Basic concepts

The full acoustic chamber is an extremely important experimental place for the acoustic experiment and the noise test. Its role is to provide a low noise test environment for a fully free field space. The main functions of the whole muffle room:

Provide an acoustic full free field environment

Low noise measurement environment

Fully free field space

The main function of the whole muffle chamber is to provide a full free field space for acoustic testing. The free field refers to the sound propagation in infinite space, there is not any reflector (surface).

In nature, the full free field space is that the sound source is suspended in the air, away from any reflection, and the sound wave can be freely propagated to the 360 degree angle.

Sound waves are propagating in a free field space with a specific physical definition:

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

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

background noise

The other function of the whole muffle room is to provide a low background noise environment to meet the requirements of the test environment. According to the.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 of sound power levels - Determination of noise sources using sound pressure Precision methods for anechoic and - hemi-anechoic rooms and GB GB6882 "acoustic noise source determination of anechoic chamber sound power level and precision method of semi anechoic chamber" in the test, the frequency range of background noise level than the measured sound source, the sound pressure level of low 6dB, low 12dB is the best.

cut-off frequency

The full acoustic room is to simulate the full free 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 100% sound absorption body that can achieve a full band (20Hz-20000Hz). Because the general materials of high frequency sound waves can easily be absorbed, and the low frequency absorption and the thickness of the material (the length of the wedge). The cut-off frequency is above the frequency, and the sound absorption system of the wall can guarantee the sound absorption coefficient of 99%. The free field in practical engineering refers to the sound wave on the cutoff frequency above the cutoff frequency is lower, the greater the length of wedge. In general, the length of the wedge for 1/4 wavelength theory.

Above the cut-off frequency, the full anechoic chamber satisfies the free field condition. Under the cut-off frequency, the full anechoic chamber can not guarantee the free field condition, and the free field correction is needed according to GB 6882.

The identification method of the whole muffle room

In practical engineering, the free space of sound field is the key basis for identifying the good or bad of the noise cancellation room. ISO 3745,.ANSI S12.55 and national standard GB 6882 provide an identification method for identifying the free space of a full acoustic chamber.

In addition, the background noise is another important indicator of the whole muffle room. The background noise obtained in the measurement is the background noise of the instrument and the background noise of the anechoic chamber.