The need for dual subwoofers

mosc · Posted: Sun Apr 16, 2006 8:17 am Post subject: The need for dual subwoofers

Last year, Robin Miller and I did a lot of research comparing single and dual subwoofers in HIFI surround systems. We became interested in this when Robin set up a dual subwoofer system with dual channel bass management that preserved the directionality of the recorded signal. We played recordings of live acoustical music - orchestras and pipe organs mostly.

We both noticed almost immediately that dual subwoofers sounded better. The music had a more luxurious texture; it was more pleasing. We didn't observer that you could hear much improved directionality; there was some other undescribable subjective quality improvement.

The theory advocated by Bose is that you can not detect directionality from very low frequencies below 100 Hz or so, so one subwoofer is all that it needed. This theory is very widely accepted - almost to the point of being considered a fact or law. Certainly, we could tell that there was something happening that Bose did not consider when you used stereo subwoofers.

We set up an experiment using the G2 as a sound souce. It makes a great audio test tone generator. The synth generated sine waves that were processed through a frequency shifter. For example, one could send the left sub a 70 Hz tone and the right sub a 72 Hz tone. This frequency difference is easilly adjustable with a knob. In our experiments this slight difference in frequency between the left and right subwoofers produces a noticeable feeling of swirling in the listener. If, by contrast, you take these two tones and mix them linearly and send the resulting mono signal to a single sub woofer, the listener can hear a beating effect, but there is no swirling experience.

The unique thing about our experiments was that we were not measuring the directionality effect, but some other effect. Clearly, if you play a single low frequency tone alternatively on two different subwoofers, you can not tell which one is playing. But we had found something quite different - somthing with the binaural perception at low frequencies.

We programmed the G2 to run a series of tests at frequencies from about 120 Hz down to 20 Hz. The swirling effect seems to work down to about 45 Hz or 40 Hz for most people.

From this work, we are convinced that dual subwoofers are superior to one. The paper that Robin presented about this at the Audio Engineering Society convention paper can be found here:

http://www.filmaker.com/papers/RM-2SW_AES119NYC.pdf

Check Robin's web site, http://www.filmaker.com for other interesting information about surround.

deknow · Posted: Sun Apr 16, 2006 1:22 pm Post subject:

i can certainly imagine that you can do things with dual subwoofers (or quad?) that you can't do with one. does one find program material that triggers this effect, or does one have to "compose" for it specifically? the other question that comes to mind is regarding where the effect happens...in the air by the two signals interfereing with each other (can you "record" this effect with a mono mic)?, or does it happen in the brain by your two ears hearing differant things?

deknow

mosc · Posted: Sun Apr 16, 2006 7:31 pm Post subject:

Robin's paper proposes an explaination. There are neurological structures in the brain that can detect the difference in arrival of pressure waves at these very low frequencies.

You don't have to compose for this effect. We noticed it when listening to recordings of orchestras and pipe organs.

I believe the swirling effect is caused by the mixing of the different frequencies in the brain, not in the air. If you play tones in binaural headphones that are different by 2 to 5 Hz, you'll experience a similar swirling effect. With headphones, there is no mixing in the air.

bigtex · Posted: Mon Apr 17, 2006 5:02 pm Post subject:

Mosc,

I would like to perform some experiments to add to this. I work at Dolby Labs as well as Recombinant Media Labs (aka Asphodel Records) I've got access to a lot of equipment, discrete channels of amplified sound, and truly "dead" listening spaces. I probably won't be able to get to it until the weekend after next, though (a friend's wedding this weekend). The picture on the front page of the RML site is the inside of the main sound room. The image shows off the 10 video projectors in there, seamlessly wrapping around the room (so nice!).

At Dolby, everybody seems to believe in the nondirectionality of bass, but I've never truly believed in that. I know it is very subtle, perhaps not even conciously discernable, but I'm sure our senses and our brains can tell, if just barely. At the RML, there is a 16.2 surround sound setup in a ~20x30 room. Right now, the two subwoofers (L Acoustics dual 18") are run in bridged mono. I'd like to see about changing that. Next weekend I'll go in there and gather some people into the room to perform listening tests. I'll do tones as well as music. The two subs are along the back wall of the room, but I'm thinking about trying them out on opposite walls, or even opposite corners.

Soon, though, we'll be installing about 18 floor-shaking transducers, so discrete subwoofer channels may become a moot point...

The surround speakers in the room are also L Acoustics, and there is one in each corner and in the middle of each wall, with eight at the top and eight at the bottom. The spatialization capabilities of this system are amazing. A virtual center can be created at the center of the ceiling or the center of the floor, and sounds can fly around the room seamlessly and in a very convincing way. The sound system has a true 120 dB dynamic range, but you need to wear earplugs if it is used to it's full potential (and even with good earplugs, with the sound cut by 33dB, it's almost painfully loud).

Anyway, if there's anything you'd like me to try in this space (to prove a point or otherwise) let me know.

mosc · Posted: Mon Apr 17, 2006 7:14 pm Post subject:

Sounds like a great room for electronic music performances. Very Happy

I find it amazing the people with all that money and supposed knowlege would run two subwoofers in bridged mono configuration. When you test it with music, make sure you have some good recording, not stuff recorded on multitrack.

I suggest you talk to your company arrange for Robin Miller to come out there. He's one of the most knowledgeable 3D sound experts. See http://filmaker.com .

diatonis · Joined: May 01, 2005 Posts: 56 Location: los angeles, ca

Hi,

A very non-technical approach is to place the sub directly behind you or as close to you as possible. If you could isolate your chair from the speaker vibrations then you could sit on it - pun intended Wink

Two is definitely better than one and the further away from you they are the more noticeable of their placement.

2.3 cents