 # Chapter 15 – Sound

1.    Describe how a vibrating object produces sound waves. (2 marks)

• Sound is produced by vibrating sources placed in a medium.

• When the object (e.g. ruler or tuning fork) vibrates, a series of compressions (high air pressure) and rarefactions (low air pressure) are produced by shifting of air layers.

• In doing so, a longitudinal wave is produced.

2.    Describe how a vibrating object produces sound waves. (4 marks)

• Sound is produced by vibrating sources placed in a medium.

• When the vibrating object moves outwards, it pushes the air layers close together and produces a region of higher pressure known as compression.

• When the object moves inwards, it pulls the air layers apart and produces a region of lower pressure known as rarefaction.

• The continuously vibrating object thus produces a series of compression and rarefactions traveling away from the vibrating source as a longitudinal wave.

• Energy is transferred away from the source via collisions of adjacent particles in the medium.

3.    Describe how the air pressure at a particular point changes when a sound wave passes that point.

• Air pressure changes from high (compression) to low (rarefaction) alternately.

4.    Describe an experiment which shows that a medium is needed to transmit sound waves.

• Before air is drawn out from the bell jar, the sound of the ringing bell can be heard.

• Evacuate air out of the bell jar slowly using the vacuum pump. The sound of the ringing bell decreases until eventually, no sound is heard, although the hammer can still be seen vibrating.

• This experiment shows that a medium is needed to transmit sound waves.

5.    Suggest briefly how measurements involving echoes can be used to find the depth of water in a sea.

• SONAR (SOund Navigation And Ranging) is used to send out sound waves until they are reflected by the bottom of the lake.

• The reflected sound is detected and the time taken, t, is recorded.

• If the speed of the sound in water v is known, then the depth of the lake, d, can be calculated by the formula:

6.    Describe how you would carry out an experiment to measure the speed of sound in air. Your answer should make clear what measurements you would take and how those measurements would be used to produce the results.

• Position observers A and B at a large known distance, d.

• The distance d should be large so that it reduces the percentage error due to human reaction.

• Observer A fires the pistol.

• Observer B uses his stopwatch to measure the time interval t1 between on seeing the flash of the pistol and hearing the sound.

• To reduce the measurement error due to the effect of the wind, repeat the experiment with A and B switched place, measuring the the time t2.. Obtain the average of the two times, <t> = (t1+t2)/2

• The speed of sound, v can be calculated by:

Speed = Distance/Time = d/<t>