A testing chamber is a special environment used to assist in acoustic measurements. It has two main purposes:
• Create an environment where the relationship between sound power and sound pressure level is known.
• To reduce or eliminate noise interference, including environmental noise, auxiliary equipment, mechanical equipment, automobiles, trucks, airplanes and rail transportation and other equipment.
Sound power measurement requires the following acoustic testing room and environment:
• Precision levels: Anechoic chamber, semi-anechoic chamber, and reverberation chamber
• Engineering level: Semi-anechoic chamber and free-field chamber
• Survey Level: Free-space rooms and natural environments
Sound intensity measurement requires a free-field room and a natural environment.
Sound quality measurement requires a free-field room and a natural environment.
Basic Concepts
Sound pressure level varies with distance from and direction from the sound source. Some of this variation is caused by the sound source itself, while other variations may be due to the testing environment. A good testing environment allows users to distinguish these variations.
The following diagram illustrates five areas related to the sound field:
• Near field: The region close to the sound source, where sound pressure fluctuates around the average value. Within the near field, sound waves at different distances from the sound source, due to their different phases, mutually reinforce or weaken each other through interference. Therefore, the near field depth is a function of the sound source geometry, the measurement location, and the sound wavelength.
• Far field: The region far from the sound source, where sound pressure gradually decreases with increasing distance. Within the far field, sound waves at different distances from the sound source are in phase. The far field begins where the near field ends.
• Direct field: The region where sound energy mainly originates from the sound source and propagates in a straight line without being reflected.
• Resonant field: The region where sound energy primarily originates from the sound source through reflection (at least once). Under the statistical constraint of a finite number of reflections, the resonant field gradually reaches the conditions of a scattering field.
• Ambient noise area: The sound energy does not originate from the sound source but mainly from areas unrelated to the test.
Sound field of the low ambient noise test chamber
Sound field of high ambient noise test chamber
1. Anechoic chamber
An anechoic chamber is required for high-precision acoustic power testing, as per ANSI S12.35 and ISO 3745 standards. Such a test chamber contains a high-transmission-loss outer enclosure (typically constructed of concrete or modular steel plates), and its ceiling, floor, and walls have internal sound-absorbing wedges. Tension cable flooring is often used to allow walking on the wedges.
Anechoic chamber structure
A noise-canceling wedge is designed to provide an absorption coefficient greater than 0.99 for all frequencies above the design cutoff frequency. After reflection from such a wedge, the intensity will attenuate by 20 dB or more. The length of a noise-canceling wedge is typically one-quarter of the wavelength at the cutoff frequency. For example, a wedge with a 100 Hz cutoff frequency is typically about 36 inches long. The design and manufacture of high-efficiency noise-canceling wedges is a highly complex task and must be completed by the acoustics room designer.
Because there is no reflection in the anechoic chamber, the sound field is direct. The relationship between sound pressure level and sound power level follows a simple inverse square propagation law:
Where LPi is the sound pressure level at the i-th microphone, LW is the sound power level, r is the distance from the sound source to the receiver in meters, and DII is the direct index of the sound source in the direction of the i-th microphone.
A well-designed and well-constructed enclosure typically reduces noise by at least numerically equal to the sound propagation loss of the enclosure components. Therefore, the internal sound intensity level at the i-th third pitch is:
The sound intensity level at each microphone location of the device under test should be 10 dB or more higher than the background sound pressure level.
verify
At this performance level, any sound-reflecting surface will affect the performance of the environment. Although many reflections are effectively suppressed, some still remain. Therefore, the standard requires verification testing, including a series of deviation tests, to identify areas where sound propagation does not conform to the above equation to some extent.
Measuring surface
Using a spherical microphone array requires at least 20 microphone positions within an anechoic chamber. The radius of the spherical surface must be at least twice the length of the device under test. Under certain conditions, up to 40 microphone positions are required.
A quarter wavelength is permissible between the microphone and the wedge tip, although in some cases (such as in broadband noise environments), it may be necessary to use a microphone positioned closer to the wedge tip for room verification.
