Acoustic Test Chambers and Environments

Publish Date: Aug 12, 2014 | 7 Ratings | 4.57 out of 5 | Print

A test chamber is a specialized environment that assists in acoustical measurements. The purpose is twofold: 

  • To create an environment in which the relationship between sound power and sound pressure is well known
  • To reduce or eliminate interference from intruding noises, including but not limited to environmental noises, operation of support equipment, mechanical equipment, automobile, truck, aircraft, and rail traffic.


Sound power measurements require the following acoustical test chambers and environment:

  • Precision Grade: Anechoic Chamber, Hemi-Anechoic Chamber, and Reverberation Chamber
  • Engineering Grade: Hemi-Anechoic Chamber and Free-Field Chamber
  • Survey Grade: Free-Field Chamber and Natural Environment


Sound intensity measurements require a Free-Field Chamber and Natural Environment.

Sound quality measurements require a Free-Field Chamber and Natural Environment.


Basic Concept
The sound pressure level varies with distance and direction from a sound source. Some of the variations are due to the sound source, and some to the test environment. A good test environment allows the user to discern the difference.

The following graphics depict five relevant regions within the sound field:

  • Near Field: A region near the sound source where the sound pressure fluctuates about a mean value. In the near field, sound waves arriving from different points on the source are out of phase and interfere constructively and destructively. Thus, the depth of the near field is a function of the geometry of the source, the measurement location, and the wavelength of sound.
  • Far Field: A region farther from the sound source where the sound pressure diminishes gradually with distance. In the far field, sound waves arriving from different points on the source are in phase. The far field begins where the near field leaves off.
  • Direct Field: A region in which the majority of sound energy arriving from the sound source at a point has traveled a straight line between them, that is, without reflection.
  • Reverberant Field: A region in which the majority of sound energy arriving from the sound source at a point has been reflected at least once. In the statistical limit of an infinite number of reflections, the reverberant field approaches diffuse field conditions.
  • Ambient Noise region: A region in which the majority of sound energy arriving at a point is not generated at the sound source, but rather is intrusive sound not associated with the test.


Sound Field in a Chamber with Low Ambient



Sound Field in a Chamber with High Ambient



Interior Acoustics of Test Chambers

1. Anechoic Chamber
An anechoic chamber is required for precision-grade sound power measurements such as ANSI S12.35 and ISO 3745. Such a chamber consists of a high-transmission loss outer envelope (usually constructed of concrete or modular steel panels) with an interior cladding of anechoic wedges on roof, floor and walls. A tensioned cable floor is sometimes included to permit walking above the wedges.


Schematic of an Anechoic Chamber


An anechoic wedge is designed to provide a normal-incidence sound absorption coefficient greater than 0.99 for all frequencies down to its design cutoff frequency. Reflected sound from such a wedge is attenuated 20 dB or more. An anechoic wedge is typically one-quarter wavelength long at its cutoff frequency. For example, a 100 Hz cutoff wedge is usually approximately 36 inches long. The design and production of effective anechoic wedges is an extremely difficult task and is best left to acoustic chamber manufacturers.

Because of the effective lack of reflections in an anechoic chamber, the direct field fills the chamber. The relationship between sound pressure level and sound power level is simply that of inverse square law spreading:



where LPi is sound pressure level at the i-th microphone, LW is sound power level, r is the distance from the source to receiving point in meters, and DIi is the directivity index of the source in the direction of the i-th microphone.

The noise reduction of a well-designed and well-constructed enclosure will typically be at least numerically equal to the sound transmission loss of the shell components. Thus, the interior sound pressure levels in the i-th one-third octave band should be:



Sound pressure levels at each microphone position for the equipment under test should ideally exceed the background sound pressure levels by 10 dB or more.

Qualification
At this level of performance, any sound-reflective surface can compromise the performance of the environment. Although most reflections are effectively suppressed, some always remain. Therefore, the standards mandate a qualification test consisting of a series of “draw-away” tests to identify areas in which the propagation of sound does not follow the above equation to an acceptable degree.

Measurement Surfaces
A spherical microphone array is used that calls for a minimum of 20 microphone positions in the anechoic chamber. The radius of the spherical surface is required to be no less than twice the characteristic length dimension of the equipment under test. Under certain conditions, as many as 40 microphone positions are required.

It is customary to allow ¼-wavelength between the microphones and the wedge tips, although in some cases (such as with a broadband noise source), it may be possible to qualify the room with microphones positioned closer to the wedge tips.

