[Duke]

Jazz and Baroque,

Under the Geddes placement paradigm, placing one of the low frequency sources closer to the ceiling than the floor is desirable, but it's not like the sound falls apart if you don't.  You'd probably still be better off than with symmetrical placement.  In fact, the remaining floor-to-ceiling standing wave mode can be addressed by placement of subs in the horizontal plane (I can describe this if you'd like). 

JoshK talks about dipole subwoofers having a better power response match-up with dipole main speakers.  I used to think the same thing, and indeed this is true if the crossover frequency is high enough.  But with a low enough crossover frequency, it's not really applicable.  The reason is this:  The ear cannot separate the first-arrival from the reverberant energy at low frequencies (this is why resarch has shown the human ear cannot detect the location of a sound source below 80 Hz or so; any subwoofer localization that you hear is due to higher frequencies leaking through).   You see, by the time the ear detects a low frequency fundamental, the localization time interval has passed so the ear classifies it as reverberant energy and doesn't hear its direction.  Thus at low frequencies it's all "power response", and the radiation pattern shapes don't really enter into the picture.  [If radiation pattern shape was a dominant factor at very low frequencies we might expect dipoles to have better "impact" than monopoles due to their higher directivity, but such is not the case.]  Fortuntely it's quite simple - when we match the levels, we have matched the power response.

It can be beneficial to operate the subs in a multisub system in some sort of mixed-phase arrangement (front subs out of phase with rear subs, left side out of phase with right side, or in an X-like configuration).  The result is not the audible nulling you'd expect, because the power response dominates.  Mixed-phase configurations can enhance the sense of spaciousness and envelopment, and further smooth the in-room response.

Rajacat,

Using mixed subwoofers is indeed acceptable.  Geddes uses three vented low frequency sources and two sealed ones in his dedicated mutichannel listening room.   He uses two vented and one bandpass (vented variation) in his two-channel upstairs living room, or did the last time I was there.  He counts his main speakers as low frequency sources; they're about 6 dB down in the mid 30's. 

Duke

[Jazz and Baroque]

And why I think that a 300-pound array of six large drivers in one cabinet is going the wrong way

I would like to revise my initial thought on vertical arrays.  I can see that a six-foot array of sub-drivers would likely suppress the floor-to-ceiling modes in a room with an 8- or 9-foot ceiling.  But a 300 pound sub is just not in my future. 

Duke, when you get a chance, I would be interested on your thoughts about horizontal placement to damp the vertical modes.

Mike

[Duke]

Jazz and Baroque,

Let me relate a story which will illustrate how to use horizontal positioning to address the vertical standing wave mode.  It's a true story, and is how I figured it out.  Sometimes a problem can be a good thing because when it's all over, you've learned something useful.

A few weeks ago I was showing a new speaker at the Rocky Mountain Audio Fest.  The bass was overly heavy - much heavier than it had been in my living room.  All day the first day of the show, the bass was too much.  From the measurements I'd made in the design phase, I knew the speakers didn't have any such bass issues.

All day long I fretted over the excessive bass, and it wasn't until that evening that I figured it out.  It sounded to me like an emphasis in the upper bass region, which would be consistent with a floor-to-ceiling standing wave mode.  Now normally either the floor or ceiling or both are somewhat lossy at low frequencies, being constructed of materials that allow some flexing and/or transmission of energy.  But in the hotel room, the floor and ceiling were extremely solid (while the walls were just sheetrocked).  The 8-foot ceiling would give me an approximately 70 Hz overemphasis, and that's what I was hearing.   Hmmmm. 

Now the way to get a low frequency notch is to position your speakers such that the energy bouncing off a wall arrives out-of-phase with the direct energy at that frequency.  So that evening I repositioned the speakers so that the bass energy bouncing off the wall behind them would arrive at the listening position out-of-phase with the direct sound at about 70 Hz.  Actually the math was less complicated than it seems; I simply placed the speakers about 4 feet out from the wall, which is 1/2 of the floor-to-ceiling distance.  Thus the round-trip path-length-difference for that reflected energy was equal to the room height.