• Advantages and disadvantages of an anechoic chamber <br />Advantages: Preserves directional information of the sound source; preserves the time history of sound; high level of measurement accuracy. Disadvantages: Requires a large, relatively expensive, and precise verification room; a 20-foot cube is needed for small sound sources (for testing 100 Hz); if measuring larger sound sources, the test frequency needs to be reduced. Requires numerous microphone positions. If lab throughput is important, multi-channel simultaneous acquisition needs to be considered.
Industry standards: ANSI S12.35, ISO 3745.
2. Semi-anechoic chamber
Anechoic chambers are used for high-precision sound power measurements, such as ANSI S12.35 and ISO 3745, as well as engineering-grade sound power measurements, such as ANSI S12.34 and ISO 3744. Many test codes require the use of semi-anechoic chambers (e.g., for computers, ECMA 74, ISO 7779, and ANSI S12.10). Such test chambers contain a high-transmission-loss shell, and their interior ceilings and walls are decorated with sound-absorbing wedges. The floor is intentionally designed to be highly reflective, with an absorption coefficient of 0.06 or less, and is typically constructed of concrete.
Semi-anechoic chamber structural diagram
Because anechoic chambers lack reflection, the interior is filled with a direct sound field. When the sound source is installed on the reflecting plane, the relationship between sound pressure level and sound power level follows the inverse square law transmission relationship with a direction of 2:
Where LPi is the sound pressure level of the i-th microphone, LW is the sound power level, r is the distance from the sound source to the receiver in meters, and DII is the direction index of the i-th microphone in the direction of the sound source. When the height of the sound source is significantly higher than the reflecting plane (approximately one-tenth of the wavelength), this relationship no longer holds: the phase of the direct sound wave and the reflected sound wave must be adjusted.
For practical reasons, when the equipment being tested is large or heavy, a semi-anechoic chamber is usually preferred.
A well-designed and well-constructed enclosure typically reduces noise at least numerically equal to the sound propagation loss of the enclosure components. Therefore, the internal sound intensity level at the i-th third pitch is:
If excessive penetration depth is used, or if the penetration is not properly configured for noise control, the test chamber enclosure isolation may be negatively affected. Detour propagation can also carry excessive sound energy into the test chamber.
Ideally, the sound pressure level at each microphone location of the device under test should exceed the background sound pressure level by 10 dB or more.
verify
The precision rating standard requires a series of deviation tests to identify areas where sound propagation does not conform to the aforementioned equations for reception. Semi-anechoic chambers allow for larger deviations compared to anechoic chambers.
For engineering-grade sound power testing, the main verification step is to complete the sound pressure measurement of the reference sound source, paying attention to any deviations in sound energy within the measurement grid. The maximum permissible environmental correction factor in this measurement is 2 dB.
Measuring surface
To perform accurate-level sound power measurements, a hemispherical grid containing 10 microphones is required. The radius of the hemisphere must be no smaller than twice the size of the sound source's characteristic dimensions.
For engineering-grade sound power measurements, a rectangular parallelepiped (shoebox) measurement surface is permitted. The grid is suitable for a location approximately 1 meter from the sound source, but smaller than the planned dimensions of the device under test. For large machines, this reduces the required test chamber size.
A quarter wavelength can be allowed at the microphone and the wedge tip, although in some cases (such as in broadband noise source environments), it may be necessary to use a microphone closer to the wedge tip for room verification.
• Advantages and disadvantages of semi-anechoic chambers:
Advantages: It preserves most of the directional information of the sound source (especially sound sources that are relatively small in wavelength).
Advantages: Preserves the time history of sound; relatively high level of measurement accuracy. Disadvantages: Requires a large, relatively expensive, and precise verification room; a 20×20×10-foot cube is needed for small sound sources (for testing 100 Hz); if measuring larger sound sources, the test frequency needs to be reduced.
Advantages: Semi-anechoic chambers are less expensive than anechoic chambers.
Disadvantages: Requires numerous microphone locations. If throughput is critical in the lab, multi-channel simultaneous acquisition needs to be considered.
Industry standards: ANSI S12.35, ISO 3745, ANSI S12.34, ISO 3744, ANSI S12.10, ISO 779, ECMA 74.
