Subwoofer Design Question

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Kirghiz
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Joined: Wed Dec 26, 2012 7:59 pm

Subwoofer Design Question

Post by Kirghiz »

How important of a factor is motor "force factor" in determining the projected performance and output of a subwoofer? As I understand it, the standard formula for determining "force factor" is (BL x BL) / RE. I have had this feeling about a certain subwoofer that it should be louder, or at least hit harder than it does even though it has a great sensitivity number, and then I ran across this about force factor, crunched some numbers, and decided that it might explain what I have been thinking about that sub.
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dwnrodeo
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Re: Subwoofer Design Question

Post by dwnrodeo »

A higher Bl subwoofer will be more efficient but won't have the xmax as a lower efficiency subwoofer and therefore lower SPL.

Taken from: http://www.bcae1.com/speaker.htm
BL:
BL is determined by the flux density (B) in the magnetic gap and the length (L) of voice coil wire in the gap. A higher BL will generally mean a speaker will have a higher relative sensitivity (efficiency). This doesn't necessarily mean that all speakers with a higher BL will produce a higher SPL. Often speakers with very high BLs have a smaller Xmax.

The diagram below shows two different motors (that's what they're called). Motor 'a' is what you might find in a speaker with a relatively low power rating and a relatively short Xmax but its efficiency will be relatively high. Motor 'b' will have a higher Xmax, higher power handling due to the larger voice coil windings and a lower efficiency. The difference in the xmax is due to the difference in overall length of the voice coil. Xmax=voice coil length minus the gap height. The difference in efficiency is due to a different number of windings in the gap. Remember that the voice coil is an electromagnet. The current passing through the coil generates a magnetic field which is distributed along its length. On the shorter voice coil, more of the generated field is in the magnetic gap producing a slightly stronger motor but with a shorter stroke.
Note:
You have to be careful when looking at reference efficiency (sensitivity). You can make a speaker really efficient by designing the voice coil to fit entirely in the magnetic gap. This would likely yield a sensitivity of 104 or so. This speaker may work very well if powered by a low powered amplifier because of the high efficiency but would not be able to produce high SPL at low frequencies because it would have a very small xmax. Actually, if the voice coil length was the same as the height of the magnetic gap, it would have no (zero) xmax.

You can also design speakers for very high power handling and high SPL but those speakers would likely have a very low reference efficiency. Speakers designed for high SPL in cars generally have a larger xmax and therefore lower reference efficiency but would easily out perform the speaker (in the previous example) with the higher reference efficiency at low frequencies.

Speakers that are designed to operate in very small enclosures are usually less efficient than speakers designed for larger enclosures. To make the speaker perform in a small enclosure, the suspension has to be stiff. This will raise the resonant frequency. To get a lower resonant frequency, they must add mass to the cone of the speaker. This added mass and the stiff suspension kill the efficiency.
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Kirghiz
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Joined: Wed Dec 26, 2012 7:59 pm

Re: Subwoofer Design Question

Post by Kirghiz »

The way I read it is that a higher BL sub will be able to play lower with more authority, but it will take more power to achieve the same decibel level, and it will actually be able to achieve higher overall decibel levels because it will likely be able to take more power in general because it will have a greater amount of xmax. A higher BL sub will basically be more likely to rattle your teeth.

That kind of confirms what I was thinking.

Thanks!
Being loud without good sound quality is pointless, but having good sound quality without being loud is also pointless.
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