Acoustic Testing Room and Environment

The test chamber is a specialized environment that supports acoustic measurements. It serves two primary purposes:

· Create an environment in which the relationship between sound power and sound pressure is known.

· To reduce or eliminate noise interference, including ambient noise, as well as noise generated by auxiliary equipment, mechanical equipment, automobiles, trucks, aircraft, rail transit systems, and other devices.

Sound power measurement requires the following acoustic test chamber and environmental conditions:

· Engineering grade: semi-anechoic chamber and free-field chamber

· Measurement levels: anechoic chamber and natural environment

· Accuracy levels: anechoic chambers, semi-anechoic chambers, and reverberation chambers

Sound intensity measurements require a free-field chamber and a natural environment.

Sound quality measurement requires a free-field chamber and a natural environment.

Basic Concepts

Sound pressure level varies with distance from and direction relative to the sound source. Some of this variation is attributable to the sound source itself, while other variations may be due to the test environment. A well-controlled test environment enables users to distinguish between these different sources of variation.

The following figure illustrates five domains related to the sound field:

· Far field: the region far from the sound source, where sound pressure decreases gradually with increasing distance. Within the far field, sound waves at different points relative to the source are in phase. The far field begins where the near field ends.

· Direct field: the region where sound energy primarily originates from the sound source and propagates in a straight line without undergoing reflection.

· Reverberant field: the region where sound energy is primarily derived from reflections (at least one) off surfaces originating from the sound source. Under the statistical constraint of a finite number of reflections, the reverberant field gradually transitions into a diffuse field.

· Near field: the region close to the sound source, where the sound pressure fluctuates around its mean value. Within the near field, sound waves at different distances from the source interfere with one another—either constructively or destructively—due to differences in phase. Consequently, the depth of the near field depends on the geometry of the sound source, the measurement location, and the sound

A function of wavelength.

· Environmental noise zone: The sound energy does not originate from the sound source but primarily from areas unrelated to the test.

 

Sound Field in a Low-Background-Noise Test Chamber

 

Sound Field in a High-Environmental-Noise Test Chamber

 

1. Anechoic Chamber

Aanechoic chambers are required for high-accuracy sound power measurements, such as those specified in ANSI S12.35 and ISO 3745. Such test chambers feature an outer enclosure with high transmission loss—typically constructed from concrete or modular steel panels—and their ceilings, floors, and walls are acoustically treated to provide sound absorption.

Internal wedges. Tension-cable flooring is often used to enable walking on the wedges.

 

Anechoic Chamber Structure

Sound-absorbing wedges are designed to provide an absorption coefficient greater than 0.99 for all frequencies above the design cutoff frequency. Reflections from such wedges result in an intensity reduction of 20 dB or more. The length of a sound-absorbing wedge is typically one-quarter of the wavelength at the cutoff frequency. For example,

Wedges with a 100 Hz cutoff frequency are typically about 36 inches long. The design and manufacture of high-efficiency acoustic wedges is a highly complex task that must be undertaken by acoustical room designers.

The sound intensity level at each microphone location on the device under test shall be at least 10 dB higher than the background sound pressure level.

Due to the absence of reflections in an anechoic chamber, the interior constitutes a direct field. The relationship between sound pressure level and sound power level is governed by the simple inverse-square law of propagation:

 

Here, LPi denotes the sound pressure level at the ith microphone, LW is the sound power level, r is the distance from the sound source to the receiving point in meters, and DIi is the directivity index of the sound source in the direction of the ith microphone.

A well-designed and properly constructed enclosure typically achieves noise reduction that is, in numerical terms, at least equal to the sound transmission loss of the enclosure components. Therefore, the internal sound intensity level at the i-th one-third octave band is:

Advantages and Disadvantages of an Anechoic Chamber

Advantage: Preserves directional information of the sound source.

Advantages: Preserves the temporal history of sound

Advantages: High-level measurement accuracy

Drawbacks: The method requires a large, relatively expensive, and precisely calibrated anechoic chamber; for small sound sources, a 20-foot cubic chamber is needed (for testing at 100 Hz), and if larger sound sources are to be measured, the test frequency must be lowered. Another drawback is the need for a large number of microphone positions. If the experiment

When throughput is critical, multi-channel synchronous acquisition must be considered.

Verification

At this level of performance, any sound-reflective surfaces will affect the acoustic performance of the environment. Although many reflections are effectively suppressed, some still remain. Therefore, code and standards mandate the completion of verification testing, which includes a series of deviation tests to identify sound transmission.

