Describe an experiment to determine the velocity of sound in air by an interference method. (06marks)

Two small loud speakers a and b about 1m apart are connected to the same oscillator so that both emit sound waves of frequency, f, in phase.

A sensitive detector is moved parallel to the line AB along PQ and it detects a maximum wave at P on perpendicular bisector MP of AB and another maximum wave when it first reaches a point Q directly opposite to B.

Constructive interference of sound wave occurs at P and Q, so the wavelength of sound wave is given by λ = AQ –BQ

The speed of sound in air is then calculated from V =fλ.

Alternative 1

A long tube is filled with water.

A vibrating turning fork of known frequency f is held over the mouth of the tube.

Water is gradually removed until a loud sound is heard.

The length l₁ of air column is measured.

Water is allowed out until a loud sound is heard again.

The length l2 of air column is measured

The velocity of sound in air is calculated from V= 2f(l₂ –l₁)

Alternative 2

• Along tube is filled with water.
• A vibrating turning fork of known frequency f is held over the mouth of the tube.
• Water is gradually removed until a loud sound is heard.
• The length L of air column is measured.
• (i) to (iv) is repeated with five other different turning forks
• The results are tabulated including values of 1/f
• A graph of L against  1/f is plotted and slope S is determined.
• The speed of sound, v = 4S.

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