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 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.