Reverberation time and Decay Curves

Reverberation time Can be measured with a noise signal or an impulse.

Say a noise source is placed In a room and have reached a stationary noise, when the noise source is suddenly turned off, the decay curve can be sketched as below for good conditions.

Good conditions means a linear response on the decay when the noise is turned off. A bad response would result in a nonlinear decay curve, which is most likely caused by the rooms dimensions (small rooms or nondiffuse sound field), where axial room modes end up “bending” the decay curve, making it difficult to determine the reverberation time.

If we assume we have good conditions and a linear decay curve, the evaluation decay is found 5 dB below the stationary noise. This 5 dB region is important to skip as it can showcase nonlinear behaviour because of the direct influence of the sound source early on In the decay.

The evaluation decay, often ranging 20 or 30 dB is then used to determine the reverbation time T20 or T30, which in short is the time it takes the decay curve to drop 60 dB determined from a 20 or 30 dB drop.

Bent Decay Curve

How can we tell if the sound field is not diffuse?

Say a noise source is placed In a room and have reached a stationary noise, when the noise source is suddenly turned off, the decay curve can be sketched as below for nondiffuse conditions.

Non Diffuse conditions usually means a nonlinear response on the decay when the noise is turned off, which is most likely caused by the rooms dimensions (small rooms or nondiffuse sound field), where axial room modes end up “bending” the decay curve and may showcase a similar decay curve as the sketched curve above.

If we try to determine the reverberation time from such a nonlinear decay curve, then we Can end up with different reverberation times depending on where we look on the decay curve (early or late decay).