Acoustical Defects

The acoustical design of an enclosed space is basically depend on the behaviour of the reflected sound. Due to the reflection of sound two main defects are developed, viz., echoes and reverberation.

1. Echoes

Echo is said to be produced when a reflected sound wave reaches the ear just when the original sound from the same source has already been reached. The sensation of sound persists for one-tenth of a second after the source has ceased. Thus an echo forms when the time lag between the two sounds is about 1/17th of the second.

Further considering the velocity of sound in the atmosphere air as 34.3 m/s, it is shown that when the distance of the reflecting surface is between 8 and 17 m, echoes are formed.

The defect of echoes also occurs when the shape of the reflected surface is curved with smooth character. Echoes are unpleasant to hear and cause disturbance to hearing.

2. Reverberation

When the surfaces of an indoor place are hard and smooth, very small energy is lost at each impact of sound and many reflections take place before the sound dies down. This repeated reflection of sound is called prolongation or reverberation. Then if sound exists too long, then successive words of a speech will overlap and confuse.

The remedy for this defect is to select a correct time of reverberation known as optimum time of reverberation. This is achieved by suitably selecting a proper absorbent or acoustical materials for different reflecting surfaces.

Acoustical level of a room and reverberation times are shown in Table 28.3, Table 28.4 shows the recommended values of the optimum reverberation time in India for halls after considering the indicated audience strengths.