The broadcast, to a certain extent, did not adhere to the aforementioned equation.

Measure the surface

The use of a spherical microphone array requires at least 20 microphone positions within the anechoic chamber. The radius of the spherical surface must be no less than twice the characteristic length dimension of the device under test. Under certain conditions, as many as 40 microphone positions may be required.

It is permissible to allow a quarter-wavelength gap between the microphone and the wedge tip; however, in certain situations—such as when dealing with broadband noise sources—it may be necessary to verify the room using a microphone positioned close to the wedge tip.

Industry standards: ANSI S12.35, ISO 3745.

2. Semi-anechoic chamber

An anechoic chamber is used for high-precision sound power measurements, such as those specified in ANSI S12.35 and ISO 3745, as well as for engineering-grade sound power measurements, such as those specified in ANSI S12.34 and ISO 3744. Many test codes require the use of a semi-anechoic chamber (for example, for computers, ECMA 74).

(ISO 7779 and ANSI S12.10). Such test chambers feature an enclosure with high transmission loss, with sound-absorbing wedges installed on the interior ceiling and walls. The floor is deliberately designed to be highly reflective, with a sound absorption coefficient of 0.06 or less, and is typically constructed from concrete.

Semi-anechoic chamber schematic

Due to the anechoic chamber’s lack of reflections, the interior is dominated by the direct sound field. When the sound source is mounted on a reflective plane, the relationship between sound pressure level and sound power level follows an inverse-square law with a directional exponent of 2:

Here, LPi denotes 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 receiving point in meters, and DIi is the directional index of the i-th microphone relative to the sound source direction. When the height of the sound source is significantly greater than the reflection plane—approximately one-tenth of the wavelength—

), this relationship no longer holds: the phases of the direct sound wave and the reflected sound wave must be adjusted.

For practical reasons, semi-anechoic chambers are typically preferred when the device under test is large or heavy.

A well-designed and well-constructed enclosure typically achieves noise reduction that is, in numerical terms, at least equal to the sound transmission loss of the enclosure components. Therefore, the internal sound intensity level at the i-th third-octave band is:

 

Using excessively large or excessive penetration depths, or failing to properly configure the penetration for noise control, can negatively impact the isolation of the test-room enclosure. Furthermore, diffraction and other secondary propagation paths may introduce excessive sound energy into the test room.

Ideally, the sound pressure level at each microphone position of the device under test should exceed the background sound pressure level by 10 dB or more.

Verification

The accuracy grading standard requires performing a series of deviation tests to identify areas where sound propagation does not conform to the aforementioned equation. Compared with an anechoic chamber, a semi-anechoic chamber permits larger deviations.

For sound power testing of engineering-grade systems, the primary verification step is to complete sound pressure measurements using a reference sound source, while carefully monitoring deviations in sound energy across the measurement grid. The maximum permissible environmental correction factor for this measurement is 2 dB.

Measure the surface

To perform precise sound-power measurements, a hemispherical microphone array consisting of 10 microphones is required. The radius of the hemisphere must be no less than twice the characteristic dimension of the sound source.

For sound power measurements of engineering-grade systems, rectangular parallelepiped (shoebox) measurement surfaces are permitted. The grid is suitable for positions approximately 1 meter from the sound source, but its dimensions are smaller than the planned dimensions of the equipment under test. For large machines, this reduces the required size of the test chamber.

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It is permissible to position the microphone at a quarter-wavelength from the wedge tip; however, in certain situations—such as when dealing with broadband noise sources—it may be necessary to verify the room acoustics using a microphone placed close to the wedge tip.

Advantages and disadvantages of a semi-anechoic chamber:

Advantages: Preserves most of the directional information of the sound source (especially for sources whose dimensions are small relative to the wavelength).

Advantages: Preserves the temporal history of sound

Advantages: Relatively high measurement accuracy

Disadvantages: The method requires a large, relatively expensive, and precisely calibrated anechoic chamber. For small sound sources, a 20-by-20-by-10-foot cubic chamber is needed (for testing at 100 Hz); if larger sound sources are to be measured, the test frequency must be lowered.

Advantages: A semi-anechoic chamber is more cost-effective than a full anechoic chamber.

Drawbacks: Requires a large number of microphone positions. If laboratory throughput is a priority, multi-channel synchronous data acquisition should be considered.

