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Homework Help: How do you build a home-made speaker using magnets, plastic, coils, etc.?

  1. Mar 23, 2007 #1
    Our teacher wants us to construct a speaker that can provide sound (that can be heard from anywhere within the classroom) when connected to wires that are hooked up to some sort of device. I'm unsure of where to start and where to get the supplies. Since we are on the magnetism course, I know that we need magnets and mabye coils. I also need some structure to hold my speaker together so that it stands by itself. Are there any links to instructions on where to build speakers from home materials (you cannot take parts from a speaker)? I found a couple of sites using google but they seem really complicated and this project is due on Monday for us. Thanks in adv..
  2. jcsd
  3. Mar 23, 2007 #2
    well it needn't be that complicated. What kind of device will it be attcahed to, in fairness you need to know that.
  4. Mar 24, 2007 #3
    Well I've searched for more information and I think all you need to build a speaker is an electromagnet, coil, wire, and a cup of some sort? You need 2 wires that are connected to the speaker (the coil?) to hook up to a device right? What device would you hook it up to? A radio, computer? Do you just stick the 2 wires in the outlet?? lol
  5. Mar 24, 2007 #4


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  6. Mar 24, 2007 #5
    Well thats what I was wondering. actually an ac outlet would work--if briefly :cry: w/o some beefy components to divide down the voltage, otherwise the wire used to form your coil will likely cause some sort of meltdown.

    If you can hook it to the outlet of a small stereo receiver--this would be your best bet, as it has a volume control that allows you control the voltage across the coil. Just make sure you have a reasonable D/C resistance in your coil before doing so otherwise you could potentially damage the stereo gear. A computer sound card designed to drive external speakers might also work depending on how effiecient your device is, a table radio usually lacks provisions for such a hook up.

    Basically you are making a small electric motor that moves back and forth along the magnetic pole piece. The link looks like a good place to start, maybe you can make some mods to the design once you have a working prototype. Remember the wire is fairly crucial--you need the maximal number of turns (thin wire) yet able to handle the current (thicker wire)--normally such wire is coated with thin, clear insulation. Might try radio shack.
  7. Mar 24, 2007 #6


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    There's no need to risk damaging a own stereo, computer, etc to test it. Hook it up to an audio frequency signal generator from your lab.

    Lab test gear should be designed so it's not damaged whatever you connect it to - but don't take that as a challenge to find a way of deliberately blowing the sig. gen. of course!
  8. Mar 24, 2007 #7
    hear, hear, assuming you have access to one and its capable of supplying at least a couple of watts. Also use a frequency somewhere between several hundred and say 1000 hz. This will be easier to hear for a couple of reasons.

    how is the project coming anyway?
  9. Mar 24, 2007 #8
    My dad is going to take me to Fry's electronics or Radioshack to buy the necessary supplies. Should I just follow the instructions on the link the other guy provided me? It seems more simple and easier to follow.
  10. Mar 25, 2007 #9
    Alright, I've decided to follow the methods used on: http://physics.mercer.edu/marone/soda/sbsintro.htm

    Thanks for the link. I went to radioshack and bought a few ceramic magnets and also 2 rare earth magnets (neodymium). After fiddling around with the magnets, coils of wire, and batteries, I was able to understand more of how speakers work. Basically, it is just a electromagnet (the solenoid or coil of wire), the permanent magnet, the structure of the speaker (cone and support), along with a power source right?

    I have one question though. I'm not sure if the procedures stated on the website will be adequate enough for me to get a good grade. In order to recieve an A, we need to project the sound loud enough so that everyone in the class room can hear it pretty distinctively. I'm seen another structure of speakers that have like an E-shaped magnet with the North pole at the middle of the E and the South poles at the top and bottom of the E structure. Is this what i'm suppose to do? or do I just put the permanent magnet inside the coil and supply a power source? thanks. :smile:
  11. Mar 25, 2007 #10


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    You will have to experiment with the coil/magnet size.
    A larger coil and magnet set up will give you a louder sound.
    You can also fit more winding on a larger coil and that will take care
    of the D/C resistance issue.
    Make sure the the coil and magnet do not rub against each other.
    Just hook up to an old radio and see what it sounds like.
    Last edited: Mar 25, 2007
  12. Mar 26, 2007 #11
    Is the direction of the magnetic field from the permanent magnet important? I can't seem to get the wire coil to move. Should the magnet be placed sideways so that the north and south poles of the permanent magnet are perpendicular to the coil?? Btw, what wires work the best: magnetic wires or insulated (the wires with thicker plastic covering)?? thanks.
  13. Mar 26, 2007 #12
    its gotta be insulated wire! But both should be so use the mag wire, designed for such. You sholld know from your class that looped wire creates a magnetic field along its length that points one way or the other down the center of the helix depending on the direction of electricity. So the loop needs to ride the magnet like a sleeve, and be in close proximity.

    Not sure what you're using for power supply. A d volt battery is fine. it will move it to one direction and stop, Does your speaker do this?
  14. Mar 28, 2007 #13
    Yes! I've finally got the magnetic field to work properly. I still have a few questions however. I'm probably going to use a cup or something as the cone but should I use thick magnetic wire or thin wire?? How is magnetic wire different from normal wires (with plastic insulation around it)?? The magnetic wire I've bought has Enamel coating around it.. Which will work better if I decide to use a cup as the cone?? The magnet that I've bought are just ceramic bar magnets that have the poles at its flatter sides (with more surface area). That means I have to place the magnet with the poles perpendicular to the flow of current.

    Can anyone suggest a type of structure that would allow my speaker to produce the most sound? I'm getting graded on the volume of sound my speaker exerts. Should I keep the structure small or big? Thanks a lot!
  15. Mar 28, 2007 #14
    You need to answer to several masters. First the moving part needs to be as light as can be. Pretty obvious. Second you need to maximize the mutual induction of such a motor. This calls for the most loops as possible. Thirdly, you wnat it to survive,
    maybe? if the sole object is to be heard in a classroom,...
    What kind of volume do you have now?
  16. Mar 28, 2007 #15
    I haven't been able to test it on a audio source yet so I don't know about volume. I've only been testing them on power sources to see if it goes one way or the other. I'm still debating on whether to use thicker magnetic wire or thinner plastic insulated wire. The thinner wire is obviously lighter than the magnetic wire but doesn't that mean its resistance is higher than the thicker wire? [tex] R= p \frac{l}{A} [/tex] right? Are we trying to get more resistance or more current? I know that you need more coils for higher magnetic fields, but what about resistance? Magnetic field inside a solenoid (coil of wire): [tex] B = \mu_{0}nI[/tex], where n= Number of turns per unit length... Does that mean the strength of the magnetic field also depends on current? So basically the magnetic field depends on current (less resistivity more current?), the number of turns, or coils, per unit length.

    Does that mean I'll be better off using the thinner (regularly plastic insulated) wire than the thicker magnetic wire? The thicker wire also contriubtes more weight to the cone or vibrating part of the speaker. However, what is the significance of magnetic wire? Does it work specifically well in speakers and these kind of projects? Would magnetic wire be more beneficial to use than the regular thinner wire? I don't know what to use here... Please help me decide, thanks!
  17. Mar 28, 2007 #16
    Both more B and I. I'd use the mag wire. The B=mu*N*I is one of the keys to efficiency. To get more I, we usually just get bigger power amps. I'd try to keep the moving patrs (coil, former and diaphragm as light as possible.
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