P1 Physics – P1.5
A Summary of Sound.
The Particle Model
We can think of a medium (stuff) as being made out of a lot of particles (tiny balls). In a gas the
particles are far apart and free to move around all over the place (unlike a solid or a liquid).
Fig 1: Particles in air, moving all over the place with random movements
When we hear a sound it is due to the movement of these particles. Imagine a loudspeaker (the
yellow cone) is vibrating back-and-forth. This causes regions where the particles are squashed
together, and other areas where they are spread further apart...
Fig 2: Speaker moving back and forwards
Where we have a lot of particles close together we have a region of high pressure, where there
are less particles we have an area of low pressure. As this wave of high and low pressure
travels through the air, from left to right in Figure 2, we have what we call a sound wave.
Sound waves are longitudinal waves and cause vibrations in a
medium, which are detected as sound.
Sound is caused by the vibration, or movement, of particles back-and-forth. If the particles are
close together in a solid or a liquid the wave travels fast. What if there are no particles? No
particles = nothing to vibrate so sound waves cannot travel. That is why, in space where there is
a vacuum, the sound of the Sun burning does not reach the Earth.
If we bounce a sound wave off a surface, such as a wall, we find that it reflects back. That is
why when you shout down a well or in a cave you get an echo – your voice is reflected back at
you.
Echoes are reflections of sounds.
Direction wave is moving in
Particles
move back
and forth
Wavelength = The length from one area of high pressure to the next
area of high pressure (metres)
Fig 3: Wavelength
If we were to measure the distance from one area of high pressure to the next area of high
pressure we would find the wavelength of the sound wave (measured in metres). If we knew
the speed the wave was travelling at (in metres per second) we could also calculate the
frequency (the number of wave cycles per second).
Wave Speed (m/s) = Frequency (Hertz) x Wavelength (metres)
In practice we cannot use a ruler to measure the wavelength of a sound wave, but we can use
an oscilloscope. This is a device connected to a microphone to give us a ‘picture’ of the sound
wave.
Wavelength = Length of one complete wave cycle
Amplitude =
Height from
centre to max
position
Fig 4: Wavelength and Amplitude
If we look at the wave above on an oscilloscope it would look a bit like this:
Fig 5: Wave on an oscilloscope
The amplitude is related to the height of the wave on the oscilloscope trace, the
taller the
wave the louder the sound. The smaller the wave the quieter the sound.
The frequency is how s t r e t c h e d or compressed the wave is. If it is more stretched
out then it means that there are less cycles per second and we hear this as a low note. A high
frequency means a high note (we say its pitch has increased).
The pitch of a sound is determined by its frequency and
loudness by its amplitude.
If we look at this on an oscilloscope we can say that:
If Trace A is the starting sound;
Trace B is about half as loud (amplitude is half),
Trace C has a higher pitch (frequency has doubled),
Trace D is half as loud and at a higher pitch than Trace A.
Trace A
Trace C
Trace B
Trace D
Quality – Shape of the waveform
The shape of the wave is important, and we call
this the quality. If we have a smooth curve then
the ‘quality’ is good. If it is all jaggedy then the
sound is not so pure.
Audible Sound – Pardon?
Sound waves are caused by the vibration of particles. Humans can generally hear a sound
when the frequency of vibration is between 20 Hertz and 20 000 Hertz (cycles per second).
If we increase the frequency even higher we get ‘sound’ waves that we cannot hear. This is the
realm of ultrasound, produced by electronic systems and filling a wide variety of roles.