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.

What the Activity is for

Investigating detectors and describing the process of detection.

This activity focuses on the design of devices to detect sounds. The structure of the ear is important here, but you will also want to be able to refer to microphones, as these transform the vibrations of the air into electrical vibrations.

What to Prepare

• a model ear or a good clear diagram
• a microphone, linked to an oscilloscope with the time-base turned off (as an alternative you might use a computer running sound-analysing software)
• a carefully chosen, large microphone opened up to show the parts

What Happens During this Activity

Lead the class through a discussion whilst looking at the model ear, or at any sound sensing system, to draw out the way in which the to and fro motion of the medium is picked up by the detector. You should also point out that the brain works through processing electrical signals, and that's why ears have to perform this transforming action (from vibrations of the air to changing electrical signals in the brain). Microphones perform exactly the same transformation.

Connecting the microphone to the oscilloscope or computer will give a large-scale, clear representation of the vibration as a vertical trace on the screen. The vertical trace is produced on the oscilloscope by turning the time base off. We strongly recommend that you do not turn the time base on, as explaining the resulting wave-like pattern is likely to lead pupils' thinking down the wrong tracks. It is better to keep things simple and to make the link: from the vibration of the sound to the vibration of the microphone diaphragm to the up and down vibration of the spot on the oscilloscope (producing the vertical line).

What the Activity is for

Finding vibrations in the medium.

The aim of this activity is to look for the vibrations between the source and detector. What happens between source and detector is not directly accessible with the senses, so now you are leading a hunt for some clues. You are trying to get children to think about the mechanism that allows them to hear. That is why we look at source and detector first.

What to Prepare

• a battery powered buzzer in a container that can be evacuated, perhaps with a flashing LED wired into the circuit
• a suitable vacuum pump

Safety note: Only a bell jar sold for use at such reduced pressure should be used. It should stand on a metal or thick glass base and be checked for chips or cracks before each use.

What Happens During this Activity

You might start by saying that now you are searching for the same to and fro motion seen in the source and detector in the medium. You can only look for clues, because the vibrations are very small and there are lots of them produced each second.

One place to start is by finding out how to cut the chain between source and detector. Place the buzzer and LED in the jar and pump the air from inside the jar. You continue to see the LED and the buzzer, but you can no longer hear anything. Emphasise that sound needs a medium to travel through – something that can move to and fro at every point.

Here's a good teaching question:

Teacher: Can sound be swept away?

Imagine the scene. You are standing on a cliff top high above the beach on a storm-tossed day. You call to your friend below, but the wind sweeps your words away.

So what is happening here? The answer is that the medium through which your words are travelling (the air) is moving wholesale as gusts of wind. You are trying to transmit a sound to the beach, but the medium is gusting along the cliff top. Your voice is swept away with the wind.

What the Activity is for

Finding the paths of sound: predicting what you can hear.

Here you can show how a simple model of the paths that sound can follow allows us to predict where things will and will not be heard. The model is less useful than the equivalent model for light precisely because the paths that sound takes are less well defined.

What to Prepare

• a small loudspeaker, driven by a signal generator
• a small microphone, connected to an oscilloscope display with the time-base turned off

or

• a computer and a data projector
• the interactive object (see below)

Use one or more of these to explore simple examples where obvious barriers either permit, or do not permit, sound to travel from source to detector. To prepare these, assume that sound travels in straight lines and reflects as light does, with the angle of reflection equal to the angle of incidence. It is best not to have too many tricky ones, where the detector is only just in, or just out of the sound shadow.

Sound travels in much less well defined beams than light, so modelling with rays is far less useful. Sound often seems to travel around corners all by itself; light does not. However, careful thinking about the paths that sound can follow, based on appropriate simplifying assumptions, does provide insight into what you can and cannot hear.

What Happens During this Activity

Ask children to use the situations that you have prepared, which could be practical, paper based or software based, to predict where the sound can be detected. There are a few ideas for possible physical arrangements in the support file.

One place where this is very important is in seismic probing. Here a controlled explosion sends off sound into the body of the Earth and detectors are used to monitor where the sound turns up and how much of it. In this technique the bending of the paths of sound due to changes in the density of the medium (the same process of refraction as encountered with light) is also important. Much of what we know about the structure of the planet under our feet is due to seismic probing.

What the Activity is for

These diagnostic questions can be used to check the pupils' understanding of sound.

What to Prepare

• print out the question sheets provided (see below)

What Happens During this Activity

The questions might be used for homework or as the basis for discussion in the class.

Glass Case question commentary: Sounds travel through a gas and bigger vibrations will just make the sound louder and therefore easier to hear. A vacuum contains no particles and therefore cannot act as a medium for sound. It is possible that the rate of vibration is too low for you to hear.

Sound Chain question commentary: You are already equipped with a detector, probably a pair of them. Therefore a much higher priority is to look for the other two elements in the source-medium-detector chain.