or surface of the call-point, intercom, or help point that is nearest to the user, which may not correspond to the position of the magnetic field source.
The semi-circular layout is ideal for small magnetic field sources, while the rectangular layout is better suited for vertical or floor loop sources For any specific system, only one of these layouts should be utilized.
An offset between the reference point and the magnetic field source ensures a uniform field pattern in areas where individuals are likely to be present.
3 area where people are expected to stand l 1 offset l 2 inner radius 300 l 3 outer radius 200 a) Magnetic field source of small dimensions
1 magnetic field source (vertical loop)
3 area where people are expected to stand l 1 offset l 3 200 l 5 700 l 2 300 l 4 424 b) Larger magnetic field source
Figure 2 – Measurement points for disabled refuge and similar call-points
The six measurement points are required at both 1,2 m and 1,7 m See Figure 3 b), but there is no requirement to measure at a height of 1,45 m.
Requirements for disabled refuge and similar call-points
The system must adhere to the requirements outlined in sections 9.3 and 8.3.7 at all designated measurement points shown in Figure 2, covering both vertical and horizontal ranges The magnetic field strength at these points should be maintained at ±6 dB relative to 400 mA/m, as measured in accordance with section 8.2 Additionally, measurements must be taken at a minimum of one specified point.
The magnetic field strength level shall not be above +8 dB ref 400 mA/m in the area where people are expected to stand
To achieve effective performance with a simple vertical loop of practical dimensions, a high field strength is essential If the signal becomes excessively loud or distorted, users can easily resolve this by moving a short distance away from the magnetic field source.
Clauses 4 and 7 address the issue of magnetic background noise levels It is impractical to set a specific requirement, as doing so could eliminate the possibility of providing a system that would still be beneficial to users.
Counter systems
Measurements should be taken at the designated points shown in Figure 3, unless otherwise stated by contractual obligations The reference point for these measurements is the face or surface of the counter nearest to the user, which may not correspond to the location of the magnetic field source.
In counter systems, managing overspill to adjacent positions is crucial, as it significantly influences design considerations This control may lead to a compromise in the uniformity of the area where individuals are expected to stand.
To ensure effective sound management, it is not essential to minimize magnetic spill between counter positions to a level that matches the acoustic spill A difference of more than 20 dB between equivalent positions at the two counters may suffice.
NOTE 2 The boundaries of the area where people are expected to stand cannot be standardized as they depend on the building layout and other factors
For vertical loops, positioning the reference point offset from the loop enhances the uniformity of the field pattern in areas where people are likely to stand However, this adjustment may diminish the effectiveness of spill control towards the adjacent counter position.
3 area where people are expected to stand l 1 300 l 4 150 l 2 300 l 5 150 l 3 offset a) Plan view
Figure 3 – Measurement points for a counter system
Measurements at the three points shown in the plan view are required at 1,2 m, 1,45 m and 1,7 m.
Requirements for counter systems
The system must adhere to the requirements outlined in sections 9.3 and 8.3.7 at all designated measurement points shown in Figure 3, covering both vertical and horizontal ranges The magnetic field strength at these points should be maintained at ±6 dB relative to 400 mA/m, as per section 8.2, with at least one measurement point achieving a minimum of ≥0 dB relative to 400 mA/m.
The field strength shall not be above +8 dB ref 400 mA/m in the area where people are expected to stand
To achieve effective performance with a simple vertical loop of practical dimensions, a high field strength is essential If the signal becomes excessively loud or distorted, users can easily resolve this by moving a short distance away from the magnetic field source.
Clauses 4 and 7 address the issue of magnetic background noise levels It is impractical to set a specific requirement, as doing so could eliminate the possibility of providing a system that would still be beneficial to users.
10 Setting up (commissioning) the system
Procedure
The commissioning procedure must involve testing sound sources, such as a talker and a CD player, in their normal positions relative to the system microphones Measurements should ensure that the amplifier controls are adjusted to achieve the specified magnetic field strength If the amplifier features a gain control before the AGC stage and an AGC operation indicator, setting it according to the manufacturer's specifications is typically adequate Additionally, the reference speech signal defined in section 6.3.3.3 can be utilized for a more objective assessment, but adjustments to the 'loop drive' control (gain control after the AGC stage) of the amplifier should generally be unnecessary.
