Microsoft Word C024012e doc Reference number ISO 9614 3 2002(E) © ISO 2002 INTERNATIONAL STANDARD ISO 9614 3 First edition 2002 11 01 Acoustics — Determination of sound power levels of noise sources u[.]
Size of sound source under test
The size of the sound source under test can be of any dimension, provided that it meets all criteria outlined in annex C The measurement surface used to evaluate the source directly determines its extent, ensuring accurate and consistent results in compliance with testing standards.
Character of sound radiated by the source
Ensuring the signal remains stationary over time is essential, as defined in section 3.12 Measurements should be conducted during periods when non-stationary extraneous noise sources are inactive, avoiding times when predictable noise events occur (see Table C.1). -**Sponsor**Need help making your article shine and SEO-friendly? As a content creator, rewriting articles to comply with SEO rules can be tough That's where [Article Generation](https://pollinations.ai/redirect-nexad/VriVvcsk) comes in! Imagine getting SEO-optimized articles instantly, saving you time and money compared to hiring a writer Get 2,000-word articles and it's like having your own content team without the hassle!
Measurement uncertainty
The sound power level value obtained from a single application of ISO 9614 procedures may differ from the true value, but confidence intervals can be established assuming that repeated measurements are normally distributed around the true value Repeated tests at the same test site under consistent conditions help assess the repeatability of the measurement, while tests across different sites and equipment help evaluate the reproducibility, which is influenced by variations in experimental techniques and environmental conditions Standard deviations reflect measurement variability but do not account for changes in the source's operating or mounting conditions, such as rotational speed or line voltage.
The estimated upper values of the standard deviations for the reproducibility of sound power levels, as per ISO 9614, are summarized in Table 1, accounting for random measurement deviations and instrument tolerances per IEC 61043 These figures do not include variations caused by source installation, mounting, or operating conditions In the absence of detailed uncertainty sources, the expanded measurement uncertainty at a 95% coverage probability, as per GUM guidelines, should be reported as twice the standard deviation of reproducibility from Table 1.
The uncertainty in determining the sound power level of a sound source is influenced by the characteristics of the sound field generated by the source, the external sound environment, the absorption properties of the tested source, and the measurement and sampling methods used for sound intensity To address these factors, ISO 9614 provides initial procedures to evaluate key indicators that describe the nature of the sound field in the measurement region, ensuring accurate and reliable sound power assessments.
8 © ISO 2002 – All rights reserved proposed measurement surface (see annex B) The results of this initial test are used to select an appropriate course of action according to Table C.1
Below 50 Hz, there is insufficient data to establish uncertainty values, with the normal A-weighted data range covering one-third-octave bands from 50 Hz to 6.3 kHz, which provides accurate results if no significantly high levels are present in the 31-40 Hz and 8-10 kHz bands Significant levels are defined as band levels that are no more than 6 dB below the A-weighted value after A-weighting; if measurements are taken over a more restricted frequency range, it should be explicitly stated When only an A-weighted determination is needed, any band level 10 dB or more below the highest A-weighted level can be disregarded, and multiple insignificant bands can also be neglected if their combined A-weighted sound power level is 10 dB or more below the highest For assessments focusing solely on the overall A-weighted sound power level, the uncertainty in bands that are significantly lower (by 10 dB or more) than the total level is considered irrelevant.
Table 1 — Estimated upper values of the standard deviations of reproducibility of sound power levels determined in accordance with this part of ISO 9614
One-third-octave band centre frequencies
Upper values of standard deviation of reproducibility dB
The A-weighted sound level is calculated using one-third-octave bands spanning from 50 Hz to 6.3 kHz This method applies to sound sources that emit relatively flat spectra within this frequency range, providing an accurate measure of perceived loudness for various environmental and industrial noise assessments.
When multiple operatives employ similar facilities and instrumentation, the variability in sound power measurements for a specific source at a designated site is typically reduced, resulting in smaller standard deviations than those outlined in Table 1.
For a specific family of sound sources of similar size and sound power spectra operating under comparable environmental conditions and measured according to a designated test code, the standard deviations of reproducibility are typically lower than those listed in Table 1 ISO 7574-4 outlines statistical methods for the effective characterization of machine batches, ensuring accurate and consistent sound measurements for quality control and noise assessment.
This section of ISO 9614 outlines procedures for measuring sound power levels on a specific source, with standard deviations outlined in Table 1 When characterizing a batch of similar sources, random sampling techniques are used, and results are expressed as statistical upper limits with confidence intervals Accurate assessment requires knowledge or estimation of the total standard deviation, which includes the production variation (standard deviation of individual machines), as defined in ISO 7574-1 These methods ensure reliable and consistent sound power level characterization across products.
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Criteria for adequacy of the test environment
The test environment must be designed to validate the accuracy of sound intensity measurements according to the principles outlined in IEC 61043 Additionally, it should meet the specific requirements specified in sections 5.2 to 5.5 to ensure reliable and compliant testing conditions.
Extraneous intensity
The level of extraneous intensity shall be minimized so that it does not unacceptably reduce measurement accuracy (see C.1.4)
When testing sources that contain substantial quantities of absorbing material, high levels of extraneous intensity can cause an underestimation of the sound power To address this issue, Annex E provides guidance on evaluating the potential errors, especially in cases where the source under test can be switched off, helping ensure accurate sound power measurements despite absorption effects.
To ensure accurate measurement results, it is essential to minimize the variability of extraneous intensity during testing through pre-test measures such as disabling non-essential sources of extraneous noise and notifying plant operators of the issue Additionally, selecting appropriate measurement periods helps maintain consistency and reliability in the data collected.
Wind and gas flows
A probe windscreen should be used when fluid flow occurs on the measurement surface to ensure accurate readings Avoid taking measurements if wind or gas flow nearby exceeds the manufacturer's specified limits, which could compromise the measurement system's performance Specifically, if wind is present, its speed relative to the probe must not exceed 1 m/s to maintain measurement accuracy.
Annex D describes the adverse effects of flow and turbulence on sound intensity measurement.
