Design and qualification of an anechoic facility in PNRPU

Design and Qualification of an Anechoic Facility in PNRPU Procedia Engineering 176 ( 2017 ) 264 – 272 1877 7058 © 2017 Published by Elsevier Ltd This is an open access article under the CC BY NC ND li[.]

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Procedia Engineering 176 ( 2017 ) 264 – 272

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the international conference on Dynamics and Vibroacoustics of Machines

doi: 10.1016/j.proeng.2017.02.317

ScienceDirect

Dynamics and Vibroacoustics of Machines (DVM2016) Design and qualification of an anechoic facility in PNRPU

I.V Belyaeva,b, I.A Korinb, E.V Sorokinb, I.V Khramtsovb, O.Yu Kustovb

a Central Aerohydrodynamic Institute (TsAGI), 105005 Moscow, Russia

b Perm National Research Polytechnic University (PNRPU), 614990 Perm, Russia

Abstract

The paper describes the anechoic facility built in Perm National Research Polytechnic University (PNRPU) in 2014-2015 with the objective of studying noise of turbulent jets, vortex rings, and airframe noise At present, the only other analogous facility in Russia is anechoic chamber AC-2 of TsAGI built in 1970s One of the most important characteristics of an anechoic facility is a region where inverse square law spreading holds within some tolerance, i.e where the free-field conditions are observed Acoustic qualification is aimed at determination of this region in the facility The present paper provides the results of acoustic qualification of the anechoic chamber in PNRPU, which show that the free-field conditions in the facility are realized within 3 m radius from the sound source for broadband noise in the frequency range 125 Hz – 20 kHz Therefore, the study demonstrates that the anechoic facility in PNRPU allows aeroacoustic experiments to be performed for obtaining quantitative results The present work restores domestic competences in design of aeroacoustic experimental facilities, which is important in view of the plans to build in Russia an anechoic wind tunnel with hot flow AKT TsAGI for large-scale aeroacoustic tests

Keywords: aeroacoustics, anechoic facility, facility qualification

* Corresponding author Tel.: +7-495-916-9091

E-mail address: vkopiev@mktsagi.ru

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

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1 Introduction

Community noise is one of the key factors of aircraft competiveness, which gives importance to investigation of noise generation mechanisms of turbulent flows, as well as development of noise reduction technologies The state-of-the-art approach to these formidable problems consists in performing experimental studies for small-scale models

in the laboratory conditions, validating the findings in large-scale experiments, and then transferring the results to full-scale ground or flight tests

Laboratory tests allow performing experimental studies in the controlled environment; in addition, they typically require less financial, time, and human resources than full-scale tests However, to ensure reliable experimental data the facility has to meet specific requirements; in particular, it has to provide the free-field acoustic conditions, i.e sound propagation as if there are no reflections from the walls Such a facility is called anechoic

In 2014-2015, an anechoic facility has been designed and built in Perm National Research Polytechnic University (PNRPU) for small-scale experimental studies on jet noise, vortex ring noise, and airframe noise It should be noted that at present the only other analogous operating facility in Russia is anechoic chamber AC-2 of Central Aerohydrodynamic Institute (TsAGI) built in 1970s, which also enables experiments with small-scale models

The work on the new aeroacoustic facility restores domestic competences in this field, which is particularly important in view of the plans to build in Russia an anechoic wind tunnel with hot flow AKT TsAGI for large-scale aeroacoustic tests

The present paper describes the anechoic chamber in PNRPU and provides the results of its acoustic qualification

2 Description of the anechoic chamber

General requirements for anechoic chambers are formulated in ISO 3745 [1] Specific recommendations to the anechoic chambers for studies of turbulent jet noise can be found in [2] These requirements and recommendations have been taken into account in the design of the anechoic facility in PNRPU (Fig 1)

Fig 1 A general scheme of the anechoic facility in PNRPU, top-view (left)

A photo of the facility with the wedges on the floor partially removed (right)

