Anechoic Chamber and Wavelengths

Anechoic Chambers are designed so that sound generated inside does not bounce back.

The phenomenon of sound not bouncing back is called absorption, and the degree which sound does not bounce back is referred to as absorption coefficient , among other terms.

Therefore, anechoic chambers are constructed with sound-absorbing materials called absorbers on all six surfaces ( 5 in a semi anechoic chamber): the floor, walls, and ceiling.

Why is it necessary to prevent sound from bouncing back?

Let’s say a company that manufactures ventilation fans measures the sound of their own fans. They conduct these measurements in a room that is not an anechoic chamber, for instance, a corner of an office.

The sound of the ventilation fan can be reflected off walls, tables, windows, and other surfaces. The reflected sound bounces back and can be picked up by the measurement device. As a result, it becomes difficult to measure the true sound of the ventilation fan.

The concept of Anechoic Chamber is often illustrated using the image of a desert. Just as a desert is an open space with no objects to block sound, an anechoic chamber is designed to be an environment with minimal sound reflections. However, measuring sound in a real desert would be impractical, so an anechoic chamber is necessary to create such a controlled, echo-free environment.

When designing an anechoic chamber, it is crucial to determine which frequency ranges of sound should not be reflected. Frequency refers to the pitch of the sound. Higher frequencies have shorter wavelengths, while frequencies have longer wavelengths. The impact of these properties on the design is detailed below.

An Anechoic Chamber and Wavelengths

As a general rule, the thickness of sound-absorbing material needs to be at least 1/4 of the wavelength of the sound it’s meant to absorb. For example, sound absorption material that is 500 millimeters thick, commonly used in anechoic chambers, can absorb frequencies of 170Hz and above. However, this is just a guideline for understanding and does not alone determine the entire design of sound absorption.

There is a formula for wavelength:Wavelength = Speed of sound ç Frequency
For a frequency of 170Hz 340÷170=2m
2m×1/4=0.5m（500mm）

When measuring ventilation fan noise at frequencies of 170Hz and above in an anechoic chamber, a thickness of 500 millimeters of sound-absorbing material is required. However, for measuring frequencies lower than 170Hz, thicker sound absorption material is necessary.

The lowest frequency that can be measured is refers to as the “measurement limit frequency”

An excessively low measurement limit frequency

Recently, we received an inquiry from an international electronics manufacturer requesting an anechoic chamber capable of measuring sounds at 5Hz.

The wavelength of a 5Hz sound is approximately 68meters.

According to the earlier calculation, a thickness of 17meters of sound-absorbing material would be needed. This would require an anechoic chamber approximately the size of Tokyo Dome.

In international projects, it is often common to set specifications for impractical low frequency ranges.

There is a practical limitation to the space available for installing an anechoic chamber, and the price increases proportionally with the size.

If the installation space is around 5 meters by 5 meters, it can typically only accommodate measurements up to about 200Hz.

In this way, anechoic chambers and wavelengths are deeply interconnected.

When considering the implementation of an anechoic chamber, it is essential to design with wavelengths in mind, requiring an accurate understanding aligned with practical realities.

At Sonora, we provide foundational knowledge about anechoic chambers to our users. Please feel free to contact us.