Temperature
The probe shall not be placed closer than 20 mm to bodies having a temperature different from that of the ambient air
Temperature gradients along the probe axis can cause time-dependent, differential modifications to microphone responses, leading to bias errors in intensity measurements Proper management of probe temperature uniformity is essential to ensure accurate and reliable acoustic data These bias errors can significantly impact the precision of intensity estimates, highlighting the importance of monitoring and controlling temperature conditions during measurements.
Configuration of the surroundings
During audio testing, the environment must remain consistent throughout the test, except for the probe holder’s position, which is crucial when testing tonal sound sources Any unavoidable changes to the test surroundings must be documented To ensure accurate measurements, operators should avoid standing on or near the probe’s axis during testing, and extraneous objects should be removed from the vicinity of the source to minimize interference.
Atmospheric conditions
Air pressure and temperature directly influence air density and the speed of sound, which are critical factors in instrument calibration It is essential to assess how these environmental conditions affect measurement accuracy, and appropriate corrections should be applied to indicated intensities according to standards like IEC 61043 Ensuring proper calibration considering these variables helps achieve precise and reliable instrumentation performance.
General
A class 1 sound intensity measurement instrument and probe that meet the requirements of the IEC 61043:1993 shall be used Adjust the intensity measurement instrument to allow for ambient air pressure and temperature in accordance with IEC 61043 Record the pressure-residual intensity index of the instrument used for measurements, as defined by IEC 61043, for each frequency band of measurement
The instrument shall have the capability of capturing time-series of intensity and squared pressure and time-averaged intensity and squared pressure (see 3.14, 6.3 and Figure 3).
Calibration and field check
To ensure compliance, verify the instrument and probe according to IEC 61043 at least annually in a calibration laboratory following relevant standards, or every two years if an intensity calibrator is used prior to each sound power measurement Document and report the calibration results in accordance with section 10 d) for quality assurance and regulatory compliance.
Before each series of measurements, verify the proper operation of the instrumentation by performing the manufacturer's recommended field-check procedure If no specific field check is provided, follow the procedures outlined in sections 6.2.2 and 6.2.3 to identify any anomalies in the measuring system that may have arisen during transportation or storage.
Determine the pressure sensitivity of each microphone of the intensity probe using a class 0 or 1 or 0L or 1L calibrator in accordance with IEC 60942:1998
To accurately measure sound intensity, place the intensity probe on the measurement surface with its axis perpendicular to the surface, positioned where the overall linear intensity exceeds the surface average Measure the normal sound intensity level across all relevant frequency bands, then rotate the probe 180° around a normal axis and re-measure, ensuring the probe remains fixed in the same position using a stand For valid measurements, the two intensity values in each one-third-octave band should have opposite signs, and their difference must be less than 1.0 dB, confirming measurement consistency and equipment accuracy.
Time-series of sound intensity and sound pressure
The instrument must continuously record time-series data of sound intensity and squared sound pressure for at least the specified scanning time T_s and overall duration to accurately determine the temporal variability indicator F_T, as per section 8.3.2 Measurement intervals should be equal to or less than 0.5 seconds to ensure precise data capture When using an FFT analyzer, the device must utilize a Hanning window and ensure at least 30% overlap between data segments, consistent with the guidelines outlined in Figure 3 and Annex G.
7 Installation and operation of the source
General
Mount the source according to the specified noise test code for the specific machinery or equipment, or, if no code exists, position it in a manner representative of normal use Identify and account for potential sources of variability in the source, extraneous sources, or testing environment to ensure accurate and reliable noise measurement results.
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Operating conditions of the source under test
When conducting noise tests, it is essential to use the operating conditions specified in the relevant noise test code If no such code exists, select appropriate test conditions from options such as testing the device under specified load and operating conditions, full load (if different), no load (idling), maximum sound level representative of normal use, simulated load under controlled conditions, or operating under a characteristic work cycle.
8 Measurement of normal sound intensity component levels
Determination of measurement surface
For accurate sound power measurements, the measurement surface should be defined around the source under test, ideally as a parallelepiped with rectangular partial surfaces when performed manually The minimum distance between the measurement surface and the source should be at least 0.25 meters unless the surface is on a component proven to emit negligible sound radiation Non-absorbent surfaces like concrete floors or masonry walls can be included in the measurement surface for convenience, but intensity measurements must not be taken on these surfaces, and their areas should be excluded from the source sound power evaluation as specified in equation (5).
Determination of scanning paths and segments
A scanning path is fundamentally composed of a single straight line that ensures uniform coverage of each partial surface at a consistent speed This method can be executed manually or using a mechanized traversing system to enhance precision It is important to measure the extraneous intensity generated by the scanning mechanism with a probe to maintain accurate and reliable results.
To ensure accurate sound measurements, the emitted noise should be demonstrably at least 20 dB lower than the source under test on the measurement surface During testing, the intensity probe must be moved continuously along specified paths on each partial surface, maintaining its axis perpendicular to the measurement surface at all times The probe’s movement should be at a uniform speed to guarantee consistent results Special attention is required when measuring areas such as cracks or openings, as these areas can significantly influence the radiated sound; therefore, careful selection of partial measurement surfaces is essential for comprehensive acoustic analysis.
NOTE In this case, the number of segments N s is 20
Figure 2 — Examples of two orthogonal scanning paths on a rectangular partial surface
According to ISO 9614, a specific method employs one of two orthogonal scanning paths across each partial surface, which must be pre-divided into segments These segments should adhere to a ratio of \(\Delta x / \Delta y\) between 0.83 and 1.2, ensuring proportionality for accurate measurements The larger side of each segment must be less than half the minimum distance between the partial surface and the source under test, optimizing measurement precision Additionally, the overall measurement area maintains a ratio of no greater than 1.5 between the largest and smallest segments to ensure uniform coverage The scanning path begins at point 1 and concludes at point 2, with the order reversible, and curved lines at corners facilitate maintaining a consistent scanning speed, thereby enhancing the accuracy of acoustic measurements.