  • Advantages/Disadvantages of the Anechoic Chamber
    + Directivity information of source preserved
    + Time history of sound is preserved
    + High degree of measurement accuracy
    - Requires a large, relatively expensive, carefully qualified chamber, on the order of a 20 ft cube (for testing to 100 Hz) for small sound sources, larger with increasing source size and decreasing test frequency.
    - Large number of microphone positions. Multi-channel simultaneous acquisition should be considered where laboratory throughput is important. Industry Standards:
    ANSI S12.35
    ISO 3745



2. Hemi-Anechoic Chamber
An anechoic chamber is used in both precision-grade sound power measurements, such as with ANSI S12.35 and ISO 3745, and engineering-grade sound power measurements made in accordance with ANSI S12.34, ISO 3744. Many test codes require the use of a hemi-anechoic chamber (such as for computers, ECMA 74, ISO 7779, and ANSI S12.10). Such a chamber consists of a high-transmission loss envelope with an interior cladding of anechoic wedges on roof and walls. The floor is intentionally made reflective, with an absorption coefficient of 0.06 or less, and is typically constructed of concrete.



Schematic of an Hemi-Anechoic Chamber


Because of the effective lack of reflections in an anechoic chamber, the direct field fills the chamber and the relationship between sound pressure level and sound power level when the source is mounted at the reflecting plane is simply that of inverse square law spreading with a directivity of 2:



where LPi is sound pressure level at the i-th microphone, LW is sound power level, r is the distance from the source to receiving point in meters, and DIi is the directivity index of the source in the direction of the i-th microphone. When the source height above the reflecting plane is significant (about 1/10-th wavelength) this relationship no longer holds: the phase of the direct and reflected waves must be accounted for.

For practical reasons, hemi-anechoic chambers are often preferred when the equipment under test is large or heavy.

The noise reduction of a well-designed and well-constructed enclosure will typically be at least numerically equal to the sound transmission loss of the shell components. Thus, the interior sound pressure levels in the i-th one-third octave band should be:



The isolation of the chamber envelope can be compromised if too large or too many penetrations are used, or if the penetrations are not properly engineered for noise control. Flanking transmission can also conduct significant amounts of sound energy into the chamber.

Sound pressure levels at each microphone position for the equipment under test should ideally exceed the background sound pressure levels by 10 dB or more.

Qualification
A series of “draw-away” tests are mandated by the precision-grade standards to identify areas in which the propagation of sound does not follow the above equation to an acceptable degree. A greater degree of deviation is permitted in a hemi-anechoic chamber than in an anechoic chamber.

For engineering-grade sound power testing, the primary qualification procedure is to perform a sound pressure measurement on a reference sound source and to note the differences in the sound power computed off the measurement grid. The maximum permissible environmental correction factor for this measurement is 2 dB.

Measurement Surfaces
For precision-grade sound power measurement, a hemi-spherical grid of 10 microphones is required. The radius of the hemisphere is required to be no less than twice the characteristic dimension of the source.

For engineering-grade sound power measurement, a rectangular parallelepiped (that is, shoebox) measurement surface is permitted. The grid fits more or less conformably around the source at a distance of (typically) 1 meter, but less than the plan dimensions of the equipment under test. For tall machines, this provides a considerable reduction in the required chamber size.

It is customary to allow ¼ wavelength between the microphones and the wedge tips, although in some cases (such as with a broadband noise source), it may be possible to qualify the room with microphones positioned closer to the wedge tips.

  • Advantages/Disadvantages of the hemi-anechoic chamber:
    + Most directivity information of source preserved (especially for small source height relative to wavelength)
    + Time history of sound is preserved
    + Moderately high degree of measurement accuracy
    - Requires a large, relatively costly, carefully qualified chamber, on the order of 20 x 20 x 10 ft (for testing down to 100 Hz) for small sound sources and larger with increasing source size and decreasing test frequency.
    + Hemi-anechoic chamber is less costly than an anechoic chamber.
    - Large number of microphone positions requires a large number of microphones. Multi-channel simultaneous acquisition should be considered where laboratory throughput is important. Industry Standards:
    ANSI S12.35
    ISO 3745
    ANSI S12.34
    ISO 3744
    ANSI S12.10
    ISO 7779
    ECMA 74


3. Reverberation Chamber
A reverberation chamber is used in precision-grade sound power measurements, such as ANSI S12.31 and ISO 3741, and in sound absorption measurements, such as ASTM C423 and ISO 354. Such a chamber consists of a high-transmission loss envelope with sound reflective interior cladding. All measurements are made in the reverberant field of the sound source.

