PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Qualification of an anechoic chamber
Anthony NASH1
1

Charles M. Salter Associates, USA

ABSTRACTAn anechoic chamber is typically used for acoustical experiments requiring a free-field environment. The
free-field properties of the chamber can be determined by using a so-called “divergence loss” (i.e., adraw-away) test to demonstrate that sound pressure decreases with increasing distance in accordance with theinverse square law. For the qualification of free-field environments, the principal governing standard is ISO
26101:2012. This standard prescribes protocols suitable for qualifying both hemi- and fully anechoic
chambers including tolerances for deviations from the inverse square law. Either random noise or pure tones
can be used for the ISO divergence loss test method, depending on the intended application of the chamber.
This paper describes an ISO qualification test for a medium-sized anechoic chamber with an internal volumeof 51 cubic meters. The chamber was equipped with a partly-open floor grate and furnished with foam
wedges having questionable sound-absorptive performance. The chamber could satisfy the ISO tolerances
when using random noise but failed to qualify when using pure tones. The paper discusses the measurements
and the likely causes of the discrepancy.
Keywords: Anechoic Chamber, Sound Absorption, Free Field, Divergence Loss, Inverse Square Law, Draw-Away

1. INTRODUCTION
The fully-anechoic chamber was manufactured by Industrial Acoustics Company (IAC). It isequipped with exposed sound-absorptive foam wedges on all six surfaces (see Figure 1). The net
volume of the chamber is 51 cubic meters (i.e., in the region between the wedge tips). An
expanded-metal mesh walking surface extends across most of its projected floor area (see Figure 1).
The walking surface is supported by slender metal posts that bear on pedestals below. Even though thefloor grating has many perforations, it is still capable of reflecting high-frequency sound; therefore,
the operational assumption is that the user would cover the walking surface with sound-absorptive
material during critical measurements. This assumption also applied to the qualification
measurements — i.e., whenever the measuring microphones were placed relatively close to thewalking surface, its semi-reflective surface was covered by 50-millimeter thick glass-fiber panels(see Figure 4).
The project specifications call for a post-construction acoustical qualification test to be conductedin accordance with Annex A of ISO 3745. The measurements involve using a qualified sound source(s)that emits sound in an omnidirectional pattern. The resulting sound pressures from the source(s) aremeasured at a number of calibrated distances to check whether the sound pressure amplitudes decrease
with distance in accordance with the so-called inverse-square law. If the chamber conforms to theinverse-square characteristic within specified tolerances, ISO 3745 presumes that its acousticalenvironment constitutes a so-called “free field” (i.e., sound waves propagate freely without
reflections).
In an ideal free-field environment, the sound pressure from a small source decreases at a rate of six
decibels per doubling of distance (i.e., a halving of sound pressure). The ratio of [decreasing] soundpressure to [increasing] distance is called the “divergence loss” or draw-away characteristic of an
anechoic chamber.

1

anthony.nash@cmsalter.com

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Deviations from the ideal draw-away characteristic represent imperfections of the acousticalenvironment within the chamber. With respect to a fully-anechoic chamber, Annex A of ISO 3745
specifies limits for deviations from the inverse-square characteristic. For frequencies between 800 and5000 hertz, the allowable deviations are plus or minus one decibel. Below 800 hertz and above 5000
hertz, the allowable deviations are relaxed to plus or minus 1.5 decibels.

Figure 1 – Plan section of 51-m3 anechoic chamber. The tips of the foam wedges are spaced apart by 3.63 meters (11 ft., 11 in.)Figure 2 – Expanded metal mesh floor grating used as a walking surface. The floor grating is removable for critical experiments
at high frequencies.

