Enquiring into sound

Note: if you are looking for science fair or science project areas, this set of Web pages may help you with ideas for techniques you might use: read with a prepared mind!

Activities

Bull roarer
Buzz button
Controlling pitch
Doppler effect
Echo effects
Locating sound 1
Locating sound 2
Low frequency sound
Seeing sound
Ultrasound
Pan pipes
Sound conduction
Simple whistle
Other pages on this site

How to do it


Doppler effect
You will need a small piezo buzzer from an electronics store, two pieces of wire, about 1.5 metres long, and a battery to match the buzzer (usually 1.5 volts).
Solder two leads to the piezo buzzer, attach the other ends to the battery, and swing it around your head. The frequency shift between "approach" and "recede" is quite audible.

WARNING : there needs to be enough room to swing the without knocking down somebody else or their work!

This will help you understand


Locating sound 1
You will need a large piece (eight metres or more) of corrugated hose, of the sort used on swimming-pool cleaners, and a small screwdriver.

Mark the centre of the length of hose, as accurately as possible, and get someone to sit with the two ends of the hose to his/her ears. The rest of the hose should be behind the subject, so your actions cannot be seen. Scratch the hose gently with the screw-driver, and see whether or not the person can locate the sound as coming from either the left of the central mark, or from the right.

By doing a large number of trials, identify that part of the pipe where people never make mistakes, sometimes make mistakes, and where they often make mistakes.

This will help you understand


Locating sound 2
This 19th century illustration is sometimes presented in modern books as an army officer trying to locate cannon by the sound of their firing, but it is also sometimes seen labelled as a ship's captain trying to locate a foghorn. From his clothing, I think the man is a mariner, but I am no expert.

Not that it matters: the challenge here is to make something like this, and see if you can make it work. You will need two large funnels, each linked by plastic tubing to your ears, and you will need some sort of support for the whole system, and mabe a compass in front, so the user can directly read off the bearing to the sound source.

If you get it to work, please e-mail me a photo of your version of the apparatus .

This will help you understand


Ultrasonic sound
You will need some lengths of 3/4" steel rod, ranging from 30 cm down to about 5 cm, a cathode ray oscilloscope, a microphone, an amplifier, and some string. (Why do I think you might have some trouble doing this one at home? No matter -- push somebody hard enough, and you may be able to borrow the gear!)

The longest bar will ring at an audible frequency when struck, but the shortest piece will vibrate at about 30 kHz. This ringing will be inaudible, but the microphone and CRO will reveal that the ringing is still there, even though we cannot hear it.

This will help you understand


Echoes and echo effects
You will need two cardboard tubes, a ticking watch (if you can find one) or other quiet noise source like a piece of paper to crackle, a piece of cloth, and flat sheets of board, cardboard, metal, and glass.

Lay one tube on a table and place the second tube at an angle to it, making a `V', then put the sound source at the top of one arm, and your ear at the top of the other. Listen for any sound.
Then put a piece of flat sheet at the base of the `V' to act as a reflector.

This will help you understand


Low frequency sound
Marin Mersenne (1588 - 1648) was interested in frequencies. If you pluck a stretched musical string and watch it, all you will see is a blur, how could you possibly count the vibrations? The answer is simple if you know that longer plucked strings give lower frequencies. Double the length of the string, and you halve the frequency.

Big may not always be beautiful, but it does have an imponderable majesty about it. Mersenne's plucked strings were a hemp rope more than 30 metres long, and a brass wire 43 metres long. With that sort of length, the vibrations were so slow that he could see each individual wave. By varying the length and the tension on these giant strings, Mersenne was able to derive a formula that tied the frequency of a string to the length, the tension, and the mass of one metre of the wire or string.

Mersenne could now predict what the frequency of a stretched wire would be, even if the frequency was too high to count. In this way, Mersenne was able to determine that the frequency of an organ pipe was 150 Hz, by tuning a wire to match the pipe, and then calculating the frequency of the wire. Writing this up, Mersenne employed this dramatic introduction: "A deaf man may tune a lute, a viol or a spinet and other stringed instruments . . . if he knows the length and the mass of the strings."

That's all you get on this one. Why should I do all the work?

