Is it necessary to make an anechoic chamber for noise testing of electroacoustic devices?

The anechoic chamber (production: static ring environmental protection) first needs to clarify the goals to be achieved, and the three most important parameters are the free field radius, cut-off frequency, and noise floor.

whether there are noise sources around and how loud the noise is; whether there is a source of vibration around the anechoic chamber; Whether the surrounding noise sources and vibration sources are in working condition during the use of the anechoic chamber; Whether there are frequent people walking around the anechoic chamber during use; All of this can have a significant impact on anechoic chambers with low noise floor requirements. Therefore, a full investigation and understanding of the target noise floor and the environment that needs to be achieved in the anechoic chamber is the prerequisite for a anechoic chamber.

Experiments that strictly avoid reflected sound interference, such as free-field reciprocal correction of microphones, directivity measurements of electroacoustic devices and sound sources, certain hearing tests, etc., must be carried out in anechoic chambers. Some experiments can be performed in either an anechoic chamber or a reverberation chamber, such as the output power and average spectrum of sound sources (including loudspeakers, various noise sources, musical instruments, etc.). If the radiation directivity of the sound source is complex, the anechoic chamber test method of sound power is very complicated.

Therefore, it is generally necessary to measure the output power and spectrum of the sound source, and do not need to know the directivity of the sound source, so the use of a reverberation chamber is a fast and good method.

The anechoic chamber is an indispensable acoustic equipment for acoustic experiments to test noise, and the acoustic free field created in it allows testers to detect the effective data and characteristic performance of the noise itself. Anechoic chambers are usually built to the size of an ordinary room, in order to ensure the silence of the acoustic free field of the anechoic chamber.

The structure of the anechoic chamber is a laboratory that can fully reflect sound energy at all boundaries and fully diffuse it to form a diffuse field with uniform energy density everywhere and random distribution in all propagation directions. It is mainly used for the detection of sound absorption coefficient of materials, the sound power test of mechanical products, etc.

Anechoic chambers are also more widely used in the electroacoustic industry, because many electroacoustic equipment are relatively small and have high sound power requirements, so anechoic chambers are needed to meet the testing needs, such as common microphones, speakers, headphones, microphones, microphones, small speakers, etc.

Anechoic chambers are extremely important experimental sites for acoustic experiments and noise testing. Its function is to provide a low-noise test environment in a free-field or semi-free-field space, and to adopt good sound and seismic isolation devices to avoid interference from the external environment.

Anechoic chambers are commonly used devices for noise cancellation, and their role is to create an environment similar to a free-field space to test the acoustic properties of equipment.

Anechoic chambers – Eiffel Acoustics, are built to provide a free-field space or a semi-free-field space for acoustic testing. The free field radius is a measure of the size of the free field, a well-designed anechoic chamber, the free field radius should be from the center point to the distance from the sharp split.

There are no principle requirements for the shape of the anechoic chamber, which is mainly determined by the content of the experiment. The size of the anechoic chamber depends on the test needs. The high-performance sound-absorbing structure is generally a sound-absorbing body in the shape of a sharp split; For anechoic chambers with low requirements, a variety of simple sound-absorbing materials or structures can also be selected.

Each interface in the anechoic chamber is equipped with a high-efficiency sound absorption structure, and more than 99% of the incident sound energy is absorbed within the required frequency range, so that the reflected wave is small enough not to exceed the specified range; At the same time, sound insulation and vibration isolation measures are taken to ensure that the outdoor noise and vibration will not affect the indoor test, and ensure that the indoor space is approximately an experimental space with no reflection, no interference and free sound field conditions.

The main purpose of the anechoic chamber (construction: Eiffel Acoustics) is to test the electroacoustic properties of the anti-noise transmitter and receiver, such as sensitivity, frequency response and directivity. The frequency range of this kind of transmitter and receiver should ensure that the language communication is clear, generally about 200-4o00Hz.

The acoustic design indicators of the anechoic chamber are as follows:

The designed anechoic chamber is a full anechoic chamber, and a working ground network is required.

The lower limit frequency is 150Hz.

In the horizontal diagonal direction, the free field range of the allowable deviation from the theoretical value of the anechoic chamber should be greater than 1m.

The noise floor in the anechoic chamber is not more than 30dba.

1. The composition is different.

The anechoic chamber is made of composite soundproof walls, sound-absorbing sharp splits and other sound-absorbing materials, which has a good sound-absorbing effect, and the anechoic chamber is divided into semi-anechoic chambers and full anechoic chambers, and the indoor background noise is generally about 25-6db(a).

