Anechoic Room Description

An anechoic, or echoless room, is a room that has total, or nearly total, absorption on all surfaces. This means that any sound that hits a wall is completely absorbed and none is reflected.

Because the anechoic room has no sound reflections, there is no reverberation or reverberation time (RT) and the impulse response is a true impulse, meaning it looks like a spike.

Anechoic rooms are used in measuring sound generated by various objects and in acoustic testing of people and animals.

Being outdoors on a calm day well away from any reflecting surfaces is a good approximation to an anechoic room.

Geometry

For these simulations an anechoic room of dimensions 10m x 10 m x 10m was created in CATT acoustics as shown in the figure below

Reverberation Time

The reverberation time (RT) in each of the six octave bands from 125 Hz to 4 kHz.  The estimated RT computed using the Sabine equation (circles) and Eyring equation (diamonds) equations is plotted along with the estimated reverberation time computed using ray tracing from 4*T-15 and 2*T-30 (up and down triangles).  T-15 is the computed time for the sound energy level to drop by 15 dB from th einitial energy level and T-30 is the time for the sound energy level to drop from -5 dB to -35 dB from the initial energy level .

Absorption

The absorption coefficients for the anechoic wedges are shown in the table below

	Absorption Coefficient
Material	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz
Wedges	0.96	0.98	0.99	0.99	0.99	0.99

Impulse Response

The impulse response for the room as computed by CATT is shown in the figure below.   Notice that even this room does not have a perfect impulse for an impulse response.  The small amount of reflection gives rise to a minor tail on the impulse respone.

These animations, auralizations, and visualizations are © 2006 by Ralph T. Muehleisen and are licenced under a Creative Commons Attribution-NonCommercial 2.5 License.