Measuring the Wavelength
and Speed of Sound Waves
Including animations from:
Schuylkill Campus (part of) The Pennsylvania State
University
Daniel A. Russell, Ph.D.
Physics Department, Kettering University
What is a Sound Wave?
• Sound is a Longitudinal Pressure Wave.
• In this experiment it will be moving through air.
• The air molecules oscillate back and forwards with
changes in pressure.
• The direction of movement of these air molecules is in
the same axis as the direction of wave travel.
The Loudspeaker
• We can conveniently create sound waves using a
loudspeaker.
The Loudspeaker
• A Loudspeaker is a Transducer that converts
Electrical Energy into Sound Energy.
• An incoming Electrical Signal causes the
Speaker Cone to Move In and Out.
• As the Cone Moves In and Out it produces
Pressure Variations.
• The Pressure Variations are Radiated as a
Sound Wave that carries Sound Energy away
from the Speaker.
The Loudspeaker
• We can Control and Monitor the Pressure
Variation Created by a Speaker Cone by
Controlling and Monitoring the incoming
Electrical Signal applied to the Speaker.
Signal Generator
~
Oscilloscope
The Microphone
• We can conveniently measure Pressure
Variations caused by Sound Waves using a
Microphone
The Microphone
• A Microphone is a Transducer that converts Sound
Energy into Electrical Energy
• An incoming Sound Wave causes the Microphone
Diaphragm to Move In and Out.
• As the Diaphragm Moves In and Out it produces
Voltage Variations.
• The Voltage Variations are given out as an Electrical
Signal
The Microphone
• By monitoring the Output Signal from the
Microphone we can Monitor the Pressure
Variations at the Diaphragm caused by the
incoming Sound Waves.
Oscilloscope
Phase-Distance Relationship
• If two points along a Wave are separated by a
Whole Number of Wavelengths, then the
passing wave will be in Phase at those two
points.
• Imagine two boats on a regular wave separated
by one, two or three wavelengths.
• The boats will be going up and down in unison.
Phase-Distance Relationship
• Notice that at distances in between Whole
Wavelengths the points are Not in Phase (They
do Not move in Unison!)
• If we relate this diagram to our sound wave the
vertical axis would represent pressure change.
• Don’t forget that sound is a Transverse Wave
and the Air Molecules move in the same Axis
(direction) as the wave travels.
The Oscilloscope
• An Oscilloscope is a device for graphically
visualising regular oscillating cycles of
electrical signals.
• Remember at the Speaker we are
Converting an Electrical Signal to a sound
Wave.
• At the Microphone we are converting the
Sound Wave to an Electrical Signal.
The Oscilloscope
• By comparing the electrical signals at the
Speaker and the Microphone we can tell if
the pressure variations are in Phase
between the Microphone and Speaker.
• Remember if the Pressure Variations are
in Phase at two points along our wave,
then we know they are separated by a
Whole Number of Wavelengths.
PC Oscilloscope (Picoscope)
• This experiment works perfectly well with
traditional cathode ray oscilloscopes.
These have been available for at least the
last ninety years.
• We use a Modern Oscilloscope that plugs
into a PC only because it allows us to use
the classroom projector so everyone can
see and visualise the signals.
Lets Do The Experiment!
• When the electrical signal to the Speaker and
the Microphone is in Phase, then we know
there is a whole number of wavelengths
between the speaker and microphone.
• If we measure the position of the Microphone at
several points when its signal is in Phase with
the Speaker, then we can deduce the
Wavelength of the Sound wave.
Wave Frequency
• We can also measure the Sound
Wave Frequency using an
Oscilloscope.
• Remember that Frequency is the
Reciprocal of Wave Period.
• Frequency = 1 / Wave Period
In Groups of Four Calculate the Speed
of Sound From Our Experiment
• We have now shown we can measure both the
Wavelength and Frequency of a sound wave in the
laboratory.
• Remember that (when using consistent units):
• Frequency x Wavelength = Wave-Speed.
• Hint. Convert wavelength into meters and frequency into
Herts.
• What figure do you get??
Did you get close to 343 ms-1
• 343 meters per second is the generally accepted
figure for dry air, at 20ºC and at sea level.
• Altitude variations across the UK have little
effect on this figure.
• Temperature however has a significant effect.
• Pressure & Humidity also have a moderate
effect on the speed of sound in air.
• Did we get between 308ms-1 and 377ms-2
• That’s within +- 10% of the accepted figure.