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discussion of an 8" sub for small subwoofers
discussion of an 8" sub for small subwoofers
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Old 19th April 2012, 03:00 PM   #1
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
Default discussion of an 8" sub for small subwoofers


I'm always on the lookout for good cheap drivers and was intrigued by this :

Aurasound NS8-385-4A 8" Woofer: Madisound Speaker Store

15mm long coil in a 25mm long gap ? That is impressive. Linear excursion
of +/- 5mm isn't particularly impressive for a sub, but what is its total
excursion capability ? That depends on distortion but its somewhere
between +/- 10mm and +/- 15mm IMO, and that is impressive.

Qts fine at 0.36, Vas small for an 8" at 17L and Fs is 40Hz.

40Hz seems disappointing and I guess it is for low bass freaks, but
it is a consequence of the low Vas, quadrupling Vas would halve Fs.

Standard closed and vented alignments yield 6L and 12L boxes.
Venting small subwoofer boxes is an exercise in frustration, here
a high quality PR would be needed, and it is simpler and cheaper
to just use two of the drivers in the same box sealed.

6L is a tiny box for an 8" given 4.5L well stuffed is about 6L effective.
Thats for Q=0.7, for Q= 0.6 10L effective 7.5L well stuffed.

For the sake of further discussion lets assume a 7L / 0.25 cuft per
driver, (with optimum stuffing = 9.8L effective box volume /driver).

Some EQ is needed obviously for extended bass.

You could simply use one driver with something like this :
Yung SD100-6 100W Class D Subwoofer Amp Module w/6dB@45Hz 301-502

For a very impressive very compact sub.

I guess for in-car use two drivers in a 14L box, mounted for force
cancelling with a 2 ohm capable amplifier would also work very well.

Domestically two of the same cabinets, one fitted with the amplifier
sounds appealing, mounted 1/3 along the back and a side wall.

As long as you have enough power to drive them, sealed subs
can be EQ'd to any sensible response you like, the only limitation
of all sealed subs is the total volume displacement, it determines
the maximum SPL at any frequency in the low bass and that SPL
drops at 12dB per octave throughout the excursion limited region.

So basically the lower you want to go with active EQ, the lower
the overall max SPL the setup will handle, a compromise that is
usually quite different for music and HT use.

Because the box alignment is well damped (Q around 0.6) an LT
type arrangement is not needed. You can simply use a PLLXO
type 1st order shelving arrangement to boost the low bass,
changing the response roll-off from 2nd order to 1st order
over its effective range, which with room gain will work well.

Very similar to Rod Elliots :

Click the image to open in full size.

Click the image to open in full size.

You will need to reduce R1 for more range and a sharper effect.

Obviously drastically increase the value (s ? switchable) of C1.

The bottom end of he curve will likely run into other bass rolls.

Haven't talked much about amplifier power, simply assuming
50W to 100W per driver. for the 4 driver, two box arrangement
200W to 300W into 4 ohms, each box 8 ohms seems sensible.

Hope this helps / inspires someone.

rgds, sreten.

The bass extension in the attachment is not impressive, until you EQ it.
Attached Images
File Type: png guff.png (22.6 KB, 437 views)

Last edited by sreten; 19th April 2012 at 03:14 PM.
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Old 26th April 2012, 11:59 PM   #3
OscarS is offline OscarS  United States
diyAudio Member
Join Date: Jan 2011
why go compact? A big ol' tapped horn should make that speaker shine!
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