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Acoustic test chamber and the environment

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semi-anechoic chamber anechoic chamber, and a total of

Free field sound intensity measurements require room and the natural environment.

Field measurement of sound quality room for freedom and the natural environment。

Basic concept

Sound pressure level with distance and direction from the sound source changes. Part of the change is a result of the sound source, it could be a result of the test environment. A good test environment for users to distinguish those changes.

The following figure shows the sound field related to five areas:

* The far field: the area away from the sound source, sound pressure gradually decreased as the distance increases. In the far field range, from different points of the sound source is the same phase. In the near-field far-field end of the place to start.

• Direct market: the sound energy is derived from the sound source in straight lines rather than by reflection of the region。

* Total sound field: The sound energy is derived from the sound source by reflection (at least once) in the region。 In the reflection of the limited number of statistical constraints, to reach a total sound scattering field conditions gradually。

* Near-field: the area near the sound source attachment, the sound pressure fluctuations around this average。 In the near field range, from the sound source at different positions as different phases, enhanced or weakened by interfering with each other。 Therefore, the depth of near-field sound source geometry, measurement location and sound

秒速时时彩 Tone function of wavelength.

* Regional Environmental Noise: Sound Energy does not come mainly from the sound source and has nothing to do with the testing area.

 

Low-noise acoustic test chamber

 

High ambient noise acoustic test chamber

 

1. Anechoic chamber

秒速时时彩 Anechoic chamber is the sound power level of precision required for testing, such as ANSI S12。35 and ISO 3745。 This test chamber containing the external envelope of the high transmission loss (usually concrete or modular steel structure), its ceilings, floors and walls with a silencer of

Internal wedge. Usually in order to walk in the wedge, but also with the tension cable floor.

 

Anechoic chamber

Silencer wedge is designed to control the frequency of higher than the design of all frequencies, providing higher than the absorption coefficient of 0.99. After such a reflection of the wedge, the intensity will be attenuated 20 dB or more. The length of silencer wedge cut-off frequency is usually a quarter wavelength. For example,

100 Hz cutoff frequency of the wedge is usually about 36 inches long. Efficient design and manufacture of silencers wedge is a very complex task, completed by the designer room acoustics.

DUT position of each microphone sound intensity level should be higher than the background sound pressure level of 10 dB or more.

Anechoic chamber because of the lack of reflection, indoor direct field. Sound pressure level and sound power level of the relationship between the spread of a simple inverse square law:

 

Which is the first i LPi microphone at the sound pressure level, LW is the sound power level, r is the point from the sound source to receiver distance in meters, DIi sound source is a microphone in the direction of i-direct index.

Well-designed and well-built enclosure for noise reduction is usually at least equal to the value of the sound transmission loss of housing components. So the first third of the pitch of the internal i-sound intensity level as:

Anechoic chamber of the advantages and disadvantages

Advantages: Save the direction of the sound source information

秒速时时彩 Advantages: Save time history of sound

Advantages: high level of accuracy

Disadvantages: large, relatively expensive, accurate verification of the room, sound source for the small cube is 20 feet (used to test 100 Hz), more if you want to measure the sound source, the test frequency to be reduced. Disadvantages: requires a lot of microphone position. If the experiment

Room throughput is important, you need to consider multi-channel synchronous acquisition.

Authentication

In this performance level, any sound reflective surface will affect the performance of the environment. Although many reflections are effectively suppressed, and some still exist. Therefore, standards required to complete verification tests, including a range of deviation from the test to find voice

Broadcast to some extent, the region failed to comply with the above equation.

Measurement surface

Requirements using spherical microphone array in an anechoic room at least 20 the microphone position。 The radius of the spherical surface of the device under test requires the length of not less than twice the size of features。 Under some conditions, take up to 40 microphone positions。

Allows the tip of the wedge between the microphone and a quarter wavelength, although in some cases (for example, broadband noise sources in the environment), may take place near the wedge tip of the microphone to the room to verify.

