Definition of Sound
- Sound is a form of energy that originates from vibrating objects. It is a longitudinal mechanical wave.
Some Sources of Sound
- Vibrating wooden strip
- Vibrating wire e.g. guitar wire being plucked.
- Vibrating drum
- Tuningfork - the prongs of tuning fork are made to vibrate by striking them against a hard surface.
- Vibrating air columns e.g. blowing air across the mouth of a test-tube.
- Air siren- it is a form of disk with a ring of equally spaced holes which are equidistant from the centre. It is rotated at a constant rate as air is blown through the holes.
- Cog - wheel and card
- Voice box (larynx)
Propagation of Sound Energy
- Sound wave is propagated in form of compressions (areas of high pressure) and rarefactions (areas of low pressure) through propagating medium. Sound requires a material medium for propagation.
- The figure below shows a vibrating fork. The time interval for the prong to go from A to B is 0.005s. Find:
- The frequency of the fork
- The wave length of the vibrations, if the velocity of sound in air is 340m/s.
- The diagram below shows sound waves passing through air. Study it and answer the questions that follow.
- Label the following:
- If the wave front takes 0.1s to travel from A to B, find:
- The frequency
- The wavelength, if velocity of sound in air is 330m/s.
- Label the following:
Aim: To show that sound requires a material medium to travel. (I.e.sound does not travel in vacuum)
- Bell glass-jar
- Vacuum pump
- Setup the apparatus as shown in the diagram below.
- Close the switch and observe what happens.
Observation: It is observed that the bell begins to ring and the sound is heard outside.
- Gradually pumpout some air and note the effect this has on sound reaching you.
Observation: The intensity of sound decreases gradually.
- Pump as much air out of the jar as possible and listen to the sound produced.
Observation: Sound is found to have almost disappeared though the hammer can be seen vibrating
The above observations show that the sound cannot travel through a vacuum. It needs a material medium for propagation.
Factors Affecting Velocity of Sound In Air
- Temperature of the air - sound travels faster in hot air than in cold air.
- Humidity of the air - the velocity of sound on air increases with humidity.
- Direction of wind - wind blowing in the same direction as sound increases the velocity of the latter.
Sound Transmission in Solids, Liquids and Gases
- Sound travels fastest in solids, followed by liquids and then gases.
- Speed of sound in materials varies from solid to solid, liquid to liquid and gas to gas depending on the density of the material. Denser material transmits sound faster.
Reflection Property of Sound Waves
- Reflected sound is called echo. Sound is reflected when it falls on hard surfaces.
- Reflections of sound waves also obey the laws of reflection:
- The angle of incidence is equal to the angle of reflection at point of incidence
- The incident sound, the reflected sound and the normal lie on the same plane.
- Reverberation refers to the effect in which original sound seems prolonged due to overlap with the echo sound. It occurs in some halls, sound waves are reflected from walls, floor and ceiling and since the echo time is short; the echo overlaps with the original sound.
- To reduce reverberation in places like broadcasting studios and concert halls, walls are made using absorbent materials like cotton wool and foam rubber.
Applications of Reflection of Sound
a) Determination of Speed of Sound
- In this case, sound is produced in front of an obstacle whose distance away is known. The time for the sound to reach the obstacle and back to the source (i.e.time for echo to be heard) is measured using a stopwatch.
Speed of sound = distance travelled, 2d/timetaken, t = 2d/t
- A man standing in a valley between two cliffs strikes a gong. He hears an echo from one cliff 0.7s later and from the other 0.2s after the first. Determine the width of the valley. (Speed of sound in air,v=330ms-1)
let distance from nearest cliff be x and that between the two cliffs d
distance = speed of sound × time
x = 115.5m
Also,2(d-x)=330m/s × (0.7+0.2)s
d = 297/2 + x
d=148.5+115.5 = 264m (this is the width of the cliff)
- A time keeper in 100m race stands at the finishing point. He starts his watch the moment he hears the sound of a gun. What error does he make in the timing of the race?(Speed of sound in air is 330ms-1)
The error is equal to the time taken for the sound from the gun to reach the time keeper, i.e. time taken for sound to travel 100m at the speed of 330ms-1 the sound takes:
T = 100m/330m/s = 0.303seconds