Re: Home-made speakers

Dear Friend,

You project is a very ambitious one.  I would like to encourage you to go ahead 
with it. But first you need to know something about the technology of receivers 
and speakers. Let us consider the most relevant example of receivers and 
speakers, which is the telephone. 
When you speak in the telephone, your sound is transformed into electric 
current pulses, with exactly the same frequency of your voice. These electric 
pulses are transformed from your phone through wires to the other phone (the 
receiver). There, the opposite process happens; where the electric pulses are 
restored to sound vibrations. This is the simple description. Let us go in 
depth.
The mechanism is like that; when you talk in the phone, you voice, which is a 
sound wave-a mechanical wave to be specific. Sound is a mechanical wave that 
propagates in air longitudinally causing compression and rarefaction in the air 
particles. So when you talk either in a microphone or in the phone, your sound 
goes to that part which is covered. You know it contains a microphone, but what 
is a microphone. The microphone consists of a delicate magnetic alloy 
diaphragm. This diaphragm vibrates when you speak, with frequency similar to 
your voice frequency. This diaphragm is attached to a coil, and this coil 
exists between a very strong magnetic poles. Hence, when you speak, vibrations 
happen, flux changes, and finally the E.M.F is induced inside the wire leading 
the electric pulses out through the wires. That was an example for a microphone 
called moving coil microphone. The other type of microphones has carbon 
granules that by their vibrations, their resistance is changed, hence the 
current passing in them also changes.

Now, my friend, you want to reflect that electric pulse to a sound pulse. Fine, 
no problem. First you need the following: strong magnet, wires (double 
bundled), magnetic alloy delicate diaphragm, and a case to put your stuff in. 
you will definitely use the wires to construct the connection from your 
recorder to the speaker. You will need also a phones plug (it is the thing in 
your headphones that you insert in the recorder). Then the wires coming from 
the recorder will end at the following: make a coil of a large number of turns 
on your magnet. This magnet is preferred to be an electromagnet and not a 
permanent magnet because the electromagnet is a one that the flux is changed as 
current frequency changes, while the permanent magnet has constant flux, except 
if you used an iron magnetized piece, which we can treat as electromagnet as 
well. The fidelity of that device you are working out depends on the large 
number of turns you are able to give in your magnet. When current passes, it 
will induce a flux. This flux is variable by the variation in the current 
frequency. Thus it will attract the delicate diaphragm in accordance with the 
variation in the current. Hence, if you put your ear close to it, you will 
listen to vibrations similar to the ones you sent. 

It is a great question. Thank you. Wishing you all the best. Please contact me 
for any further inquiries.

References:

Abbott, A.F. Ordinary Level Physics. Heinemann Educational Books, London. 
Fourth edition 1984.

Moataz Attallah
Undergraduate Mechanical Engineering student
The American University in Cairo-Egypt
Email: Mizoa@aucegypt.edu