Question

Write short notes on reflection of sound waves from plane and curved surfaces.

Answer 1

When the sound waves hit the plane wall, they bounce off in a manner similar to that of light. When a loudspeaker is kept at an angle with respect to a wall (plane surface), then the waves coming from the source (assumed to be a point source) can be treated as spherical wave fronts. Hence, the reflected wave front on the plane surface is also spherical, such that its center of curvature can be treated as the image of the sound source (virtual or imaginary loud speaker) that can be assumed to be at a position behind the plane surface. These are shown in figures.

Reflection of sound through the plane surface

The behaviour of sound is different when it is reflected from different surfaces-convex or concave or plane. The sound reflected from a convex surface is spread out and so it is easily attenuated and weakened. Whereas, if it is reflected from the concave surface it will converge at a point and this can be easily amplified. The parabolic reflector (curved reflector) that is used to focus the sound precisely to a point is used in designing the parabolic mics which are known as high directional microphones.

We know that any surface (smooth or rough) can absorb sound. For instance, the sound produced in a big hall or auditorium or theatre is absorbed by the walls, ceilings, floor, seats etc. To avoid such losses, a curved sound board (concave board) is kept in front of the speaker, in such a way that the board reflects the sound waves of the speaker towards the audience. This method will minimize the spreading of sound waves in all possible direction in that hall it also enhances the uniform distribution of sound throughout the hall. Hence a person sitting at any position in that hall can hear the sound without any disturbance.

Reflection of sound through the curved surface