Electromagnets in speakers

[Hayden]

At school many years ago we where taught that the more turns the more magnetic field there was but my research says the lower number of turns and higher gauge has better fields; is this true?
Can someone explain how all this works?

[AndrewT]

Hi,
I think the the product Ampere Turns is the controlling factor for electromotive force.

Large wire may be easier to pack together and get neat windings. Less air space means more copper.

Reduce the turns and you must increase the current and if you do some sums I think you will find the answer for the best turns / amps compromise.

[johnnyx]

It depends on the voltage of the supply too, I had a radio set where the speaker's electromagnet was supplied by the HT. A 300v coil requires lots of turns. Look at relay specs; the mechanical switch parts are the the same for different coil voltages, but the resistance of the coil is different.

[Hayden]

My boss said that it’s the surface area that counts too.
For subwoofers they have large gauge wire and less turns they use a lot of power 1000w rms so they must produce a very strong field. The theory my school had must be wrong otherwise the subwoofers would use lots of turns with smaller gauge wire.

What are the formulas of coils?
Is there a good website that explains this more in-depth than www.howstuffworks.com that website is too basic for me.
Thanks

[Nappylady]

The amount of force on a voice coil is related to the length (LENGTH! Not the total amount of copper) of wire in the voice coil gap, the strength of the fixed magnetic field in the gap, and the amount of current being pushed through the wire.

As everything else in this business, it's a tradeoff: more turns on the coil gets you a higher BL product, but it also gets you higher resistance, higher impedance, and in underhung motors, only gets you more field strength at the limits of excursion.

Larger wire decreases resistance, allowing more current to flow (and thus, stronger fields) but it also adds to the moving mass, and can limit the number of turns you can fit on the coil. Also, you can only reduce resistance so much before amplifiers become unstable. (Car amps are sometimes designed to handle loads of half an ohm--this is extreme. Most amps today are designed for 4 ohms minimum.)

A bigger magnet adds to field strength, but also adds to total driver weight (not mms) and cost.

The people designing drivers spend a lot of time choosing how many turns are right for the driver they're designing, and the "best" answer is different for every driver you could think to make.

[Hayden]

thanks Nappylady
What does BL stand for?

I had a nice 3 inch midrange I turned it into a mini sub, made a sealed box for it and recoiled it with .5mm gauge wire 2 layers down the former and up again and also did some miner modifications to it, when assembled again the coil measured .4ohms so I hooked it up to my amp and gave it a run with my bass for bassheadz cd and my results weren’t bad it rattled the filing cabinet about 70cm away but my amp’s protection circuit kicked in when I turned over 50% gain because of the .4 ohm load.
It Handled about 50w rms through it. teehee good experiment and good fun too.

[Nappylady]

The BL product is the length of wire in the gap (L) times the strength of the magnetic field (B) in Teslas. BL products are listed as Tesla-Meters on spec sheets.