Dual sub-woofers??

[Richard Greene]

Almost all two channel music has "mono" bass.

Bass may be panned right or left, but there is no out-of-phase bass information under 80Hz. in 98 to 99% of popular recordings. Bass output above 80Hz. coming from the left and right speakers will create the correct soundstage even when one mono subwoofer is used as long as the mono subwoofer is located near one of the main speakers.

However classical music two-channel recordings made in large halls usually include some stereo room ambience at low frequencies that may be audible with left-right "stereo" subwoofers located in the front of the room. Best for reproducing low frequency room ambience, however, would be subwoofers located to the right and left of the listener hooked up out of phase ! Not good locations for reproducing music however, just for reproducing room ambience.

There are three advantages of using two subwoofers.

(1) Two subwoofers placed near each other (+6dB) allows you to reduce output of each subwoofer so their drivers stroke half as far as before to produce the same bass SPL.
Two subwoofers on opposite sides of a room (+3 to +4dB) offer a little less of an improvement. Shorter strokes of the cone mean less harmonic distortion.
If the harmonic distortion from one subwoofer was over 5% (minimum for audibility to most people while listening to music), then lower distortion is likely to be an audible improvement. The louder the bass, and the larger the listening room, the more likely adding a second subwoofer will make an audible improvement by reducing harmonic distortion.

(2) Left-right subwoofers are easier to integrate with the front speakers -- they form a phantom bass center image that's very important for directional bass over 80Hz. -- this can't be be done with one subwoofer unless it's located half way between the main speakers ... which is rarely a good location to optimize bass frequency response.
If you use one mono subwoofer located off center, it becomes much more important to sharply limit output over 80Hz. to make the subwoofer sonically invisible.

(3) Left-right subwoofers produce a more consistant bass frequency response across
a row of seats, with the one exception being a listening seat located halfway between the side walls. This is obviously important for home theater, but not important for two-channel music where the listener is usually located halfway between the side walls !

Details:
Left-right subwoofers would be located out-of-phase for a specific side-wall-to-side-wall
standing wave, so cannot excite it. The center frequency of that standing wave is approximately 565/room width in feet = standing wave center frequency in Hz.
Bass output at that center frequiency (+/-3Hz.) varies significantly as one walks across a listening room -- loudest near the side walls and weakest halfway between the side walls.

Most people would initially think: Who wants to excite a standing wave ?
Well any listener who sits halfway between the side walls is already in or close to a null for that standing wave and probably needs MORE output at that narrow band of frequencies. Using one (or two) subwoofer located left (or right) of center will excite that standing wave so the bass won't be so weak at those frequencies. Exciting that standing wave will actually make maximum bass output under 80Hz. 1 to 3dB louder. That's why maximum bass output is higher when two subwoofers are stacked, or located
side-by-side, on the same side of the room.

Assuming anyone is still awake:

In summary two subwoofers are usually better than one:
- For home theater and some two-channel classical recordings left-right subwoofers have some advantages.
- For two-channel popular music (studio recordings) two subwoofers located on the same side of the room have some frequency response advantages as long as the subwoofers are made sonically invisible by sharply restricting output above 60 or 70 or 80Hz.

It's up to you to decide whether the money spent on a second subwoofer is a better investment than some other upgrade.

[Geoffcin]

I use two Velodyne FSR 15" subwoofers, but it's strictly for extending the response of my main speakers, which fall off below 35hz. With my two subwoofers engaged the response is now 14hz-40khz, with a Max SPL of about ~108db or so in room.

The woofers are set inboard of the mains, and are approximately 6' apart center to center, and about 3' in from the front wall. This is to try to match the woofer plane with the mains. In this position I have found that the standing waves are much reduced from a corner placement. The output is also slightly reduced in this placement, but that is not a problem since there is so much unused gain.

I have used my dual subs in a HT experiment. LF effects were quite pronounced, (read earthshaking) but I didn't feel it was absolutely necessary, or "better" to have two subwoofers for effects, as much as I like them for the audio. To quality this; My HT mains, CSW T500's, are capable of prodigious bass, and although there is more distortion compared to the Velodynes, I didn't feel it detracted from the LF effects at all.

In my view, if you have the room, you can't go wrong with two subs.

[etude]

Firstly, what are we trying to achieve with using more than one subwoofer?