The anechoic chamber is located within the building of Aerospace Faculty of PNRPU and has the dimensions 11.8 x 8.2 x 5.3 m (length, width, and height, respectively) The walls from aerated concrete have thickness 40 cm, which ensures sound insulation from outside sources The ceiling and the walls are lined with sound-absorbing wedges to provide the free-field conditions inside the chamber A wedge is made of acoustically transparent glass-cloth E1/1 - 100 filled with basalt superthin fibers and has the width 20 cm, length 100 cm, and total height 80 cm (the height of the taper is 70 cm, and the height of the rectangular base is 10 cm) (see Fig 2) The optimal density of basalt fibers has been chosen in a series of experiments with the wedges in reverberation chambers of TsAGI [3] The wedges are assembled in blocks of five wedges inserted into a thin metal wireframe (diameter of the wire is

2 mm) to preserve the geometrical parameters of the structure Although the wireframe deteriorates the sound-absorbing characteristics of wedges, it has been considered as a necessary compromise to their long-term preservation of the shape

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Fig 2 A scheme of the metal wireframe for a block of wedges; the dimensions are given in mm (left) A photo of the block of wedges (right)

The blocks are equipped with hooks that allow their fixation to the railings on the ceiling and the walls of the chamber, which facilitates their installation and removal, as well as provide a gap between the wall surface and the wedge base; the blocks are placed in a chess order (see Fig 1) The concrete floor of the chamber may either be covered with blocks of wedges or left hard (reflective) In the latter case, the chamber is called semi-anechoic and realizes the conditions of sound reflection from the ground that take place, for instance, during static engine tests at a ground rig This configuration also allows installation and tests of heavy pieces of machinery in the chamber To provide the free-field conditions, the floor is covered with blocks of wedges, which are equipped with wheels to facilitate their movement along the chamber This selection of movable floor covering allows easy access to the equipment during tests in the anechoic conditions, and makes the facility more versatile as a whole

Besides, the chamber has to meet the requirements on fire safety and lighting Lighting inside the anechoic chamber is provided with 12 incandescent lights DSO 01-33-40 installed on the ceiling between the wedges The materials used for the acoustic treatment (basalt fibers, glass-cloth etc) are inflammable, thus ensuring fire safety of the facility

3 Acoustic qualification of the anechoic chamber

The objective of qualifying an anechoic chamber consists in determination of the maximum allowable radius between a test source and a measurement location where inverse square law spreading holds, within some tolerance This radius determines the region in the anechoic chamber where quantitative acoustic measurements can be performed without suffering from the reflection of sound from the walls of the chamber, i.e in the free-field conditions The maximum allowable difference in anechoic chambers between measured and theoretical free-field levels according to ISO 3745 [1] is provided in Table 1

Table 1 Maximum allowable difference in anechoic chambers between measured and theoretical free-field levels

One-third octave band center frequency (Hz) Allowable difference (dB)

To study the acoustic quality of the anechoic facility in PNRPU, three sound sources were used (see Fig 3):

 - low-frequency 15”-loudspeaker (100 - 2000 Hz);

 - omnidirectional source Bruel & Kjaer 4295 (low and medium frequencies 125 - 5000 Hz);

 - high-frequency sound source based on the compressor driver JBL Selenium D408 Ti (5000 – 20000 Hz)

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Fig 3 The low-frequency sound source near a wall of the chamber (left), the mid-frequency sound source in the center of the chamber (center),

the high-frequency sound source near a wall of the chamber (right)

With these sound sources, two types of experiments were performed (Fig 4) In the first case (Configuration 1), the source (a loudspeaker) was placed near a wall, in the region of the expected noise source of a turbulent jet, which will be present in the chamber The measurements were performed for three radial directions 0о, 45о and 90о from the source The acoustic data were obtained by moving microphones in discrete steps along a radial, with the microphones motionless during data acquisition (30 s) at each microphone location Then the source was replaced and the measurements were repeated