Measurements
The procedure for achieving the desired accuracy is detailed in Annex C and summarized in Figure C.1, ensuring a clear understanding of the process During measurement, one of two orthogonal paths on each partial surface is used, following ISO 9614 standards Each partial surface is scanned twice along the chosen path, and the averaged normal intensity levels over the scan time are compared If the difference between these levels is within a specified tolerance (criterion 1), they are temporarily accepted as the average intensity for that surface Once measurements for all partial surfaces are completed, criteria 2 to 5 are evaluated; if satisfied, the intensity levels are used to accurately determine the sound power of the source, ensuring precise results.
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When determining the A-weighted sound power level, criteria 1 to 5 do not need to be met if the combined A-weighted sound power levels across the relevant bands are at least 10 dB lower than the highest A-weighted band level, ensuring an efficient assessment process in noise evaluations.
8.3.2 Test of the temporal variability of the sound field and the determination of the scanning time
To assess whether the sound field is stationary, select a measurement position with high intensity on the surface and set the instrumentation to "instantaneous mode" to record sound intensity Iₙq over more than 100 seconds Calculate the time-averaged sound intensities Iₙm for multiple periods (T ≥ 1.0 s, with typically M=10) and determine the temporal variability indicator using equation (B.1) with incremental averaging times (ΔT ≤ 0.5 s) Identify the averaging time T_F where the variability indicator F_T is less than 0.6 for each one-third-octave band, utilizing both the direct method and FFT method for comprehensive analysis.
Figure 3 — Calculation of time-averaged sound intensity I n m and squared sound pressure p m 2 , m = 1, 2, 3, M with the averaging period T from the measurement of time-series of sound intensity I nQ and squared sound pressure p Q 2
The scanning time T s for each scan shall be equal to or greater than the maximum value among those of N s T F
For individual one-third-octave bands, ensure that T < 0.6, where N_s represents the number of segments on a partial surface If the calculated T_s is not feasible as the minimum scanning time, follow the alternative actions outlined in Table C.1 to determine the appropriate scanning duration.
Manual scanning speed must not exceed 0.5 m/s, ensuring proper compliance (see 5.3) For automated scanning using a traversing system, the speed can be adjusted freely, provided that the scanning time requirements and noise level standards are met.
8.3.3 Measurement of intensities and pressures on a partial surface and check of repeatability of the scan
Perform two separate scans along the same path, each with a scanning time T s, and record the time-series data of intensities and squared sound pressures twice Ensure that the actual scanning time T s′ remains within ±20% of the desired time T s; otherwise, discard the data and repeat the measurement For each scan, extract the intensity levels L_I1 and L_I2 to analyze sound pressure and intensity characteristics accurately.
L I averaged over the scanning time T s ′ Evaluate n (1) n (2)
L −L for all frequency bands of measurement and introduce values into criterion 1 given in C.1.2
If this criterion is satisfied for the two scans, record the averaged normal intensity levels for all frequency bands of measurement:
To assess field non-uniformity, first obtain time-averaged intensities and squared sound pressures for each segment on the partial surface, following the procedure outlined in Annex G for each scan Next, calculate the average values for each segment across the two scans These averaged measurements are essential for computing the field non-uniformity indicator, F S, ensuring accurate evaluation of sound field uniformity.
In cases where criterion 1 is not satisfied, attempt to identify the causes of the difference and suppress them by taking action as given in Table C.1
8.3.4 Evaluation of the instrument capability
Evaluate the signed pressure-intensity indicator F pI n across the entire measurement surface for all frequency bands using equation (B.6) Incorporate these values into criterion 2 as specified in section C.1.4 If the criterion is not met, implement the prescribed actions outlined in Table C.1 to address the issue effectively.
8.3.5 Evaluation of the presence of strong extraneous noise
Evaluate the unsigned pressure-intensity indicator p I n
Calculate the F value for the entire measurement surface across all frequency bands using equation (B.3), and include this value along with FpIn into criterion 3 as specified in C.1.5 If the criterion is not met, follow the specified actions outlined in Table C.1 to ensure compliance.
8.3.6 Evaluation of the effect of field non-uniformity
Evaluate the field non-uniformity indicator F S across the entire measurement surface for all frequency bands, using equation (B.8) Incorporate these results into criterion 4, as specified in section C.1.6.1 If the criterion is not met, take corrective actions outlined in Table C.1 to ensure compliance.
Further actions
When criteria 1 to 4 are met across all frequency bands for the measurement surface, the initial sound power determination is considered final If these criteria are not satisfied, appropriate actions should be taken based on section C.2, involving measurement of normal sound intensity component levels and sound pressure levels using a modified measurement setup Subsequently, indicators such as F T and p I n should be recalculated to ensure accurate results.
F , F pI n , and F S and assess according to C.1 Take actions according to C.2 Repeat this procedure until the criteria according to C.1 are fulfilled
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When criteria 1 to 3 are met but criterion 4 is not, increase the scanning density by at least a factor of 2 If the ratio of field non-uniformity indicators between previous and current measurements meets criterion 5 (see C.1.6.2), the current scanning density is deemed sufficient, allowing the measured average intensity levels for each partial surface to be used for radiated power calculations.
If the ratio of the field non-uniformity indicators of the previous measurement F S(1) to the present measurement
F S(2) does not satisfy criterion 5, the scanning density should be increased more
If repeated actions, such as increasing scanning density, do not meet the required criteria, record a null test result and specify the reasons Alternatively, follow ISO 9614-2 standards to measure radiated power using engineering or survey-grade equipment for accurate assessment.
9 Determination of sound power level
Calculation of partial sound powers for each partial surface of the measurement surface
Calculate a partial sound power in each frequency band for each partial surface of the measurement surface by equation (5).
Calculation of normalized sound power level
Calculate the sound power level of the sound source under test, L W , in each frequency band according to equations (8) and (9) Then calculate the normalized sound power level according to equation (10)
If the sound power P is negative in any frequency band, the method given in this part of ISO 9614 is not applicable to that band
To determine the A-weighted sound power level, the averaged normal sound intensity levels (L In) are derived from measured one-third-octave band levels with frequency weighting per IEC 60651 standards Additionally, the weighting factors specified in IEC 60651 should be applied at standard centre frequencies in accordance with IEC 61260, ensuring accurate and compliant sound level measurements.