There are two basic things:

1. sound staging
2. smooth room response
3. increase dynamic range / output / lower distortion

1. The first is preserve the soundstage. If the subs are crossed high, then you would want them with the mains. As you cross them lower and/or with a higher order lowpass filter this becomes less important, as you may not be able to localise them. However, you must also consider that distortion enables you to localise the subs.

Many have stated, including in particular one guy who does recording, that in the past recordings were mono for the bass prior to digital as it had to do with the limitations of records. Apparently it is quite common for bass to be stereo down to the limit of audibility, if I recall correctly. Richard Green, I can't say I can validate if that is true or not, and I wish I could find the email he sent, but if you can indicate a source of some more concrete info, perhaps we can get a better grasp of what is actually the case.

2. The second is smoothing room response. This doesn't get nearly enough attention in the setup of sound systems. Below say 200 Hz is the "modal region" where the in room response is dominated by room modes. Here is an example of a room 4.2 x 6 m with 3m ceiling:

10Hz +10 db
18Hz -15
28Hz +12
35Hz -7
40Hz +8
45Hz -5
50Hz +5
.... etc

Above about 150 Hz the room acoustics moves into "geometric acoustics" where sound travels like light rays. This point is called the Schroeder frequency. Comercial cinemas have this point below the audible range usually, but in HT you need to address room modes in some way. There are 3 ways that I know of to do this:

A. experiment with placement, richard made some good suggestions.

Also consider placing subs on either side of your couch - nearfield listening which increases the ratio of the direct to reverberant field, which reduces the impact of room acoustics for the subs.

Two subs smooth room response better than one. If I understand correctly, they excite more room modes, and the result is a smoother response.

B. room treatment - bass traps, as well as wall construction that causes the walls to act as acoustic absorbers in the bass range, tuned absorbers (helmholz resonators)

This is often neglected and not well understood IMO.

C. equalisation - using a unit like the behringer feedback destroyer

There is a heap of info on this approach on the DIY audio forum.

3. A second sub will give you more dynamic headroom. You don't have to worry as much about those loud booms in movies that you don't know about, or rush over to your sub to turn it down before it bottoms out. Sure, you may not want to listen loud, but a comercial theatre is usually a lot more dynamic than a home system, and if you want serious HT, then considerable dynamic headroom is worth having.

Doubling the radiating cone area yields +3db sensitivity normally, but with subwoofers depending on how they are set up, I believe you can get +6db. Also, if each is a powered subwoofer, then you effectively get another +3db due to twice the total power being applied to the system. Thus you get theoretically 6 - 9db increase in max SPL. However, room placement has a bigger impact on max SPL than this.

Almost all two channel music has "mono" bass.
If the harmonic distortion from one subwoofer was over 5% (minimum for audibility to most people while listening to music), then lower distortion is likely to be an audible improvement. The louder the bass, and the larger the listening room, the more likely adding a second subwoofer will make an audible improvement by reducing harmonic distortion.

There is some interesting and new research that has been done on distortion perception that may challenge your view on this:

http://www.gedlee.com/distortion_perception.htm

The link includes samples that demonstrate that there is no correlation between current distortion measurements and actual perceived distortion. Harmonic distortion is not the issue. The linearity of the system is what is important.

Most people would initially think: Who wants to excite a standing wave ?
Well any listener who sits halfway between the side walls is already in or close to a null for that standing wave and probably needs MORE output at that narrow band of frequencies. Using one (or two) subwoofer located left (or right) of center will excite that standing wave so the bass won't be so weak at those frequencies. Exciting that standing wave will actually make maximum bass output under 80Hz. 1 to 3dB louder. That's why maximum bass output is higher when two subwoofers are stacked, or located
side-by-side, on the same side of the room.

I'm not sure how relevant standing waves are to this discussion. IIRC, standing waves will form between 6m walls about about 60 Hz. There may not be a lot of response in the range where standing waves exist in the room. If the width is 4m, then they will form above about 90 Hz.

Could you elaborate a little more on your comment about placing subs together. I don't quite follow your logic on this.

I recently bought a book on HT which challenged many things I previously believed, and discusses some of the points I've mentioned above. It's worth checking out:

http://www.gedlee.com/Home_theatre.htm

I have no affiliation of course. If you are building a HT system, I'd consider it a MUST read.