3. Resonance Chamber
Resonance chambers are used for high-precision sound power measurements, such as ANSI S12.31 and ISO 3741, and sound absorption measurements, such as ASTM C423 and ISO 354. This test chamber consists of a high-transmission-loss enclosure with an internal sound-reflecting veneer. All measurements are performed in the reverberant region of the sound source.
For sound absorption testing
All surfaces are intentionally made reflective, with an absorption coefficient of 0.05 or lower at all frequencies, allowing for partial air absorption. The test chamber volume must be larger than 125 cubic meters, 200 cubic meters or more. The room dimensions cannot be 1:1, and the ratio of the long side to the short side cannot exceed 2:1. Sound-reflecting material needs to be suspended inside the chamber, and its movement during testing is strongly recommended. The test chamber and scattering surface area may be determined based on practical experience using a validation process. Furthermore, the surface area of the scattering area (both sides) needs to be at least 25% of the main test chamber.
The ideal dynamic range for each third of a octave is 45 dB. Note that in order to measure all frequency bands simultaneously, the dynamic range requirement must be met in all frequency bands at the same time.
Sound power test
The requirements are similar, but the room volume must be 100 times the volume of the sound source used for precise level testing. Additionally, sound absorption is required at frequencies below 200 V<sub>1/3</sub>, where V is a volume expressed in cubic meters.
Construction issues
Test chambers constructed of concrete are useful for sound absorption testing, but require the addition of low-frequency sound-absorbing materials for sound power level testing. Test chambers built of modular steel plates are useful for sound power level testing because steel plates are very flexible at low frequencies, providing accurate low-frequency absorption capabilities. However, steel plate test chambers are not the most effective for sound absorption testing: the increased low-frequency sound absorption may reach the limits of standard tolerance, making it difficult to resolve typical low-frequency sound absorption values.
Sound pressure/power relationship
The test chamber is designed for use with a large number of room modes, and at the statistical limit of an extremely high number of modes, the following simple relationship between sound pressure level and sound power level will be produced:
Where LP is the sound pressure level, LW is the sound power level, and A is the Sabine absorption of the test chamber. Several corrections are provided in the standard based on temperature and static pressure. For sound power testing, a Waterhouse correction is introduced to compensate for the higher-than-average sound energy density of the walls.
If the number of modes is sufficient, the sound pressure level is essentially uniform in the central part of the test chamber. Note that the number of modes is proportional to Vf3, therefore the number of modes decreases rapidly as the frequency decreases. Doubling the volume of the test chamber effectively extends the low-frequency performance by one-third of the octave.
Sound isolation
A well-designed and well-constructed enclosure typically reduces noise numerically much less than the sound propagation loss of the enclosure components. This is because any sound penetrating the test chamber will remain inside within the test chamber after several reflections, thus increasing the energy intensity. Therefore, the internal sound intensity level at the i-th third pitch is:
In the presence of active broadband noise sources, the sound pressure level will be 45 dB or more higher than the ambient sound level (including impulse events) across all frequency bands of interest.
Verification <br />Regarding sound absorption:
• Sound absorption coefficient of an empty room.
• The change in microphone position attenuation rate in the test chamber was measured in the absence of a test specimen.
• Measure the change in decay rate in the presence of a test specimen. Use a reference test specimen.
• Measure the change in attenuation rate when the sound source is present.
For sound power:
• Sound absorption coefficient of an empty room.
• For broadband noise sources, the average sound pressure level varies with the location of the reference sound source in the test chamber.
• For a pitch source, the sound pressure level varies with the microphone position relative to the speaker's sound pressure level at the intermittent pitch.
Advantages/Disadvantages <br />Advantages/Disadvantages of a reverberation chamber for sound power: Disadvantages: Loss of directional information from the sound source; Loss of the time history of the sound. Advantages: Relatively high measurement accuracy. Disadvantages: Requires a large, carefully validated test chamber, approximately 200 cubic meters. Advantages: Measurements can be completed quickly using a single microphone.
• Industry standards:
ANSI S12.31/S12.32
ISO 3741/3742
ASTM C423/ISO 354
4. Free space room
A free-field chamber is similar to a semi-anechoic chamber. Instead of using sound-absorbing wedges in the walls and ceiling, it uses several inches of fiberglass or similar sound-absorbing material in the walls and ceiling of the test chamber. Smaller rooms and those used for low-frequency testing require thicker layers of material, while larger rooms and those used for high-frequency testing sometimes contain only three inches of sound-absorbing material.