When setting up a system for the first time or after significant changes, it is beneficial to have a few hearing-aid users present to verify that subjective experiences align with objective measurements Additionally, it is crucial to assess the proper functioning of their hearing aids and confirm that users comprehend what they are intended to listen to.
It is essential that the trained persons(s) specified in Clause E.5 are present, with the receivers they will use for normal system checking
Some hearing aid users often set their volume controls excessively high, while older hearing aids may overload at lower field strengths When there are notable differences in opinions regarding a system's performance among users, it is essential to verify the settings of the hearing aids.
Magnetic noise level due to the system
Explanation of term
The magnetic field strength is measured using a vertical pick-up coil, accounting for both background fields and amplifier noise, with all signal inputs muted.
NOTE This value cannot be measured correctly until the commissioning procedure has been carried out.
Method of measurement with a speech signal
Method of measurement with pink noise
Method of measurement with a sinusoidal signal
Method of measurement with a combi signal
Method of measurement – Other (no input signal)
The magnetic field strength must be measured according to Clause 7, using A-weighting, at multiple points within the useful volume while the system is powered on and all signal inputs are muted.
NOTE If the signal is derived from sound system equipment, the muting is applied at the inputs of that equipment.
Requirements
If the reference signal-to-noise ratio exceeds 47 dB, the magnetic field strength must remain below –47 dB when the system is active Conversely, if the reference signal-to-noise ratio is below 47 dB, the magnetic field strength with the system on cannot exceed the level when the system is off by more than 1 dB.
Amplifier overload at 1,6 kHz
Explanation of term
Applying frequency response correction to the amplifier can help compensate for metal loss, enabling it to achieve the necessary maximum magnetic field strength at 1 kHz However, this adjustment may lead to overload issues at higher frequencies Therefore, testing with a signal at 1.6 kHz is deemed adequate.
Methods of test
To achieve a magnetic field strength that is 1 dB below the desired value, apply a 1 kHz sine wave signal and adjust its level accordingly Limit the application of this signal to the shortest time possible to avoid overheating the amplifier Subsequently, change the frequency to 1.6 kHz while maintaining the same signal level.
NOTE 1 The magnetic field strength is intentionally increased by the frequency-response compensation
To determine whether the amplifier is overloaded, apply one of the following tests:
• observe the 'clip indicator' on the amplifier, if one is provided;
• compare the measured output voltage with the manufacturer's specified value;
• examine the output current waveform with an oscilloscope
NOTE 2 The current can be checked by including a low-value resistor, such as 0,22 Ω, in series with the loop, but neither end of the resistor is likely to be at earth potential For many amplifiers, a measurement can be made between the 'cold' loop output terminal and the amplifier signal earth.
Requirements
The maximum value of the magnetic field strength obtained from the reference speech signal (see 6.3.3.3) shall normally be 400 mA/m, measured with a meter as specified in 6.1
The measured r.m.s levels of a reference signal and real sound sources are influenced by the characteristics of the AGC circuit and the signal source, leading to potential deviations from the target value With a sufficiently long measurement time to capture true maximum levels, the system typically maintains an accuracy of ±3 dB relative to 400 mA/m, ranging from 283 mA/m to 566 mA/m.
Changes to the field strength of 400 mA/m are prohibited if the system is accessible to the general public Adjustments can only be made for a closed group of hearing-aid users who report that the current signal level is inadequate, as the system designer cannot control the gain settings of individual hearing aids Certain conditions may lead a close community of users to find a field strength of 400 mA/m unsatisfactory.
To ensure compliance, if a field strength of 400 mA/m ± 3 dB is not attained with actual signals, the measurement must be redone using the signal outlined in section 6.3.3.3 Should the requirement remain unmet, it is essential to reassess the system specifications and the setup procedure detailed in section 4.1 to verify the correct specification of both the system and the amplifier.
The field strength varies by location, meaning it may match the value specified in section 8.2 in some areas, while being higher or lower in others Additionally, the subjective loudness experienced by users is influenced by the volume control settings of hearing aids, which are beyond the control of manufacturers or installers Consequently, it is not appropriate to enforce a specific field strength value when there is agreement that adjustments can be made.
Systems for small useful magnetic field volumes
Overview
There is often a requirement to supply an induction loop signal to a hearing aid user in specialised circumstances These can normally be divided into three major categories.