The following information, if applicable, shall be compiled and reported for all measurements that are made according to this part of ISO 9614 a) Test
1) date and location of test b) Sound source under test
7) description of the source under test (including its major dimensions and surface texture);
8) qualitative description of the character of the source under test, including tonal or cyclic character and variability;
16 © ISO 2002 – All rights reserved c) Acoustic environment
1) description of the test environment:
if indoors, a description of the geometry and nature of the enclosure surfaces,
if outdoors, a sketch showing the surrounding terrain, including physical description of the test environment;
2) air temperature in degrees Celsius, barometric pressure in pascals, and relative humidity;
3) mean wind speed and direction, where relevant;
4) any source of variability in the test environment; description of any devices/procedures taken to minimize the effect of extraneous intensity and/or excessive reverberation;
5) qualitative description of any gas/air flows and unsteadiness d) Instrumentation
1) equipment used for measurements, including names, types, serial numbers and manufacturers and probe configuration;
2) method(s) used for checking calibration and field performance;
3) places and dates of calibration and verification of test equipment;
5) the pressure-residual intensity index in accordance with IEC 61043 e) Measurement procedure
1) description of the mounting, or support system, of the scanning mechanism, and of the intensity probe;
2) description of the scan including geometry and speed;
3) quantitative description of the measurement surface, partial surfaces and their segment numbers; a drawing of the scanning paths shall be presented;
4) scanning time on each partial surface;
5) description of any steps found necessary to improve measurement accuracy f) Acoustical data
1) tabulation of the field indicators F T , n,
F p I F pI n , and F S in each frequency band of sound power determination, calculated from each set of measurements on each partial surface used;
The article emphasizes the importance of presenting the normalized sound power level of the test sound source across all relevant frequency bands in a clear tabular format When determining the A-weighted sound power level, contributions from frequency bands that do not meet criteria 1 to 4 or criteria 1 to 3 and 5 should be omitted unless their impact is negligible as per section 4.3 Additionally, it is necessary to include a statement indicating the omission of these frequency bands to ensure transparency and compliance with measurement standards.
3) presentation of the results of the probe-reversal field checks specified in 6.2.3, if appropriate;
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List of symbols used in this part of ISO 9614
Symbol Term Unit Cited in p(t) instantaneous sound pressure Pa 3.2
( ) u tG instantaneous particle velocity m/s 3.2 ρ density of the air kg/m 3 Annex H c speed of sound m/s Annex H ρc characteristic impedance of air Pa.s/m Annex H θ air temperature °C 3.6.4, annex H
B barometric pressure Pa 3.6.4, annex H t time s 3.2
T s scanning time s 3.13.3, 6.3 nG unit normal vector directed out of the volume enclosed by the measurement surface — 3.4
S i area of the partial surface i m 2 3.6.1
N s number of segments on a partial surface — 8.3.2
N total number of segments on the measurement surface — B.2.2 q 2 p time-series of squared pressure, where q = 1, 2, 3, Q Pa 2 3.14.3 m 2 p time-averaged squared pressure, where m = 1, 2, 3, M Pa 2 3.14.4
2 j p time-averaged squared pressure measured on each segment Pa 2 B.2.2 p 0 reference sound pressure (= 20 àPa) Pa B.2.2
I ni signed magnitude of the partial surface average normal sound intensity measured on the partial surface i of the measurement surface W/m 2 3.6.1
I nq time-series of intensity, where q = 1, 2, 3, Q W/m 2 3.14.3
Symbol Term Unit Cited in n m
I time-averaged sound intensity, where m = 1, 2, 3, M W/m 2 3.14.4 n j
I time-averaged unsigned normal sound intensity measured on each segment W/m 2 B.2.2
I nj time-averaged signed normal sound intensity measured on each segment W/m 2 B.2.3
L p averaged sound pressure level dB B.2.2
L normal sound intensity level dB 3.5
L averaged signed normal sound intensity level dB 3.6.1, B.2.3
L I δ level of residual intensity I δ dB 3.10
L W0 normalized sound power level dB 3.6.4 pI 0 δ pressure-residual intensity index dB 3.10
F unsigned pressure-intensity indicator dB B.2.2
F pI n signed pressure-intensity indicator dB B.2.3
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General
Evaluate the field indicator F T at a point on the measurement surface and the indicators p I n
Sound power level indicators, including F, F pI n, and F S, are calculated over the measurement surface using equations (B.1) to (B.9) across each frequency band These indicators are derived from time-series data by configuring the measurement instrument to operate in instantaneous mode, as outlined in section 3.14.1.
Definition of field indicators
Temporal variability indicator, F T
To evaluate the temporal variability indicator, F_T, of the sound field, select an appropriate measurement position on the surface Record the time-series data of intensity, I_nq, and compute the time-averaged intensities, I_nm, over the period T Use these values to calculate F_T according to equation (B.1), which provides a quantitative measure of the sound field's temporal variability.
− ∑ (B.1) where I n is the average value of I n m , m = 1, 2, 3, M calculated from equation (B.2): n n
Adjusting the averaging period T to satisfy the condition F T < 0.6 allows for accurate determination of the minimum scanning time T_s, which must be at least N_s times T_F T < 0.6, where N_s represents the number of surface segments Typically, the value of M is set to 10 to facilitate calculations When evaluating F T, it’s essential that two consecutive intervals, each with period T for I_n m and I_n (m + 1), do not overlap and may be separated to ensure precise measurement.
NOTE The symbol F 1 is used for F T in ISO 9614-1 (see annex I).
