Sound: what does it look like? How does it work?

Sound is a wave traveling through the air.

Can we see it? How does it move from a wire to real sound, and back to a wire again for recording?

Equipment

Ideally, for this project, you will have three things – or their equivalents. They sound expensive, but there’s lots of ways of doing them on the cheap.

  1. A frequency generator that generates signal in the audio frequency – (20 to 20,000 Hz)
  2. Some kind of oscilloscope display to show the sound wave
  3. Some powered speakers to crank up the tone and make it nice and loud. Any set of PC speakers will work fine.

There are lots of ways to do this.

All-in-one USB scope/signal generator

Our favourite is to pick up a cheap USB oscilloscope/tone generator. It displays the waveform and everything else on your PC screen – or even better on an overhead projector or big screen TV. Hook the tone output to a nice speaker.

This   has the advantage of being able to run off your laptop PC without external power. Add some battery powered speakers or hook it up as an input to a portable stereo, and you can run this off the grid if your laptop batteries hold out long enough.

PC web sites/applications – require Internet connection

This audio tone web site will generate audio tones for you from your PC speaker.

This one is even better. We put both in case a link disappears.

This site has a great oscilloscope that works through your PC microphone to display the sound waves. You may need to put the sound out to external speakers or from a separate PC if you’re also using one of the tone generators above.

This site puts both into one, but has more limited frequency. You can start with it, then move to the tone generators.

A google search will show a lot of on-line virtual oscilloscopes. We gave up after trying a few of them. They all seem to require Java and/or Flash and some security settings that we can’t recommend you use. Search and try at your own risk.

Old lab equipment

Noting is better than old-school lab gear with big knobs to turn and buttons to push.

If you can get access to it, old lab equipment is awesome. We’ve been able to borrow some for a local maker space that in turn picked it up from a university lab that was throwing it away.

Reach out to your local amateur (Ham) radio club. They can be extremely supportive of groups teaching radio and technology to youth, and often have access to a lot of old geat.

Demonstrating basic sound

We like to use external powered speakers for two reasons.

  1. You can send the sound to a wire – without the speakers hooked up.
    • With the scope displays, you can show that the signal is going out
    • Why can’t you hear it?
      • It’s going out at as an electromagnetic wave, at a frequency
      • But it’s not being converted to a sound wave.
  2. Now hook up a speaker
    • The speaker gets the electromagnetic wave from the wire
    • That wave makes the speaker vibrate – you can touch it and feel it, especially if the speaker is not covered by a case or grill. You might even be able to see it at low frequencies.
    • That speaker “vibrates” the air – which transmits the sound wave!

A microphone is the reverse – when you vibrate the internal parts, they generate the electrical waveform. The oscilloscope web site above does this nicely.

Demonstrate harmonics

If you double the frequency, you’re moving up an octave – like going from low C to middle C to high C on a piano. Look up the frequencies and try it. Match the tone to a musical instrument someone brings in. Those multiples of the base frequency are called harmonics.

Demonstrate amplitude – which is loudness/volume.

Make the tone louder. Watch how the frequency stays the same – but the height of the wave – the amplitude – changes. In the web page when you can add two sound waves.type identical  frequency values into the two boxes. Watch the amplitude of the combine wave exactly double.

Hearing test and power requirement – youth vs leaders

Here’s a good reason to have nice powered speakers.

Ask the youth if there’s something they notice as the frequency increases. It gets quieter. Why? Because generating a higher frequency takes more power. Unless you raise the power as you raise the frequency, it will get quieter and quieter.

And this is where you compare youth and leader hearing.

As we age, we hear higher frequencies less and less.

  • Have everyone raise their hands
  • Start raising the frequency. Tell everyone to drop their hand when they can no longer hear. You’ll soon see all the leader hands to done.
  • The youth can hear the higher frequencies, and it doesn’t bother them too much, because that power thing keeps the volume dropping.
  • Now crank up the volume on those external speakers. Watch the cringe. And if you’re an old coot of a leader, it doesn’t bother you because you can’t hear it!

Additional science and math stuff

From the sound wave displays, show them what one wavelength is.

Explain how frequency is the number of waves per second, measured in hertz (symbol Hz)

What’s the speed of sound in the air? Have them research it. If it’s in km or miles per hour, have them convert it to metres per second.

Once you know how many metres per second, as them to calculate the wavelength of one wave at a given frequency – say, 686 Hz.. (Hint: at sea level, sound travels at 343 metres/second).

 

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