Dual sub-woofers??

[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.