CONCLUSION:

1. First experiment with placement with 2 subs - nearfield, with mains, corners (front or opposite) and many different combinations.
2. If you have the ability (ideally this would involve some construction / demolition / addition), use acoustic damping methods that work for the bass. Room modes will be further reduced.
3. To really travel first class use eq as well.

Note: this is a LOT of work, some of the above are quite difficult, and you need to do considerable research first if you want to be sure that you are actually making an improvement. I would also get the book I suggested if you are really serious about all this, and then some of my comments might actually make sense.

[Richard Greene]

"Also consider placing subs on either side of your couch - nearfield listening which increases the ratio of the direct to reverberant field, which reduces the impact of room acoustics for the subs. "

RG comments:
For a subwoofer, "near field" means only one or two feet from your ears -- beyond that point the majority of bass energy is reflected off room surfaces rather than going directly from the subwoofer driver to your ears.

Subwoofers located close to your ears are worth trying, but there may be two problems:
(1) You may need a time delay if the main speakers are much further away.
(2) Uneven frequency response:
If the subwoofers end up near the middle of the room, bass will be considerably weaker than subwoofer locations near walls ... except for the floor-to-ceiling standing wave which will be fully excited by any subwoofer sitting on the floor and may be very audible because "middle-of-the-room" subwoofers don't excite other standing waves strongly.
The floor-to-ceiling standing wave center frequency will be:

565/room height in feet = Center frequency of standing wave in Hz.

(71Hz. for the typical 8 foot ceiling = a frequency frequently excited by the program material)
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"Two subs smooth room response better than one. If I understand correctly, they excite more room modes, and the result is a smoother response."

RG:
Tom Nousaine did research presented September 1997 in an AES paper preprint 4558.
He found that room modes don't change much from using multiple subwoofers and
that using multiple subwoofers usually made the bass frequency response worse
than a single corner subwoofer.



"http://www.gedlee.com/Home_theatre.htm"
"I have no affiliation of course. If you are building a HT system, I'd consider it a MUST read."

RG comments:
Dr. Earl Geddes was a former Ford engineer and local Michigan audio club member who many years ago was designing a listening room in his home that had flexible walls to reduce bass reflections and standing waves. A true scientist of audio.
... However, the distortion audibility study you referenced seemed to include intermodulation distortion which is not relevant for a limited-band subwoofer (whose distortion is mainly lower order harmonic distortion). The audibility of harmonic distortion from subwoofers is very subjective because the sounds coming from the main speakers mask the distortion coming from the subwoofer. Low-order harmonic distortion from subwoofers is not that unpleasant - it simply makes a bass sinewave "note" sound like a chord. My point was that adding a second subwoofer would lower harmonic distortion (assuming the same SPL as before), but there could only be an audible improvement if the harmonic distortion from one subwoofer was audible to begin with.

[etude]

For a subwoofer, "near field" means only one or two feet from your ears -- beyond that point the majority of bass energy is reflected off room surfaces rather than going directly from the subwoofer driver to your ears.

Richard, could you elaborate a little more on this? Surely the characteristics of the room must be taken into acount here. As a general rule, the dominance of the direct and reverberant field in a room is determined by the liveness of the room and the location of speakers and listening position. This is generally applied to a full range source, but I expect that in the bass there would also be a similar relationship. In particular the nearfield region, IMO, would be extended if the walls were designed to absorb bass energy. It should also be impacted by the size of the room.

Time delay should not be a problem for HT, receivers normally have this function.

My experiments in an average sized room, although not extensive, have shown that the bass is subjectively stronger this way than placed in corners at a greater distance. The increase in output for the same input signal and power is quite noticeable. I have not tried this in other rooms yet however.

Tom Nousaine did research presented September 1997 in an AES paper preprint 4558.
He found that room modes don't change much from using multiple subwoofers and
that using multiple subwoofers usually made the bass frequency response worse
than a single corner subwoofer.

I haven't seen that paper so I can't really comment. Where can it be obtained?
My understanding that multiple subwoofers smooths room response is based on comments from many including notably Dr Earl who recommends the use of 2 or 3 subwoofers in a home theatre to smooth the response.

My point was that adding a second subwoofer would lower harmonic distortion (assuming the same SPL as before), but there could only be an audible improvement if the harmonic distortion from one subwoofer was audible to begin with.