Industry standards: ANSI S12.35, ISO 3745, ANSI S12.34, ISO 3744, ANSI S12.10, ISO 779, ECMA 74.

3. Resonance Chamber

The reverberation chamber is used for high-precision sound power measurements, such as those specified in ANSI S12.31 and ISO 3741, as well as sound absorption measurements, such as those specified in ASTM C423 and ISO 354. This test chamber consists of a highly sound-insulating enclosure with sound-reflective interior linings. All measurements are conducted with the sound source

to be conducted in the reverberation area.

 

For sound absorption testing

All surfaces are deliberately made reflective, with an absorption coefficient of 0.05 or less across all frequencies, allowing for partial air absorption. The test chamber must have a volume greater than 125 cubic meters, preferably 200 cubic meters or more. The room dimensions must not be in a 1:1 ratio; the ratio of the longer side to the shorter side

The ratio must also not exceed 2:1. Sound-reflective materials need to be suspended within the room, and it is strongly recommended that they be moved during the testing process. The dimensions of the test chamber and the scattering-surface area may be determined through a verification procedure based on practical experience. In addition, the surface area of the scattering region (on both sides)

At least 25% of the main test rooms are required.

The ideal measured dynamic range for each third-octave band is 45 dB. Note that, in order to measure all frequency bands simultaneously, the dynamic-range requirement must be met across all bands at the same time.

Sound Power Testing

The requirements are similar, but the room volume must be 100 times the sound source volume used for precision-level testing. In addition, sound absorption is required at frequencies below 200 V¹/³, where V represents the volume in cubic meters.

Construction Issues

Test chambers constructed from concrete are highly effective for sound absorption testing; however, low-frequency sound-absorbing materials must be incorporated for sound power level measurements. Test chambers built from modular steel panels, on the other hand, are particularly well suited for sound power level testing, as steel panels exhibit excellent performance at low frequencies.

It is highly flexible and provides adequate low-frequency sound absorption. However, steel-plate test chambers are not the most effective for sound-absorption testing: the enhanced low-frequency absorption may approach the limits of standard tolerances, making it difficult to accurately determine typical low-frequency sound-absorption values.

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Sound Pressure/Power Relationship

The test chamber is designed for use with a large number of room modes; in the statistical limit of extremely high mode density, the following simple relationships hold between sound pressure level and sound power level:

Sound isolation

A well-designed and properly constructed enclosure typically achieves noise reduction that is, in numerical terms, significantly less than the sound transmission loss of the enclosure components. This is because any sound that penetrates into the test chamber will, after a few reflections, become trapped within the chamber, thereby increasing the sound energy level.

degree. Therefore, the internal sound intensity level of the i-th one-third octave band is:

Here, LP denotes sound pressure level, LW denotes sound power level, and A represents the Sabine absorption of the test room. The standard provides several corrections based on temperature and static pressure. For sound power measurements, the Waterhouse correction is introduced to account for the wall’s sound energy density exceeding the average value.

Correction.

When the number of modes is sufficiently large, the sound pressure level is essentially uniform in the central region of the test chamber. Note that the number of modes is proportional to Vf3; therefore, as frequency decreases, the number of modes drops rapidly. Doubling the volume of the test chamber effectively extends the low end by one-third of an octave.

Frequency performance.

 

In the presence of active broadband noise sources, the sound pressure level shall exceed the ambient sound level (including impulsive events) by 45 dB or more across all frequency bands of interest.

Verification

Regarding sound absorption:

Sound absorption coefficient of an empty room.

Measure the variation in the microphone position attenuation rate within the test chamber in the absence of a test specimen.

Measure the change in decay rate in the presence of test specimens, using reference test specimens.

Measure the variation in the decay rate when the sound source location is known.

Regarding sound power:

Sound absorption coefficient of an empty room.

For broadband noise sources, the average sound pressure level varies with the position of the reference sound source within the test room.

For a pitch-based sound source, the sound pressure level varies with the microphone position relative to the sound pressure level of the intermittent-pitch speaker.

Pros/Cons

Advantages and Disadvantages of a Reverberation Chamber for Sound Power

Disadvantage: Loss of directional information about the sound source.

Disadvantage: The temporal history of the sound is lost.

Advantages: Relatively high measurement accuracy

Disadvantages: Requires a large, rigorously validated test chamber, approximately 200 cubic meters in size.

