• Reflection of Sound

If a sound is not absorbed or transmitted when it strikes a surface, it is reflected. The law for reflection for sound is the same as that of light.

When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound. The sound waves that travel towards the reflecting surface are called the incident sound waves. The sound waves bouncing back from the reflecting surface are called reflected sound waves.

A perpendicular drawn on the point of incidence is called the normal. The angle which the incident sound waves makes with the normal is called the angle of incidence, "i". The angle which the reflected sound waves makes with the normal is called the angle of reflection, "r".

The following two laws of reflection of light are applicable to sound waves as well:

1. The angle of incidence ∠i is equal to the angle of reflection ∠r.

2. The incident wave, the normal to the reflecting surface and the reflected wave at the point of incidence lie in the same plane.

Echo

The repetition of sound caused by the reflection of sound waves is called an echo. We can hear echo when there is a time gap of 0.1 second in original sound and echo (reflected sound). Echo is produced when sound reflected from a hard surface (i.e., brickwall, mountain etc.) as soft surface tends to absorb sound. To calculate the minimum distance to hear an echo:

Speed = Distance / Time

Reverberation

A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. In an auditorium or big hall excessive reverberation is highly undesirable.

Reverberations do wonders when it comes to musical symphonies & concert halls, when the right amount of reverberation is present, the sound quality gets enhanced drastically. This is the reason why sound engineers are appointed during the construction of these halls.

If a room has about nearly no any sound absorbing surfaces like wall, roof and the floor, the sound is said to bounce back between the surfaces and also it takes a very long time as the sound dies. In such a room, the listener will then have a problem for registering the speaker. This is because he tends to hears both the direct sound as well as echo. And also if these reverberations will be more excessive, the sound is said to run together with a mere loss of articulation, and it becomes muddy and also garbled.

To reduce reverberation, the roof and walls are generally covered with sound-absorbent materials (Porous materials) such as mineral wool, compressed fibreboard, rough plaster or draperies.  The seat materials are also selected on the basis of their sound absorbing properties.

Applications of Multiple Reflection of Sound

(i) Megaphone, loudspeakers, bulb horns and trumpets, shehnai etc. are designed to send sound in a particular direction without spreading all around. All these instruments have funnel tube which reflects sound waves repeatedly towards audience. In this amplitude of sound waves adds up to increase loudness of sound.

(ii) Stethoscope : It is a medical instrument used for listening the sounds produced in human body mainly in heart and lungs. The sound of the heartbeats reaches the doctor’s ears by the multiple reflection of the sound waves in the rubber tube of stethoscope.

(iii) Sound Board : In big halls or auditoriums sound is absorbed by walls, ceiling, seats etc. So a curved board (sound board) is placed behind the speakers so that his speech can be heard easily by audiences. The soundboard works on the multiple reflection of sound.

(iv) The ceiling of concert halls are made curved, so that sound after reflection from ceiling, reaches all the parts of the hall.