Body-worn audio systems
Body-worn systems typically utilize a neck loop, resembling a necklace, which connects to standard audio equipment or mobile devices The pick-up coil's position in the hearing aid is easily adjustable, allowing for a clearly defined listening space for performance measurement Expected performance standards are outlined in this document, referencing IEC 62489-1 for further details.
Small volume, defined seating, mainly in households
In a household environment, small volume systems can include neck loops, specialized cushions, or loops integrated into chairs, all typically powered by a dedicated amplifier It's important to consider the listener's head position, as it can vary significantly with height While performance should generally align with specified standards, there may be instances where field strength requirements are not met at the edges of the hearing aid's operational space.
Specific locations such as help and information points, ticket and bank counters, etc
When installing information points in fixed locations, defining the listening space can be challenging due to varying head heights among users—1 m for children, 1.2 m for wheelchair users, and 1.7 m for taller individuals Additionally, horizontal displacements from the optimal position can occur, and the presence of significant metal can hinder system performance It is crucial to consider background noise, signal strength, and frequency response, recognizing that a well-functioning system, despite its limitations, is often preferable to having no system at all.
Counter system loops come in various sizes and configurations, with three-dimensional loops presenting analytical challenges While vertical loops are straightforward to install, their conventional arrangement results in a less-than-ideal field pattern, characterized by a significant volume of low field strength along the horizontal axis, as illustrated for a typical 70 cm square loop in Figure A.1.
Figure A.1 – Field pattern of a vertical loop
The optimal range of field strengths for listening heights is between 12 cm and 62 cm above the center of the loop, considering the distance variations from the loops The loop conductor is positioned 35 cm above the center, which influences the upper lobe of the field pattern Therefore, the center of the loop should be strategically placed within this range to maximize effectiveness.
For optimal placement, the lower edge should be positioned 73 cm above the floor, while the upper edge should be at 143 cm Alternatively, setting the center of the loop at 182 cm above the floor effectively utilizes the lower lobe of the field pattern.
NOTE These loop positions are for a loop 70 cm square: for other dimensions the optimum loop positions can be determined from a field pattern plot similar to that in Figure A.1
Figure A.2 illustrates that the initial arrangement provides complete side-to-side coverage The measurements are in meters, and the inverted diagram confirms that this coverage also applies to the subsequent arrangement.
V er tic al di st anc e fr om c ent re m
−36 Relative field strength level dB (ref field strength at 0,0,0)
Figure A.2 – Contour plot of field strength of vertical loop
This is for a 70 cm by 70 cm loop, at a distance of 0,3 m The field strength level is relative to that at the centre of the loop and in its plane
The measurement heights of 1.2 m and 1.7 m pertain to users rather than systems, making them applicable to counter systems as well However, it is crucial to include an additional measurement at a height of 1.45 m, as a poorly designed installation may result in a null at this level At 1.45 m, the field strength level referenced at 400 mA/m may reach up to +12 dB, but should not exceed this value.
In a plan view, a more relaxed standard is suitable for a refuge system, as multiple individuals may gather around it to listen In contrast, a counter system should ideally allow only one person to hear at a time.
Overview
To achieve the primary goal of the standard, which is to ensure the proper design, installation, and setup of induction-loop systems, it is essential to simplify the technical requirements for measuring equipment If only expensive equipment is deemed acceptable, many installations may not have their performance accurately measured Additionally, any deviation from these recommendations must be justified.
Signal sources
Real speech
For optimal results, it is advisable to use CD recordings of speech that are free from data compression While other sources may be utilized, it's important to recognize that variations in speech signals can lead to significant differences in measurement outcomes.
When utilizing recorded speech, it is essential to use appropriate equipment for playback This equipment must have an adjustable output voltage, capable of operating within the ranges of 0 mV to 10 mV and 0 V to ensure optimal sound quality.
1 V The output source impedance should be 1 000 Ω or less
If local speech sources are used, several different speech samples should be tested to ensure that variability between speakers does not invalidate the measurements.
Simulated speech
The recommended sources are as follows
• The recording on CD supplied as a supplement to the ITU P.50 standard [2] The male speech should be used
• The reference speech signal (ISTS) See 6.3.3.3
To ensure optimal playback of the recording, it is essential to use appropriate equipment that can adjust the output voltage within the ranges of 0 mV to 10 mV and 0 V to 1 V, with a source impedance of 1,000 Ω or lower.
