Wedge Anechoic Chamber

Free-Field Space

The main function of an anechoic chamber is to provide a free-field space for acoustic testing. A free field refers to sound wave propagation in an infinite space without any reflecting surfaces.

In nature, a free-field space is achieved by suspending the sound source in the air, far from any reflecting objects, allowing sound waves to propagate freely in all 360° directions.

Sound wave propagation in free-field space has specific physical definitions:

1. For point sources, sound pressure decreases with distance according to the inverse square law;
2. Under standard temperature and pressure, the sound pressure level equals the sound intensity level. This is the theoretical basis for measuring sound power in anechoic chambers.

Background Noise

Another function of the anechoic chamber is to provide a low background noise environment to meet testing requirements. According to ISO3745-2003, ANSI S12.55-2006, and GB6882 standards, within the test frequency range, the background noise level should be at least 6dB lower than the sound source being measured, with 12dB being preferable.

Cut-off Frequency

An anechoic chamber simulates free-field space indoors, requiring absorption coefficients above 99% on all surfaces. Designing a 100% absorptive material across the full frequency range (20Hz-20000Hz) is impossible. Common materials easily absorb high-frequency waves, while low-frequency absorption relates to material thickness (wedge size). The cut-off frequency defines the point above which free-field conditions are met.

Anechoic Chamber Verification Methods

In practical engineering, free-field space verification is crucial for determining chamber quality. ISO 3745, ANSI S12.55, and GB 6882 provide methods for verifying anechoic chamber free-field conditions.

Additionally, background noise is another crucial indicator, measured as the combination of instrument and chamber background noise.