Free-field chambers are used for sound power testing at both engineering and survey levels. They are also used for sound intensity testing, primarily to control ambient noise and improve the accuracy of intensity tests by reducing sound reflections.
For engineering-grade sound power testing, a parallelepiped test surface is required. The main verification step involves performing sound pressure level tests on a reference sound source, noting that sound power differences are calculated based on the measurement grid. A maximum allowable difference of 2 dB is permitted. If the difference is too large, the room's sound-absorbing materials cannot be improved; the solution is to use more microphones positioned near the sound source.
Precautions for sound intensity testing
The theory of sound intensity states that testing methods can completely eliminate the effects of indoor sound reflections and environmental noise. The key point behind this theory is:
• Use an unlimited number of sampling points
• The source output is completely stable or all points are sampled simultaneously.
Practical implementation requires sequential sampling of a finite number of points over a period of time. Therefore, while this method can suppress reflection effects to a certain extent, it cannot completely eliminate them. Free-field rooms typically provide sufficient sound absorption and sound isolation, thus yielding perfect results from the sound intensity method.
• Advantages/disadvantages of free-field rooms:
Advantages: Acceptable measurement accuracy; requires a moderately sized, relatively inexpensive, and validated testing room.
Disadvantages: A large number of microphone locations require a large number of microphones. If throughput is critical in the lab, multi-channel simultaneous acquisition needs to be considered.
Disadvantages: In addition to sound intensity, it has lower precision compared to a semi-anechoic chamber.
• 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" encompasses all remaining categories, including suitable conditions, outdoor testing, testing in non-dedicated areas (such as conference rooms), and so on. In most cases, these are used for survey-level sound power testing. Engineering-level measurements may require a large, very quiet space.
For sound quality measurements, the effect of the environment on sound may be a relative part of the test, such as sound recording in a car. In such cases, internal acoustics are not the issue, but potential interference from ambient sound needs to be considered.
Internal acoustics
Many “other” environments lack sound-absorbing materials on walls or ceilings, making it difficult to perform engineering-grade sound power measurements. Larger areas may perform better because they offer more space for absorption. Outdoor testing clearly has advantages for certain applications because it provides ample space and can approximate the performance of a semi-anechoic chamber (an infinitely large parking lot).
The sound intensity method still applies in such environments. However, it's important to remember that this method is not entirely "bulletproof," and measurement accuracy can be affected in highly resonant environments. Extra care and sampling are required to achieve test environments that deviate from a semi-anechoic state.
Sound isolation
"Other" environments typically offer at most moderate sound isolation. Noise from buildings, mechanical systems, and surrounding activities can cause interference. The acceptability of these conditions depends on the relative intensity of the noise emitted by the device under test and the ambient sound intensity. Therefore, louder noise sources are more likely to be acceptable in such spaces compared to quieter sources.
High ambient noise levels can even interfere with sound intensity measurements. In reality, steady-state ambient noise can be effectively suppressed to within approximately 10 dB of the sound pressure level of the device under test. Impulsive environments can be suppressed to the same limit, but this is not possible if measurements are taken simultaneously at all locations. In high ambient noise levels, a quieter environment is necessary.
The biggest challenge in outdoor testing is controlling environmental noise. Therefore, outdoor testing is usually carried out on large machines.
• Other advantages/disadvantages of the environment:
Disadvantages: Lower measurement accuracy. Advantages: No special testing room required.
Disadvantages: It may require measuring more locations, thus extending the testing time.
Disadvantages: May result in more environmental noise interference.
Industry Standards:
ANSI S12.12
ANSI S12.36
ISO 3746
More information on related NI products:
• Useful website for sound and vibration toolkits:
• Acoustic Systems
• Eckel Industries
• Industrial Acoustics
Information contributor: David A. Nelson, PE, INCE Bd. Cert., from Nelson Acoustical Engineering, Inc., specializing in noise and vibration control, sound quality, laboratory facilities, test and control systems, and related instructions for equipment, buildings, laboratories, products, and machines.
