[JohnR]

As far as I'm aware, multiple subs won't do anything to solve the problem of excessive decay times near and below the room fundamental. Careful positioning of one or two dipole subs will.

[*Scotty*]

According DS-21's comments in the blog he posted a link to, he is also a proponent of multi-sub installations as recommended by Dr. Earl Geddes.
It should be noted that the benefits of a multiple sub setup, in particular a CABS implementation, accrue regardless of the polar response pattern of the loudspeaker above the Schroeder frequency.
Scotty

[*Scotty*]

John, if the reflection from the rear wall behind the listener does not occur, than there are no excessive decay times below the Schroeder frequency. This is what placing sub-woofers operating out of phase on the rear wall behind the listener accomplishes.
Eliminating the primary reflection associated with the rooms longest dimension solves the problem.
 The key is to generate a planer wavefront that travels from the front wall where the main stereo speakers are located towards the rear wall where it is canceled out before reflection can occur. A bass planer wave propagating along the rooms longest dimension avoids creating reflections associated with the rooms height and width dimensions. This is the only multiple sub approach that does not generate a complex series of reflections and a large spectrum of multiple decay times.
Scotty

[oboaudio]

oboaudio,

The rotary woofer is most impressive, but is there an application for music?

For the $24,000 stated cost plus creation of a rigid infinite baffle (that wouldn't transmit vibration throughout the house, cracking drywall/etc.) there'd better be a dramatic effect.

I did all the work myself, which brought the price down to half, around $12,000.     
As I stated, pipe organ music, especially the Mormon Tabernacle choir with that enormous pipe organ
is simply stunning.

Please read Peter Moncrieffs report on the rotary woofer (International Audio Review) where he
shows the benefits of this sub.     

One does not have to listen at loud levels to get the benefit of the rotary sub (transients are awesome
and the extension it provides it superb.     Rolling off the Magneplanar Tympani IV bass panels below
25 hz lowers distortion dramatically.

 While expensive and requires a bit of work, if one wants low distortion and stunning transient bass
response, this is my recommendation.

I have not cracked any walls with this sub, as I only use about half its total output. 

[rollo]

John, if the reflection from the rear wall behind the listener does not occur, than there are no excessive decay times below the Schroeder frequency. This is what placing sub-woofers operating out of phase on the rear wall behind the listener accomplishes.
Eliminating the primary reflection associated with the rooms longest dimension solves the problem.
 The key is to generate a planer wavefront that travels from the front wall where the main stereo speakers are located towards the rear wall where it is canceled out before reflection can occur. A bass planer wave propagating along the rooms longest dimension avoids creating reflections associated with the rooms height and width dimensions. This is the only multiple sub approach that does not generate a complex series of reflections and a large spectrum of multiple decay times.
Scotty

   I'm with Scotty on this one. Why ? Well after visiting Scotty and hearing the results myself it was a no brain-er. When I got home hooked up a sub in the rear of the room put it out of phase and experienced the most affective bass in my system. No measurements justtried it.
    Adding the rear sub along with two Fried transmission line subs for each channel up front Organ music was intense. The room fill was uncanny. It seemed we were immersed in the Organ. Thanks Scotty.
    So multiple subs at least three will make your system sound as full as needed. If one could incorporate a time delay to the rear sub depending on distance oh my !

charles

[DS-21]

As far as I'm aware, multiple subs won't do anything to solve the problem of excessive decay times near and below the room fundamental. Careful positioning of one or two dipole subs will.

Oh, but they do. (In in the first mode region, all that dipoles do is fail to excite modes because they have no material output. "Dipole" here means "open baffle" or "vented box operating below its tuning frequency." Those two are the same thing.)

Below the Schroeder frequency, frequency response and time domain performance are just two ways of looking at the same thing. So get the FR flatter, and the time domain problems are fixed, too.

See Toole, Sound Reproduction, at 245. ("However, because we know that low-frequency room resonances generally behave in a minimum-phase manner, we know that if there are no prominent peaks protruding above the average spectrum level, there will not be prominent ringing in the time domain. It is this indirect, inferential knowledge that permits us to confidently use frequency responses as a primary source of information about room behavior at low frequencies.")


Though that said, subjectively I've found merely equalizing one sub "flat" does not subjectively eliminate ringing and hang-over like multisubs do. Except in the extreme nearfield (like "subwoofer used as an armrest" nearfield). I don't honestly know why.

