[klh]

Do any of you know of or where I might get one? I've been searching the web for a simple phase adjust for a subwoofer but haven't had any luck finding one. I don't need a unit with EQ. I don't need high or low pass filters. Just a simple phase adjustor.

[Ethan Winer]

Maybe this?

http://www.littlelabs.com/ibp.html

[klh]

Thanks Ethan! The price is a little steep for what I need ($600). I'm thinking about a second sub and putting it in the rear of the room to even out the frequency response a la the Harman white paper. For that kind of money I could get a good plate amp and be able to adjust the volume, phase, low pass etc. Certainly not the best sound, but this will only accentuate the bass and even the response, not add low end authority.

[JRace]

You could always just wire the second sub out of phase....

[Brucemck]

Ethan, what are the relative merits of some of the alternatives I've heard proposed ...

(1) If you're running two (or more) subs as a single mono sub and have placement flexibility then just place them where the combined signal yields a fairly flat response across the primary listening position(s).  This is the trial and error augmented with measurement method.  (I've tried this and it appears to work reasonably well in my room.)

(2) Place your two subs where they "make sense" and then play with the relative phase on one of the units until you get the smoothest response, particularly around the crossover frequency.  Sometimes this is suggested as a "fine tuning" step to augment (1) above.

(3) Add "time alignment" to (1) above or even to (2) above.  I can do this with the sub crossover if I choose.

All a long winded way of asking, when running multiple subs in mono is time alignment across the subs a big plus?  Is the ability to adjust phase a big plus?

[Duke]

My understanding, from a reading of several sources that advocate multiple subwoofers distributed around the room, is that a variable phase control is not necessary.   I don't think either Welti or Geddes advocate it. 

But maybe I missed something.  Klh, why do you feel that you need a variable phase control for that second subwoofer?

Duke

[ACS Gordon]

Are the two subwoofers going to be identical? If so the long wavelengths of bass frequencies make exact phase adjustment not very important at all.  If the two subwoofers are different then all bets are off since they will have different phase responses and a phase control will be useless. Overall, even with 2 subs the room standing waves will still dominate the response but it will be better and it will play louder too!

[Ethan Winer]

The price is a little steep for what I need ($600).

Oh, you want something affordable!

I have plans for a phase shifter you can build, but it's not for the faint of heart:

http://www.ethanwiner.com/St-Synth.html

In practice, $600 invested in bass traps will do infinitely more than tweaking the phase.

--Ethan

[Ethan Winer]

when running multiple subs in mono is time alignment across the subs a big plus?  Is the ability to adjust phase a big plus?

The short answer is I don't know because I've never played with more than one sub in a room.

--Ethan

[klh]

Duke...

I think I'm getting things confused with Double Bass Arrays.

For those that don't know, the idea with a Double Bass Array is if you have a multi driver two dimentional array of subs along the front wall such that the distance between each driver in the vertical and horizontal merideans is twice the distance between the outermost driver and the wall, you effectively get an infinitely wide and tall plane of low frequency waves coming at you. Imagine the right and left walls, the floor and the ceiling are mirrors... looking out, you would see the bass array having the same spacing going out as far as you can see. If you put the exact same array of drivers on the back wall, wire them exactly out of phase and delay them appropriately, then they will essentially catch the low frequency waves as they hit the back of the room (the coupling of the output would cause perfect cancellation). The Harman White Papers attemp to address room modes but only account for the four walls. This addresses the four walls as well as the floor and ceiling. The one requirement for both configurations is a rectangular room and no bass absorption. If set up as described, the frequency response should be the same as one of the drivers in an anechoic chamber no matter where you are sitting. Theoretically, low frequency modes are completely removed (at the expense of room gain). The closer the subs are spaced, the higher in frequency this phenomenon continues. So, the idea is to have enough drivers to cause this unaltered frequency response to go up to about 80 Hz. And, if you have enough large drivers, you can go as low as you want and dial in whatever curve you want, and since all modes are theoretically removed, that dialed in curve is the same throughout the entire room (not just the sweet spot). Depending on the amount of space you have, you can go small sealed with a Linkwitz transform curcuit (LTC), large sealed (IB) or LLT. Obviously the more drivers you have, the less each one must move to have the desired output at the desired frequency, and the less each one moves, the less distortion you get.

As an example, in a room that is 20' wide and 8' tall, you could have 8 drivers on the front and rear walls (so 16 total); basically, there would be two rows of 4 drivers on the front walls and two rows of 4 drivers on back wall. Horizontally, they would be spaced 5' apart (so the outer drivers would be 2.5' from the side wall) and vertically, they would be 4' apart (so the drivers would be 2' from the floor and ceiling). Sure, that's a lot of drivers, but the only need for a PEQ would be dial in your preferred house curve... and with less EQ comes less distortion. Obviously an LLT would need the least EQ, but having ports could mess with the mirrored effect; to what extent is unknown. Doing the same with sealed boxes (whether small or extremely large) would require a bit more EQ, but it's still less and easier to implement than with traditional subs. Obviously each design (LTC, IB or LLT) has it's own pros and cons.

For those with money to spend or those who just want the best or most even bass response, this looks to be a solid answer to taming our in room bass response. It would also look pretty impressive (to say the least)!!!!!! It would also be damn expensive!