Pink noise
The source must generate pink noise with a peak-to-peak voltage to true r.m.s voltage ratio of at least 8, as measured by an oscilloscope It should maintain a one-third-octave band spectrum that is flat within ±1 dB from 100 Hz to 5 kHz, while being band-limited to –3 dB or lower at 75 Hz and 6.5 kHz Additionally, the output voltage should be adjustable within the range of 0 mV to 10 mV.
0 V to 1 V The output source impedance should be 1 000 Ω or less
For effective signal processing, band-limiting filters must be at least third-order and utilize one operational amplifier section This band limitation ensures that the signal interacts with Automatic Gain Control (AGC) systems in a way that resembles speech dynamics.
Sine wave
Magnetic field strength level meter
General recommendations
The coil must have a cross-sectional area of less than 100 mm² and an axial length greater than its mean diameter Additionally, its placement within the instrument and the orientation of its maximum sensitivity should be clearly indicated.
The ideal measurement range for increased resolution should span from –62 dB to +8 dB relative to 400 mA/m, although a range of –52 dB to +8 dB is often sufficient for many applications and can be achieved with low-cost display driver devices The recommended ranges and indicator markings for both types of meters pertain to the r.m.s values of a sinusoidal signal A resolution of ±1 dB or better is required within the levels of –3 dB to +6 dB relative to 400 mA/m, and meters must be calibrated to read 0 dB in a sinusoidal magnetic field at 1 kHz with a strength of 400 mA/m r.m.s.
Indications can be provided through various displays, including moving-coil meters, LED dot or bar displays, and LED or LCD digital displays Some devices may feature a 'peak hold' function, allowing the measured peak hold values to exceed the 60-second average values by approximately.
2 dB A preset control for setting the sensitivity may be provided (see Annexes E and F)
Output connectors must be included for connecting headphones and measuring equipment like spectrum analyzers Headphone outputs should meet IEC 61938 standards, while external measuring devices typically require an output of around 1 V r.m.s at maximum level The source impedance should be 1,000 Ω or lower, and connecting a load that meets the meter's specifications should not alter the measured result by more than 0.2 dB.
Peak-programme meter (PPM) type
The peak-programme meter is a specialized instrument that features a full-wave peak rectifier, providing dynamic characteristics akin to those of the Type II meter outlined in IEC 60268-10.
Simplified specifications for the meter dynamics, derived from IEC 60268-10, are:
• a 10 ms tone burst at 5 kHz should produce a reading of −2 dB ±1 dB below that produced by a continuous 5 kHz sinusoidal signal;
The duration from the removal of a 1 kHz sinusoidal signal, which initially shows an indication of 0 dB, to the point where the indication drops to −20 dB should be 2.3 seconds ± 0.5 seconds If a moving-coil meter is not used, this can be assessed by monitoring the time-dependent variation of a relevant internal voltage within the equipment, utilizing a storage oscilloscope.
True r.m.s meter type
A true r.m.s meter can be a specialized instrument or a sound level meter modified with a magnetic pickup coil and an equalizer to achieve a flat frequency response in unweighted measurements It is essential for the meter to include a true r.m.s rectifier and comply with the Class 2 sound level meter standards outlined in IEC 61672-1, with specific exceptions noted in B.3.1.2.
Field strength level meter calibrator
A calibrator must generate a magnetic field strength of 400 mA/m r.m.s at 1 kHz, covering the entire volume of the pickup coil of the intended meter It should also support additional frequencies of 100 Hz and 5 kHz for response verification Refer to Annex F for more details.
Spectrum analyzer
A spectrum analyzer should provide one-third-octave band analysis over at least the frequency range 100 Hz to 5 kHz The filter characteristics should conform to those specified in IEC 61260 [6]
Where the spectrum analyser is part of the field strength meter, filters with centre frequencies
100 Hz, 1 kHz and 5 kHz only need to be provided Additional filters enable a better analysis of system performance when assessing losses due to metal
General
The requirements outlined aim to provide essential information to end users, installation personnel, and equipment manufacturers, ensuring that induction-loop systems function in accordance with the established standards.
The installer should provide at least the following information.
Information to be provided to the hearing aid user
A clearly visible sign must be positioned near the entrance(s) of an area equipped with an induction loop, ensuring it is large enough for easy readability and made from durable materials Additionally, the same symbol used for the induction loop sign should be applied to indicate induction coupling on telephone handsets.
