Reverberation means the prolonged reflection of sound from walls, floor or roof of a hall. In simple language it is nothing but persistence of sound even after the sources of the sound has stopped.

Echo:

• An Echo is produced when the reflected sound waves reach the ear just when the original sound from the same source has already been heard.
• Echo is the repetition of sound due to the reflection from an obstacle.
• The sensation of sound persists on human ear for l/10th of a second after the sound source has stopped. So, to hear an echo, the reflected sound must reach to the listener 1/10 th of a second later than the original sound. Assuming the velocity of sound in air as 340 m/s the reflected sound must travel a total distance of 34 meters (to and fro) before reaching to the listeners ear for the echo to be heard.

Reverberation time:

• The time gap between the initial direct note and the reflected note upto a minimum audibility level is called reverberation time.
• More precisely, the interval of time taken by a sustained or continuous sound to fall to an intensity level equal to one millionth of its original value, (i.e. fall by 60 db in loudness) is called reverberation time.
• In a good auditorium it is necessary to keep the reverberation time as small as possible. The intensity of the sound as received by listener is shown graphically in Fig. 1.
• When a source emits sound, the waves spread out and the listener is aware of the commencement of sound when the direct waves reach his ears. Subsequently the listener receives sound energy due to reflected waves also. If the note is continuously sounded, the intensity of sound at the listener’s ears gradually increases. After sometime, a balance is reached between the energy emitted per second by the source and energy lost or dissipated by walls or other materials.

Fig. 1

• The resultant energy attains an average steady value and to the listener the intensity of sound appears to be steady and constant.

• This is represented by a portion BC of the curve ABCD.

• If at C, the source stops emitting sound, the intensity of sound falls exponentially as shown by the curve CD.

Fig. 2

• When intensity of sound falls below the minimum audibility level, the listener will not get the sound.
• When a series of notes are produced in an auditorium each note will give rise to its own intensity curve with respect to time. The curve for these notes are shown in Fig. 5.2.2.
• In order to maintain distinctness in speech its is necessary that:

(a)     Each separate note should give sufficient intensity of sound in every part of the auditorium.

(b)     Each note should die down rapidly before the maximum average intensity due to the next note is heared by the listener.

• Intensity of sound is energy transported by sound waves per unit time through a unit area perpendicular to the direction of propagation of sound waves.

A lecture hall or auditorium should satisfy the following conditions in order to be acoustically good.

1. The initial sound from the source should be of adequate intensity.
2. The sound should spread evenly with proper loudness everywhere is the hall
3. The sound of speech or music should be clear and words of or musical notes must be distinctly audible to all.
4. All undesired or extraneous noise must be reduced to the extent that it will not interfere with normal hearing of speech or hearing.
5. Any distortion due to shape and size must be absent.

Avoid noisy places like railway tack, roads with heavy traffic, airports, industrial vicinity for auditorium.

1. Volume

•  Size of the hall/ auditorium should be such that it remains optimum.

•  Small halls leads to irregular distribution of sound because of formation of standing waves.

•  Too big halls may also create a weaker intensity and larger reverberation time which is a very serious issue.

2. Shape

•  It is one of the most important parameter to be considered for acoustically correct hall.

•  As the reflections are created by roof and side walls, they should be designed in such a way that echos are not allowed to generate.

•  In place of parallel walls, splayed side walls are preferred. Curved surface on walls, ceilings or floor produce concentration of sound into particular region and absence of sound in other regions.

•  Hence curved surface must be designed with proper care.