Industry Standard: ANSI S12.35, ISO 3745.

2。 Semi-anechoic chamber

秒速时时彩 Anechoic chamber for high-precision measurement of sound power levels, such as ANSI S12。35 and ISO 3745, and engineering measurement of sound power levels, such as ANSI S12。34 and ISO 3744。 Many test code requires the use of semi-anechoic chamber (for example, for computer, ECMA 74

, ISO 7779 and ANSI S12.10). Such high transmission loss test chamber shell contains within it the ceilings and walls decorated with a silencer wedge. Ground specially designed for high reflectivity and absorption coefficient of 0.06 or less, usually constructed of concrete.

Semi-anechoic chamber structure

Because the lack of reflection anechoic chamber and indoor full direct sound field, when the sound source installed in the reflective surface, the sound pressure level and sound power levels for the direction of the relationship between the inverse square law for the transfer of 2 relations:

Which is the first i LPi microphone sound pressure level, LW is the sound power level, r is the point from the sound source to receiver distance in meters, DIi the direction of the sound source is a microphone in the direction of i-index. When the sound source height above the reflecting plane is large (about one-tenth of the wavelength

), This relationship no longer holds: direct sound and reflected sound waves phase must be adjusted。

For practical reasons, large and heavy test equipment, they usually prefer to use semi-anechoic chamber。

Well-designed and well-built enclosure for noise reduction is usually at least equal to the value of the sound transmission loss of housing components。 So the first third of the pitch of the internal i-sound intensity level as:

 

If you use too large or too much penetration depth, or if not for the noise control through the appropriate settings, the test case isolation room may be adversely affected. Roundabout will be too much communication may sound energy into the test room.

The location of the device under test each microphone sound pressure level in the ideal case, should not exceed the background sound pressure level 10 dB or more。

Authentication

The precise level of standard deviations required to complete a series of tests to identify the sound propagation equation for failing to receive the region. Anechoic chamber in comparison, the semi-anechoic chamber to allow a large number of deviations.

Sound power levels for engineering testing, validation step is the completion of the main reference source of the sound pressure measurement, and pay attention to measuring the deviation of sound energy grid. The measurement of the maximum allowable environmental correction factor is 2 dB.

Measurement surface

秒速时时彩 In order to accurately measure sound power levels, need to use a microphone in the hemisphere containing 10 grid. Requirements better than the hemisphere radius twice the sound source, the small feature size.

秒速时时彩 Sound power levels for engineering measurements, allowing the rectangular parallelepiped (shoebox) measurement surface. From the sound source for the grid and the location of about 1 meter, but the plan than the size of a small device under test. For large machines, this reduces the size of the required testing room

。

Allows the tip of the microphone and a quarter wavelength of the wedge, although in some cases (for example, broadband noise sources in the environment), may take place near the wedge tip of the microphone to the room to verify.

Semi-anechoic chamber of the advantages and disadvantages:

Advantages: Save most of the direction of the sound source information (in particular in terms of smaller relative to the wavelength of the sound source)

Advantages: Save time history of sound

Advantages: relatively high level of accuracy

Disadvantages: large, relatively expensive, accurate verification of the room, sound source for the needs of small 20 × 20 × 10-foot cube (used to test 100 Hz), more if you want to measure the sound source, the test frequency to be reduced.

Advantages: Semi-anechoic chamber anechoic chamber than the lower cost.

Disadvantages: requires a lot of microphone position. If the laboratory throughput is important, you need to consider multi-channel simultaneous acquisition.

Industry Standard: ANSI S12。35, ISO 3745, ANSI S12。34, ISO 3744, ANSI S12。10, ISO 779, ECMA 74。

3. A total of anechoic chamber

Anechoic chamber were used for high-precision measurement of sound power, such as ANSI S12。31 and ISO 3741, and sound absorption measurements, such as ASTM C423 and ISO 354。 The testing room with the sound reflection from the interior shell composed of the high transmission losses。 All measurements in the sound source

Echo area.