Unsigned pressure-intensity indicator,
Calculate the unsigned pressure-intensity indicator, p I n
F , for the measurement surface from equation (B.3): n p I n
F p I =L −L (B.3) where L p is the averaged sound pressure level, in decibels, calculated from equation (B.4):
2 j p is the time-averaged squared pressure measured on each segment; p 0 is the reference sound pressure (= 20 àPa);
N is the total number of segments on the measurement surface
L is the average unsigned normal sound intensity level, in decibels, calculated from equation (B.5): n n 0
I j is the time-averaged unsigned normal sound intensity measured on each segment
NOTE 1 The unsigned pressure-intensity indicator, p I n
F , is called the surface pressure-intensity indicator, F 2 , in ISO 9614-1 (see annex I)
NOTE 2 Although areas of segments for each partial surface can differ by at most 50 % (see 8.2), the effect of this difference is neglected in applying equations (B.3), (B.4) and (B.5) The same idea is also applied in B.2.3 and B.2.4
NOTE 3 In applying equations (B.3), (B.4) and (B.5), the total number of segments is considered to be 2N because of the repetition of scan on each partial surface.
Signed pressure-intensity indicator, F pI n
Calculate the signed pressure-intensity indicator, F pI n , for the measurement surface from equation (B.6): n n pI p I
L p is the averaged sound pressure level, in decibels, calculated from equation (B.4);
L is the averaged signed normal sound intensity level, in decibels, calculated from equation (B.7): n n 0
= ∑ dB (B.7) where I n j is the time-averaged signed normal sound intensity measured on each segment
NOTE 1 The signed pressure-intensity indicatorF pI n is called the negative partial power indicator F 3 in ISO 9614-1 This indicator is equivalent to the sound field pressure-intensity indicator F pI specified in ISO 9614-2 in the special case of uniform segment area
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F −F is equivalent to the negative partial power indicator F + − / specified in ISO 9614-2 in the special case of uniform segment area.
Field non-uniformity indicator, F S
Calculate the field non-uniformity indicator, F S , for the measurement surface from equation (B.8):
NOTE The symbol F 4 is used for F S in ISO 9614-1
Procedure for achieving the desired accuracy
Qualification requirements
General
In applying ISO 9614, the sound field conditions at measurement points on the initial measurement surface can vary significantly To ensure accurate sound power level measurements, it is essential to verify that the instrumentation and measurement parameters—such as the measurement surface, distance, and path—are appropriate for the specific sound field and environmental conditions This helps in maintaining the uncertainty limits and achieving reliable results.
The general procedure is summarized in Figure C.1.
Check for the adequacy of the averaging time
An averaging time, T, that satisfies the condition F T < 0.6 (expressed as T F T < 0.6), is used to determine the scanning time, T_s The value of T_s must be equal to or greater than N_s multiplied by F T < 0.6 If this requirement cannot be practically met, alternative actions should be taken as outlined in Table C.1.
Check for the repeatability of the scan on a partial surface
During the scanning process, each partial surface is scanned using the same scanning path, ensuring consistency The averaged intensity levels in each frequency band of measurement must remain within the specified tolerance limits to guarantee accurate and reliable results.
The formula L −L u (C.1) defines the difference between normal intensity levels, where L I n (1) and L I n (2) represent the intensity levels obtained from two scans, and s denotes the uncertainty specified in Table 1 To ensure accurate assessment, this criterion must be satisfied; if not, specific actions outlined in Table C.1 should be taken, as illustrated in Figure C.1 Following these guidelines is essential for maintaining measurement precision and adhering to quality standards in imaging analysis. -**Sponsor**As a content creator, rewriting your article with SEO in mind is my specialty! Need help crafting coherent paragraphs that pack a punch and follow SEO rules? With [Article Generation](https://pollinations.ai/redirect-nexad/CbqA4MnQ), you can effortlessly create high-quality, SEO-optimized articles in minutes, saving time and money Perfect for startups and businesses looking to boost their online presence without breaking the bank! Let's transform your content into something truly remarkable.
Check for the adequacy of the measurement equipment
The dynamic capability index L d of the measurement instrumentation shall be greater than the indicator, F pI n , determined in accordance with annex B in each frequency band of measurement:
If a chosen measurement surface does not satisfy criterion 2, take action as given in Table C.1 (see Figure C.1)
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Check for the presence of strong extraneous noise
F and F pI n , and check if the following condition is satisfied for each frequency band of measurement:
If this criterion is not satisfied, take action as given in Table C.1 to reduce the effect of the extraneous noise (see Figure C.1).
Check for the field non-uniformity
C.1.6.1 Initial check for the field non-uniformity
Calculate the field non-uniformity indicator F S for the measurement surface, and check if the following condition is satisfied in each frequency band of measurement:
If all previous criteria are met but this specific criterion is not satisfied, appropriate action should be taken according to Table C.1 to minimize the impact of field non-uniformity Refer to Figure C.1 for visual guidance on reducing field non-uniformity effects, ensuring optimal performance and compliance with quality standards.
C.1.6.2 Check for the adequacy of the scan-line density
When the scan density on a partial surface is increased by a factor of two or more, it is essential to compare the current and previous field indicators Ensure that, across each measurement frequency band, the following condition is satisfied to maintain optimal data accuracy and consistency This process helps verify the reliability of the scan results and ensures precise assessment of the surface.
If criterion 5 is satisfied, the result is qualified as the final result even if F S(2) W 2.
Action to be taken to increase the grade of accuracy of determination
When criteria 1 to 5 are not met, it is essential to follow the procedures outlined in Table C.1 for each specific condition to improve the accuracy of the sound power level measurement under test Refer to Figure C.1 for detailed guidance on implementing these corrective actions to ensure reliable and precise results.
Table C.1 — Actions to be taken to increase accuracy of determination of sound power level
If T s is not practicable A Increase the scanning time, and/or reduce the temporal variability of extraneous intensity, or measure during periods of less variability
Modify the scanning speed, time, and/or path
Modify partial surfaces and/or measurement surface
To ensure accurate measurements, minimize the average distance between the measurement surface and the source—reducing it to at least 0.25 meters in environments with significant extraneous noise or strong reverberation Conversely, if there is minimal external noise and reverberation, increase the distance up to a maximum of 1 meter for optimal results.