In the second case (Configuration 2), the source was placed in the center of the anechoic chamber, which corresponds to the typical tests of acoustic characteristics for pieces of machinery The measurements were also performed along the radial directions 0о, 45о and 90о from the source (see Fig 4) in the same manner as for Configuration 1

During these tests, possible reflective surfaces in the chamber (such as the jet nozzle, the vortex ring generator orifice, and the collector) were covered with sound-absorbing wedges; such a configuration corresponds to the measurements in anechoic chambers without flow Measurements with flow (turbulent jet, vortex rings etc) require additional qualification and validation tests and will be reported later

Fig 4 A scheme of the qualification tests (top-view): Configuration 1 (left), Configuration 2 (right)

Positions of the microphones are summarized in Table 2; their height above the chamber’s floor was fixed during the tests At every measurement point, the distance between a microphone and the tips of the sound-absorbing wedges exceeded 1 m

The measurements were performed with 4 microphones (¼” Bruel & Kjaer 4961) connected to Bruel & Kjaer LAN-XI 3055-В-120 Three of these microphones were moved along the radial directions, while one was placed at a fixed point inside the anechoic chamber and served as a reference signal to check the stability of the source over the time span of the measurement The data were acquired and post-processed with Bruel & Kjaer PULSE system (version 15)

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Table 2 Positions of the microphones during the qualification tests

Point No

Distance from the source, m Distance from the source, m Direction 1 Direction 2 Direction 3 Direction 1 Direction 2 Direction 3

1 0.50 0.50 0.50 0.50 0.50 0.50

2 0.59 0.59 0.59 0.59 0.59 0.59

3 0.71 0.71 0.71 0.71 0.71 0.71

4 0.84 0.84 0.84 0.84 0.84 0.84

5 1.00 1.00 1.00 1.00 1.00 1.00

6 1.19 1.19 1.19 1.19 1.19 1.19

7 1.41 1.41 1.41 1.41 1.41 1.41

8 1.68 1.68 1.68 1.68 1.68 1.68

9 2.00 2.00 2.00 2.00 2.00 2.00

10 2.38 2.38 2.38 2.38 2.38 —

11 2.83 2.83 2.83 2.83 2.83 —

12 3.36 3.36 3.36 3.36 3.36 —

13 4.00 4.00 — 4.00 — —

14 4.76 4.76 — 4.76 — —

The deviation of the measured data from the inverse square law was calculated with the optimal reference method

[4] According to it, the theoretical free-field decay at distance r from the source is given by

0

( ) 20lg

p

a

L r

r r



where a represents the apparent strength of the source and r0 is an offset distance between the physical location of

the source and its effective acoustic center The a and r0 parameters are computed from the measured sound pressure

levels as

2

0

,



Here, N is the number of measurement points along the radial direction, q i = 10-0.05L pi , L pi is the sound pressure level

at distance r i from the source The difference (in dB) between the measured and theoretical free-field levels for

distance r i is then defined by

( )

In what follows, the differences ΔL pi are used for the assessment of the acoustic quality of the chamber

It should be noted that the inverse square law spreading takes place in the acoustic far field As a result, there is a

minimum distance rmin corresponding to the near field, so that for r < rmin the inverse square law spreading might fail

to hold It can be assumed [4], that

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95 m/Hz max 0.5m,

r

f

The region in the anechoic chamber where the free-field conditions are realized is therefore determined for distances

r > rmin from the sound source

4 Results of acoustic qualification tests

Fig 5 provides illustrative results of anechoic chamber qualification for low frequencies as plots of the deviation

from the free-field decay (ΔL pi ) versus linear distance r from the source for 1/3-octave frequency bands The plots

correspond to Configuration 1; the low-frequency loudspeaker was used as a source producing broadband noise in

the pertinent frequency range It is seen that the deviations are within the tolerance bounds for the distances less than

3 m from the source

100 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

Dir# 1 Dir# 2 Dir# 3 Tolerance limit 125 Hz

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

Dir# 1 Dir# 2 Dir# 3 Tolerance limit

160 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

400 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

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Fig 5 Deviation from the free-field decay versus linear distance from the source for different 1/3-octave frequency bands

