One or two subs?

[Mash]

You connect a mic to the sub, place the mic where you will listen, and press EQ on the sub's remote. The sub then emits a sequence of 10 frequency sweeps. The mic measures the sweep's response where you sit and the sub then corrects the output levels for the next sweep. I have one of these subs in our kitchen. It keeps the fresh salmon jumping.

The servo feedback feature cancels out amplifier and speaker cone distortion by correcting the sub's final output to match the sub amp's input signal in real time. This is the same feature that is used on the Mackie HR824Mk2 speakers.

If you should ever use loudspeakers like Magnepans, a servosub is a must. A non-servo sub will usually sound rather 'soft' next to the Maggies.

[Bigmoney]

Mash.... If I understood you correctly the velodynes will self correct themselves for a flat frequency response. Thats amazing. Can only imagine what those subs cost. What are the cheapest velodynes with that function and is it as effective, or more effective than hooking up and separate eq such as the one suggested by e stat and adjusting manually?

[blackraven]

The velodyne SPL series comes with the automatic 6 band equalizer and sells for about $800 to $1500 depending upon the size of the sub. The more expensive Digital Drive subs with the servo control and 8 band equalizer are over $1800.

[Mash]

Bigmoney, I always preferred the 15" Velodynes as being the best choice for a home sound system. The usual difference between the 18" and 15" Velodynes was the 18" driver and a larger box. The units smaller than 15" did not ever seem to cut the mustard.

An yes, you are correct that "...the Velodynes with EQ will self-correct themselves for a flat frequency response."

Audio Advisor was selling the 15" SPL for $1000 a while back. This was about $200 below the next best price. The 15" servo-controlled DD will likely cost about $2000. The servo-controlled sub will be more distinct when reproducing a (struck) bass drum, string bass, and other instruments with sharp attacks. A pipe organ will sound about the same on either the servo-controlled DD or the non-servo SPL, since pipe organs generate sound using resonating air columns in pipes, i.e. their sound issues as sine waves.

The servo feature will shine with speakers such as Maggies and the Mackie HR824.

[Sir Terrence the Terrible]

You connect a mic to the sub, place the mic where you will listen, and press EQ on the sub's remote. The sub then emits a sequence of 10 frequency sweeps. The mic measures the sweep's response where you sit and the sub then corrects the output levels for the next sweep. I have one of these subs in our kitchen. It keeps the fresh salmon jumping.

The servo feedback feature cancels out amplifier and speaker cone distortion by correcting the sub's final output to match the sub amp's input signal in real time. This is the same feature that is used on the Mackie HR824Mk2 speakers.

If you should ever use loudspeakers like Magnepans, a servosub is a must. A non-servo sub will usually sound rather 'soft' next to the Maggies.

Non servo's also sound soft and less impactful with horn loaded speakers as well(really just my opinion). I know when I got the servo option added to my subs, it really drastically changed the low end response. No overhang, super quick transient response, and the sucker could play about 6db louder than before the servo's were added with far less distortion to boot.

The problem I have with auto setup/equalizing circuitry lays with the fact they do not take into account early first reflections rather loud amplitude, and the lower level late arriving reflections. Audyssey auto setup/EQ is the only one that takes into consideration the reflection "pattern", not just the loudest modes in the room. With most other setup and EQ routines, you get in the ballpark, but no homerun. I can go back and still find issues with a good old fashion lab grade microphone and a RTA at 1/6, 1/10, and 1/20th resolutions. The audyssey is pretty much on the mark, and only requires very little tweaking when its all done.

[Sir Terrence the Terrible]

Sir T, I humbly defer to your experience in this area.

I can only speak from my own experience of two subs with corner placement - I didn't get anywhere near 9 dB extra and the in room response. If memory serves around 5 dB, I would have to check. Moving even a foot or so from the corners would diminish the gain rapidly.
Also, I noticed larger peaks at a few frequencies, but the "dips" were alleviated. Net effect being a smoother response overall, but trading one set of problems for another. The nice thing for me is the BFD could kill the peaks, I couldn't do much about the dips before. I have noticed that it is much easier to tell when the 2 subs are out of phase with the main speakers near the crossover region though and this was the complexity of which I speak. Integrating well took a lot longer (or maybe I was fussier?). But overall I'm much more satisfied.

May I ask, why does THX recommend placing the subs across from each other near the center of the walls as opposed to the corners?

Kex,
The earlier THX certified subs had a low frequency cutoff at about 35hz, and relied on corner placement and room gain to extend the response down to 20hz. It used the rapid roll off of the ported systems in combination with the low frequency support that corner placement gave you to keep distortion and excessive cone motion and overloading to a minimum.

Subwoofers certified as THX ultra 2 are required to have a low frequency cut off of 20hz with no room gain needed to support the low frequency response. The subs are quite a bit larger and more powerful than their earlier approved models, so they recommend a placement that avoids exciting the rooms modes and nodes. The problem with this placement is once again, it places more demands on the subs themselves to provide the necessary gruntage to reach the second octave cleanly. Some do okay with this, and some have to struggle a bit depending on the size of the room they sit in.