Precision full silence room
FIG. 1: precision full silence room (full silence room, anechoic room, anechoic room)
Some standard models for reference:
1。 Basic concept of total silence room
Complete anechoic chamber is a very important experimental site in acoustic experiment and noise test。 Its function is to provide a low noise testing environment in full free field space。 Main functions of the full silence room:
Provides acoustic full free field environment
Low noise measurement environment
Full free field space
The main function of the full anechoic chamber is to provide a free field space for acoustic testing. Full free field means that there is no reflector (surface) when sound wave propagates in infinite space.
The whole free field space in nature is to hang the sound source in the air, away from any reflection, the sound wave can be freely 360? Omnidirectional Angle propagation。
The propagation of sound waves in the full free field space has a specific physical definition:
Opposite the point sound source sound pressure attenuates with distance, which is the inverse square law of sound energy;
Pressure level is equal to intensity level at ambient temperature。 This is the theoretical basis for measuring sound power in the anechoic chamber。
Another function of the full anechoic chamber is to provide a low background noise environment to meet the requirements of the test environment. According to... iso3745-2003 Acoustics-Determination of sound power levels of noise sources using sound pressure. Precision methods for anechoic and hemi-anechoic rooms.ANSI s12.55-2006 Acoustics- Determination of sound power levels of noise sources using sound pressure-precision methods for anechoic and hemi-anechoic rooms and national standard GB6882 Precision method for measurement of sound power levels of acoustic noise sources anechoic and semi-anechoic Chambers Preferably low 12dB.
The full anechoic chamber simulates the full free field space in the room, so the sound absorption coefficient of each surface in the room is more than 99%. It is impossible to design a sound absorber that can achieve 100% of the full frequency band (20hz-20000hz). Because universal materials are easily absorbed by high-frequency sound waves, while low-frequency absorption depends on the thickness of the material (the length of the wedge). Cut-off frequency refers to above this frequency, the sound absorption system of the wall can guarantee 99% of the sound absorption coefficient. In practical engineering, the full free sound field space refers to the sound wave above the cutoff frequency. In general, the length of the spike applies to the 1/4 wavelength theory.
Above the cut-off frequency, the complete anechoic chamber meets the condition of free field. Below the cut-off frequency, the complete anechoic chamber cannot guarantee to meet the condition of free field.
Method for identification of complete anechoic chamber
In practical engineering, the free space of sound field is the key basis for the identification of anechoic chamber。 ISO 3745,。ANSI S12。55 and GB 6882 provide identification methods for the identification of free space in a full anechoic chamber。
In addition, background noise is another important indicator of total noise elimination room. The background noise is the background noise of the instrument plus the background noise of the anechoic chamber.
2. All acoustic performance parameters of the anechoic chamber
1. The size and main technical indicators of the total silence room
ISO 3745, ANSI S12.55 and GB 6882
Cut-off frequency 125Hz (can be extended to 50Hz as required)
Background noise: 16dB(A) (11dB(A) with special design
Internal dimensions are designed according to needs and standards
2. Acoustic performance parameters
2.1 sound absorption performance
The provided wedge for installation meets the sound absorption coefficient of 0.99 on the cutoff frequency (take the cutoff frequency 125Hz for example)
Frequency (Hz) sound absorption coefficient
2.2 sound attenuation
The full anechoic chamber can provide an approximate full free field environment with frequency range from 125Hz to 8000Hz, and its inverse square law performance deviation meets the requirements of ISO 3745, ANSI S12。55 and GB 6882 as follows:
The measurement requirement is to use single frequency signal, choose the center frequency of 1/3 frequency range