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zenmasterbrian 27th August 2006 08:57 AM

operation below fs
This is an extension of a thread Giant Subwoofer.

I had asked about drivers bigger than 18". But a great deal was said about running below fs.

So, I want to separate these two issues so they both can be explored more fully.

Some folks have been extremely helpful here already.

But still, who is running below fs now, or has thought about it, or has some information about it?

zenmasterbrian 27th August 2006 09:00 AM

I am especially intriqued by the above. He uses a small sealed box enclosure to raise the resonance of a sub woofer.

So he operates only below fs, never above or at it.

He uses eq to contend with the downward slope.

I find this approach very intriquing.

It solves one issue. Previously, you could add more drivers and more power, but it does not lower fs.

Here, he starts below fs. So more drivers and more power do help.

richie00boy 27th August 2006 09:31 AM

And the Linkwitz Transform (P71) is even better. It has the best of both worlds and then some - it equalises below Fs yet also makes use of above Fs so is more efficient and can use a bigger box. Also the upper cutoff frequency is not set in stone.

AKN 27th August 2006 10:30 AM

This may be of interrest:

Btw, sorry to say that seems to be lifeless nowdays but there is still some good stuff there.

REK 27th August 2006 06:17 PM

Subwoofer operation below fs
There are several listing of giant subwoofer systems that operate below Fs.

They are called infinitely baffled subwoofer, and work this way: If you measure the free air resonance (impedance peak) of a subwoofer, in my case an 18” JBL Professional 2245H, it is approximately 20Hz. When you mount the sub in a manifold, as pictured above, and measure the impedance plot of the system, the impedance peak goes down, in my case to 12.5Hz. This is using 4 of the 2245H’s in a 41” wide by 55” high by 12” deep box, with the top opening in the first floor living room covered by a 12” by 42” open mesh iron grate, and the backs of the subwoofers opening into the basement, hence the name “Infinite Baffle”. The basement is 28’ by 33’ and almost 8’ high, so it is reasonably large. The system is powered by an Ampzilla power amp, feeding two of the 2245’s in parallel on each channel that delivers nearly 400 Watts per channel into 4 Ohms. The system in capable of providing 126dB SPL at 16Hz, at less than 10% THD, at the listening position in the living room, which is 14’ by 22’ by 9’, and has several open doorways into other parts of the house. As mentioned in a previous post, Tom Nousaine wrote up this system in Audio Magazine, in December 1999, in an article. Incidentally, the first link above is on his Infinite Baffle Subwoofer system that Tom Nousaine installed in his house.

zenmasterbrian 27th August 2006 07:34 PM

I'm going to check out all the links posted.

Richie00boy, might you have a link for yours? It does sound highly relevant.

We all talked about this on the Giant Subwoofer thread. I'm intriqued, but I'm still reluctant to beleive you can slew through it with no problems.

So I like the Elliot sound approach. But of course I understand that if you don't have to push the resonance up with a small box, then the possibilities are still greater.

richie00boy 27th August 2006 08:06 PM

P71 is on the ESP site, where you have already linked from ;)

zenmasterbrian 27th August 2006 09:22 PM

I'll check it out. Thanks:D

soho54 27th August 2006 09:38 PM

I don't understand what you are looking for? There is a 12db/octave roll-off under Fs. These effects can be negated by EQ, in the form of enclosure size and tuning, or electronic signal manipulation.

EBS and IB subs use the enclosure method. They use large enclosures, which decrease the air spring pressures on the drivers, and drastically increase the power to SPL ratio. Enabling increased output below Fs, with little power needed. The trade-off is max SPL achievable per sweep area, which is why you want high xmas 15"s or greater in this application.

ELF, EAS, and LT circuits electronically modify the input signal. In this case you add EQ to negate the drivers natural roll-off. These designs use smaller enclosures to increase the air spring, which lowers the power to SPL ratio. Massive amounts of power and EQ are used to produce SPL level near a normal sealed sub. The lower you try to go the more power required. +1K levels of power for high SPL applications. A driver with high power handling and high xmas is needed to make the most of these types of subs.

There is nothing special about running a driver under Fs. You just have to pick your poison. A large enclosure or huge amps and extra electronics.

soho54 27th August 2006 11:38 PM


It solves one issue. Previously, you could add more drivers and more power, but it does not lower fs.
Here, he starts below fs. So more drivers and more power do help.
I do not understand this. The Fs is not lowered in a EAS sub either. You just use a box with an Fsc above the passband. That way you know the roll-off will be 12db/octave, and you can just use the generic EQ for any box and driver combo. The EAS method is the easy way of creating LT like gains.

More drivers always help with output above and below the drivers Fs.

You can accept a sub operating below an abnormally high Fsc, but not at normal levels?

EDIT: Not trying to flame you. I'm trying to understand where you are coming from.

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