Sound Power
Sound Absorption



For sound absorption testing
All surfaces are intentionally made reflective, with an average absorption coefficient of 0.05 or less required at all frequencies after allowance for air absorption. The volume of the chamber must be greater than 125 m3 and should be 200 m3 or greater. Room dimensions shall not be ratios of small integer numbers, nor shall the ratio of the largest to smallest room dimension exceed 2:1. Sound-reflective diffusing elements are required to be suspended in the chamber, and it is strongly recommended that some of them be kept moving during tests. The number and surface area of diffusers may be determined empirically using a qualification procedure. Otherwise, the surface area of the diffusers (both sides) should be at least 25% of that of the host chamber.

The ideal dynamic range of the measurement in each one-third octave band is on the order of 45 dB. Note that in order to be able to measure simultaneously in all bands, the dynamic range requirement must also be met in all bands simultaneously.

For sound power testing
The requirements are similar, but the room volume should be 100 times the source volume for precision-grade testing. Also, additional sound absorption is required at frequencies below 200 V1/3, where V is the volume in cubic meters.

Construction Issues
Chambers constructed of concrete are very useful for sound absorption testing, but require that low-frequency sound absorbers be added for sound power level testing. Chambers constructed of modular steel panels are very useful for sound power level testing because the panels are flexible at low frequency and provide just about the correct amount of low-frequency absorption. Steel panel chambers are, however, not exceptionally effective for sound absorption testing: the elevated sound absorption at low frequencies may approach the limits tolerated by the standards, and may make it difficult to resolve low sound absorption values which are typical at low frequency.

Pressure/Power Relationship
The chamber is designed to produce an extremely large number of room modes which, in the statistical limit of extremely high mode count, result in the following simple relationship between sound pressure level and sound power level:



where LP is sound pressure level, LW is sound power level, and A is the Sabine absorption of the chamber. Various corrections are given in the standards for temperature and static pressure. For sound power testing, a Waterhouse Correction is introduced that accounts for greater-than-average sound energy density near the walls.

The sound pressure level is essentially uniform throughout the central portion of the chamber if the mode count is high. Note that mode count is proportional to Vf3, so that the mode count decreases dramatically with lower frequency. Doubling the chamber volume effectively extends the low frequency performance by one-third octave.

Sound Isolation
The noise reduction of a well-designed and well-constructed enclosure will typically be significantly less than the sound transmission loss of the shell components. This occurs because any sound penetrating into the chamber interior persists in the chamber over a number of reflections, raising the energy density. Thus, the interior sound pressure levels in the i-th one-third octave band should be:



Sound pressure levels with the broadband noise source(s) active should exceed the ambient sound levels (including impulsive events) by 45 dB or more simultaneously in all bands of interest.

Qualification
For Sound Absorption:

  • Empty room sound absorption coefficients.
  • Measurement of the variation of the decay rate with microphone position in a chamber with no test specimen present.
  • Measurement of the variation of the decay rate with test specimen position. A reference test specimen is used.
  • Measurement of the variation of the decay rate with sound source position.


For Sound Power:

  • Empty room sound absorption coefficients.
  • For broadband noise sources, variation of the average sound pressure level with the position of a reference sound in the chamber.
  • For tonal noise sources, variation of the sound pressure level with microphone position for a loudspeaker playing a succession of discrete tones.


Advantages/Disadvantages
Advantages/Disadvantages to Reverberation Chamber for sound power

- Directivity information of source lost
- Time history of sound is lost
+ Moderately high degree of measurement accuracy
- Requires a large, carefully qualified chamber, approximately 200 cubic meters
+ Measurements can be performed rapidly with one microphone
  • Industry Standards:
    ANSI S12.31/S12.32
    ISO 3741/3742
    ASTM C423/ISO 354



4. Free-Field Chamber
A free-field chamber is an approximation of a hemi-anechoic chamber. Rather than carrying anechoic wedges on the walls and ceiling, the chamber walls and ceiling are treated with several inches of glass fiber or a similar sound absorbing material. Small rooms and those intended for low frequency testing require a thicker layer, large rooms, and those intended for high frequency sometimes have sound absorption treatments as thin as 3 in.