[DS-21]

I did all the work myself, which brought the price down to half, around $12,000.     
As I stated, pipe organ music, especially the Mormon Tabernacle choir with that enormous pipe organ
is simply stunning.

I bet that's a pretty awesome effect.

[cheap-Jack]

Hi.

As far as I'm aware, multiple subs won't do anything to solve the problem of excessive decay times near and below the room fundamental. Careful positioning of one or two dipole subs will.

Yes, I agree. Read my notes below.

 
 Organ music was intense. The room fill was uncanny. It seemed we were immersed in the Organ.    So multiple subs at least three will make your system sound as full as needed.

My poll is: it depends on room acoustical conditions.

I got only one 100W sub, placed a few inches against the right front corner walls of my listen area in my basement, about 6ft behind my right channel 2-way bookshelvers. It is front firing 10" driver, oriented to beam direct at my sweet spot.

The drop ceiling (with standard 4'x2" glasswool panels) gets only 7 ft clearance from the wall-to-wall carpet flooring on the concrete basement. All walls are covered with thin wooden panels on standard 4"x3" wood stud structure behind with voids filled with glasswool quilts. No other added acoutical treatment at all.

Strangely enough, the active sub, placed right at the wall corner withOUT adding any bass traps, does deliver clean solid subsonic bass notes of pipe organs. So powerful & room filling I was virtually awed when I played for the first time the first sound track of the famous Bach Toccata & Fuge in D minor from the LP: "Andrew Davis plays the organ at Roy Thomson Hall". It sent my basement space shaking!
First time with an active sub!

Only a couple nites ago, I played the LP: Richard Strauss "Also Sprach Zarathustra" by Boston Symphony Orchestra conducted by William Steinberg. The clean & solid starting bars of subsonic bass marched right onto me, unbeliverable. The organ bass pedal notes near the end of the first track were even more overwhelmingly all over me. I got to find back my jaw I dropped bigtime.

No bass resonance or distortion whatever so ever I can hear, period. No added acoustical stuffs ever added, I repeat.

So it all depends on the room acoustics. Mind you, I am playing on my all tube system with 15W+15W tube power amp No expensive gear at all. What makes it happen is the 100W sub, & that'a all.

Do I need to add more subs? No! Unless I want to annoy my neighbours.

c-J

[DS-21]

My poll is: it depends on room acoustical conditions.

I got only one 100W sub, placed a few inches against the right front corner walls of my listen area in my basement, about 6ft behind my right channel 2-way bookshelvers. It is front firing 10" driver, oriented to beam direct at my sweet spot.

The drop ceiling (with standard 4'x2" glasswool panels) gets only 7 ft clearance from the wall-to-wall carpet flooring on the concrete basement. All walls are covered with thin wooden panels on standard 4"x3" wood stud structure behind with voids filled with glasswool quilts. No other added acoutical treatment at all.

Drop ceiling with damping material, false walls with more damping material...that sounds about as good as a room can be to reproduce bass. (And given that it's a basement, with all of that extra insulation, I bet your energy costs to keep it comfortable are extremely low, no matter where you live.)

Lucky, and smart.

You've done real room treatments, not just superficial room mutilation, there. People who can do such treatments can definitely get away with relatively less sub, and less EQ.

[JohnR]

Below the Schroeder frequency, frequency response and time domain performance are just two ways of looking at the same thing. So get the FR flatter, and the time domain problems are fixed, too.

See Toole, Sound Reproduction, at 245. ("However, because we know that low-frequency room resonances generally behave in a minimum-phase manner, we know that if there are no prominent peaks protruding above the average spectrum level, there will not be prominent ringing in the time domain. It is this indirect, inferential knowledge that permits us to confidently use frequency responses as a primary source of information about room behavior at low frequencies.")

I believe I understand the concept, but something's missing. Consider a single mode - depending on where you place the microphone, the steady-state response may see a peak or a dip. Somewhere in-between, the response will seem to be flat. However, the mode is still being excited. In a decay or waterfall plot, you then see the mode show up.

Does that mean that the room is not minimum phase? Regardless, it doesn't seem to be as clearcut as suggested by Toole, as your comment about EQing flat also indicates.

[JohnR]

@*Scotty*, I was thinking about the "random" approach. Sorry, I wasn't being specific enough. Having said that, I'd really like to see some decay and/or waterfall measurements of a CABS-type approach in a real room. If I can get enough time while SO is away I might be able to try it (her study is the only room in the house that is close to rectangular and has enough space for such an experiment).