This will help you understand


Bull roarer
You can make a bullroarer with some string, a hand drill, a 30 cm (1 foot) ruler, and a place with enough room to swing the bullroarer. Drill a small hole in one end of the ruler, insert the string in the hole, tie a knot, and then swing around your head.

This works better with a small fishing swivel in the string close to the ruler. These seem to have been common in most cultures around the world, and bull roarers were used by the Australian aborigines for ceremonial purposes.

This will help you understand


A buzz button
You will need a large button, and some tough thin thread or string. Put the two ends of the string through two holes in the button (if it is a four-hole button, use diagonally opposite holes), and tie the two ends of the string securely.

Loop two fingers of each hand through the string with the button in between your hands, and flip the button to make it turn once or twice. By alternately pulling and releasing the string, the button will start to rotate, producing a characteristic roaring noise. Harder pulling produces faster rotation, and a higher tone.

This will help you understand


Seeing sound
You will need a piece of large diameter (90 mm stormwater is good) plastic tube about 120 mm long, a balloon, some rubber glue, a small piece of mirror, a source for a light beam, such as slide projector with a "pinholed" aluminium foil cover on the lens, (or better still, a pinholed slide: use any slide that failed to come out, and drill a tiny hole through it).

Place a piece of the balloon rubber over one end of the tubing and pull it tight: if you cut off the lower half of a balloon, this will do the job well. Glue the mirror in place. When the glue is set, clamp the tube horizontally, with the light beam shining on the mirror at an angle. Have somebody speak, shout or whatever into the tube from the other end, and watch what the reflected light beam does.

This will help you understand


Exploring pitch
This is another 19th century illustration, this time showing a way of exploring pitch. All you need is a small gear wheel of some sort, and a piece of card to hold against the teeth of the wheel as it turns, so that the card clicks each time a tooth of the gear slips past.

If you have a gear with 30 teeth, making two rotations a second, there will be 60 clicks per second, and that will give you a sort of 60 Hz tone.

This will help you understand


One-tube Pan pipe
When you blow across the top of a closed tube, it makes a note, rather like a Pan pipe, which is not surprising, since the Pan pipes are a set of closed tubes.

If you blow across the top of a drinking straw when the other end is in a liquid, you can get different notes, depending on how much of the straw is under the surface of the liquid, since this controls the length of the open part of the tube.

That's all -- you do the rest!

This will help you understand


Listen to the strings
Use a rubber band to attach a metal spoon to the midpoint of a 60 cm string. Wrap the ends of the string around your index fingers. Rest your index fingers in your ears. Rock your body so that the spoon taps against the side of a table. What do you hear?

With a bit of imagination, you may be able to relate this to a common toy, used by children, and involving two empty tins and a single piece of string ( which rules out a pair of stilts :-)
There are more experiments with sound

This will help you understand Back to the index


Whistle blower
This is an easy way to make a reed pipe, using a plastic drinking straw.

First, flatten the end of the straw by pinching it between your fingers.

Then cut off the corners of the flattened part, put that part in your mouth past your lips and begin to blow.
If there is no sound, you may need to blow a bit harder or a bit more gently, or you may need to flatten the reed part out more by pulling it gently between your teeth.

You can experiment with cutting small parts off the other end of the straw while you are blowing it, or cutting holes in the straw to play a tune, or you may be able to work out ways of joining five or six straws together, to make a very low note. This will help you understand Back to the index


And now for some help

How the Doppler effect works
Sound is made of pressure waves and non-pressure waves, sometimes called rarefactions or decompressions. The frequency of a sound that we hear depends on how quickly the successive compressions reach you.

If the buzzer is moving towards you, each successive wave comes from a bit closer, and so the compressions reach you more often, and the tone sounds higher.

When the buzzer is swinging away from you, each successive compression has further to travel, and so the tone sounds lower.

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How the ultrasound works
Shorter objects vibrate at a higher frequency. This effect will depend on the steel selected, so some experimentation will be needed before you perform this exercise successfully.
It will probably help if you strike each piece of steel in its centre with a small rubber or wooden mallet.