The reverberation chamber is made and decorated with soundproof materials such as soundproof walls, diffusers, and sound-absorbing panels, and is different from the anechoic chamber in that the reverberation chamber has a reverberation effect to form a diffuse sound field, while the anechoic chamber produces a free sound field.

2. The functions are different.

It can be used to determine the sound absorption coefficient of sound-absorbing materials and sound-absorbing structures, to measure the noise level and power level of equipment and products, the frequency response characteristics of electroacoustic components such as microphones and loudspeakers, the acoustic frequency characteristics of musical instruments or precision instruments, and the noise level.

The reverberation room, also known as the audio-visual room, is to improve the environment for audition evaluation, so as to make a correct judgment on the electroacoustic period and audio equipment, it is necessary to establish a room that conforms to the acoustic characteristics of the audition.

Description of the acoustic effect of the anechoic chamber:The anechoic chamber is a free sound field, and the sound is absorbed after it is emitted, and the anechoic chamber is a silent environment.

Anechoic chamber test products:Testing electronic equipment, household appliances, automobiles, auto parts and other products;

Description of the acoustic effects of the reverberation chamber: The reverberation chamber is a diffuse sound field, and the sound is reflected after it is emitted, which produces an echo effect.

Reverberation chamber test products: The reverberation room is generally used in the electroacoustic industry, testing musical instruments, audio, piano and other products.

Both anechoic chambers and reverberation chambers belong to a special kind of acoustic laboratory (anechoic: Eiffel Acoustics), and the distinction between the two is mainly based on acoustic effects. The main function of the anechoic chamber is to eliminate the interface acoustic reflection of the sound source, that is, to form a free sound field in the anechoic chamber; The main function of the reverberation chamber is to form a diffuse sound field, which is also known as the reverberation sound field.

The anechoic chamber is divided into a full chamber and a semi-chamber, and the chamber is to simulate the free and non-reflective boundary, and the material with a large sound absorption coefficient is used to make sharp splits to absorb sound waves.

Anechoic chamber – eliminates all non-direct sounds.

Reverberation chamber – try to keep all non-direct sounds.

The two are opposites, one is the strong sound absorption of the wall, and the other is the strong reflection.

Anechoic chambers can measure the parameters of electroacoustic equipment, such as microphones, speakers, horns, etc. The reverberation chamber is used to measure acoustic parameters such as the sound absorption coefficient of the sound building material.

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Reverberation chambers, Eivel Acoustics, analysis are rarely used due to the complexity of testing and calculation in the process of testing noise levels and power levels of noisy products. Most of them use anechoic chambers for noise testing and analysis of electromechanical products. Anechoic chambers are divided into two types: full anechoic chambers are to establish a free sound field (six-sided sound absorption) in the measurement space, and semi-anechoic chambers are to establish a semi-free sound field (five-sided sound absorption) in the test space.

Acoustic laboratories can be divided into three categories, namely reverberation chambers, soundproof chambers, and anechoic chambers. For the laboratory, it should have good sound and seismic isolation performance for the surrounding environment, and the purpose of this is actually obvious, in order to avoid the interference caused by external noise during experimental work.

Structurally speaking, the general practice is to make the room into a separate concrete box, put on the spring foundation, the spring foundation method is divided into several kinds, such as: steel springs, air springs, cork, etc., the specific use of which practice should be determined according to the actual situation.

The sound power and spectral characteristics of the specimen are measured.

The design of the acoustic anechoic chamber is not only a special laboratory for acoustic testing, but also an important part of the testing system, in fact, it is also one of the acoustic testing equipment, and its acoustic performance indicators directly affect the accuracy of the test. The main purpose of the anechoic chamber is to test the electroacoustic performance of the anti-noise transmitter and receiver, such as sensitivity, frequency response and directionality. The frequency range of this kind of transmitter and receiver should ensure that the language communication is clear, generally about 200-4000Hz.

The following is a specific case study.

According to the use of the acoustic anechoic chamber and the original room conditions, the acoustic design indicators are as follows:

1) The design of the anechoic chamber is a full anechoic chamber, and requires the setting of a working ground network.

2) The lower limit frequency is 150Hz.

3) In the horizontal diagonal direction, the free field range of the allowable deviation from the theoretical value should be greater than 1m.

4) The noise floor in the anechoic chamber is not more than 30dba.