Your point was understood. It makes perfect sense that a 2nd sub will reduce distortion for the same output level. My point was simply that the measure of harmonic distortion does not correlate to the perception of distortion. You mentioned that 5% THD was audible. If I understand the study correctly, then this is not true. eg. a music sample containing 0.1% THD is shown to have much higher subjective distortion than the same track with 9.6% THD. This is a significant study as it shows that THD does not relate to the subjective distortion level. One could be convinced to buy one amplifier over another as it has a slightly lower THD. It is the shape of the nonlinearity, not the actual level that the ear perceives most.

[Sir Terrence the Terrible]

Richard, could you elaborate a little more on this? Surely the characteristics of the room must be taken into acount here. As a general rule, the dominance of the direct and reverberant field in a room is determined by the liveness of the room and the location of speakers and listening position.

The closer you are to the source reproducing the signal, the more it dominates what you hear. The farther away you are, the more the room dominates what you hear via the various reflections. If you sit closer to the sub than it is from the nearest surface, then the subs direct output will dominate what you hear. If the sub is placed closer to a surface than to your ears, its reflected output will dominate what you hear

This is generally applied to a full range source, but I expect that in the bass there would also be a similar relationship. In particular the nearfield region, IMO, would be extended if the walls were designed to absorb bass energy. It should also be impacted by the size of the room.

The walls will not absorb ALL of the bass frequencies. Some will be reflected back into the room.

Time delay should not be a problem for HT, receivers normally have this function.

The time delay found in most receivers does not apply to the sub, it mostly applies to the surround and center channel. Any time difference that must be applied has got to be physical(i.e moving the sub rather than electrical)

My experiments in an average sized room, although not extensive, have shown that the bass is subjectively stronger this way than placed in corners at a greater distance. The increase in output for the same input signal and power is quite noticeable. I have not tried this in other rooms yet however.

Ahhh, but the proof is not subjective, its objective. Using measuring instruments you will find the bass VERY much louder with corner placement, than a nearfield position. Each surface the sub "see's" will add approximately 3db to its output without any increase in volume directly from the sub. A corner placement will yield approximately 9db's additional output. A sub in a corner will have to work ALOT less to acheive a given SPL than one sitting in the nearfield for just that reason.



I haven't seen that paper so I can't really comment. Where can it be obtained?
My understanding that multiple subwoofers smooths room response is based on comments from many including notably Dr Earl who recommends the use of 2 or 3 subwoofers in a home theatre to smooth the response.

It all depends on where those multiple subwoofers are placed, and there is a penalty in the low bass for the optimal placement. According to Dr Toole research, the only way multiple subs smooths the frequency response of the bass is if four subs were placed in the nulls of the room(center wall location that do not excite the rooms modes and nodes). However this placement has a low bass penalty of rolling off rapidly below 50hz. Two subwoofers placed in seperate corners only benefitted a single row of seats across the front(for multiple seating setups), provided about 4.5 db boost in the low bass, but measured not quite as well as a single sub in a corner.. A single sub in a corner provided the loudest bass, measured slightly better than two subs, and had a boosted LF output.

His conclusions are that multiple subs do smooth the bass response but only when placed at null positions, and with a LF penalty.

[Richard Greene]

I'll be on vacation until Feb. 26 and may not see a computer for a while ...


ETUDE WROTE:
Richard, could you elaborate a little more on this? Surely the characteristics of the room must be taken into acount here. As a general rule, the dominance of the direct and reverberant field in a room is determined by the liveness of the room and the location of speakers and listening position.

TTT WROTE:
The closer you are to the source reproducing the signal, the more it dominates what you hear. The farther away you are, the more the room dominates what you hear via the various reflections. If you sit closer to the sub than it is from the nearest surface, then the subs direct output will dominate what you hear. If the sub is placed closer to a surface than to your ears, its reflected output will dominate what you hear[/b]

RG puts in his two cents:
The sub is usually on the floor and the floor is much closer to the driver than your ears.
When one builds subs and later measures them, the mike is placed one inch from the
driver's dust cap to eliminate room effects even though the microphone's "proximity effect" boosts the upper bass somewhat (placing the mike one foot away or three feet away reduces the proximity effect but also means the measurements will be heavily influenced by the listening room). In a typical room with the sub near the main speakers 85-90% of the bass energy that reaches your ears is reflected off a room surface first -- sort of like those pesky Bose 901 speakers that we all love to hate.
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ETUDE WROTE:
This is generally applied to a full range source, but I expect that in the bass there would also be a similar relationship. In particular the nearfield region, IMO, would be extended if the walls were designed to absorb bass energy. It should also be impacted by the size of the room.