A plan indicating the useful magnetic field volume should be placed beside the above- mentioned sign or incorporated in it
The name, or position, of the person responsible for the proper operation of the loop system and how to contact them should also be given
For small area induction-loop systems e.g window counters, a sign should be placed in a prominent position where the hearing aid user is expected to be normally situated
Clear instructions on how to use the induction-loop system should be available to hearing aid users on request
Figure C.1 – Graphical symbol: inductive coupling
Information to be provided to system installers and by them to users
The following information should be provided:
• the plan specified in Clause C.2;
• the specifications for the amplifier and associated equipment as given in Clause C.4;
• the field strength set up as described in 10.4 (including the notes);
• the setting of the control positions to provide the required field strength in the specified magnetic field volume;
• the method by which the magnetic field strength can be monitored to ensure consistent day-to-day operation of the system;
To achieve optimal sound quality, it is essential to position microphones correctly, meet the signal requirements for external playback devices, and adjust the necessary controls This ensures that the specified magnetic field is generated during normal operation.
• the effect of other electrical equipment used in the area where the loop system is installed.
Information to be provided by the manufacturer of the amplifying equipment
The previous version of this standard used a 'long-term average level' of speech signals as a reference value, but this term lacks a formal definition Measurements of speech signals with a true r.m.s meter across various averaging times illustrate this ambiguity.
Averaging time s Relative level at maximum reading dB
A meter with a long averaging time is impractical for adjusting the system gain control to achieve a long-term average of 100 mA/m at the reference point Even with a 15-second averaging time, this process remains challenging and prone to inaccuracies for several reasons.
After making any adjustments, it is essential to allow a stabilization period of at least 45 seconds, equivalent to three time-constants, for the voltages in the time-constant circuit to reach the new input level This stabilization effect is observable on a meter when measuring a pink noise signal.
Despite a 15-second averaging time, the meter readings remain inconsistent, necessitating a decision between estimating an 'average' reading or recording the maximum reading over an unspecified time period.
• The measurement does not determine whether the induction-loop system can produce the higher field strengths necessary for the reproduction of the speech signals without unacceptable amplitude distortion
Using a 0,125 s time-constant, the reading is, of course, variable from moment to moment, so that the user also has to use some subjective averaging in this case
The peak programme meter is engineered to provide accurate maximum level readings while minimizing operator fatigue Experiments indicate that a Type 2 peak programme field strength meter, as defined in IEC 60268-10 and calibrated in r.m.s values for sine-wave signals, registers 560 mA/m when the short-term r.m.s field strength of a typical speech signal is 400 mA/m.
Induction-loop systems, which utilize minimal or no compression unlike automatic gain control, are proven to provide excellent signal levels in practice The meter quickly reacts to any adjustments made to the gain control settings.
If the amplifier is overloaded on programme peaks, this is shown by the failure to achieve
Basic theory and practice of audio-frequency induction-loop systems
Properties of the loop and its magnetic field
An induction loop consists of a conductor that carries an audio-frequency current, encircling the area designated for reception This current generates a magnetic field, measured in amperes per meter, with its strength varying significantly at different locations both inside and outside the loop.
Detectable magnetic fields are generated by such loops outside the intended volume, and size limitations can greatly influence their design There are methods available to minimize signal leakage beyond the desired area and to effectively cover extensive regions.
Figure E.1 illustrates a loop with a diagram of magnetic vectors, which follow circular lines, resulting in both vertical and horizontal components The magnetic field strength varies significantly in space, as depicted in Figure E.2 Along line Z1, located in the loop's plane, the field strength is extremely high near the wire Moving away from the loop plane, as indicated by line Z2, helps achieve a more uniform field distribution The 'system null line' indicates that points where the vertical component of the magnetic field is zero are found at increasing distances from the loop perimeter as the height of the observation point above the loop plane rises.
In audio-frequency induction loop systems, listeners typically stand or sit, aligning the telecoils in hearing aids vertically to respond to the vertical component of the magnetic field However, in environments like hospitals and places of worship, some telecoils may be positioned horizontally or at various angles, making the relevant magnetic field component crucial The magnetic field strength \( H \) (in A/m) at the center of a single-turn square loop with side length \( d \) (in m) is influenced by the current \( I \).