Now, thinking back, I would expect one to not shift the phase in second sub when they are in opposed midwall locations. By having them out of phase, peaks and troughs are offset, which evens the response, and is obviously the goal. In this case, the Harman white paper supports a "quick fix" that helps modal response and minimizes variation from seat to seat, while still re-enforcing the really low frequencies. Because of the minimal variation from seat to seat, a PEQ would also help multiple locations... it would just require more PEQ than the Double Bass Array. Very interesting.

To add to this discussion... I've seen many people discount the benefit of multiple subs because they say integrating the subs with the mains becomes much more complex. They basically say the phase is thrown out of whack and multiple subs become more of a problem than a problem solver. For those with more experience, what do you think?

[klh]

BTW, I have a dual Ascendant Audio Avalanche 18" IB sub in the ceiling centered right to left and adjacent to the front wall above my HT screen. It would be fairly easy to add a second IB in the ceiling centered right to left, but have it adjacent to the back wall instead. My problem is locating more Avalanche 18" drivers. A similar driver (though probably a bit better) is Kevin Haskin's Maelstrom-X. It's T/S parameters pretty close, so I might be able to make due with them.

I have one more question. With two subs, each midwall and opposite the other, are they supposed to put out the same volume? I ask because if they are centered in the front and rear wall (as would be the case in my HT), the rear sub is much closer to the seated area than the front sub (since the couch and chairs are towards the back of the room). If we are optimizing this for where people actually sit, should the rear sub be turned down a bit so it's less localizable? If that's the case, since the front sub is twice the distance from the seated position as the rear, could I go with half the number of drivers in the rear sub, but keep the wattage to each driver the same? In theory, that would keep the volume of the rear the same as the volume of the front at the seated position. It would also cost less

[klh]

I exchanged a couple e-mails with Todd Welti (the author of the white paper) and he suggested equal raw SPL out of both subs is necessary. So, if possible, same drivers, same number of drivers, same amp, equal power. He said in this specific circumstance tactile sensation is the least localizable sense, with auditory second and visual first. Since the rear sub is overhead and slightly behind (thus between the ears), it shouldn't be localizable at all (tactile, auditory or visual). As such, the only real draw will be the screen itself, and that should greatly overpower any localized sensation coming from above.

[Duke]

Hi klh,

Well the Double Bass Array you describe is new to me.   I can't quite visualize how it would accomplish this:

"If set up as described, the frequency response should be the same as one of the drivers in an anechoic chamber no matter where you are sitting."

So you exhanged e-mails with Welti!  Good for you!  He's one helluva nice guy, and obviously he knows a thing or two about in-room bass characteristics.

One thing you should consider, in my opinion, when distributing multiple subwoofers around a room is the crossover frequency and slope.  You don't want the subwoofers giving away their position by passing upper bass and lower midrange energy.  I suggest crossing over the sub no higher than 80 Hz, and using a 4th-order low-pass filter.   

An alternative to Welti's symmetrical placement suggestions is Geddes' asymmetrical suggestion.  Here's a link to a brief informal study comparing four subs in corners to four subs asymmetrical:

http://www.gedlee.com/downloads/sub%20study%20.pdf

Best of luck with your project,

Duke

[klh]

Duke,

Thanks for the link. Interesting read.

The double bass array is very conceptual. I heard about it over on AVS. The guy who posted on it says it's all the rage over in northern Europe. If you think about it only in the horizontal meridian, if the distance between each sub is twice the distance between the outermost sub and the wall, you have them properly located. Because the distance between the outermost woofer and the wall is half the distance between each sub, if the right and left walls were mirrors, it would appear as if the array goes on to infinity if you looked off to the right or left. Think about it. Then you apply the same principle in the vertical meridian. Then you do the same on the back wall and delay it such that the forward movment on the front woofers has perfect cancellation by the backwards movement on the woofers on the back wall. This effecively takes room modes and room gain out of the equation for lower frequencies (if no bass escapes the room and no absorption occurs within the room). The closer the woofers are in proximity, the higher up the frequency spectrum this phenomenon continues... so if you get them close enough that this phenomenon goes beyond 80Hz, you're golden. Of course this is all theoretical, but this guy over on AVS said in practice it works pretty damn good.

I think it's pretty interesting, and probably works, but it also would be pretty expensive. Probably the best way to do it would be to have a second wall a couple feet behind the front and rear wall that vent up into the attic, thus your front and rear arrays are infinite baffle enclosures. That would look good, not take up much space, and simplify the construction.

BTW, my crossover now is 80Hz, but I plan on getting new speakers (seven) to go around and hopefully I'll be able to drop the crossover to 60Hz. I think that would be ideal.

Krister

[Duke]

Thanks for the additional information, klh.  Okay, it sounds to me like the bass array at the front of the room approximates a planar source, and the identical array at the rear of the room is delayed such that it cancels out the front array's output when it arrives at the rear of the room.

Very innovative!

Assuming it works as intended, here is the problem that I see:  The ear needs to hear more than one cycle of a deep bass tone before it perceives the pitch, and often the room itself provides the sustain in the bass region (and sometimes too much of it).  Depending on how the recording is miked, bass pitch definition could suffer with the dual bass array because the bass tones would be extinguished before the ear had time enough to properly interpret them.  There's a happy medium area in between anechoic bass and over-reverberant bass, and I think the dual array might well take us too far in the direction of anechoic bass.   

Duke

[klh]

Huh. I don't know enough about psychoacoustics to add anything to your point, but it is interesting. Accurate in room bass sure is an animal that's tough to tame .