 

Testing for sound absorption

All reflective surfaces are deliberately made, and its absorption coefficient at all frequencies of 0.05 or less, to allow some air absorption. Test chamber volume must be large than 125 cubic meters, 200 cubic meters or greater. Room size can not be 1:1, long side and short side than the

Cases can not exceed 2:1. Requires sound reflecting material hanging in the room, and strongly recommended to move in the testing process. Test chamber and scattering surface area may be used in accordance with the verification process to determine the actual experience. Furthermore, the scattering region surface area (both sides)

秒速时时彩 Requires at least 25% of the main test chamber.

秒速时时彩 One-third of the ideal scale for each measurement dynamic range is 45 dB. Note that in order to simultaneously measure all bands, all bands must meet dynamic range requirements.

秒速时时彩 Sound Power Testing

Demand is similar, but the room size must be used to accurately grade the test 100 times the capacity of the sound source. In addition, also need less than 200 V1 / 3 of the frequency of the sound absorption, where V is the volume expressed in cubic meters.

秒速时时彩 Construction issues

Test chamber constructed using concrete, very useful for sound absorption testing, but testing needs to add the sound power level low frequency sound absorption materials. Modular construction with steel plate sound power level of the test chamber testing is very useful because plate in the low-frequency

Under is flexible, provided the correct low-frequency absorption. Steel test chamber in the sound absorption test is not the most effective: improving the low-frequency sound absorption may be up to standard tolerance limit, so that the resolution of typical low-frequency sound absorption values ​​under difficult

。

 

Sound pressure / power relations

Test chamber is designed to use a lot of room modes, the number of statistical models in the high limit, will produce sound pressure level and sound power levels of the following simple relationship:

Sound isolation

Well-designed and well-built enclosure for noise reduction is usually much less than the value of the housing component of the sound transmission loss. This is because any puncture into the testing room within the sound will be reflected in within a few remain in the test room to improve the energy intensity

Degrees. So the first third of the pitch of the internal i-sound intensity level as:

Where LP is the sound pressure level, LW is the sound power level, A is the Sabine absorption of the test chamber. According to the temperature and static pressure, the standard gives a number of amendments. For sound power measurement, to compensate for the sound wall energy density is greater than the mean introduced Waterhouse

Amendment.

If a sufficiently large number mode, the sound pressure level is essentially a central part in the test room unity. Note that mode is proportional to the number and Vf3, so as reduce the frequency, mode number decreases rapidly. Doubling the size of the test chamber effectively extend the lower one-third scale

Frequency performance。

 

Noise sources in the presence of active broadband sound pressure level of interest in the band all over the ambient sound level (including pulse events) High 45 dB or more.

Authentication

For sound absorption:

Empty room sound absorption coefficient.

In the absence of the presence of the test specimen, the location of the test chamber's microphone to measure the decay rate changes.

秒速时时彩 Where the presence of the test specimen, measurement of the decay rate changes。 With reference to the test specimens。

Sound source location in case of change of decay rate measurements。

For sound power:

Empty room sound absorption coefficient.

For the broadband noise source, the average sound pressure level with reference to the sound source position changes in the test room.

The loud sound source, sound pressure level position with the microphone, speakers for playing continuous sound pressure level of pitch changes.

Advantages / disadvantages

A total of anechoic chamber for sound power advantages / disadvantages

秒速时时彩 Disadvantages: the direction of the sound source information is lost

Disadvantages: the sound of the time history is lost

Advantages: relatively high precision

Disadvantages: large, carefully validated testing room, about 200 cubic meters

Advantages: You can quickly complete the measurement using a microphone

Industry standards:

ANSI S12.31/S12.32

ISO 3741/3742

秒速时时彩 ASTM C423/ISO 354

4。 Free-field room

Free-field room is semi-anechoic chamber of the approximation. Arrangement of the walls and ceiling are not silencers wedge, but in the testing room, decorated walls and ceiling a few inches of glass fiber or similar sound absorbing material. Small room and a room for the needs of low-frequency testing of materials

Layer thick, large rooms and a room for high-frequency test sometimes contains only three inches of sound absorbing material。

Room free field surveys for engineering level and sound power level of the test. Free-field room is also used to test the sound intensity, and its main function is to control environmental noise, sound reflection by reducing the accuracy to improve the strength test.