The key is to generate a planer wavefront that travels from the front wall where the main stereo speakers are located towards the rear wall where it is canceled out before reflection can occur. A bass planer wave propagating along the rooms longest dimension avoids creating reflections associated with the rooms height and width dimensions.

The planar wavefront part is a tough assumption. That's why DBA uses an array of four subs on each (front and back) wall. The approach with just one sub won't help with the height and width modes - correct?

[*Scotty*]

Square or rectangular doesn't matter, the important part is to have some solid walls without huge openings. The front and the rear walls need to complete walls. Doors that close are fine, open doorways are going to be a problem, an ideal implementation assumes for solid walls.
Scotty

[JohnR]

Well, that room might not work then, because the front wall is not that solid (mostly glass).

Do you have any measurements of your setup? Not just static frequency response but also decay/waterfall plots.

[*Scotty*]

That's  alright, it might not be perfect but it should be good enough for experimental purposes. The glass wall gives you a higher loss factor at long wavelengths but you ought to be able to at least give it a try and see what happens.
Scotty

[TONEPUB]

Build a bigger listening room, so the bass has somewhere to go...

[amuser]

Listen to the mains extensively with out a sub first. Integration of the two speakers is paramount and having a good idea of how the mains can improve will benefit in picking the right sub and xover.

[DS-21]

I believe I understand the concept, but something's missing. Consider a single mode - depending on where you place the microphone, the steady-state response may see a peak or a dip. Somewhere in-between, the response will seem to be flat. However, the mode is still being excited. In a decay or waterfall plot, you then see the mode show up.

Waterfall plots are pretty pictures, but I can't say I've ever seen information on one that wasn't obvious from the FR plot.

[JohnR]

Waterfall plots are pretty pictures, but I can't say I've ever seen information on one that wasn't obvious from the FR plot.

it's easier to see in the decay plot. Here's an example - see the (0 1 0) mode at 27 Hz.



The plot is done with the parameters recommended by Paul Spencer in the Bass Integration Guide and I've added the -20dB target line.

[cheap-Jack]

Hi.

Drop ceiling with damping material, false walls with more damping material...that sounds about as good as a room can be to reproduce bass. (And given that it's a basement, with all of that extra insulation, I bet your energy costs to keep it comfortable are extremely low, no matter where you live.)

Lucky, and smart.

You've done real room treatments, not just superficial room mutilation, there. People who can do such treatments can definitely get away with relatively less sub, and less EQ.

Yea, you can say I am "lucky" as the basement was finished way before I moved in my pre-owned house
22 years back. I did not spend a single penny on it acousically & decoratively.

It just happens this way. Sheer luck for me or what???

What I want to point out is:- one does not NEED to get too paranoid or fussy on room acoustics UNLESS it really hurts the sound. Frankly I seldom run into such bad acoustic situation that really needs treatement after auditioning so many home audios for many years now as I am a casual sound adviser

Don't get blinded by such so called acoustic 'science'. One's bread may be another's poison. Beware of the hidden agena behind those boosting acoustic treatment.

c-J

 

[Soundminded]

There is no one thing that will give you great bass. It's a combination of multiple things. First and foremost is to buy or build a speaker system capable of reproducing very low frequencies without distortion. Without that you have no possibility whatever else you do of achieving that goal. IMO no sound system can claim to be truly high fidelity if it cannot reproduced all audible octaves including the lowest one. Personally I favor acoustic suspension speakers, especially the AR 12" model. Second is to read Floyd Toole's treatise on uniformity of bass. His experiments show to achieve uniform bass you need 4 subwoofers in a typical room. Corner placement for all 4 is optimal, mid-wall placement for all 4 is also said to work well. Third is to have an amplifier with enough power and bandwidth, that will be able to drive the speakers at low frequecies witout distortion. Fourth is to use an equalizer. Because residential construction often absorbs bass frequencies and because the small dimensions of residential listening rooms causes a cutoff below certain frequencies, bass output that is flat doesn't necessarily translate into bass tones that sound flat. Even more deep bass energy must be pumped into a room to make it sound flat.

When you achieve deep low distortion bass you will experience problems that are new. These include low frequency disturbances on both analog and digital sources you never heard before. There's also the problem of not just the potential for acoustic feedback of turntables but actually knocking the laser in a disc player off track. And then there are the shattered windows.