sound production

1. Waves, sound &
music
(in one-dimension, mainly)

2. Making sounds
Many sources of sound are very familiar:
the human voice
animal sounds
straw oboe
whistling tubes
musical instruments (string, woodwind, brass, percussion,
electronic)
electric & combustion motors, rolling wheels & tyres, other noise
What words are commonly used to describe these sounds?

3. A hearing test
What is the lowest frequency that you can hear? the
highest?
At what frequency does the sound seem loudest?

4. Learning outcomes
• illustrate that all sounds are produced by vibrations
• distinguish between transverse and longitudinal waves
• describe the propagation of sound in terms of density waves
• relate pitch of a sound to wave frequency and volume of a sound to
wave amplitude
• recall and use the relationship between wave speed, frequency and
wavelength to make quantitative predictions
• use concepts of phase and phase difference to explain superposition of
linear waves
• describe how a standing wave is created in musical instruments
• make links with music to motivate learning about waves
• interpret and use graphical representations of waves
• develop confidence in using signal generator & oscilloscope
• use units of frequency, Hz and kHz

5. Teaching challenges
Students can have difficulty visualising how a wave moves through
space i.e. that wave motion differs from the motions of particles in a
medium through which a wave travels.
Representing waves:
– Sound waves are often displayed on an oscilloscope, which
involves representing a longitudinal wave with a transverse wave.
– Many students confuse displacement - distance graphs with
displacement - time graphs.
The fact that wave speed depends entirely on the medium and not
on the sound source (i.e. frequency or amplitude) is not generally
understood.
Standing waves: Typically it takes many examples for students to
grasp the concept of ‘phase’. They find it difficult to appreciate
that standing waves are a superposition of two identical waves
travelling in opposite directions.

6. Radiation
‘any process in which energy emitted by one body travels
through a medium, or through space, ultimately to be
absorbed by another body.’
Includes
• sound
• light (and the whole electromagnetic spectrum)
• nuclear radiation (, positrons, neutrons)

7. SOURCE MEDIUM DETECTOR
A general model for radiation
journey: may involve transmission, reflection,
refraction, partial absorption
detector: absorption at the journey’s end
SPT simulation

8. Vibrations make sounds
Making a sound source visible
• vibrating tuning fork touched into a beaker of water, or against a
suspended ping pong ball
• candle in front of a vibrating loudspeaker
• rice or polystyrene balls on a vibrating loudspeaker cone that
faces upwards
• using a ‘wobbleboard’
You can also feel your larynx vibrate.

9. Two types of wave travel
Particles in a medium vibrate about their mean
positions, transferring energy but not matter.
longitudinal wave – vibration along the direction of energy
transfer
transverse wave – vibration perpendicular to the direction of
energy transfer

10. Longitudinal waves
can be thought of as ‘density waves’ in a material
medium (solid, liquid or gas).
SPT simulation 2
AKA ‘pressure’ or ‘compression’ waves, because
compressions alternate with rarefactions.
• slinky spring
• PP experiment Waves along a line of students

11. Detectors of sound
• ear – structure, range of hearing, locating a sound
source by comparing arrival times
• microphone
• sound level meter

12. Visualising sounds
• Faroson software
• standard oscilloscope
• Soundcard Oszilloscope

13. Measuring sound
Frequency (pitch) – vibrations or cycles per second (Hz, KHz)
Amplitude – size of the vibration
Loudness – perceived strength of a sound (frequency dependent)
Intensity – energy carried by a sound (dB scale)

14. Wave changer
An oscilloscope displays any longitudinal wave
as a transverse wave.
Why?
• oscilloscope display: voltage – time
• microphone diaphragm vibrates, producing an a.c. waveform
So how do you explain this at an introductory level?
• use a ‘wave changer’

15. displacement – distance graph shows where the
particles are at one instant of time. A snapshot.
Gives wavelength, .
Representing waves graphically

16. Representing waves graphically
displacement – time graph shows what happens to
a single particle in the medium over time.
‘Period’, T, is time for one complete cycle. Gives frequency,
T
f
1


17. Wave speed
distance wave travels in a second (m/s)
= wavelength (m) x number of waves each second (s-1)
In symbols,
• To find the speed of sound, measure a distance and a time.
• speed of sound in a solid > speed in a liquid > speed in a gas
• The speed of sound in air depends on temperature & humidity.
• Medium, not source, generally determines wave speed (though
both water & glass are ‘dispersive media’ – v depends on f).

f
v 

18. Measure the speed of sound
… with a double beam oscilloscope
…or use echoes from an exterior school wall.

19. Diffraction, at edges & gaps
Diffraction: spreading of wavefronts as
they pass the edge of an object.
• only noticeable when the gap/obstacle size is
similar to the wavelength
Examples:
• Sound waves diffract through a doorway
• Radio waves diffract around hills & buildings
• Visible light with gap/obstacle < mm
hole
pinhead
ripple tank

20. Refraction
As a wave travels from one medium to another, its
speed changes at the boundary.
The frequency of waves arriving and departing is the
same.
… so wavelength changes at the boundary.

f
v 

21. Wave interference
Two waves can pass through a common point without affecting
each other.
Superposition principle: The displacement at any point will be the
vector sum of the displacements caused by each wave.

22. Phase relationship
Wave phase at a point describes the stage in a cycle of the
vibrating particle.
Where two waves cross, what happens depends on the phase of
each wave, i.e. their phase relationship.
in phase: constructive interference
out of phase: destructive interference

23. Workshop session in the lab
Experiments:
• signal generator and oscilloscope
• Waves with trolleys
• speed of sound
– in air, using Audacity software
– in a metal bar, using Picoscope
– using the clap-echo method
… plus demonstration experiments
Questions
C21 Activity 6.8 Questions on v = f
C21 Activity 6.9 Do you understand wave motion?

24. Beats
Two waves with slightly
different frequencies.
Their phase relationship
constantly changes.
This effect is used to tune musical instruments (reduce the beat
frequency to zero).

25. Reflections at a boundary
Transverse pulse in a string:
When the pulse reaches a rigid support, the reflected
pulse is upside down.
When the pulse reaches an open end, the reflected
pulse is upright.

26. Standing waves
Watch two waves pass through each other.
W.Fendt simulation: Standing Wave
Phet simulation: Wave on a string
Note that amplitudes vary along the wave path.
• Any point of maximum amplitude is called an
antinode.
• Any point of zero amplitude is called a node.
All musical instruments involve standing waves.

27. Musical standing waves
In stringed instruments, there is a node at each
end of a string.
In wind instruments, there is an antinode at
each end of an air column.

28. Web-based resources
Institute of Sound and Vibration Research, University of
Southampton
Acoustic, Audio and Video Engineering, University of
Salford
Voicebox SEP booklet and UCL interactives

29. Applications
Speech and hearing (speech therapy,
voice coaching, hearing impairment, audiology)
Sound transmission and production (mobile phone, radio, TV, film)
Control of sound in the environment (acoustics, noise reduction,
health and safety)
Music (instruments, recording, performance)
Microphones and loudspeakers
Sonar (echo sounding e.g. mapping the sea bed)
Ultrasound (medical (ultrasound images), non-destructive testing
(flaw detection), cleaning jewellery), distance measurement)
Include
information on
careers
29

30. Support, references
www.talkphysics.org
SPT 11-14 Light & Sound
Ep1 Describing sound
Ep2 Quantifying sound
Ep3 Using sound
David Sang (ed., 2011) Teaching secondary physics ASE / Hodder
Phet simulations Sound and waves
Use Google Earth for examples of refraction & diffraction