P3aL9 Listen!  Look!

Key Words

Amplitude - height of wave from the crest to the rest position

Compression - region of high pressure.

Decibels - measurement of the sound intensity (loudness).

Frequency - number of waves per second.

Hertz (Hz) - unit for frequency.

Longitudinal wave - wave in which the displacement is parallel to the direction of travel.

Oscilloscope - an electronic instrument that can display sound waves as electrical waves.  Often called a CRO.

Pitch - the frequency.

Quality - the "shape" of the sound wave.  Tells us what instrument is being played.

Rarefaction - region of low pressure.

Time base - a control on the CRO that enables the period of the waves to be measured.

Vacuum - a space where there is no material.

Vibrate - move forward and backward at a high frequency.

 

Test Yourself

Homework
Physics GCSE
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Grade E

Sound is a mechanical wave.  This means that it needs a material to travel in.  A sound wave has a source and spreads out from that source.  Sound waves arise because of the vibration of a source.

 

 

A water wave also needs a material (water) to travel in.  A water wave is like this:

 

 

A water wave is called a transverse wave.

 

However a sound wave is not like a water wave, but travels as a series of pulses of high and low pressure.  This can be shown in this picture.

 

 

 

Note the following:

  • The regions of high pressure are called compressions.

  • The regions of low pressure are called rarefaction.

  Rarefaction is NOTHING to do with refraction.

 

A sound wave is a longitudinal wave.

 

There are all sorts of different ways of measuring the speed of sound.  In air the speed of sound is usually considered to be 340 m/s.  In water it is about 1500 m/s, while in steel the speed is 6000 m/s.

 

Sound needs a material to travel in.  It cannot travel through a vacuum.  This can be shown in this experiment.

 

 

 

The bell is set ringing.  You can hear it ringing before the vacuum pump is turned on.  The sound of the bell gets fainter until you cannot hear it at all.  When the air is let back in, you start to hear it again.

 

Grade C

Properties of Sound

We can investigate these using the oscilloscope (CRO):

 

 

 

 

The microphone turns the sound waves into electrical waves, which the CRO displays on the screen.

 

There are three main properties that we can display on the screen:

  • The frequency (pitch), the number of waves per second.

  • The amplitude (volume), the size of the waves.

  • The quality (timbre), the shape of the waves.

We will look at how these properties look on a CRO screen. The CRO plots a voltage-time graph with the voltage on the vertical axis and time on the horizontal axis.

 

Amplitude

The amplitude or loudness (volume) is represented by the size of the wave.  The bigger the wave, the louder the sound.

 

 

The number of waves on the screen is the same.  The loud wave has a bigger amplitude (is higher) than the quiet wave.

Frequency

The frequency (pitch) is shown by the number of waves on the screen.  The more waves shown, the higher the frequency.

 

 

These sounds have the same loudness, but different frequency.  There are more waves across the screen with the higher pitched wave.  The amplitude of both waves is the same.

 

 

Quality of Sound

The pictures above show sine waves, which give pure sounds.  However these are very boring to listen to.  Musical instruments give more complicated waves, which makes them sound more interesting.  The quality of the sound is shown as different wave shapes.  The quality of the sound allows us to tell what instrument is playing the note.

 

 

The two waves have the same frequency and the same amplitude.  The shape of the two waves is different.

Different musical instruments have a different quality of sound, so each one sounds unique.  This is because of the presence of harmonics, which are small waves with frequencies a whole number of times the frequency of the notes (fundamental frequency).  The repeating pattern of the sound of an instrument is pleasing to most people (even if not all).

When noise is played into a CRO, the waveform is very jagged and random.  Such sounds are not pleasant to listen to.

Grade A

Sound levels are measured in decibels (dB).   The scale is not linear, but logarithmic.  For each bel of sound (10 dB) the  sound level goes up 10 times.  If we go from a quite library (20 dB) to a loud classroom (70 dB), the sound level has not gone up by 3.5 times.  It has gone up by 50 dB (5 bel) which is 100 000 (105) times louder.