The cutoff frequency is a key indicator for describing the acoustic performance of the sound-absorbing structure on the walls of an anechoic chamber, representing the ability of the anechoic chamber to provide a space close to a free field or semi free field. In the design of anechoic chambers, the cutoff frequency is usually defined as the lowest frequency at which the sound absorption coefficient is equal to or greater than 99%.
 
The relationship between sound absorbing spikes and cutoff frequency
 
The sound-absorbing wedge is the main sound-absorbing material in the anechoic chamber, and its design directly affects the setting of the cutoff frequency. The cut-off frequency of the sound-absorbing wedge is approximately equivalent to or slightly lower than the frequency at which the length of the sound-absorbing structure is 1/4 wavelength. For example, if the total length of the sound-absorbing wedge is 600 millimeters, plus a cavity of 100 millimeters, its cut-off frequency is approximately 121 hertz, which is the sum of the sound velocity (340 meters/second) and the length of the sound-absorbing structure multiplied by 4. The actual cut-off frequency may be lower than this estimated value, and the actual value needs to be determined through measurement data from the metrology institute.
 
The importance of cut-off frequency
 
The cut-off frequency is crucial for the functionality of the anechoic chamber. When sound waves are vertically incident on a sharp edge, if the sound pressure reflection coefficient is less than 0.01 (10%), that is, the sound absorption coefficient is not less than 0.99, then this frequency is considered the cutoff frequency. Measurements above the cutoff frequency can be corrected according to international standards. In practical engineering, the lower the cutoff frequency, the longer the required length of the sharp edge, in order to more effectively absorb low-frequency sound waves.
 
Setting of cut-off frequency
 
The choice of cut-off frequency depends on the test object and test content. For example, a fully anechoic chamber requires that all six interfaces of the room be covered with sound-absorbing materials, while a semi anechoic chamber only requires five interfaces to be covered with sound-absorbing materials. The cut-off frequency is usually around 100 Hz, corresponding to a spike length of approximately 700 millimeters.
 
Conclusion
 
In summary, the cutoff frequency of an anechoic chamber is a key acoustic parameter that reflects its ability to provide a space close to a free field or semi free field. The setting of the cutoff frequency is closely related to the design of the sound-absorbing wedge, including factors such as the length of the wedge, filling material, and cavity depth. In practical applications, the selection of cutoff frequency needs to be determined based on specific testing and acoustic requirements.