More SPL From a Smaller Box

[Patrick Bateman]

The sub box in my car takes up a lot of space, so I've been thinking about ways to downsize it without giving up the bass.

I stumbled across a box type that should allow me to do this, so I thought I'd 'walk through' how this works.

Here's a Dayton RSS390HO in a conventional ported box. The box requires 3.9 cubic feet of airspace.

If you feed the subwoofer 1600 watts, it's good for about 123dB from 25hz on up. Pretty nice performance for a $200 15" woofer.

Unfortunately, 1600 watts will destroy the sub, because it only has 12mm of xmax.

 

So...

 

What's the solution?

Stay tuned...

[Patrick Bateman]

If you reduce the box size by 28% and raise the tuning, you can get the excursion down by about 30%. Instead of maxing out at 21mm, it's maxing out at 16mm.

But I'm not done yet...

 

[DiBo]

Smaller box gives you more cone control which in turn let's you saturate the sub with more power which in turn gives you more response. Pretty common knowledge I think. 

[Patrick Bateman]

The last part of this "trick" is that you apply a high pass filter.

What this does is that the high pass filter, when applied carefully, eliminates the 'peak' that you get from putting the woofer in a vented box that's too small. So the combination of the "too small box" and the high pass filter yields flat response.

The FUN part is that your power handling just goes off the charts. The filter is set so that the woofer will never ever exceed it's xmax. So if you get it all 'dialed in' properly, you'll never be able to blow the woofer up, no matter how hard you play it. The filter is preventing over-excursion.

Another neat thing is that the filter is reducing output at resonance by about 3dB. This means that at resonance, the woofer is getting half as much power as it would be in a "conventional" ported box. 3dB doesn't sound like a lot, but that reduced the power by HALF at resonance. So this should help with the long term health of the woofer and it should help with thermal power limits and also raise the headroom a bit by reducing power compression. This is because a woofer in a ported box is getting absolutely brutalized at the resonance frequency, because at resonance the full output of the amplifier is getting poured into the woofer. Due to swings in the impedance curve, a woofer in a ported box will be getting much less power *above* resonance than *at* resonance.

 

  

[Patrick Bateman]

6 minutes ago, DiBo said:

Smaller box gives you more cone control which in turn let's you saturate the sub with more power which in turn gives you more response. Pretty common knowledge I think. 

This is definitely not my idea, I stole it from here: http://sound.whsites.net/qb5align.htm#s10 

 

But I've never built a box like this.

From the looks of it, you have to select your woofer carefully. The combination of a high pass filter and a small box meant that a lot of woofers simply wouldn't work. For instance, I tried doing it with some prosound eighteens and I wound up with a box that crapped out at 60-70hz. It was basically a really big and loud midbass.

To make it work, I had to find a woofer with a really low FS and a fairly large QES and QMS. Basically a home theater sub.