Eiffel acoustic anechoic chamber construction anechoic structure:

1) Sound insulation and vibration isolation Because the anechoic chamber is located in the room on the ground floor of the test building, the west side is adjacent to the stairs and elevators, and the south side is the corridor of the building, which is greatly disturbed by noise, especially the footsteps of going up and down the stairs and the solid sound generated when the elevator is running. In order to improve the sound insulation effect of the anechoic chamber on solid sound and air sound, the "room-in-room" sound insulation structure is completely separated from the original building. (2) In order to isolate the solid sound caused by impact, the floating ground with elastic cushion is used for vibration isolation.

The method is to lay a layer of 15cm thick (10cm after compaction) glass wool insulation board as a vibration isolation elastic cushion on the original ground, and make a layer of 20cm thick reinforced concrete floor on top of it, leaving a 5cm gap with the external wall to prevent the rigid connection with the external wall and isolate the solid sound transmission in the building and outdoors.

3) The partition wall is built on the floating ground with a layer of 24cm thick brick wall as the inner wall, leaving a gap of 20cm between the outer wall, and the brick joints of the wall are required to be full of mortar to prevent leakage of the gaps.

Anechoic chamber is a laboratory room for acoustics, which is widely used in the acoustic performance testing of electroacoustic products and the measurement of noise and sound power level of electromechanical equipment. The design of the anechoic chamber should deal with the relationship between the use requirements and the acoustic environment around the laboratory and the construction cost on the basis of following the acoustic principle.

Whether the performance of the anechoic chamber (reference: static ring environmental protection) meets the requirements of use is generally tested by the method of verifying the free field, that is, the sound pressure generated by the point sound source in it should be inversely proportional to the distance to the sound source, and the deviation between the measured sound field and the ideal free field is the main index used to measure the performance of the anechoic chamber. In general acoustic testing, this deviation is required to be no more than 1dB; For microphone calibration, it is required that this deviation is not greater than in the vicinity of the calibration distance.

In addition to meeting the requirements of free field, the anechoic chamber is also required to have a low noise floor. Therefore, between the anechoic chamber and the foundation, certain vibration isolation measures need to be taken.

The vibration isolation system should be set up in the structure of the full anechoic chamber and the category of vibration isolation system should be selected according to the requirements of the test, including metal spring vibration isolators and anyloids, rubber vibration isolators or rubber vibration isolation pads, rock wool boards and their products.

The relative humidity of the anechoic chamber is not more than 70%, and the noise reduction and silencing design of the ventilation and air-conditioning system should be determined according to the allowable noise level in the anechoic chamber, the noise value of the equipment, the vibration magnitude, the frequency characteristics and the noise of the transmission system.

The noise test of the speaker speaker and the design of the anechoic chamber crossover is computer-aided to manual adjustment, and then to the precision test of the anechoic chamber and then adjusted a number of meticulous links, so as to make the acoustic environment of the crossover circuit suitable for the selected speaker for testing and testing.

When designing an anechoic chamber to test the noise of the horn speaker, attention should be taken:

1) The requirements for the interface sound absorption coefficient are quite different between the test items of the pure tone signal and the test items of the wideband noise signal.

When the interface absorption coefficient a=, i.e. |r|=, the range of l=1db is r<, and the range of l=-1db is r<. The test of the electroacoustic parameters of the loudspeaker is mostly based on pure tone signals, so it is generally required that the interface sound absorption coefficient is greater than that in order to meet the requirements of less than 10% of the sound field deviation at a large enough test distance. If ln<1dB is required, then even if r=l 2, that is, the measurement point is close to the interface, it is enough to have a=.

In particular, the measurement of the power level of the machine's radiated noise in a hemi-anechoic chamber allows for a deviation of 2 dB or 3 dB in the free sound field, so that the interface absorption coefficient does not have to be 0 99.

The sound-absorbing materials used in the anechoic chamber require the sound absorption coefficient to be greater than. Generally, a gradient absorbing layer is used, a sharp split or conical structure is commonly used, glass wool is used as a sound-absorbing material, and soft foam plastic is also used. When the test is carried out in the anechoic chamber, the object or sound source to be tested is placed on the nylon wire mesh or steel wire mesh, because the weight of this kind of mesh can be limited, so it can only test the sound source with light weight and small size.

Whether the performance of the anechoic chamber meets the requirements of use is generally tested by the method of verifying the free field, that is, the sound pressure generated by the point sound source should be inversely proportional to the distance to the sound source, and the deviation between the measured sound field and the ideal free field is the main index used to measure the performance of the anechoic chamber. In general acoustic testing, this deviation is required to be no more than 1dB; For microphone calibration, it is required that this deviation is not greater than in the vicinity of the calibration distance.