What is Reverberation?

Reverberation Time

Reverberation time is defined as that time required for the sound in an air space to decay by 60dB. This represents a change in sound intensity or sound power of 1 million (10log 1,000000 = 60dB), or a change of sound pressure or sound-pressure level of 1,000 (20 log 1,000 = 60dB). In very rough human terms, it is the time required for a sound that is very loud to decay to inaudibility.

Pressing the accelerator of a car results in acceleration of the vehicle to a certain speed. If the road is smooth and level, this speed will remain constant. With this accelerator setting the engine produces just enough torque to overcome all the friction losses and a balanced (steady-state) condition results. It is the same with sound in a room.

When the switch is closed, a loudspeaker arranged to produce random noise into a room will produce a sound that quickly builds up to a certain level. This is the steady state or equilibrium point at which the sound energy radiated from the loudspeakers is just enough to supply all the losses in the air and at the boundaries of the room.

A greater sound energy radiated from the loudspeaker will result in a higher equilibrium level; less power to the loudspeaker will result in a lower equilibrium level. When the loudspeaker switch is opened, it takes a finite length of time for the sound level in the room to decay to inaudibility. This “hanging-on” of sound in a room after the exciting signal is called reverberation and it has a very important bearing on the acoustic quality of an air space.

Influence of Reverberation on Speech

Let us consider what happens to just one tiny word in a reverberant space. The word is back. It starts abruptly with a “ba…” sound and ends with a consonant “…ck”, which is much lower in level. As measured on a graphic-level recorder, the “ck” sound is about 25dB below peak level of the “ba” sound and reaches a peak about 320 milliseconds after the “ba” peak.

Both the “ba” and “ck” sound are transients that build up and decay after the manner. Sketching these various factors to scale yields. The “ba” sound builds to a peak at any arbitrary level of 0dB at time = 0, after which it decays according to the reverberation time of the room, which is assumed to be 0.5 seconds (60dB decay in 0.5 seconds).

The “ck” consonant sound, peaking 0.32 seconds later, is 25dB below the “ba” sound peak. It too decays at the same rate as the “ba” sound according to the assumed 0. seconds RT60. Under the influence of the -.5 second reverberation time, the “ck” consonant sound is not masked by the reverberation time of the “ba”. If the reverberation time is increased to 1.5 seconds, as shown by the broken lines, the consonant “ck” is covered completely by reverberation.

The primary effect of excessive reverberation is to impair the intelligibility of speech by masking the lower level of consonants. In the word “back” the word us unintelligible without a clear grasping of the “ck” part. Understanding the “ck” ending is the only way to distinguish “back” from bat, bad, bass, ban or bath. In this oversimplified way, we can understand the effect of reverberation on the understanding of speech intelligibly and the reason why speech is more intelligible in rooms having lower reverberation times.