Anechoic is defined as “free from echoes or reverberations”. An ideal anechoic chamber would contain no reflections of sound from its walls, ceiling, or floor and existence of an acoustical free-field condition. Temperature, pressure, humidity, and wind in an anechoic chamber must be taken account as these parameters could create significantly and unpredictably disturbance to the uniform radiation of sound waves.
The inverse square law would function perfectly in an ideal free-field environment. This indicates that the sound pressure level from a sound source would decrease six decibels for each doubling of distance from the source.
To set-up an anechoic chamber to meet the various standards’ requirements, a number of acoustical, mechanical, electrical, and aerodynamic considerations must be considered:
- Test object size
- Anechoic treatment selection
- Cut-off frequency
- Test object noise levels
- Parent room noise levels
- Vibration isolation
- Ventilation system requirements
- Structural requirements
- Physical access requirements
- Lighting and electrical requirements
- Visual requirements
- Housekeeping requirements
- RF shielding requirements
99% of sound waves are absorbed by the absorption lining or surfaces. The background sound pressure level inside the anechoic chamber should be very low. To fulfil that requirement, the anechoic chamber needs to be insulated from external noises which implies an excellent walls, floor, ceiling and absorption lining or surface design.
A practical anechoic chamber is expected to deliver an acoustical free field and an environment which meets other requirements, including the control of temperature, pressure, humidity and ambient sound pressure level. This to enable precise acoustical measurements of the sound output and frequency content of a source and its directivity pattern to be performed.
Related standards are:
ISO 3745:2012 Acoustics — Determination of sound power levels and sound energy levels of noise sources using sound pressure — Precision methods for anechoic rooms and hemi-anechoic rooms
ISO 26101-1:2021 Acoustics — Test methods for the qualification of the acoustic environment — Part 1: Qualification of free-field environments
ISO 13347-3:2004 Industrial fans — Determination of fan sound power levels under standardized laboratory conditions — Part 3: Enveloping surface methods
EN ISO 3745 IEC 60268-1 IEC 60268-5 EN 60534-8-2, Armature EN 60534-2-3 ISO 13347