(Configuration 1, low-frequency loudspeaker)

The frequency ranges of the loudspeakers are overlapping, which allows comparison of deviations from the free-field decay for different sources in the same 1/3-octave frequency bands Such a comparison is shown in Fig 6 for low-frequency and mid-frequency loudspeakers, which demonstrates that the results for different sources are in a good agreement

800 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

Fig 6 Deviation from the free-field decay versus linear distance from the source, Configuration 1:

the low-frequency loudspeaker (left), the mid-frequency source (right)

Illustrative results for high frequencies are provided in Fig 7 and demonstrate that inverse square law spreading is observed for all measurement points of Configuration 1 in the high-frequency range

2500 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

10000 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

Fig 7 Deviation from the free-field decay versus linear distance from the source for different 1/3-octave frequency bands (Configuration 1)

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Similar data are shown in Fig 8 for Configuration 2 Again, the region where deviation from the free-field decay

is within the tolerance limits has been found at distances less than 3 m from the source

125 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

500 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

2500 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

10000 Hz

-2.5

-1.5

-0.5

0.5

1.5

2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

-2.5 -1.5 -0.5 0.5 1.5 2.5

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

r, m

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Fig 8 Deviation from the free-field decay versus linear distance from the source for different 1/3-octave frequency bands (Configuration 2)

The results for the other 1/3-octave frequency bands, which have not been provided here for brevity, also confirm the conclusion that the region where inverse square law spreading holds is within 3 m radius from the source

5 Conclusions

The paper describes the anechoic facility built in Perm National Research Polytechnic University (PNRPU) in 2014-2015 with the objective of studying noise of turbulent jets, vortex rings, and airframe noise

The results of acoustic qualification of the anechoic chamber are provided The performed measurements have shown that the anechoic facility in PNRPU realizes the free-field conditions The radius of the region where the inverse square law spreading is observed equals to 3 m for broadband noise in the frequency range 125 Hz – 20 kHz, provided that microphones are in the far-field of the sound source and at least at the distance of 1 m from the wedge tips

These results demonstrate that the anechoic facility in PNRPU allows the aeroacoustic measurements to be performed to obtain quantitative results

Acknowledgements

The work has been performed with the financial support of the Russian government under grant “Measures to Attract Leading Scientists to Russian Educational Institutions” (contract No 14.Z50.31.0032)

References

[1] ISO, ISO 3745, Acoustics—Determination of sound power levels of noise sources—Precision methods for anechoic and semi-anechoic rooms, International Organization for Standardization, Geneva, Switzerland, 2003

[2] K.K Ahuja, Designing clean jet-noise facilities and making accurate jet-noise measurements, Int J Aeroacoustics 2 (2003) 371-412 [3] I.V Belyaev, A.Yu Golubev, A.Ya Zverev, S.Yu Makashov, V.V Palchikovskiy, A.F Sobolev, V.V Chernykh, Experimental Investigation

of Sound Absorption of Acoustic Wedges for Anechoic Chambers, Acoust Phys 61 (2015) 606-614

[4] K.A Cunefare, V.B Biesel, J Tran, R Rye, A Graf, M Holdhusen, A.M Albanese, Anechoic chamber qualification: Traverse method, inverse square law analysis method, and nature of test signal, J Acoust Soc Am 113 (2003) 881-892

Tiêu đề	Design and qualification of an anechoic facility in PNRPU
Tác giả	V. F. Kopieva, V. V. Palchikovskiy, Yu. V. Bersenev, S. Yu. Makashova, I. V. Belyaeva, I. A. Korin, E. V. Sorokin, I. V. Khramtsov, O. Yu. Kustov
Trường học	Perm National Research Polytechnic University
Chuyên ngành	Aeroacoustics
Thể loại	Conference paper
Năm xuất bản	2017
Thành phố	Moscow

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