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Qualifying the chamber required several draw-away measurements of sound pressure generated bya fixed source having uniform directionality. The specifications called for measuring sound pressure ata discrete points spaced 25 millimeters apart along five independent traverses, each of which would be
500 millimeters long. The measurements were to be obtained at the center frequency of eightcontiguous octave bands extending from 125 Hz to 16 kHz.
The project specifications read in part:“....The source signal used during the testing shall be single or multiple discrete
frequency sinusoidal pure tones in each octave band over the qualification range.
Broadband noise, such as pink noise or white noise, is not allowed...”.
As discovered during the measurements, the requirement for discrete tonal test signals would havesignificant ramifications when attempting to qualify this particular anechoic chamber.

2. MICROPHONE ARRAY
The large matrix of position and frequency data required by the specifications leads to 800 discrete
field measurements of sound pressure. For this reason, it was decided to “mass produce” the field dataacquisition by simultaneously recording the acoustical signals from an array of four or seven matchedmicrophones (see Figure 3).

3. SOUND SOURCE SIGNALS
In addition, the tonal source signals were also “mass produced” by creating a pair of complex toneclusters based on the respective centers of each fractional octave band. Each tone cluster represented alow- or high-frequency range, respectively.
The signal frequencies were expressly selected so a) they were aligned precisely with the bin
centers of a fast Fourier transform (FFT) analyzer and, b) potential harmonic distortion components
from a given tone would not coincide with the fundamental of any other tonal signal. In this way, an
FFT spectrum of the recorded complex tonal signal would represent the true received sound pressure ateach tonal frequency of interest.
Since the number of discrete tones within each of the frequency clusters did not affect the fieldmeasurement time, it was decided to decrease the frequency spacing of the complex tonal signal fromoctave-band intervals to one-third-octave-band intervals. With this improved spacing of the test
signals, the low-frequency tone cluster now contained 12 tones and the high-frequency tone cluster
contained 13 tones as shown in Table 1 below.
Table 1 – Tone clusters used with low- and high-frequency sound sources
Frequency, hertz
Low Range Cluster

High Range Cluster

100
125
160
206

824*
1000*
1248*
1600

257
325
388
485
611
824*
1000*
1248*

2064
2576
3248
3880
4848
6112
8240
10000
12504

*duplicated tonal signals

By intent, three of the tonal signals within each cluster were duplicates of tones in the other cluster;thus, the low- and the high-frequency sound sources each reproduced three duplicate tonal signals.

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4. SOUND SOURCES
Given the eight-octave frequency range, it was necessary to use two types of sound sources — one
for low frequencies and the other for high frequencies. The low-frequency source was a Bruel & Kjaer
OmniSource Type 4295 as shown in Figure 3. The Type 4295 was used to produce signals in the range
from 100 Hz through 1250 hertz.

Figure 3 – Left photo: checking the one-meter separation between low-frequency source and closest microphone.
Right photo: array of seven microphones installed along a diagonal traverse beginning one meter from the low-frequency
source. The 50-millimeter-thick vertical glass-fiber panels help eliminate undesirable reflections from the floor grating.

The high-frequency source was a custom-built design conforming to the arrangement suggested in
ISO 3745. It comprised a 1.5-meter length of 1/8-inch, Schedule 80 steel pipe coupled via tapered
adapters to a JBL Type 2426J compression driver. The open end of the steel pipe faced upward asshown in Figure 4. This high-frequency source satisfied the ISO 3745 directivity requirements up to
90 degrees off axis. The high-frequency source was used to produce signals between 800 Hz and12.5 kHz (the upper frequency limit of the compression driver).

Figure 4 – Left photo: array of four microphones along a diagonal traverse beginning one meter from the high-frequency source.
Right photo: array of seven microphones installed along a traverse normal to the high-frequency source

The microphones and their pre-amplifiers were inserted into aluminum tubes fitted over spaced
aluminum dowels installed in a square wood member. The wood member, in turn, was supported by anadjustable tripod head. Figure 4 depicts the fixture used for supporting the microphones at
25-millimeter intervals. For each traverse measured using tone clusters, an array of seven microphone
tubes was moved three times along the wood member until data were measured at all 20 microphone
positions.
These multi-microphone and multi-tone signal configurations enabled the field data matrix
to be reduced from [5 traverses x 20 microphone positions x 8 bands =] 800 recordings to [5 traverses
x 3 microphone array positions x 2 frequency ranges =] ~30 recordings.

