Experiment to measure the speed of sound wavesCopy

Learning Objectives

• Describe how the reflection of sound may produce an echo
• Describe an experiment to determine the speed of sound in air

Contents to learn

Do you know, what is the echo?

When sound is incident on a big surface, part of the sound energy is reflected.

The reflected sound waves heard after a silence, is called an echo.

Humans can distinguish an echo if the silent period is more than one-tenth of a second. Clear good echos are produced if;

(a) the distance between the reflector and the sound source is more than 30 meters.

(b) the area of a reflector is large compared to the wavelength of the incident sound.

(c) the incident sound is high pitched i.e. of a high frequency.

Echo sounding is used to survey the depth and nature of the seabed. Knowing the speed of sound in a particular medium, one can calculate the distance between the sound source and the reflector, by timing the echo.

Calculating the speed of sound:

Medium to transmit sound waves

• Sound waves need a medium to transmit (to travel).
• Sound waves cannot travel through a vacuum.
• The speed of sound waves in different media is different.

Speed of sound:

Sound waves have a definite velocity of propagation in a given medium.

A thunder of a cloud is usually heard a few seconds after lightning flash is seen.

When a distant gun is fired, the flash of discharge is seen some time before the sound is heard.

The time interval between the event occurring and it being heard by the observer is simply due to the high velocity of light as it reaches the observer almost instantaneously.

The simplest way of finding the velocity of sound is by arranging for a gun to be fired at a measured distance from an observer who measures the time between seeing the flash and hearing the sound. By using the following relation speed of sound can be measured

$Speed = \frac{\mathrm{distance}}{\mathrm{time}}$

The velocity of the sound in air at

$0^o$

C is 331.4 m / s.

The velocity of sound increases with the increase in temperature.

Some important data about the speed of sound:

Measuring the speed of sound in air

Experiment 1: Determination of speed of sound in air in the school laboratory

Apparatus – two wooden blocks, two microphones, measuring tape

The wooden blocks and the two microphones are arranged in a straight line. The bang from the blocks is picked up first by microphone 1 and then by microphone 2. The first microphone activates the timer, and the second microphone stops it. The speed of sound is calculated from the distance between the two microphones and the time taken by the sound to travel between them.

The equation used to calculate the speed of sound will be

$speed = \frac{dis\tan ce }{time}$

Experiment 2: Determination of speed of sound in air in the playground