Basic concept of the hottest anechoic chamber and

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Basic concept of anechoic room (semi anechoic room)

according to the special requirements and product characteristics of anechoic room customers, Sedley acoustics division can provide complete measurement systems, laboratories and customized non-standard measurement devices

I. Basic Concept of anechoic chamber (or semi anechoic chamber)

anechoic chamber (or semi anechoic chamber) is a very important experimental place in acoustic experiments and noise testing. Its function is to provide a low-noise testing environment in free field or semi free field space. The main functions of the anechoic chamber:

● provide an acoustic free field environment

● low noise measurement environment

on the basis of the traditional anechoic chamber (or semi anechoic chamber), it combines the original technical advantages and strength. Over the past few years, the mature design and good reputation guarantee make it a friend trusted by more and more users at home and abroad

anechoic chamber

free field space

the main function of anechoic chamber is to provide a free field space or semi free field space for acoustic testing. Free field means that when sound waves propagate in infinite space, there is no reflector or reflector. The free field space in nature is to hang the sound source in the air, away from any reflector, and the sound wave can propagate freely to 360 degrees. Half free field space means that there is a total reflection surface in free space, and sound waves can only propagate freely in the 180 hemisphere direction. Common semi free field spaces, such as large open-air parking lots on cement floors, outdoor basketball courts, parking aprons, etc

wave propagation in free field or semi free field space has a specific physical definition as:

● the sound pressure of point sound source attenuates with distance, which is the inverse square law of sound energy

● the sound pressure level is equal to the sound intensity level at room temperature. This is the theoretical basis for measuring sound power in anechoic chamber

background noise

although large open-air parking lots on cement floors, outdoor basketball courts, parking aprons, etc. can also provide semi free field space, the measurement will be affected by background noise. Imagine that the noise of the refrigerator is 30dB (a) and the background noise of the environment is 45dB (a). It is impossible to measure the noise of the refrigerator. Therefore, another function of the anechoic room is to provide a low background noise environment to meet the requirements of the test environment

according to the national standard GB "precision method of anechoic chamber and semi anechoic chamber for the determination of sound power level of four aluminum foil noise sources in one experiment", within the frequency range of viscosity test of thickened oil, the sound pressure level of background noise is at least 6dB lower than that of the measured sound source, and the best is 12dB lower

cut-off frequency

anechoic chamber is to simulate free field or semi free field space indoors, so the sound absorption coefficient of the wall is required to be more than 99%. The reflection coefficient of the semi free field ground is more than 95%. It is impossible to design a 100% sound absorber that can achieve the full frequency band (20hz-20000hz). Because common materials are easy to absorb high-frequency sound waves, and low-frequency absorption is related to the thickness of the material (the length of the wedge) that promotes the film blowing machine to move towards the high-end level. The cut-off frequency means that above this frequency, the sound absorption system of the wall can ensure 99% of the sound absorption coefficient. In practical engineering, the (semi) free sound field space refers to the sound wave above the cut-off frequency. The anechoic chamber is (semi) free field space. The lower the cut-off frequency is, the greater the length requirement of the wedge is. Generally speaking, the length of the wedge is applicable to the 1/4 wavelength theory

above the cut-off frequency, the anechoic chamber meets the free field conditions. Below the cut-off frequency, the anechoic chamber cannot meet the free field conditions, and the free field correction should be carried out according to GB 6882 during measurement

identification method of anechoic chamber

in practical engineering, the free space of sound field is the key basis for identifying the quality of anechoic chamber. The national standard ISO 4745 and GB provide the identification method for identifying the free space of anechoic chamber

in addition, background noise is another important index of anechoic chamber. The background noise obtained in the measurement is the background noise of the instrument plus the background noise of the anechoic chamber

II. The acoustics of anechoic chamber (semi anechoic chamber) is based on the parameter

1 Size and main technical indicators of semi anechoic chamber

※ design standards of semi anechoic chamber ISO 3745 and GB

※ cut off frequency 125Hz (can be extended to 50Hz according to requirements)

※ background noise 16dB (a) (with special design, it can reach 11db (a)

※ external size

※ internal size is designed according to needs and standards

※ available space

2 Acoustic performance parameter

2.1 sound absorption performance

the wedge provided for installation meets the sound absorption coefficient of 0.99 at the cut-off frequency (taking the cut-off frequency 125Hz wedge as an example)

frequency (Hz) Sound absorption coefficient

125 0.99

250 0.99

500 0.99

1000 0.99

2000 0.99

4000 and the high performance of new materials 0.99

8000 0.99

2.2 silencing performance

the semi anechoic chamber can provide an approximate free field environment with a frequency range from 125Hz to 8000Hz, and its inverse square law performance deviation meets the requirements of the following international standard ISO 3745:

the measurement requires a single frequency signal, The allowable deviation (DB) of the center frequency selected in the 1/3rd octave band

