Introducing sources of sound

What the Activity is for

This activity focuses on the idea that anything producing a sound must be vibrating. We think it best to start off with sources that involve obvious vibrations and then look at more subtle movements later.

What to Prepare

You will need a collection of things that vibrate along with the means of showing that they do vibrate. We suggest:

• a large panel of hardboard (approx: 1 metre by 1 metre) popularised as the wobble board
• a tuning fork and a bowl of water, together with a table tennis ball suspended on a long piece of fishing line
• a large loudspeaker, a signal generator and a collection of ball bearings or dried peas
• a strobe light
• a laser and a small piece of plastic mirror

To take it further:

a small wind-up music box and access to a table to use as a sounding board a small loudspeaker (50 millimetre diameter) and access to a pane of glass

Safety note: Signal generators driving loudspeakers have been known to produce unpleasant feelings, even nausea, in susceptible individuals at a variety of frequencies from as low as 4–15 hertz to above 20 kilohertz.

Safety note: Strobe light: photosensitive epilepsy is very rare but anyone suffering from it must not be exposed to flickering light. The same applies to those prone to migraine attacks.

Safety note: Lasers approved for school use should be used. Ensure that the beam cannot enter anyone's eye either directly or by reflection. A laser pointer can be used under the careful control of a teacher but be aware that some are marked with an incorrect power rating and so are more hazardous than they might appear.

What Happens During this Activity

Making sources

Start with the wobble board – just wobble it to and fro slowly in an exaggerated action. As you increase the rate of wobbling, start to talk about the vibrations of the board, connecting this use of vibration with the to and fro action. Ask what the board is doing to the air particles next to it. Slow the action right down so you can emphasise the push on the particles, and their bouncing back when this push is no longer acting. Emphasise that the vibrating board sets huge numbers of particles into this to and fro motion. You might also try to count the vibrations in a set interval of time, setting the scene for the introduction of the concept of frequency.

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Introduce the tuning fork as something which makes a sound, but which we cannot see moving. (In fact our eyes cannot react fast enough to catch it moving, added to which it is not moving very far.) You can show that the prongs of the tuning fork are moving by dipping the end in a beaker of water. Try drawing the pupils in close as you demonstrate this on the pretext of trying to see the very small vibrations. The pupils will be wetted by the water particles as they are displaced by the to and fro action of the fork.

Alternatives

An alternative is to suspend a table tennis ball on a long piece of fishing line beside the vibrating fork, so that the fork collides with it at the extremity of its vibration.

A further way of seeing the vibrations is to polish up one end of the tuning fork (right at the end) and then bounce a laser beam off the outside of the end at a glancing angle (a laser pointer is enough), so that the reflected spot ends up on the wall a long way off.

The laser beam set-up amplifies the vibration of the fork. If you cannot polish up the end sufficiently, attach a small piece of plastic mirror to the end with a piece of Blu-tack. You might want to model what you are doing, perhaps by monitoring small movements on a large object such as the laboratory door.

To make lots of particles vibrate (as you did with the wobble board), just place the stem of the tuning fork on a wooden table and listen out as the table is set vibrating. A nice alternative is to have a music box playing, first with it held in the air and then with it resting on the table.

A further activity at this stage involves setting up a circus of vibrating objects (including a range of musical instruments) and getting pupils to spot the vibration that is acting as the source of the sound.

With the loudspeaker, you could use a strobe light flashing slightly more or less often than the number of loudspeaker vibrations a second to give snapshots. We think, however, that it would be better to keep things simple. Place the loudspeaker so that it faces upwards and then put (small) ball bearings or (dried) peas on the surface. With the loudspeaker connected to the signal generator, the peas or ball bearings jump about as the speaker cone vibrates.

Two extensions to discuss:

• Rooms can be bugged by bouncing laser beams from the windows (health and safety is not so much of a concern for spies, but we don't suggest you try this), detecting any to and fro motion of the window pane. What would create such a motion? This is the stuff of spy stories.
• Small loudspeakers are available that can be clamped to window panes. The loudspeaker sets the whole window pane vibrating to get lots of particles moving. This produces a much louder sound.