Sound power level for engineering tests, need to use the parallelepiped test surface. Validation step is the main source in the reference sound pressure to complete the test, pay attention to differences in the sound power of grid computing is based on measurement. 2 dB difference between the maximum allowed. If the difference had

Large, indoor sound absorbing material can not be improved, the solution is to use more sound sources deployed in the vicinity of the microphone.

Sound level testing on the Notes

Test the theory that the sound intensity method can completely eliminate sound reflection and indoor environmental noise. The theory behind the key points are:

Using an infinite number of sampling points

Source output completely stable or simultaneous sampling of all points。

Actual implementation of the requirements of the order of sampling in the limited period of time and more points. Thus, although this method reflection effect can be suppressed to a certain extent, it can not completely eliminate it. Free-field room usually provides adequate sound absorption and sound insulation, which can be

Sound intensity method is perfect results。

Free-field room, advantages / disadvantages:

Pros: acceptable accuracy

Advantages: the need for moderate size, relatively inexpensive, proven testing room.

Disadvantages: a lot of microphone position takes a lot of microphone。 If the laboratory throughput is important, you need to consider multi-channel simultaneous acquisition。

秒速时时彩 Disadvantages: Apart from the sound intensity, semi-anechoic chamber compared to low precision.

Industry standards:

ANSI S12.34

ISO 3744

ANSI S12。10

ISO 7779

ECMA 74

ANSI S12。12

秒速时时彩 ANSI S12.36

ISO 3746

5. Other environmental

秒速时时彩 "Other Environment" is the rest of the class of all types, including appropriate conditions, the test outdoors, test non-specific areas (such as conference rooms) and so on. In most cases, they are used to investigate the level of sound power measurement. May require very, very quiet place

To complete the project level measurements。

The sound quality measurements, the role of environment on the sound may be testing the relative part of the sound recording in such vehicle。 In such cases, the internal acoustics is not a problem, but the potential interference from environmental sounds needs to be considered。

Internal acoustic

 

Sound intensity method is still used in such an environment。 However, the need to remember that this method does not completely "bulletproof", and measurement accuracy at high sound environments were likely to be affected。 Test environment in order to achieve the degree of deviation from the semi-mute state, the amount needed

Outside attention and sampling.

Many "other" environmental wall or ceiling, the lack of sound absorption materials on, so the completion of sound power level measurement is very difficult. Larger area may have better performance, because it has a greater absorption of space. Clearly, the outdoor test with

Some of the advantages of a particular application, because it has a lot of space, can approximate the performance of semi-anechoic chamber (infinitely large parking lot.)

Sound isolation

If the surrounding higher grade, or even would interfere with sound intensity measurements。 In fact, the steady-state can effectively suppress ambient noise than the sound of the device under test and high pressure near 10 dB。 Pulse environment can be suppressed to the same limit, but if measured in

All locations simultaneously. Higher levels in the environment, the need to find a more quiet environment.

"Other" environment is usually moderate at best to provide sound isolation. The construction, mechanical systems and peripheral activities may be conducted noise interference. Acceptability of these devices depends on the occurrence of the measured relative intensity and the sound intensity of the surrounding environment. Because

This, less than the sound source, the relatively loud sound sources are more likely to be accepted in this space。

The greatest difficulty in the outdoor test is to control environmental noise。 Therefore, the outdoor test is usually done on large machines。

秒速时时彩 Other environmental advantages / disadvantages:

Disadvantages: lower accuracy

Advantages: No special testing room。

Disadvantages: may need to measure the position more, to extend the test of time。

Disadvantages: may be more environmental noise

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