Advantages: Measurement can be completed quickly using a single microphone.

Industry Standard:

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 semi-anechoic chamber. Instead of installing anechoic wedges on the walls and ceiling, a few inches of fiberglass or similar sound-absorbing material are applied to the walls and ceiling of the test room. Small rooms, as well as rooms used for low-frequency testing, require specific materials.

The layer is relatively thick; large rooms and rooms used for high-frequency testing sometimes contain only three inches of sound-absorbing material.

An anechoic chamber is used for engineering-level and survey-level sound power measurements. It is also employed for sound intensity measurements, with its primary function being to control ambient noise and enhance measurement accuracy by minimizing sound reflections.

For sound power testing at the engineering grade, a parallelepiped test surface must be used. The primary verification step is to conduct sound pressure measurements on the reference sound source, bearing in mind that the difference in sound power is calculated based on the measurement grid. A maximum allowable difference of 2 dB is permitted. If the difference exceeds

In large indoor spaces, sound-absorbing materials cannot be effectively improved; the solution is to deploy more microphones positioned close to the sound source.

Precautions for Sound Intensity Testing

The theory of sound intensity posits that the testing method can completely eliminate the effects of indoor sound reflections and ambient noise. The key premise underlying this theory is:

Use an infinite number of sampling points

The source output is completely stable, or all points are sampled simultaneously.

The practical implementation requires sequentially sampling a finite number of points over a period of time. Therefore, although this method can suppress the reflection effect to a certain extent, it cannot eliminate it entirely. An anechoic chamber typically provides ample sound absorption and sound isolation, thereby enabling

The perfect result of the sound intensity method.

Advantages/Disadvantages of a Free-Field Room:

Advantages: Acceptable measurement accuracy

Advantages: A test chamber of appropriate size, relatively inexpensive, and proven effective is required.

Drawbacks: A large number of microphone positions necessitates the use of a correspondingly large number of microphones. If laboratory throughput is a critical factor, multi-channel synchronous data acquisition should be considered.

Disadvantages: Aside from sound intensity, its accuracy is lower compared with a semi-anechoic chamber.

Industry Standard:

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 category that encompasses all remaining types, including conditions that are appropriate, outdoor testing, and testing in non-dedicated areas (such as conference rooms), among others. In most cases, these are used for sound power level measurements under survey conditions. Such tests may require a large, very quiet space.

The measurement of the project’s completion grade is now complete.

In sound-quality measurements, the influence of the environment on sound can be a relative factor in the testing process—for example, when recording sound inside a vehicle. In such cases, internal acoustics are not the primary concern; rather, potential interference from ambient noise must be taken into account.

Internal Acoustics

 

The sound intensity method is still used in such environments. However, it should be remembered that this method is not entirely “bulletproof,” and its measurement accuracy may be compromised in highly reverberant environments. To achieve a degree of deviation from a semi-anechoic test environment, it is necessary to…

Pay attention to the outside and perform sampling.

Many “other” environments lack sound-absorbing materials on walls or ceilings, making it extremely difficult to perform engineering-grade sound-power measurements. Larger spaces may perform better because they offer more absorption area. Clearly, outdoor testing has

It offers advantages for certain applications due to its large footprint, enabling performance that approximates that of a semi-anechoic chamber (an infinitely large parking lot).

Sound isolation

If the surrounding environment is highly reflective, it can even interfere with sound-intensity measurements. In fact, steady-state ambient noise can be effectively suppressed to levels approximately 10 dB below the sound-pressure level of the device under test. Impulsive noise can also be suppressed to the same limit, but if the measurement is conducted in

All positions are completed simultaneously. However, when the environmental level is high, it is necessary to seek a quieter environment.

“Other” environments typically provide only moderate sound isolation. Noise from building structures, mechanical systems, and surrounding activities may cause interference. The acceptability of such conditions depends on the relative intensity of the noise generated by the equipment under test and the ambient noise level in the surrounding environment. Because

Therefore, compared with quieter sound sources, louder ones are more likely to be perceived in such environments.

The greatest challenge in outdoor testing is controlling ambient noise. Consequently, outdoor tests are typically conducted on large-scale equipment.

Advantages/Disadvantages of Other Environments:

Disadvantages: Lower measurement accuracy

Advantages: No special test chamber is required.

Disadvantages: It may be necessary to measure more locations, thereby extending the test duration.

Disadvantages: Increased environmental noise interference may occur.