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How we locate sounds
We locate sounds by distinguishing very small differences in the time of arrival, or by differences in the loudness of the two sounds, nobody is quite sure.
This experiment seems to suggest to me that we use the timing more than the strength of the sounds.
How could you test this by sticking a handkerchief down one side?

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Why do the echoes bounce?
The compression waves of sound bounce off the flat surface, and like a ball on the ground, they bounce off at about the same angle as they approached (we call these angles the angle of incidence and the angle of reflection).

Probe

What is the best reflector? Experiment with different reflector materials, and try the effect of covering the reflector with cloth. What sorts of surfaces reflect sound best?

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Low frequency sound
The frequency n of a stretched string is inversely proportional to the length, and it is directly proportional to the square root of (the tension divided by the (mass per unit length)).
So you will double the frequency if you do any one of

  • halve the length,
  • quadruple the tension, or
  • quarter the mass per unit length (= halve the diameter).

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How a bull roarer works
The bull roarer spins around as you swing it, producing a compression as part of the wood swings forwards, and a rarefaction as it swings back -- at least, that's my story until I can find something more satisfactory.

Well, it's still not entirely satisfactory, but the edited transcript of an Internet science list discussion may be of some help.

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The buzz button
The main thing to keep in mind here is that the button is spinning slightly irregularly, and that these irregularities come at regular intervals (are you still with me? :-).

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Why you can see the sound
This seems to work well sometimes and to fail at other times, but I have still not worked out why: persevere! Sound is a vibration, and the rubber diaphragm vibrates as the sound hits it (maybe the tightness of the rubber is important here??) This makes the mirror vibrate, and the light beam is forced to move a larger amount, amplifying the small movement in the mirror.

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Find that foghorn/gun
The idea is that two separated points for sound collection will give better separation and definition, a bit like a hammerhead shark having its eyes on the ends of extensions of its head, to give it more powerful stereo vision.
I can't say that I'm entirely convinced, but I hope somebody will be able to let me know how they get on , and how effective it is.

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Gear and pitch
The tone will not be completely pure, as the card itself will produce a click that has a distinct frequency, but the effect will still be audible. There will probably also be a bit of a tone imposed on the clicks by the gear wheel.

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Pan pipes
The main thing that decides what note a pipe plays is its length, so if you experiment carefully, you may even be able to work out the lengths for a tuned set of Pan pipes, made from bamboo.

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Why do blocked ears still hear noises?
When the metal spoon taps against the table, it sends a vibration up the string, through your fingers, and into your ears. Your eardrums pick up the vibrations and send them to your brain where they are translated into sound.

Sound travels almost as well in solids, liquids and gases, so why is it much clearer here? That happens because sound travelling in a solid bounces back into the solid each time it reaches the surface. The string acts like a tunnel, guiding the sound waves along and keeping them together, instead of spreading out, so nearly all of the sound gets to your ears. Back to the question

Why does the pipe make that sound?
Sound is a set of regular compressions and decompressions (which are sometimes called rarefactions ). When you blow down the pipe, pressure makes the two halves of the reed close up, then they bounce apart, then they close again, and so on. Each opening sees another compression race down the pipe, while each closing of the pipe causes another rarefaction as the pressure suddenly drops. There is more to it, but that might just get you started on a project.

By the way, if you make a slit along one side of a straw for about 1 cm, you can slip that end inside a second straw to make a deeper note. People seem to find something very funny about the noise you get from about four straws linked together, but I can't think why . . .

Back to the question

This file is http://www.ozemail.com.au/~macinnis/scifun/sound.htm, first created on August 19, 1997. Last recorded revision (well I get lazy and forget sometimes!) was on July 11, 2001.
Worried about copyright? You need to go look at my fine print . Well, maybe you don't after you read the next paragraph, but do it anyhow . . . because if you go there and look around, you will find out what other activiies there are in this series, if you came here through a back door.


©The author of this work is Peter Macinnis -- macinnis@ozemail.com.au , who asserts his sole right to the product as it is packaged here, recognising that many of the ideas are common. Any non-profit educational or home use is completely acceptable without let or hindrance. Copies of this whole file or site may be made and stored or printed for personal or educational use. The work used here derives from on-going research and development which will one day lead to a book on brain food ideas.
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