TTT WROTE:
The walls will not absorb ALL of the bass frequencies. Some will be reflected back into the room.

RG puts in his two cents"
Those reflections cause standing waves at frequencies whose wavelengths are related to room dimensions and comb filtering (deep narrow nulls) at many other frequencies.
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ETUDE WROTE:
My experiments in an average sized room, although not extensive, have shown that the bass is subjectively stronger this way than placed in corners at a greater distance. The increase in output for the same input signal and power is quite noticeable. I have not tried this in other rooms yet however.

TTT WROTE:
Ahhh, but the proof is not subjective, its objective. Using measuring instruments you will find the bass VERY much louder with corner placement, than a nearfield position. Each surface the sub "see's" will add approximately 3db to its output without any increase in volume directly from the sub. A corner placement will yield approximately 9db's additional output. A sub in a corner will have to work ALOT less to acheive a given SPL than one sitting in the nearfield for just that reason.

RG puts in his two cents:
In 1994 Tom Nousaine presented a paper to our local audio club with measurements showing one subwoofer in a corner usually provides a better bass frequency response
than two subwoofers in non-corner positions. So I immediately went home and measured my two "stereo" subs used as speaker stands located about six feet from the front walls ... and then measured again with the two subs stacked in the left hand corner.
The output in the octave from 20 to 40 Hz. was 9dB louder -- even more than I expected. Unfortunately the corner was located too far from my main speakers ... and my active crossover at that time (100Hz. 18dB/octave) allowed male voices to be heard from the corner subs = the result of using corner subs was poor integration with my main speakers.
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ETUDE WROTE:
I haven't seen that paper so I can't really comment. Where can it be obtained?
My understanding that multiple subwoofers smooths room response is based on comments from many including notably Dr Earl who recommends the use of 2 or 3 subwoofers in a home theatre to smooth the response.

TTT WROTE:
It all depends on where those multiple subwoofers are placed, and there is a penalty in the low bass for the optimal placement. According to Dr Toole research, the only way multiple subs smooths the frequency response of the bass is if four subs were placed in the nulls of the room(center wall location that do not excite the rooms modes and nodes). However this placement has a low bass penalty of rolling off rapidly below 50hz. Two subwoofers placed in seperate corners only benefitted a single row of seats across the front(for multiple seating setups), provided about 4.5 db boost in the low bass, but measured not quite as well as a single sub in a corner.. A single sub in a corner provided the loudest bass, measured slightly better than two subs, and had a boosted LF output.
His conclusions are that multiple subs do smooth the bass response but only when placed at null positions, and with a LF penalty.

RG puts in his two cents:
I believe the "Dr. Toole research" you refer to is a (misleading) study by someone else at the Harman website that is focused on average bass frequency response in a large portion (many seats) of a listening room. It does not seek to optimize any specific seating position. Read the assumptions at the beginning of the white paper. I believe the paper is misleading because "averages" mask the reality of what any one person hears.

The bass in most listening rooms is uneven due to the effects of standing waves.
Those standing waves can be addressed with:
(1) Lots of bass traps
(2) Parametric EQ of subwoofer (most effective on bass peaks heard at one listening position)
(3) Dipole subwoofers (less excitation of about half of the typical 5 or 6 axial standing waves under 80Hz.)

[piece-it pete]

Ever feel like a midget in a land of giants ?

Sir Terrence, have you tried stereo subs? I'm not a snake oil person and I'll say again that mine affect soundstage - no question. I've had them for about a year now & have tried all kinds of crazy things with them - Richard knows some of it! - but that's all part of the learning, and now have it figured out now thusly:

They're actively crossed-over @ 80hz, 24db slope (curtesy of Marchand), with my current mains getting the high side. I've found different speakers like different points, some like full signal.

Joey mentioned his experience of starting to pick up directionality at about 70hz. Maybe that has something to do with it, maybe in combo with the mains getting almost nothing down there.

Otherwise, I bow to your superior knowledge, O Terrible one .

Pete

[the hand of boredom]

Doc,

(Or Terrible) If you were on a budget, would you get a few bass traps or EQ for the best bass response you could afford?

THOB