Shield the measurement surface from extraneous noise source or take action to reduce sound reflections to the source
Same actions as for criterion 2)
Increase the average distance of the partial surface from the source
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25 Figure C.1 — Scheme for achieving the desired accuracy
Effects of airflow on measurement of sound intensity
Sound intensity probes can be affected by airflow during measurement, especially in windy outdoor conditions or near cooling fans While the theoretical basis for intensity measurement is invalid in steady fluid flow, errors are generally negligible at low Mach numbers (Ma < 0.05), except in highly reactive environments However, unsteady airflow turbulence can cause more significant measurement errors.
Turbulence in flow impinging on a probe can originate from the flow itself or be caused by the presence of the probe, with fluctuations in fluid momentum leading to pressure variations that are non-acoustic and often indistinguishable from sound signals when registered by pressure-sensitive transducers Since turbulence is convected at speeds close to the mean flow and contains small-scale eddies, pressure gradients and particle velocities in turbulent flows can significantly exceed those in sound waves, resulting in strong pseudo-intensity signals The use of carefully designed windscreens can effectively suppress turbulence caused by the probe, but turbulence generated by external sources like wind flows, cooling fans, or blowers remains challenging to eliminate entirely, especially near the measurement surface While throttling can reduce the mean flow from fans, it does not eliminate turbulent pressure fluctuations, which can complicate sound power measurements Therefore, to ensure accurate sound intensity measurements, it is crucial to use windscreens and conduct meticulous experiments to differentiate true acoustic signals from pseudo-sound or turbulence-induced fluctuations.
A probe windscreen is designed to divert airflow away from pressure transducers, ensuring accurate measurements Due to the low convection speed of turbulence, turbulent pressure and velocity fluctuations on the windscreen's outer surface do not effectively reach the central region where the transducers are located In contrast, sound waves are less attenuated by the windscreen, allowing for clearer measurement of acoustic signals This selective attenuation underpins the windscreen's principle of discriminating between turbulent fluctuations and sound waves, enhancing measurement precision in aerodynamic testing.
Discrimination methods have limitations, as extremely intense turbulent fluctuations cannot be fully excluded, and large-scale, low-frequency turbulence is less effectively attenuated than small-scale, high-frequency turbulence Since wind- and fan-generated turbulence generally exhibits a frequency spectrum that decreases rapidly at higher frequencies, low-frequency turbulence (typically below 200 Hz) tends to have the most significant impact on intensity measurements Accurate turbulence assessment requires understanding these spectral characteristics to mitigate measurement interference.
The scale and frequency of turbulence are highly dependent on the generation process, making it impractical to specify rules for every unsteady flow scenario encountered during field intensity measurements Since the root mean square (r.m.s.) of turbulent pressure fluctuations increases with the square of the mean flow speed, a conservative blanket limit is applied to the mean flow speed to ensure accurate and safe measurements.
A key indicator of low-frequency sound contamination is the persistent or increasing one-third-octave intensity and particle velocity levels below 100 Hz, especially when sound pressure levels do not similarly rise, suggesting the source does not strongly radiate in this range High unsteadiness in measured sound intensity and particle velocity levels can also indicate turbulent pseudo-intensity contamination However, inter-microphone coherence may not reliably detect turbulence, as large-scale low-frequency pressure fluctuations can be highly correlated over typical microphone separation distances.
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Effect of sound absorption within the measurement surface
When a source exhibits significant sound absorption, indicated by a measurement of F pI n exceeding 3 dB, it is essential to evaluate the impact of absorbed sound power on total sound measurements If the source can be switched off without altering the extraneous noise level, the absorbed sound power (L W,abs) can be directly measured using sound intensity measurements around the source, following ISO 9614 guidelines and applying equation (9) In cases where switching off the source affects extraneous noise levels, a rough estimate of absorbed sound power can be obtained by using an artificial extraneous sound source that replicates the original noise levels on the measurement surface.
The effects of absorption may be neglected if the following condition is satisfied:
L W is the level of the total sound power with the source running [according to equations (8) and (9)];
L W,abs is the level of the absorbed sound power with the source switched off
Otherwise, actions should be taken in order to reduce the level of the extraneous intensity or to shield the measurement surface from the extraneous noise sources
Measurement surface and scanning procedure
The core principle of sound power measurement via the intensity technique involves capturing the intensity component perpendicular to a measurement surface that fully encloses the sound source Key uncertainties affecting measurement accuracy stem from instrumentation, signal analysis errors, and non-ideal field sampling methods To address these challenges, this guideline provides comprehensive procedures for effective field sampling, ensuring minimal uncertainty when adhering to the specified scanning parameters in ISO 9614 Following these guidelines helps achieve the designated accuracy levels outlined in Table 1.
For accurate measurements, the measurement surface must be easily scannable and designed to minimize the influence of extraneous intensity and near-field effects from the source Keeping scan lines straight ensures consistent data collection, with the probe’s orientation maintained throughout each linear scan to enhance precision and reliability.
The measurement surface, the partial surfaces and the scan pattern should be selected to suit the source geometry and its environment according to 8.1 and 8.2 (see Figure F.1)
Figure F.1 — Recommended measurement surfaces for curved sound sources
To ensure accurate surface measurement, each partial surface must be clearly defined for easy and comfortable scanning at a constant speed with uniform line density Maintaining the probe perpendicular to the local surface axis is essential for precise data collection Care should be taken during turns at the end of scanning lines, as these can introduce errors by overestimating edge contributions Consistently maintaining a steady scanning speed throughout the entire scan path is crucial for reliable surface-averaging results.
To optimize processor efficiency, it's important to minimize the interval between completing the scan of a partial surface and stopping the operation, especially when processor operation times are set in discrete steps Reducing this delay enhances overall performance by ensuring swift transitions between tasks Implementing strategies to decrease these intervals can lead to more efficient processing and improved system responsiveness.
To ensure accurate measurements, it is essential to focus equally on following the selected scan path, maintaining consistent scanning speed, ensuring uniform line density, and properly orienting the probe axis Overemphasizing any single aspect can negatively impact measurement accuracy, so balanced attention to all these factors is crucial for optimal results.
