Prediction and simulation

There is an excellent sound simulation on Physics education technology (Phet). The basic set up is:

From the evidence in the vibrations section, sound appears to be a form of energy which travels in waves and is generated by vibrating objects within certain frequency limits. It seems to be a low energy phenomenon which can yet be detected by our ears or a microphone.

If this is the case then we predict that sound should show the wave behaviours described by scientists for water waves.

A two pronged approach should work well. Devise and perform experiments then refer to the Phet sound simulation which acts as a working model of the theory of the physics of sound. All Phet simulations can be downloaded for offline use and were distributed on the Junior Science software CD in Autumn 2005.

Reflection

Echoes and speed of sound measurements shed light on this but devise an experiment of your own. Use what you know of ball bounces off walls and light reflection off mirrors. What variables are involved? What measurements should you make? How could you channel the sound? How could you measure it?

The Phet simulation may be useful. Try changing the wall position and angle or the frequency. Do these effect the sound pattern much?

 

Refraction

Sound bends around doorways and we can hear around corners. Set up a sound intensity meter to gather data to map the intensity of sound near an obstacle. Is the meter better than listening? Can this or another method confirm refraction of sound? For light, a denser object than air ,e.g. glass or a round bottomed flask of water, can refract the light to a focal point. How could you make a sound lens? What would you need and how could you get it? Try it out.

Diffraction

Set up a speakers emitting a single pitch sound. Walk around it. What do you notice? Setup a sound intensity meter to map the sound intensities around the speaker.

Interference

This and diffraction are expected when two speakers are placed close together so that the wavelength of the sound is similar to the distance between the speakers: the speakers are like point sources of sound from which sound spreads outwards in a cone.

Devise and perform an experiment to map the sound intensities around a pair of speakers: computers equipped with sound software and two speakers can be used or a signal generator connected to two small speakers. If you walk in front of the speakers what do you hear?

Use the Phet sim to model what happens for different frequencies, amplitudes and distances between the speakers.

Does the experiment match the model? Is this convincing evidence that sound behaves as a waveform?

 

It can be shown that objects vibrating with simple harmonic motion (most objects can do this) generate sine waves as a result. What implications does this have? Calculate some frequencies for captured sound waves. List a few.

Now we have looked at the basic behaviour of sound we may be able to better understand sound in action: its speed, music, hearing and some peculiar properties explored in other pages of this resource.