Why you need 4 subs

[Dr_jitsu]

Please discuss:

http://www.soundandvisionmag.com/features/2006/11/why-you-need-four-subwoofers

[HopefulFred]

Articles like this always come up a little short for me. It's not that I disagree - I don't. It's mostly that the authors don't do enough explaining to convince me. As with everything audio, the bottom line seems to be, "listen, and you'll understand." Well that's OK for him; he works in the home theater industry and listens to different systems all the time. I'd appreciate a more complete theoretical approach.

For instance: "To get an even more consistent bass response over a large seating area, move up to four subs. (Why not three? Because symmetry is key to making this work. Every sub needs to have a counterpart placed in the opposite position.)"

This seems counterintuitive to me. If the following grid is the response in a room, 20 by 25, normalized around reference with the sub placed midway up the left wall (third row, first column)...

+3:+2:-1 :+3
+1:-1 :-2:+1
+5:-3 :+1:+2
+1:-1 :-2:+1
+3:+2:-1 :+3

...is this a reasonable made-up response for a single sub (again, placed at the midpoint of the left wall (+5 in the grid)? You'll notice that I've assigned high values for the corners and for near the sub. I've also placed a relative null near the center.

So this would be the same room, with the same sub, in the opposite location (Just written backwards):

+3:-1 :+2:+3
+1:-2 :-1:+1
+2:+1 :-3:+5
+1:-2:-1 :+1
+3:-1 :+2:+3

Now can we add them like this for when both subwoofers are used in opposing positions?

+3:+2:-1 :+3........+3:-1 :+2:+3........+6:+1:+1:+6
+1:-1 :-2:+1........+1:-2 :-1:+1........+2:-3:-3 :+2
+5:-3 :+1:+2...+...+2:+1 :-3:+5....=..+7:-2:-2 :+7
+1:-1 :-2:+1........+1:-2 :-1:+1........+2:-3:-3 :+2
+3:+2:-1 :+3........+3:-1 :+2:+3........+6:+1:+1:+6

It seems to me that this is the way the simple rules of wave interactions would play out, but I have a feeling this is wrong, as the sum of the outputs is less smooth than each was individually. Also, notice the symmetry of the sum. I created each individual output symmetrical front to back, as the sub was placed midway in a symmetrical (rectangular) room, but the sum becomes symmetrical in an additional dimension (presumably two additional dimensions as this data set could only describe response on a single plane).

How does this look to you? I feel like I have overlooked some basic assumption.

Fred

[diamonddelts]

I know I into go for a quad sub system in one month when I get the money saved up.

[tesseract]

How does this look to you? I feel like I have overlooked some basic assumption.

The basic assumption these theories make is that the room is sealed, rectangular and without dimensions that are divisible by 4. Most rooms do not fit this criteria.

But you have to start somewhere and this is the baseline that has been used to form the Welti/Devantier theory.

Another popular theory uses subs semi-randomly placed around the room.

Both methods contend that 3 or more subs create enough peaks spaced close enough together that people cannot detect the individual room modes (room interactions below the Schröder frequency). The brain/ear mechanism works to fill in those modes. Indeed, even measurements show that the sum of many peaks results in a flatter room response, as demonstrated at the end of the Mehlau link I posted.

[tesseract]

Please discuss:
http://www.soundandvisionmag.com/fea...our-subwoofers

Dr. jitsu, that link posted to you over on AVS is a good one. It summarizes the link Harman link above. I have posted both links in my post above for you in the past, but it never hurts to review once some understanding of the basics has been reached.

Here are some other really good guides to understanding how subs work and how to get the most out of them in your room.

http://forum.blu-ray.com/subwoofers/95817-guide-bass-management-part-i.html

http://forum.blu-ray.com/subwoofers/48286-guide-subwoofers-part-ii-standing-waves-room-modes.html

There are even more links at the bottom of those. Dig in! :applause:

Finally, here is a great discussion pondering these methods.

Read these and get your Black Belt in bass. :smoke1:

[sbdman]

Here was what seemed to me a very scientific method to predict the optimum number and placement of subs. It's written in a power point type presentation side by side with the an oral description.

http://www.harman.com/EN-US/OurCompa...s/multsubs.pdf

The final results mirror Dr_Jitsu's post, but use some known mathematical tools and formulas.

[tesseract]

Thanks Sbdman,

I posted that link already, then added "Dr. jitsu, that link posted to you over on AVS is a good one. It summarizes the link Harman link above."

But after he plows through all the other links I piled onto him, it would be a good exercise for him to review Welti/Devantier again. :biglaugh:

[sbdman]

Sorry about that - I remembered Harmon (which you were probably the one that linked it some time ago as well when I first saw it), but not Welti/Devantier.

I should probably play around with placement and equalization of mine, but it just sounds so much better than my old sub, that I'm completely satisfied with the sound now. Impresses me every time it's called upon.

[tesseract]

Sbdman,

I hear you. Lots of time was spent optimizing, and I am sure I could get more out of my subs, but they are doing so well there is no point in pushing it further. I've thought about getting a REW, mic, mic preamp and a BMS. But further improvement would most likely come from two more subs, so I'll spend my money there, first. :)

Someday I will get more to measure with than the test tones and SPL meter I have now, just for fun and to show off (and help) to friends and family.

[Dr_jitsu]

Here is the Welti conclusion:

One subwoofer at each wall midpoint is the best in terms of
Std, Max-ave and Max-min but does not support low
frequencies particularly well. Two subwoofers, at opposing
wall midpoints, performs very nearly as well as four at the
midpoints and gives a much better LF factor. One
subwoofer in each corner also has good low frequency
support, but does not perform quite as well as one
subwoofer at each wall midpoint, in terms of Std, Max-ave
and Max-min. If cost and aesthetics are considered,
subwoofers at 2 wall midpoints is preferred.

[tesseract]

If cost and aesthetics are considered, subwoofers at 2 wall midpoints is preferred.

Yes, two subs will get you 90% of what three or four subs will. It is the final 10% you are chasing. I dare say that you will get 110% or more. You are now in a Bass Elite club and will soon have a foundation that most of us can only dream of. I will be doing the same thing as soon as my budget allows.

I don't think costs or aesthetics were your prime considerations when you purchased 6 CraigSub drivers powered by 3000 watts. :biglaugh:

[Dr_jitsu]

Actually, the route I am taking, while certainly not cheap, gives me excellent bang for buck factor.

However I like to think that I am getting that extra 20%:D

[tesseract]

Not to mention that you are only 90% of the way there with two subs if they are set perfectly setup.

Going with 3-4 subs or more give you more wiggle room as far as setup, placement can be more random and still have good results.

[diamonddelts]

Not to mention that you are only 90% of the way there with two subs if they are set perfectly setup.

Going with 3-4 subs or more give you more wiggle room as far as setup, placement can be more random and still have good results.

I can't wait to hear what a quad sub setup sounds like. But I must admit that my six towers sound damn good on their own with music. So much so that I usually just turn the subs off because you can not tell when they are own with most songs.

Now the CHT subs deep extension becomes evident in the HT department. This is where I am looking forward to the bass immersion that I'm hoping a quad sub setup will bring.