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Procedure for obtaining time-averaged intensities and squared pressures from a sequence of short-time averaged intensities and squared pressures
The scanning time (T_s′) is calculated by multiplying the sampling interval (Δt) by the number of averaged intensities and squared pressures obtained in one scan (N_x), expressed as T_s′ = Δt · N_x During the scanning process, as illustrated in Figure G.1, the last sampling data may be interrupted and discarded, but this has a negligible impact because the sampling interval (Δt) is typically much smaller than the total scanning time (T_s′).
General
Calibrating the sensitivity of microphones used in sound intensity probes under the actual meteorological conditions ensures accurate measurement of sound pressure p (in Pa) relevant to real-world environments The measured sound pressure is directly applicable when properly calibrated, allowing for precise assessment of acoustic intensity Additionally, the particle velocity u (in m/s) is derived from these calibrated measurements, enabling comprehensive analysis of sound fields in various meteorological conditions Proper calibration under actual weather conditions is essential for reliable sound intensity measurements in environmental acoustics.
∫∂ (H.1) where ρ is the density of the air (in kg/m 3 )
Using the air density specific to the actual measurement conditions ensures that the measured particle velocity and sound intensity (in W/m²) accurately reflect real-world conditions This approach guarantees that sound intensity measurements are both precise and representative of the environment being analyzed.
ISO 9614 mandates the use of a Class 1 instrument as specified in IEC 61043:1993 for accurate sound intensity measurements According to IEC 61043:1993, Class 1 processors must allow input of ambient atmospheric pressure and temperature or correction factors derived from these parameters This ensures that sound intensity measurements reflect true conditions by accounting for current meteorological variables, leading to more accurate and reliable results.
informative) Field indicators used in ISO 9614-1, -2 and -3
List of symbols used in this part of ISO 9614
Symbol Term Unit Cited in p(t) instantaneous sound pressure Pa 3.2
( ) u tG instantaneous particle velocity m/s 3.2 ρ density of the air kg/m 3 Annex H c speed of sound m/s Annex H ρc characteristic impedance of air Pa.s/m Annex H θ air temperature °C 3.6.4, annex H
B barometric pressure Pa 3.6.4, annex H t time s 3.2
T s scanning time s 3.13.3, 6.3 nG unit normal vector directed out of the volume enclosed by the measurement surface — 3.4
S i area of the partial surface i m 2 3.6.1
N s number of segments on a partial surface — 8.3.2
N total number of segments on the measurement surface — B.2.2 q 2 p time-series of squared pressure, where q = 1, 2, 3, Q Pa 2 3.14.3 m 2 p time-averaged squared pressure, where m = 1, 2, 3, M Pa 2 3.14.4
2 j p time-averaged squared pressure measured on each segment Pa 2 B.2.2 p 0 reference sound pressure (= 20 àPa) Pa B.2.2
I ni signed magnitude of the partial surface average normal sound intensity measured on the partial surface i of the measurement surface W/m 2 3.6.1
I nq time-series of intensity, where q = 1, 2, 3, Q W/m 2 3.14.3
Symbol Term Unit Cited in n m
I time-averaged sound intensity, where m = 1, 2, 3, M W/m 2 3.14.4 n j
I time-averaged unsigned normal sound intensity measured on each segment W/m 2 B.2.2
I nj time-averaged signed normal sound intensity measured on each segment W/m 2 B.2.3
L p averaged sound pressure level dB B.2.2
L normal sound intensity level dB 3.5
L averaged signed normal sound intensity level dB 3.6.1, B.2.3
L I δ level of residual intensity I δ dB 3.10
L W0 normalized sound power level dB 3.6.4 pI 0 δ pressure-residual intensity index dB 3.10
F unsigned pressure-intensity indicator dB B.2.2
F pI n signed pressure-intensity indicator dB B.2.3
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Evaluate the field indicator F T at a point on the measurement surface and the indicators p I n
F, F pI n, and F S are measured over the surface according to equations (B.1) to (B.9) across various frequency bands used to determine sound power levels These indicators are derived from time-series data by configuring the measurement instrument to operate in instantaneous mode, as outlined in section 3.14.1.
To evaluate the temporal variability indicator, F_T, of the sound field, select an appropriate position on the measurement surface Record the time-series of intensity data, I_nq, and compute the time-averaged intensities, I_nm, over the measurement period T Using these data, calculate F_T according to equation (B.1).
− ∑ (B.1) where I n is the average value of I n m , m = 1, 2, 3, M calculated from equation (B.2): n n
By adjusting the averaging period T to achieve a condition where F T < 0.6, an optimal value for T is determined This value, T F T < 0.6, is then used to calculate the minimum scanning time T_s, ensuring T_s ≥ N_s · T F T < 0.6, where N_s represents the number of segments in the partial surface Typically, M is set to 10 to standardize evaluations To accurately assess F T, two consecutive intervals of period T for Iₙ^m and Iₙ^(m+1) must be non-overlapping and can be separated to prevent interference.
NOTE The symbol F 1 is used for F T in ISO 9614-1 (see annex I)
Calculate the unsigned pressure-intensity indicator, p I n
F , for the measurement surface from equation (B.3): n p I n
F p I =L −L (B.3) where L p is the averaged sound pressure level, in decibels, calculated from equation (B.4):
2 j p is the time-averaged squared pressure measured on each segment; p 0 is the reference sound pressure (= 20 àPa);
N is the total number of segments on the measurement surface
L is the average unsigned normal sound intensity level, in decibels, calculated from equation (B.5): n n 0
I j is the time-averaged unsigned normal sound intensity measured on each segment
NOTE 1 The unsigned pressure-intensity indicator, p I n
F , is called the surface pressure-intensity indicator, F 2 , in ISO 9614-1 (see annex I)
Note 2 indicates that while the segments for each partial surface can vary by up to 50%, this difference is neglected when applying equations (B.3), (B.4), and (B.5) This approach is also consistently used in sections B.2.3 and B.2.4 to simplify calculations without significantly affecting accuracy.