The magnetic field strength \( H \) in amperes per meter (A/m) is expressed by the formula \( H = \frac{2\sqrt{2} I}{\pi d} \), where \( H \) is vertical and perpendicular to the plane of the loop This formula is valid when the dimensions \( d_1 \) and \( d_2 \) of a rectangular loop are relatively similar, allowing \( d \) to be approximated as \( \sqrt{d_1 d_2} \).
For information desks, the loop is typically arranged in a vertical plane, utilizing the effective vertical component at the top wire level to power hearing aids It is crucial to maintain the listening position above the loop's axis, as lowering it can lead to a lack of usable signal.
The optimal vertical field strength within the projected area of a rectangular loop is achieved at a distance of 0.12 to 0.16 times the loop's width, provided the length-to-width ratio is not excessively large Figure E.2 illustrates the vertical component distribution across a loop based on the position and elevation of the listening point, calculated for a loop 1.5 times longer than its width, with minimal variation observed as the aspect ratio increases to 4 Additionally, the figure indicates that increasing the distance from the loop plane or using a smaller loop can reduce field strength It is important to note that the least effective position for a loop is at head height, particularly when considering routing a loop conductor from floor level over a doorway The curves in the figure are marked with distances from the loop as a percentage of its width, while the horizontal axis represents position as a percentage of the loop width, with the vertical scale showing field strength variations in decibels.
Directional response of the telecoil of a hearing aid
The directional response of a telecoil in hearing aids is influenced by metal components, yet it adheres to a cosine law This indicates a minimal reduction in response, with only a 3 dB decrease at a 45° angle to the magnetic axis and a 9.3 dB decrease at 70° off axis.
Figure E.1 – Perspective view of a loop, showing the magnetic field vector paths
I a) Geometry of the loop and positions of measurement of the field patterns shown in b) b) Patterns of the vertical and horizontal components
Figure E.2 – Strengths of the components of the magnetic field due to current in a horizontal rectangular loop at points in a plane above or below the loop plane
Figure E.3 illustrates the spatial variation of the vertical field, indicating that at significant distances from the loop plane, the null points are located well beyond the loop's perimeter Additionally, the field strength outside the loop can be sufficiently high to be either usable or potentially disruptive to nearby loop systems, or both.
Percentage of loop dimension Fi el d s trengt h lev el dB rel at iv e t o t he fiel d s trengt h at the cent re o f t he l oop and in i ts pl ane
Horizontal field strength measured along a line parallel to this and 1,2 units above or below the loop plane
Vertical field strength measured along a line parallel to this and 1,2 units above or below the loop plane
The 0 dB reference for field strength is the field strength at the centre of the loop and in its plane
Loop layout 15 units by 10 units
Figure E.3 – Field patterns of the vertical component of the magnetic field of a horizontal loop
Figure E.4 illustrates the field patterns generated by a vertical coil, commonly utilized in information point systems These coils are typically small, resulting in a height that is significantly greater compared to horizontal loops in a room To achieve adequate field strength at the height of an average standing person, the field strength must be exceptionally high for children or individuals in wheelchairs This requirement can lead to increased magnetic noise levels from the system, as discussed in section 9.3 Additionally, the elevated field strength may interfere with the proper functioning of some hearing aids, although it does not cause permanent damage.
The field strength near the horizontal axis and the plane of the loop significantly changes with height, making it essential for users to maintain a safe distance from the loop.
1 Distance from the centre divided by the loop width
R el at iv e v er tic al fi el d st rengt h dB loop plane height 0,1 height 0,14 height 0,2 height 0,5 height 1 +3 dB
The term 'Distance' refers to the horizontal distance between the pick-up coil (telecoil) and the plane of the loop, with the field strength level being measured in relation to the center of the loop.
Figure E.4 – Field patterns of the vertical component of the magnetic field of a vertical loop 0,75 m square
Figure E.5 illustrates the field pattern of a typical loop system from a perspective view at the optimal height above the loop It clearly demonstrates the increase in field strength as one approaches the loop conductor from its center, along with a distinct null at points just outside the loop perimeter, where the field vectors align horizontally.
