Core Physics Topic 11 - Sound

Nature of Sound

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.  One such source is a loudspeaker.

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, which we saw in Topic 9.

 Question 1 How does the wave vibration compare with the direction of the spreading of the 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.

 Question 2 How does the wave movement of a sound wave compare with the direction of the spreading of the wave?

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.

Like all waves, the wave equation applies:

 Learn this for the exam: Wave speed (m/s) = frequency (Hz) × wavelength (m)   In physics code: c = fl

In triangle form:

See Topic 9 if you don't remember the symbols.

 Question 3 A musical note of 440 Hz is played.  What is its wavelength?

 Question 4 What is the frequency of a sound of wavelength 4.0 m in water, where the speed of sound is 1500 m/s?

The human ear can pick up sounds that have a frequency of 20 Hz to 20 000 Hz (20 KHz).  Outside this range we have:

• Infrasound (below 20 Hz).  This is often used by very large animals (e.g. elephants) for communication over long distances.

• Ultrasound (above 20 kHz).  This is used by bats for navigation by SONAR (sound navigation and ranging).  It is also used by a non-destructive investigation equipment.  We will look at this in the next topic.

 Question 5 What are the wavelengths of the extremes of human hearing (20 Hz and 20 kHz)?

The  human ear is most sensitive to 3000 Hz, the frequency of a human scream.  Babies cry at this frequency and alarms sound at this frequency.  3000 Hz sounds are very penetrating and will make you feel very uncomfortable.   As you get older, the upper limit of hearing goes down.  Young people can hear bats; middle aged people cannot.

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.

 Question 6 What happens when the vacuum pump is turned on?  (Assume that the vacuum pump is quite quiet in operation.)

Properties of Sound

Sound has a number of properties that we can investigate using a 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 pictures show a note of the same frequency, but different loudness.

 Question 7 How can we tell that the note has the same frequency, but different loudness?

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.

 Question 8 How can we tell that the loudness is the same, but the frequency is different?

A high pitched note has a high frequency, NOT a loud volume.

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.

 Question 9 Compare the two waves.  What features are the same?  Which feature is different?

Reflection and Refraction of Sound

Like all waves, sound waves can be reflected and refracted

Echoes are examples of reflected sound.  A ship using SONAR (sound navigation and ranging) sends pulses (pings) of high frequency sound to the bottom of the sea.  It has microphones that pick up the echoes, and the depth of the water can be worked out from the time taken between sending the pulse and receiving the echo.

 Question 10 It takes 1.5 s for a SONAR to receive an echo after it has sent a pulse.  How deep is the sea?  Speed of sound in water is 1500 m/s.

Many animals navigate using pulses of sound.  They make a sound picture in their brains.  It's an effective way of hunting when conditions are murky underwater, or dark.  The most obvious animals that do this are dolphins and bats.

Geophysicists can tell the structure of rocks by setting off explosions and listening to the echoes with a network of microphones.  The sound waves both reflect and refract as they pass through different rock types.  The idea is shown in the picture below.

The pattern of reflections is quite complicated to interpret, but nowadays there are computer programs that can give a detailed read out of the rock structure within seconds.

 Question 11 What is the speed of sound in Rock A compared to that in Rock B?  (Don't worry about the numbers!)  How can you tell?

 Summary Sound is a mechanical longitudinal wave. It needs a material to travel in. It has wave properties.  It can be reflected or refracted. The frequency is the pitch The amplitude is the loudness The quality allows us to know the source of the sound. Humans can hear in the range from 20 Hz to 20 kHz