Acoustic Chamber (or Semi-Acoustic Chamber) Introduction:

An anechoic chamber is a room with a free-field sound, meaning it has only direct sound without reflections, though in reality, reflections within an anechoic chamber are kept as minimal as possible. To achieve the effect of a free-field, the six surfaces of the room must have a high absorption coefficient, greater than 0.99 within the operating frequency range. Typically, acoustic wedges are installed on all six surfaces, with steel rope nets mounted over the ground-level wedges.

Another structure is the semi-anechoic chamber, which differs from the standard anechoic chamber in that the floor is not treated with sound-absorbing materials. Instead, the floor is covered with tiles or terrazzo to create a reflective surface. This semi-anechoic structure is equivalent to half the height of a standard anechoic chamber, hence the name "semi-anechoic chamber."

An anechoic chamber (or semi-anechoic chamber) is an extremely important experimental facility for acoustic experiments and noise testing. Its purpose is to provide a low-noise testing environment with a free or semi-free field space. The main functions of an anechoic chamber:

Provide an acoustic free-field environment

2. Low-noise Test Environment

Open Field Space:

The primary function of an anechoic chamber is to provide a free field or semi-free field space for acoustic testing. A free field refers to the propagation of sound waves in an infinitely large space without any reflective bodies or surfaces.

The free-field space in nature involves suspending a sound source in the air, far from any reflective surfaces, allowing sound waves to propagate freely in all 360 degrees. A semi-free-field space refers to a free space with a complete reflecting surface, where sound waves can only freely propagate in a 180-degree hemispheric direction. Common semi-free-field spaces include large outdoor parking lots with concrete surfaces, outdoor basketball courts, and airports.

The propagation of waves in free field or semi-free field spaces is defined physically as:

1. The sound pressure of a point source decreases with distance, which is known as the inverse square law of sound energy.

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

Background Noise:

Large outdoor parking lots with concrete surfaces, outdoor basketball courts, aprons, etc., can also provide semi-open spaces, but measurements are affected by background noise. Imagine, the noise of a refrigerator at 30dB(A) while the environmental background noise is at 45dB(A); it is impossible to measure the refrigerator's noise. Therefore, another function of the soundproof room is to provide a low background noise environment to meet the requirements of the testing conditions.

In accordance with the national standard GB/T6882-2016 "Acoustics - Precision methods for the measurement of noise source sound power and sound energy levels in anechoic and semi-anechoic rooms using the sound pressure method," within the test frequency range, the sound pressure level of the background noise should be at least 6dB lower than that of the measured noise source, ideally 12dB lower.

Acoustic specifications of the anechoic chamber:

1. The background noise in the anechoic chamber. The measured background noise is the sum of the instrument's background noise and the anechoic chamber's background noise.

2. Cut-off Frequency

An anechoic chamber is designed to simulate free field or semi-free field spaces indoors, thus requiring wall absorption coefficients of over 99%. The reflection coefficient for the semi-free field surface should be over 95%. It is impossible to design a 100% absorbent material that can achieve full frequency range (20 Hz – 20,000 Hz) absorption. General materials are easily absorbed by high-frequency sound waves, while low-frequency absorption is related to the material's thickness (the length of the wedge). The cutoff frequency refers to the frequency above which the wall absorption system can ensure an absorption coefficient of 99%. In actual engineering, the (semi-)free field space refers to the space within the anechoic chamber where sound waves above the cutoff frequency are treated as (semi-)free field. The lower the cutoff frequency, the longer the required length of the wedge. Generally, the length of the wedge is applicable to the 1/4 wavelength theory.

3. Meets the requirements of the international standard ISO 3745-2012 "Acoustics - Determination of sound power levels and sound energy levels of noise sources using sound pressure. Precision methods for anechoic and hemi-anechoic rooms" and the national standard GB/T6882-2016 "Acoustics - Precision methods for determining the sound power and sound energy levels of noise sources using sound pressure. Anechoic and hemi-anechoic rooms."

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