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5. RECORDING INSTRUMENTATION
G.R.A.S. Type 40AE pre-polarized, one-half-inch condenser microphones were installed on
Brüel & Kjær Type 2671 constant-current line drive (CCLD) microphone pre-amplifiers. Eachmicrophone pre-amplifier was connected to one channel of a Brüel & Kjær Type 5963 eight-channel
accelerometer power supply system. The Type 5963 featured a DeltaTron ® constant-current supply
combined with a custom amplifier having a gain of 40 decibels. The amplified output signals from theType 5963 were recorded on an eight-channel Sony Type 208Ax digital audio tape (DAT) machine —the eighth channel of the DAT machine recorded the source test signal as a timing reference. The
frequency response of the entire acoustical measuring/recording system was qualified to >10 kHz.

6. DATA ANALYSISThe calibrated recordings were later played back into a fast Fourier transform (FFT) dynamic signal
analyzer using a flattop window. This analysis resulted in 210 individual FFT spectra, each containing
an array of either 12 or 13 tones, depending on the frequency range of the source signal cluster. Eachtonal amplitude was extracted from the FFT spectrum and placed in a spreadsheet so a series ofdivergence loss (i.e., draw-away) data could be plotted at each respective frequency.
In addition to tonal signals, broad-band (“pink”) noise was also used as a supplemental
high-frequency source signal for one of the five traverses. In this case, only four microphones were
deployed so the noise data could be measured onsite using a four-channel one-third-octave-band
analyzer. The microphones were spaced 100 millimeters apart — for this measurement, the length of
the four-microphone array was 300 millimeters. The array was successively moved to nine
overlapping measurement positions along the fixture, resulting in 36 measurements (including 14
duplicates). The broadband noise test (with its duplicated measurement positions) was intended asa further quality control technique in case of discrepancies found in the tonal measurements.

7. DISCUSSION OF MEASURED DATA
Figure 5 is a scatter plot of one-third-octave-band data from the broadband noise test. The abscissa
axis represents the log of the distance ratio in the region between the near and far ends of the
500-millimeter traverse (the closest microphone is one meter from the source). The ordinate axis
represents the sound pressure level.

Sound Pressure Level, decibels re: 20 micropascals

40

2 kHz band

Anechoic Chamber Draw-Away Test
pink noise sound source
measured in one-third-octave bands

y = -19.8x + 36.6
R² = 0.935

4 kHz band

y = -16.2x + 32.1
R² = 0.8

6.3 kHz band

y = -16.5x + 28.0
R² = 0.9

8 kHz band

y = -18.4x + 23.2
R² = 0.8

30

25

20
0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Log of Distance Ratio (relative to one meter from source)

Figure 5 – Draw-away data from a random noise test signal as measured in one-third-octave bands. At first glance, thestraight-line fits appear reasonable except that the downward slopes for the 4 kHz and 6.3 kHz bands are too shallow for a freefield. Many of the scatter points in this chart have multiple values — these data represent duplicated microphone positions.

For an ideal anechoic chamber, the draw-away characteristic should appear as a straight line
defined by the expression,y = -20x + intercept
where:y is the sound pressure level in decibels
x is the common logarithm of the distance ratio
intercept is the sound pressure level at the point where the line intersects the y-axis

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For a distance ratio of 1.5, an ideal free field should exhibit a reduction in sound pressure of
precisely 3.522 decibels (equivalent to 6.0206 decibels per doubling of distance). In Figure 5,
the fitting equation for the 2 kHz band nearly attains this ideal draw-away slope. For this same band,the coefficient of determination (the squared correlation coefficient, R 2 ) is 0.9, indicating that the data
exhibit relatively minor fluctuations about the straight-line approximation.
For the higher frequency noise bands, the slopes of the fitting lines do not conform to the model. In
the 4 kHz and 8 kHz bands, the slopes vary from -16.2 to -18.4 and the squared correlation coefficients
have fallen to 0.8.
Figure 6 is a scatter plot for the same traverse using high-frequency tones. Compared to bands ofnoise, the use of discrete tones is more revealing of residual acoustical reflections arising from thewalls, floor, and ceiling surfaces. It is now quite obvious that the acoustical environment in thechamber is not behaving like a free field.
60
y = -10.0x + 57.4
R² = 0.5