1/3rd octave band (Hz)

<630 ± 2.5

± 2.0

≥ 6300 ± 3.0

2.3 sound isolation performance

the sound insulation characteristics of the semi anechoic room are determined according to the design requirements and the surrounding environment. The composite sound insulation structure can meet the following sound insulation characteristics:

octave frequency range (Hz) 125 250 500 1000 2000 4000 8000

transmission loss (DB) 2831

III. anechoic chamber (semi anechoic chamber) components

3.1 perforated wedge

perforated sound-absorbing wedge, the surface of the wedge is made of perforated galvanized steel, the perforation rate is ≥ 20%, and the inner core is made of high sound-absorbing formed glass wool, providing an approximate free field that meets the standard of semi anechoic chamber, Ensure that the normal sound absorption coefficient of sound wave reaches 0.99 above the specified cut-off frequency

the dust-proof treatment technology is adopted between the glass wool and the perforated plate, which makes the wedge surface easy to clean and ensures long-term stable acoustic performance parameters

the surface of the wedge is sprayed with plastic, and the color can be selected by the customer

3.2 strong sound-absorbing wedge

sampling super sound-absorbing cotton, and the surface is treated with glue curing

3.3 wedge door

the wedge door adopts a metal perforated sound-absorbing wedge structure, so that the wedge door and the internal wedge form a complete sound-absorbing surface. The wedge door adopts a special hinge, so that the door can be opened and closed easily without deformation for a long time. When the wedge door is opened, it can be designed to be staggered with the wedge teeth of the inner wall, saving space

3.4 sound insulation door

in the design of anechoic room (semi anechoic room), door is a very important factor. A good sound insulation door should have good sound insulation performance and optimized edge effect

the structure of the sound insulation door adopts 1.5mm thick cold-rolled steel plate, with formed sound-absorbing cotton inside. The hinge adopts imported products, and the whole door floats when opening the door. When returning the door, the high-density magnetic edge banding is completely closed to ensure that its acoustic treatment can match the acoustic treatment of other surrounding parts to eliminate edge leakage as much as possible, the intensity meets the operating requirements, and there is no deformation after long-term use. At the same time, the outer surface is sprayed with plastic to make the color consistent with the wedge

3.4 interior wall

the structure of interior wall and ceiling can be designed according to user requirements. The commonly used structures are:

● concrete structure - this structure has good sound insulation effect, but it is heavy. High requirements for floating ground

● "noise shield" composite sound insulation structure - patented design, consisting of two layers of steel plates, the inner layer is perforated, and the interlayer is filled with sound absorbers with high sound absorption coefficient to ensure sound insulation effect. The weight is light, but the price is a little expensive

3.5 floating ground

when the external noise is high or vibration measurement is required, floating structure is usually used to isolate the external influence. As a damping material that plays a key role in floating structure, our company has accumulated a lot of experience on floating ground. The vibration reduction methods adopted are:

◆ spring structure: the vibration isolation frequency can be achieved (1Hz, this structure is used when the low noise requirement is very high. But the disadvantages are: high cost, difficult maintenance, rust prevention of spring.)

◆ rubber damping pad structure: the vibration isolation frequency can reach 7Hz. This structure is used when low noise requirements are high

◆ glass wool felt vibration damping structure: the vibration isolation frequency can be 15Hz, and the background noise is (a)

◆ no treatment: in the built plant, due to the height limit, the anechoic room is directly built in the plant

in the actual project, what kind of vibration damping is used depends on:

● the required background noise

● the surrounding environment, noise sources and vibration sources

3.6 lighting

providing incandescent lamps that can meet the indoor lighting, The background noise and sound reflection of incandescent lamps should be minimized. At the same time, the consistency and aesthetics between the lamp and the overall indoor design should also be considered

3.7 circuit system

except for lighting, all circuit conduits, wires and controls are required to be installed behind the wedge to avoid sound reflection

3.8 wiring pipeline

the wiring pipeline meets the requirements that the line passes without any impact on the test

3.9 control room

the design of the control room should fully consider the aspects and functions of users, and at the same time, you can choose to install the monitoring system to reduce the human impact during measurement

3.10 special design requirements

carry out specific special requirements according to the customer's industry characteristics: including air conditioning system, dehumidification system, working condition system, transmission system, ventilation, monitoring system, etc. (end)

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