−6 Relative vertical field strength dB
V er tic al di st anc e fr om th e ax is of the lo op m
Distance 15 cm Distance 30 cm Distance 40 cm Distance 50 cm Distance 60 cm Distance 70 cm
Figure E.5 – Perspective view of the variation of the vertical field strength level at an optimum height above a horizontal rectangular loop
M ean f iel d- st ren gt h of s qu ar e
8,0 dB 6,5 dB 5,0 dB 3,5 dB 2,0 dB 0,5 dB –1,0 dB –2,5 dB –4,0 dB –5,5 dB –7,0 dB –8,5 dB –10,0 dB –11,5 dB –13,0 dB a) Directional response, linear amplitude scale b) Directional response, decibel amplitude scale
Figure E.6 – Directional response of the magnetic pick-up coil (telecoil) of a hearing aid
Supplying the loop current
The loop has resistance and inductance, both of which can normally be calculated with sufficient accuracy for design purposes Both the resistance and the inductance are
The resistance of a loop is directly proportional to the number of turns, while the inductance is roughly proportional to the square of the number of turns Additionally, a factor of 0.25 is proportional to the perimeter of the loop rather than its area.
The resistance R of a single turn square loop of side d (in m), with conductor area a (in m 2 ) and resistivity ρ (in Ωãm) is given by R = 4ρd/a (in Ω)
The inductance \( L \) of a single turn can be approximated by the formula \( L = 8d \) (in µH) for loops with an area greater than one square meter, using a conductor of standard thickness Additionally, copper foil may offer lower inductance compared to round copper wire, and the presence of magnetizable materials near the loop can affect the inductance.
Inductance increases the loop's impedance at high audio frequencies, reaching a level 1.4 times the resistance at the frequency where the inductive reactance \$2\pi fL\$ equals the resistance \$R\$ For single-turn square loops with sides up to 5 m, a conductor with high resistance can still support the required loop current for speech, music, or pink noise, even if the impedance rise becomes significant above 5 kHz However, for larger loops, maintaining a flat frequency response up to 5 kHz necessitates compensating for the increased loop impedance.
To achieve optimal performance, it is common to use a current-drive amplifier with a high output source resistance, as outlined in IEC 60268-3 This type of amplifier effectively mitigates the impact of loop inductance and maintains a consistent loop current, even when the loop impedance fluctuates with frequency.
The output source resistance of an amplifier, as outlined in IEC 60268-3, should not be excessively large; typically, a resistance of only a few ohms is sufficient for most loops An output source resistance that is ten times the loop resistance is generally adequate However, excessively high output source resistance can lead to stability and electromagnetic compatibility (EMC) issues.
An amplifier must generate sufficient output voltage to drive the necessary current through the loop impedance At low frequencies, this relationship is defined by the equation \$U = IR\$.
At higher frequencies, a voltage \$U_h = I\sqrt{R^2 + (2\pi f L)^2}\$ is necessary, but since speech energy diminishes at these frequencies, values of \$f\$ between 1.5 kHz and 2.5 kHz are typically sufficient For systems designed to transmit music signals, a frequency close to 2.5 kHz is generally ideal It's important to note that while the maximum achievable field strength may be relaxed, the requirements for frequency response remain stringent.
Signal sources and cables
Microphones
Choosing the right microphone types and positions is crucial for minimizing reverberation in the signal sent to the loop Directional microphones, particularly cardioid and boundary-layer types, are often the best options The goal is to capture desired sounds while reducing room reverberation and ambient noise, which may necessitate the use of highly directional microphones While expensive microphones are typically unnecessary, it's advisable to avoid electret types that require batteries due to maintenance concerns Dynamic microphones are generally not recommended due to their low sensitivity and potential for magnetic feedback; however, with careful design and proper placement away from the loop cable, they can be effectively utilized.
Other signal sources
Musical instruments equipped with magnetic pickups can sometimes function as induction-loop transducers, potentially causing harmful electronic feedback To mitigate this issue, it may be necessary to experiment with the positioning of these instruments in relation to the loop system.
Cables
Precautions are necessary to prevent malfunctions due to current induced in cables by the magnetic fields See [12].
Care of the system
Regular checks by a trained professional are essential to ensure the system operates correctly, both at set intervals and prior to use This verification can be performed using a portable receiver that displays field strength indicators, such as LEDs, at minimum levels of –6 dB and 0 dB Additionally, the system should include a headphone output equipped with a gain control feature.
To ensure a comfortable listening experience, the headphone amplifier's maximum gain should be adjusted so that the 0 dB indicator is illuminated Setting the gain too high can lead to an inaccurate perception of background magnetic noise and an exaggerated evaluation of the system's magnetic field strength, in addition to potentially dangerous sound pressure levels.