Sound Pressure Level, decibels re: 20 micropascals

1600 hertz

55
824 hertz

y = -15.7x + 54.2
R² = 0.73880 hertz
y = -10.7x + 51.5
R² = 0.2
50

y = -26.4x + 49.8
R² = 0.3

2056 hertz
45

Anechoic Chamber Draw-Away Test
high-frequency sound source

40
0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Log of Distance Ratio (relative to one meter from source)

Figure 6 – Draw-away data from the high-frequency tone cluster. The unusual behavior of the 2056-Hz trace indicates that thechamber is not anechoic.

The draw-away for the 2056-Hz tone bears no resemblance to the 2 kHz one-third-octave band of
random noise shown in Figure 5, above. In Figure 6, the slope of the straight-line fit for the 2056-Hz
tone is minus 26.4 (eight decibels per doubling) — a value that is physically impossible. The severenotch in the draw-away for the 2056-Hz tone suggests an acoustical cancellation occurring between thearriving wave front and a strong reflection from a nearby surface.
Figure 7 shows draw-away data measured with the low-frequency sound source and its associatedtone cluster. Here, the slope for 125 hertz is much too shallow and the slope for 824 hertz is much too
steep. The squared correlation coefficients appear to be acceptable; however, this appearance isdeceiving since the 25-millimeter spatial sampling interval is only a small fraction of a wavelength.
70

Sound Pressure Level, decibels re: 20 micropascals

125 hertz

y = -7.6x + 69.3
R² = 1.0
y = -20.5x + 65.3
R² = 1.0

257 hertz
65

325 hertz

Anechoic Chamber Draw-Away Test
low-frequency sound source

y = -22.6x + 62.3
R² = 1.060824 hertz

y = -28.7x + 56.1
R² = 1.0

55

50
0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Log of Distance Ratio (relative to one meter from source)

Figure 7 – Draw-away data measured with a low-frequency tone cluster. The downward slopes of the 325-Hz and 824-Hz test
signals are too steep for a free-field environment.

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Sound Pressure Level, decibels re: 20 micropascals

65

low

Anechoic Chamber Draw-Away Test
low-frequency sound source
versushigh-frequency sound source(SPL normalized at one meter)

1248 hertz

60
highy = -14.9x + 60.1
R² = 0.6low551000 hertz

highy = -12.5x + 53.7
R² = 0.6low
50824 hertz

y = -15.7x + 48.9
R² = 0.7

high

45
0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Log of Distance Ratio (relative to one meter from source)

Figure 8 – Comparison between the low- and high-frequency sources radiating the three duplicated signals from the pair of toneclusters. The slopes are much too shallow for a free field.

As shown in Figures 8, both sources exhibit significant problems with respect to the slopes of thestraight-line fits being far from the ideal value of -20; nevetheless, one would still expect thedraw-away performance of the two sources to match one another more closely.

8. CLOSING REMARKSFor this particular anechoic chamber, it is likely that the sound-absorption properties of the foam
wedges were deficient. There was no evidence that the wedges were ever qualified by testing them in
an impedance tube.
The slopes of the straight-line fits are affected by reflections from the walls, floor, and ceiling of thechamber. These phase-dependent reflections lead to narrow-band acoustical cancellations at highfrequencies and non-ideal slopes at low frequencies.
In short, the chamber is absorptive but not anechoic.

ACKNOWLEDGEMENTS
Discussions with Douglas Winker of ETS-Lindgren were very much appreciated as he helped
convince the author that the measured draw-away characteristics of a well-designed anechoic chambershould closely resemble an ideal free-field.

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