Sound Absorbency of Anechoic Box

Sonora Technology may receive enquiries about the sound absorption properties of anechoic box, so we’d like to explain as followings:

1. An anechoic box and an anechoic chamber are something different.

An anechoic box is a simple measurement device, which is not defined by ISO standards. Therefore, its properties are different from those of an anechoic chamber and the concept of the measurable “lower frequency limit” required for an anechoic chamber does not apply.

2. Sound absorption coefficient data for an anechoic box

The anechoic box has a sound insulation layer of 75 mm, a sound absorption layer of 150 mm and a total thickness of 225 mm.The sound absorption layer is an interior material, made of high-density glass wool + AST sound-absorbing material (sound-absorbing material of our own brand).
We have not disclosed the sound absorption coefficient data of this sound absorption layer, since it has practically no meaning and could be misleading.

Thus, the sound absorption coefficient data is used in order to compare different materials, however, in reality, it depends on the equipment and environment of the measurement laboratory, so even if the same material is measured in different laboratories, or even in the same laboratory under different conditions, such as temperature and humidity, the measurement data would be different and difficult to compare each other realistically.

In other words, even if the sound absorption coefficient data of our anechoic box is disclosed, the sound absorption coefficient may be judged as “higher” or “lower” compared to the sound absorbing materials measured under different conditions.

Furthermore, adding jigs, clamps, etc. inside the anechoic box will create a reflective surface for sound waves, and the sound absorption properties inside would vary depending on the size of the anechoic box.

3. Actual sound absorption of an anechoic box

So how is the degree of the sound absorption of an anechoic box actually? It would be better for you to measure in practice or to try out on one of our demonstration units.

As a guideline, a suitable frequency range for measurement is the mid to high frequency range. When the thickness of the sound absorption layer is multiplied by λ/4, the target frequency range should be 570 Hz and above.

(a general theory: when the thickness of the sound absorption material is more than λ/4 of the wavelength, the sound incident on that material will not bounce back.)

However, this does not mean that sound waves below 570 Hz or in the low frequency range will not be absorbed.

Thank you for your understanding.