Desiging a new Sub Driver - what parameters? - diyAudio
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Old 18th October 2002, 05:11 AM   #1
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Default Desiging a new Sub Driver - what parameters?

Hi All,

I thought I might throw this question to the Forum on the off chance someone else has already done this.

We are designing a new range of Subwoofer kits and have secured a local (Australia) manufacturer to build the drivers for us. He uses mostly "off the shelf" components in the driver so our choice of design parameters will be limited to some extent.

Our kit design requirements are:

1) Vented Box of approx 55 Litres
2) Available power of 150W RMS into 4 ohms
3) Target in room F3 response of 105db SPL @25Hz
4) Option for using the driver in a smaller 35 Litre sealed enclosure

So, what are the ideal parameters to design into this woofer? given that we won't have infinite adjustability on these parameters what are the key ones to get right? The woofer we will be using has a 4 Ohm voice coil and is likely to have a high (91dB) efficiency.

It has a polyglass cone, large voice coil and power handling to spare. I have initially spec'ed a unit with an Fs of 22 Hz, a VAS of 150L and a Qts of 0.38. Anything else I should be thinking about?

Thanks

Marcus
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Old 18th October 2002, 12:09 PM   #2
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It's always nice to have a high Xmax in a sub, 6 or 7 mm perhaps?
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Old 19th October 2002, 12:58 PM   #3
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This is not a parameter, but I strongly suggest that you insist on a symmetrical magnetic field. This is achieved by either :
A) a shorting ring in the magnetic system
B) A somewhat elogated pole piece, which puts just as much magnetic material in front of the voice coil as behind it.

Either method is used satisfactorialy by several manufacturers.

Without a symmetrical magnetic field, when the cone approaches resonance frequency, it goes all the way to the end of it's travel and moves back and forth from there-clipping off one half of it's wave form. I have seen this happen, and there are several audio writers that have described this phenomenon-"suck-in" or "suck-out"-as well, notably DB Keele in his reviews for Audio magazine.

It is supposed to occur in both sealed and ported boxes, but I personally have only seen it happen in ported-which is the kind of box you plan to build.
forward diaphragm displacement--have you heard of this?
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Old 19th October 2002, 01:07 PM   #4
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How about a coil exclusively for motional feedback? It need only be wound with fine wire and not take up much space. Actually actually actually (!) if you are using an aluminium voice coil former, it is already a "coil" of almost 1 turn. Just attach a pair of flexible leads to it and you are done! What's more, seeing it's length far exceeds the airgap depth, a constant amount of "coil" is bathed in flux at any one time! This would be ideal!! I want one.

GP.
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Old 19th October 2002, 03:28 PM   #5
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Default long throw!! -> highest xmax!

pinkmouse is on the money there! the biggest barrier to decent sub performance is the prohibitively large amount of air the driver has to move when wavelengths get really long.

obviously the cone area is important in this but having a huge wide cone is really a waste of space if it can't move in and out a decent amount to compress those big fat wavefronts!

I'm not sure how far you can go with this before it gets too expensive unless you are building for the very high-end. remember that occasionally you get fudged xmax values in datasheets where they quote the distance of a compression plus a rarefaction (cone's movement in both directions) but it's more correct to just quote one direction. with one of these fudged values, rather than 6 or 7mm as pinkmouse suggested, the manufacturer would claim 12 or 14mm respectively. it'd be fantastic if it was that far on one direction but it's not likely unless you're looking at a $500+ driver.
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