Foil speaker

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Aluminium foil strip is used to create a speaker.
This is a simple to construct example of the motion of a conductor in a magnetic field.
It is useful to illustrate electromagnetic induction or sound waves. It may also be of sufficient interest to act as an open-day idea as the students are often impressed with the simplicity of the set-up.
This is more likely to be used as a demo.


A thin strip of aluminium held between the poles of a magnet can be used to make an effective speaker.


The required equipment consists: Aluminium foil strip (from baking foil) (of ca. 25mm width by 75cm to one and a half meters long),
"Horseshoe" magnet or ceramic magnet held on yoke,
Resistor for protection (optional if staff are aware of low current requirement-of around 5 ohms 2x 10ohm 5W in parallel),
Two x boss, clamps and retort stands,
Two pair of half corks (or other insulating holders,
Connecting wires,
Crocodile clips x 2,
and signal generator/amplifier.

Cutting the foil:

This is probably the hardest part. It can be done with a SHARP craft knife or using the straight edge of a metal rule as a guide and pulling, allowing the edge to cut the foil.

A better solution is to get a length kitchen foil and lay some scrap plain paper over the top.
Then, as neatly as possible, fold the foil in two so that there is a foil-paper foil sandwich.
Add more paper and continue until the width will fit into a guillotine.
Cut several strips of about half inch (13mm) width. Select one of the cut strips unfold and remove the paper to give a long foil strip.
Roll the other strips and store for later use.


The foil is held at each end in the clamps by the cork halves. A small portion of the foil is left at the "spare end" to allow electrical contact
The foil is then extended across the bench so as to be parallel to the bench. This is done so that the foil is as straight and flat as possible. Angling the stands may allow a "flatter" strip. The crocodile clips are used to make contact between the foil and the signal generator (via resistor).
The magnet is placed so that the vibrations are in the thin plane and give greatest deflection. I.e. placed at the centre and across the width of the foil, so that (using the motor rule) the "flat" of the ribbon of foil moves up and down.
Voltage should be kept small.


Best results are achieved at higher frequencies(>500Hz although if voltages are small low frequencies work reasonably). The longer length of foil gives, to some extent a better frequency response.
By use of the am modulation (excuse tautology) facility of the sig.-generator. an i-pod or radio signal source can be used on the speaker. Requires a 3.5mm jack to 2 or 3 x 4mm lead to be made up.

A youtube video shot on a mobile phone of the equipment in action. Note: lead to possibly connect to a transistor radio, i-pod or similar where the signal generator has an amplification function
--D.B.Ferguson 12:49, 12 September 2008 (UTC)

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