So the net result was this:  The reflection off the wall behind the speakers caused a "zig" at the same frequency where the floor-to-ceiling mode was causing a "zag".  It wasn't a perfect cancellation of the 70 Hz emphasis, but I'd say it was 80% effective.  The bass was much better balanced after that. 

This same principle can be applied to a multisub system.  By positioning each sub such that the path length difference of a primary reflection is equal to the room height, the one will tend to cancel out the other.

Duke

[Jazz and Baroque]

Duke,

very nice.  I will remember your solution. 

Mike

[JohnR]

Could someone post a link to a picture of a "W frame dipole sub" for those of us who have no idea what this is??

See http://linkwitzlab.com/woofer.htm

[Jazz and Baroque]

thanks, JohnR

this thread has let me think through a lot of my questions about subwoofers.

Mike

[Mike Dzurko]

Some dipole owners try stereo subwoofers, and these people are more likely to keep the subwoofers in their system - not because they get twice as much bass, but because the blend is much better.  There's a reason for it.

Multiple subs will suppress room modes.  The easiest example is one sub located against a wall.  This sub will excite half-wavelength resonances that bounce back and forth against the opposite wall. 

Setting up a second sub on the opposite wall will tend to cancel out this half-wavelength mode. 

However, this model assumes that the same bass signal is coming out of both subs. 

If the recording engineer has close-miked the bass instrument, and the mixing engineer puts all the bass on one channel, then the advantage of the multiple subs disappears.  This was done on some great jazz LPs from the '60s.

This is a good reason to mix the left and right channels of the low bass, even with a set-up that uses multiple subs. 

It is a why I am interested in finding small subs with 8-inch drivers. 

And why I think that a 300-pound array of six large drivers in one cabinet is going the wrong way, unless, of course, you can afford two of them and have no trouble moving them around the room to find the best set-up.   

Just my thoughts on the matter,
Mike

Mike:

Good points.  We've found that the recording techniques employed sometime will determine whether mixed mono sounds better or stereo bass sounds better. In practice, few will want to be switching back and forth. As always, I recommend each individual listen in his setup to see which sounds best in that particular room and setup.

[JoshK]

Here is the info for the W frame dipole woofer.
http://www.linkwitzlab.com/woofer.htm

I understand Duke's retort, but I don't happen to agree.  I think there is a bit more to it because a lot of the room modes are in the 80-300hz range and a good part of this is where that is crossing over so there is some interactions that let a constant dipole dispersion through that range "blend" better.  I also think that one typically can locate the sub because of other factors including distortion products, box resonance, etc. 

[bushbison]

My 2 cents:
The Vandersteens would do the job just fine (I've heard this exact combo and been quite pleased); if you are on a low budget and stick to music,  2 Martin Logan Dynamos are hard to beat for the price (& they're small and nice and fast; the rest of the M.L. line is great as well but go up in price QUITE quickly).

However, if you are into both music and movies, needing speed AND extension, I would look to ACI and especially TBI as that is what we have running with some planer hybrids and have been never disappointed.  As well the TBI products are quite discrete in size.

At the very least, audition as many as you can in your home... SO many factors there... and if local dealers (?) don't allow you to do so, the online companies may be a good way to go.  Both ACI and TBI have great customer service.

Again, just my H.O.   

[Mike Dzurko]

It is easy to get wrapped up in the pursuit of absolute sonic perfection. It’s a lot of fun in fact.  However, how many people do you know that will likely add four subs to their systems? Or be hanging subs from their ceiling or placing a sub on a tall stand?  Okay, okay, around here it’s not that uncommon, and it is pretty cool for sure. . . . I’ve done those kinds of things, and will again. And yes we have customers with four and even more of our subs, but that is a small percentage of satisfied sub owners.

So, a cautionary note: There will be those who read through this thread and think “I can’t get a sub(s) to sound good in my system unless I can buy four/hang them from the ceiling”.  Not so, many thousands of music lovers are enjoying what quality sub(s) can do in their system. A sub(s) can often yield some of the most significant sonic improvement per dollar spent in an audio system.  If you haven’t tried sub(s) or have come up less than satisfied, I recommend you look for the right sub(s) for your system and then take the time to work the setup. You will be amazed at the results.