Maintenance should be necessary only at infrequent intervals, but the system components should be inspected regularly so that any damage can be repaired as soon as possible.
Magnetic units
A current in a closed circuit generates a magnetic field around it, with the field strength directly proportional to the current For circular or rectangular loops with a consistent length-to-width ratio, the field strength is inversely related to the perimeter of the circuit, measured in amperes per meter In the case of multi-turn loops, the field strength is further amplified by the number of turns.
In electrostatics, a voltage applied between two conducting plates creates an electric field in the surrounding area The strength of this electric field is directly proportional to the voltage and inversely proportional to the distance between the plates, measured in volts per meter.
This standard outlines the requirements related to magnetic field strength, while also acknowledging the use of other magnetic units It is essential to describe the relationships between these units Additionally, some quantities expressed in these units have undergone official name changes in the past, yet the old names continue to be commonly used.
• Magnetic field strength (formerly 'magnetomotive force') was expressed in oersted in the CGS magnetic system For practical purposes, 1 Oe = 79,58 A/m
• Magnetic induction (formerly 'flux density'); this is now expressed in tesla (T) It is related to the field strength by the equation, B = à 0 à r H, where à 0 is the permeability of free space
The magnetic permeability of free space is given as \(4\pi \times 10^{-7} \, \text{H/m}\), with the relative permeability (\(à_r\)) in air being 1 for induction-loop systems This results in a magnetic induction of 1,256 \(àT\) for a field strength of 1 A/m In the CGS magnetic system, magnetic induction is measured in gauss (Gs), where 1 Gs equals 100 \(àT\) Notably, in this system, \(à_0 = 1\), meaning that an induction of 1 Gs in air corresponds to a field strength of 79.58 A/m.
Effects of metal in the building structure on the magnetic field
The magnetic field generated by the loop induces currents in the building's metal work, which alters the spatial pattern of the field strength in a frequency-dependent way Theoretical analysis of this phenomenon is highly complex, except in certain idealized scenarios.
In a closed metal loop within a building, the current flow diminishes the magnetic field strength inside its perimeter, influenced by a larger enclosing loop This reduction, driven by mutual inductance between the loops, becomes more pronounced at higher frequencies The effect is particularly significant when the metalwork is situated in floors or ceilings near the loop conductor.
The presence of metal in walls can significantly influence magnetic field strength, making its effects challenging to anticipate Specifically, metal located within the loop's perimeter may lead to an increase in the magnetic field strength beyond that perimeter.
High-frequency loss increases with distance from the loop conductor The effect of metal loss can thus be counteracted by the use of arrays of small loops
Figure F1 shows the field pattern of a typical loop system without nearby metal, while Figure F.2 shows the effect of metal in the floor below the loop
Figure F.1 – Magnetic field pattern of a 10 m by 14 m loop, 1,2 m above its plane
NOTE There are areas of decreased field strength inside the loop and areas of increased field strength outside it
Figure F.2 – Magnetic field pattern of a 10 m by 14 m loop, 1,2 m above its plane, showing the effect of metal (iron) in the floor
Calibration of field-strength meters
Regular calibration checks are essential for sound level meters to ensure accurate microphone sensitivity, especially due to varying ambient conditions While magnetic field strength meters do not require frequent calibration, having a calibrator is still beneficial Acceptable types of calibration coils include various options that meet industry standards.
• 1 m or 0,5 m diameter calculable loop – big and unwieldy;
• 30 cm diameter calculable single-turn loop;
• 30 cm diameter multi-turn loop (needs calibration check but can be driven from an audio signal generator)
It is also practicable to use square coils of similar dimensions:
• Helmholtz coil (IEC 60268-1) [13], see Figure G.1
These calibrators can be used to check both sensitivity and frequency response
Figure G.1 – Triple Helmholtz coil for calibration of meters
100 n 1 , n 2 , n 3 Numbers of turns on the coils 1, 2 and 3 d Diameter of the spherical volume within which the field strength is uniform
The relationship between the magnetic field strength in the central spherical volume and the coil current is influenced by the structure's dimensions For specific dimensions, this relationship can be calculated using the formulas provided in [11] It is recommended to verify the results through measurements with an appropriate magnetic field strength meter.
Effect of the aspect ratio of the loop on the magnetic field strength