NOTE 3 In applying equations (B.3), (B.4) and (B.5), the total number of segments is considered to be 2N because of the repetition of scan on each partial surface
B.2.3 Signed pressure-intensity indicator, F pI n
Calculate the signed pressure-intensity indicator, F pI n , for the measurement surface from equation (B.6): n n pI p I
L p is the averaged sound pressure level, in decibels, calculated from equation (B.4);
L is the averaged signed normal sound intensity level, in decibels, calculated from equation (B.7): n n 0
= ∑ dB (B.7) where I n j is the time-averaged signed normal sound intensity measured on each segment
The signed pressure-intensity indicator, denoted as F pI n, is known as the negative partial power indicator F 3 in ISO 9614-1 This indicator is equivalent to the sound field pressure-intensity indicator F pI specified in ISO 9614-2, particularly in cases involving a uniform segment area.
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F −F is equivalent to the negative partial power indicator F + − / specified in ISO 9614-2 in the special case of uniform segment area
Calculate the field non-uniformity indicator, F S , for the measurement surface from equation (B.8):
NOTE The symbol F 4 is used for F S in ISO 9614-1
Procedure for achieving the desired accuracy
To ensure accurate sound power level measurements in accordance with ISO 9614, it is essential to verify that the instrumentation and measurement parameters—such as measurement surface, distance, and path—are appropriate for the specific sound field and environmental conditions Variations in sound field conditions across measurement positions can significantly impact measurement uncertainty, making it necessary to evaluate the adequacy of the setup to maintain acceptable upper limits for these uncertainties.
The general procedure is summarized in Figure C.1
C.1.2 Check for the adequacy of the averaging time
An averaging time T that satisfies the condition F T < 0.6 is used to determine the scanning time T_s The scanning time T_s must be equal to or greater than N_s multiplied by the averaging time, expressed as T_s ≥ N_s ⋅ T_F T < 0.6 If this requirement cannot be practically met, appropriate measures should be taken as outlined in Table C.1.
C.1.3 Check for the repeatability of the scan on a partial surface
During the inspection process, the scan is repeated on each partial surface using the same scanning path to ensure consistency The averaged intensity levels in each measurement frequency band must remain within established tolerance limits, confirming the reliability and accuracy of the scan results This approach guarantees precise surface analysis and maintains quality control standards.
The equation L − L_u (C.1) relates to normal intensity levels obtained from two scans, where L_I_n(1) and L_I_n(2) represent these levels, and s denotes the uncertainty specified in Table 1 If this criterion is not met, corrective actions should be taken according to the guidelines outlined in Table C.1, as illustrated in Figure C.1.
C.1.4 Check for the adequacy of the measurement equipment
The dynamic capability index L d of the measurement instrumentation shall be greater than the indicator, F pI n , determined in accordance with annex B in each frequency band of measurement:
If a chosen measurement surface does not satisfy criterion 2, take action as given in Table C.1 (see Figure C.1)
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C.1.5 Check for the presence of strong extraneous noise
F and F pI n , and check if the following condition is satisfied for each frequency band of measurement:
If this criterion is not satisfied, take action as given in Table C.1 to reduce the effect of the extraneous noise (see Figure C.1)
C.1.6 Check for the field non-uniformity
C.1.6.1 Initial check for the field non-uniformity
Calculate the field non-uniformity indicator F S for the measurement surface, and check if the following condition is satisfied in each frequency band of measurement:
If all previous criteria are met but this specific criterion is not satisfied, take corrective action as outlined in Table C.1 to mitigate the effects of field non-uniformity, as illustrated in Figure C.1.
C.1.6.2 Check for the adequacy of the scan-line density
When the scan density is doubled or more over the same partial surface, it is essential to compare the current and previous field indicators Ensure that, for each measurement frequency band, the following condition is met to maintain accuracy and consistency in the analysis This approach helps verify the reliability of the scan results across different frequency ranges.
If criterion 5 is satisfied, the result is qualified as the final result even if F S(2) W 2
C.2 Action to be taken to increase the grade of accuracy of determination
When criteria 1 through 5 are not met, it is essential to follow the specified actions outlined in Table C.1 for each condition to improve the accuracy of determining the sound power level during testing Refer to Figure C.1 for visual guidance on the testing process Implementing these corrective actions ensures more reliable and precise sound power level measurements.
Table C.1 — Actions to be taken to increase accuracy of determination of sound power level
If T s is not practicable A Increase the scanning time, and/or reduce the temporal variability of extraneous intensity, or measure during periods of less variability
Modify the scanning speed, time, and/or path
Modify partial surfaces and/or measurement surface
When measuring in environments with substantial extraneous noise or strong reverberation, position the measurement surface as close to the source as possible, with an average distance of at least 0.25 meters to ensure accurate results Conversely, in quieter settings with minimal noise and reverberation, increase the average distance to a maximum of 1 meter to maintain optimal measurement accuracy.
Shield the measurement surface from extraneous noise source or take action to reduce sound reflections to the source
Same actions as for criterion 2)
Increase the average distance of the partial surface from the source
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25 Figure C.1 — Scheme for achieving the desired accuracy
Effects of airflow on measurement of sound intensity
Sound intensity probes can be affected by airflow during measurement, particularly in windy outdoor environments or near cooling fans While the theoretical foundation of intensity measurement may be compromised in steady fluid flows, the resulting errors are minimal at low Mach numbers (Ma < 0.05), except in highly reactive environments However, unsteady airflow turbulence can introduce more significant measurement inaccuracies.
Turbulence can occur in the flow impinging on a probe and may also be generated by the probe itself, leading to pressure fluctuations that are non-acoustic but detectable by pressure-sensitive transducers These turbulence-induced signals can be indistinguishable from genuine acoustic pressures, as turbulence is carried by the flow at speeds close to the mean flow and involves small-scale eddies that create steep spatial pressure gradients and high particle velocities, potentially producing strong pseudo-intensity signals While turbulence caused by the probe can be mitigated with appropriate windscreen design, turbulence from external sources like wind flows or fans often persists despite flow reduction methods such as throttling Turbulent pressure fluctuations near fans and blowers are particularly challenging to suppress with windscreens, making careful experimental procedures essential Accurate sound power measurements in such environments require the use of windscreens and meticulous testing to ensure that the readings reflect true sound intensity rather than turbulent pressure fluctuations or pseudo-sound.