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SOUNDSOUND

• Different sounds give us different kinds of feelings, e.g. relaxation, excitement, annoyance.

• Noise can be useful too. Fire alarms and car horns warn us of danger.

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All sounds are produced by vibrations.

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Sound (tuning fork)• It is produced by It is produced by vibrating vibrating

sources placed in a mediumsources placed in a medium

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PRODUCTION OF SOUNDPRODUCTION OF SOUND• Sound waves are produced when a vibrating

object causes small but rapid changes to the air pressure around it.

• When layers of air are pushed close together, a compression of the air particles is formed.

• When the air layers are pulled apart, a decompression or rarefaction of the air particles is formed.

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Compressions are regions in the medium where theparticles are closer together. Compressionsare regions where the air has a slightly increased pressure, as a result of the particles being closer together.

Rarefactions are regions in the medium where the particles are spread out. This results in a slight decrease in air pressure.

The wavelength (λ) for longitudinal waves is the distance between the centres of adjacent compressions (or rarefactions), or the distance between adjacent regions where the particles that are moving are in phase.

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CC

C

RR

Rloudspeakerdiaphragm

vibration

loudspeaker

distanceundisturbedair pressure

C C CR R R

p

Sound waves travelling through air

C = compression

R = rarefaction

Refer to TB pg. 211

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The frequency of the sound produced will be the same as the frequency of the vibrating object.

As time goes by, the positions of the compressions and rarefactions move away from the source at the speed of sound.

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Waveform of a sound wave:Waveform of sound which we see on the oscilloscope screen is really a graph showing how the air pressure in front of the microphone diaphragm changes with time.

pressure

normal pressure

time/s

compression

rarefaction

compression

rarefaction

The crests of the wave on the screen correspond to compressions of the sound wave and the troughs correspond to rarefactions.

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rubber stopper

bell jarfine flexible wires

rubber cord

vibrating hammer

to vacuum pump

bell

switchbattery

Sound waves need a medium to travel

Refer to TB pg. 212

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Procedure:

Set up the apparatus as shown in the diagram.

Switch on the electric bell. Loud bell sound can be heard continuously.

Switch on the vacuum pump.

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Observation:

The sound becomes softer and softer during the process of pumping.

When almost all the air is being pumped out, no bell sound can be heard but the hammer still vibrating which shows that the bell is working properly.

Turn off the pump and let air return to the bell jar. The bell sound can be heard loudly again.

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Conclusion:

The experiment shows that sound waves produced by the bell vibration need a medium(air) to travel to our ears.

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Qus 1

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Qus 2

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The Speed of Sound Sound travels in air at 330 m/s (1180 km/h).

Sound travels in fresh water at 1 500 m/s (5400 km/h).

Sound travels in metal(copper or iron) at 5 000 m/s (18 000 km/h).

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Speed of Sound

Speed of sound in air at normal temperature is about 330 m/s.

Speed of sound increases as temperature of medium decreases.

Sound wave travels faster in a denser medium than that in a less dense medium.

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The speed of sound in a medium depends on the density of the medium.

The speed of sound increases as the density of the medium increases.

Solids liquids gases

Speed of sound decreasing

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To determine speed of sound in air

One person fires a gun into the air.

The time keeper starts a stopwatch when he sees the flash from the gun and stop the stopwatch as he hears the gun shot.

1 km

Method I

Refer to TB pg. 218

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Calculate the speed of sound using the formula below:

speed of

sound =distance between the two observers

time taken

What is the assumption made in this experiment?

There is no wind affecting the speed of sound.

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Measuring the speed of sound by using echoes(Echo is a reflection of sound.)

100 m

hard wood

vertical wall

Nature of wall: tall, hard, flat, large area, stand vertically

The surroundings must not have any other wall.

Method II

Refer to TB pg. 219

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Procedure:1 Measure a distance of 100 m or more at

right angle to a large vertical wall.

2 Make a sharp clapping sound by banging two blocks of hard wood together. Repeat the sound at regular time intervals to coincide exactly with the echoes.

3 Starting at zero as a stopwatch is started, count the number of claps and stop the stopwatch at 50 or 100 claps

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4 Calculate the speed of sound using the formula below:

speed of

sound =

(distance between observer and wall ) x 2

total time taken

= 200 x number of claps

total time taken(m / s )

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Advantages of the method compared with other method not using echoes

1 The accuracy will not be affected by wind.

The time taken is for the sound to travel to and fro hence the effect of the wind in direction will be balanced off by the opposite direction.

2 The distance used for the experiment is short.

3 The time taken can be extended easily.

4 The experiment can be repeated easily.

5 The co-ordination between time keeper and the person who is clapping is easier.