Free-field chambers are used for sound power testing to engineering and survey grade. Free-field chambers are also used for sound intensity testing, where their primary function is to control ambient noise and to increase the accuracy of intensity measurements by reducing sound reflections.

For engineering-grade sound power testing, the parallelepiped measurement surface is used. The primary qualification procedure is to perform a sound pressure measurement on a reference sound source and to note the differences in the sound power computed off the measurement grid. A maximum permissible difference of 2 dB is prescribed. If the deviation is too great and the sound absorption treatment of the chamber cannot be improved, the solution is to use a larger number of microphones positioned closer to the sound source.

Note regarding sound intensity testing
The textbook theory of sound intensity indicates that the method can completely eliminate the effect of sound reflections and ambient noise within the chamber. The assumptions behind this theory are:

  • An infinite number of sampling points are used, and
  • Source output is completely steady or all points are sampled simultaneously.


Practical implementation requires a non-infinite number of sampling points made successively over time. Therefore while the method suppresses the effects of reflections to a great degree, it cannot do so completely. A free-field chamber usually provides sufficient sound absorption and sound isolation to permit excellent results with the sound intensity method.

  • Advantages/Disadvantages to Free-Field Rooms:
    + Tolerable degree of measurement accuracy
    + Requires a moderate sized, relatively less expensive, qualified chamber.
    - Large number of microphone positions requires a large number of microphones. Multi-channel simultaneous acquisition should be considered where laboratory throughput is important.
    - Reduced precision and accuracy relative to hemi-anechoic chamber, except for sound intensity

  • Industry Standards:
    ANSI S12.34
    ISO 3744
    ANSI S12.10
    ISO 7779
    ECMA 74
    ANSI S12.12
    ANSI S12.36
    ISO 3746


5. Other Environments
“Other environments” is a catch-all category that covers in situ conditions, testing outdoors, and testing in non-dedicated spaces (such as conference rooms), and so on. In most cases these are useful for survey-grade sound power measurements. It may be possible in very large and very quiet spaces to qualify for engineering-grade measurements.

For sound quality measurements, the effect of the environment on the sound may be a relevant part of the test, such as for sound recordings within automobiles. In such a case, interior acoustics is not at issue, but potential interference from ambient sound should still be considered.

Interior Acoustics
Most “other” environments lack sound absorption treatments on walls and ceilings, and therefore, have a hard time qualifying for engineering-grade sound power measurements. A larger space tends to perform better because of the larger absorption footprint. Obviously, testing outdoors has merit in certain applications because it is the largest “room” possible, and approximates the performance of a hemi-anechoic chamber (the “infinite parking lot”).

Sound intensity methods are also appropriate in such an environment. It should be remembered, however, that the method is not completely “bulletproof,” and that the accuracy of measurements made in a highly reverberant environment may be compromised. To the degree that the test environment departs from hemi-anechoic conditions, additional care and additional sampling points may be required.

Sound Isolation
“Other” environments usually provide a moderate degree of sound isolation at best. Noise from building mechanical systems and nearby activities intrude to some degree. The degree of acceptability of the situation depends on the relative levels generated by the equipment under test and the prevailing ambient. Thus, it’s more likely that such a space will be acceptable for a loud sound source than for a quiet one.

If the ambient level is too high, it can even interfere with sound intensity measurements. In practice, it appears that steady state ambient sound can be effectively cancelled only up to the point where it is 10 dB greater than the sound pressure level of the equipment under test. An impulsive ambient can be cancelled up to the same limit, but only if measurements are performed simultaneously at all positions. If ambient levels are too high, a quieter environment should be sought.

The greatest difficulty in testing outdoors is controlling the ambient. Thus, outdoor tests are typically performed on large machinery.

  • Advantages/Disadvantages to Other Environments:
    - Poor measurement accuracy
    + Requires no special test chamber.
    - A greater number of measurement positions may be required, extending test time.
    - Greater interference from ambient noise may be expected. Industry Standards:
    ANSI S12.12
    ANSI S12.36
    ISO 3746

Additional References

Related NI Products:

Information Contributed By: David A. Nelson, P.E., INCE Bd. Cert. Nelson Acoustical Engineering, Inc. specializes in noise and vibration control, sound quality, laboratory facilities and test control systems, and instruction related to plants, buildings, laboratories, products, and machinery.

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