Sub Location With Respect to Room Treatments

ajzepp · « **on:** 15 Dec 2007, 01:15 am »

So I'm getting ready to make my second trip out to GIK in the next couple of weeks with the intention of picking up four corner tri-traps. I'm also planning to upgrade my subwoofer, so I need to make sure I understand how the sub will interact with the bass traps.

In a 16x12 rectangular room, with bass traps in all four corners, where are the good and bad locations for a subwoofer? Does it matter if its sealed or ported? Forward or downward firing? Do I need to place it a certain distance from the traps?

Thanks, guys!

youngho · « **Reply #1 on:** 15 Dec 2007, 06:50 pm »

Bass traps will help reduce the effects of standing waves or modes. That's how they interact with the sub. If you haven't already read Floyd Toole's white paper on bass, click http://www.harman.com/wp/pdf/LoudspeakersandRoomsPt3.pdf.

If your ceiling is 8' tall, then a bad location will probably be in the midpoint of either 16' wall. However, what the listener perceives is also dependent on his or her position in the room, as well as the position of the subwoofer, so the answer is complicated. The effects of sealed vs ported or forward vs downward firing are virtually insignificant as factors in determining optimal subwoofer position, which should not be significantly affected by trap proximity.

Without any measurement, one way of placing a subwoofer is to put in the listening position, then walk around the room listening for the smoothest-sounding bass and then placing the subwoofer there. With measurement at the listening position, most people typically like to look for the smoothest low frequency response and will go through multiple iterations of placement and measurement accordingly. However, with two channel listening, one caveat is that many listeners will find that the subwoofer integrates with the speakers better if it's located closer to one of them.

Young-Ho

ajzepp · « **Reply #2 on:** 15 Dec 2007, 11:23 pm »

Thanks, Y-H!

bpape · « **Reply #3 on:** 16 Dec 2007, 02:41 pm »

Since you have very interactive dimensions (2x12=3x8, 2x8=16, etc.) you need to pay even more than normal attention. I'd start out at around 2/7 (use a prime number as the denominator) of the room width and 1/7 of the room length. Also (and this can be important in this type of situation) get the sub up off the floor at least a foot - especially if it's a down firing sub.

People completely forget about the height of the sub. It's just as critical as the other two.

Bryan

ajzepp · « **Reply #4 on:** 16 Dec 2007, 09:39 pm »

Bryan: I've never even thought about having my sub off the floor. Are there any products you could recommend as far as something stable to mount it on?

Alwayswantmore · « **Reply #5 on:** 16 Dec 2007, 09:45 pm »

Interesting points. Interesting discussion.

IronLion · « **Reply #6 on:** 16 Dec 2007, 09:50 pm »

I recently moved my sub about 11" off the floor with an extra 4 post Skylan stand that I had laying around; the difference in my small room was noticeable. Everything tightened up and a lot of the overhang or boominess that I had been hearing went away. ASC makes a product called the Sub trap that is basically a sub stand with a small bass-trap component to it, but for my purposes it was too big in size.

bpape · « **Reply #7 on:** 16 Dec 2007, 10:44 pm »

Quote from: ajzepp on 16 Dec 2007, 09:39 pm

Bryan: I've never even thought about having my sub off the floor. Are there any products you could recommend as far as something stable to mount it on?

Watch this space for upcoming announcements...

Bryan

ajzepp · « **Reply #8 on:** 16 Dec 2007, 10:51 pm »

Ahhh, very clever

T.O. may love him some "him", but I love me some GIK

IronLion: Thanks for the ASC info...I'd never seen or heard of anything like that before!

zybar · « **Reply #9 on:** 16 Dec 2007, 10:55 pm »

Quote from: IronLion on 16 Dec 2007, 09:50 pm

I recently moved my sub about 11" off the floor with an extra 4 post Skylan stand that I had laying around; the difference in my small room was noticeable. Everything tightened up and a lot of the overhang or boominess that I had been hearing went away. ASC makes a product called the Sub trap that is basically a sub stand with a small bass-trap component to it, but for my purposes it was too big in size.

Altering the height off the floor changes the Floor/Ceiling room modes and can certainly make the differences you mention.

More people should play with sub height and not just where they place the sub in the room.

George

Vinyl-Addict · « **Reply #10 on:** 16 Dec 2007, 11:06 pm »

Quote from: bpape on 16 Dec 2007, 02:41 pm

People completely forget about the height of the sub.

Bryan

Excellent point Bryan!
I have both of my Rel Storm III subs placed on top of Sound Anchor stands. The open frame design of the stand(s) really compliment(s) the downward firing Rel subs.

youngho · « **Reply #11 on:** 17 Dec 2007, 01:48 am »

I think the reason that the height of the sub often escapes consideration is that fact that most people listen to music and watch movies while seated, which puts their ears an average of 30 inches off the floor, which is obviously dependent on the height of the listener. An 8' ceiling has a first-order mode of approximately 70 Hz, which would have a node or relative null at 48 inches. The significance of the higher order height modes is minimized by crossing over at reasonable frequencies.

What does this mean? Lifting the subwoofer off the floor is particularly helpful if you are 8' or 4' tall and listen to music while standing up or if you listen to music while lying down on the floor, although there may be effects on ringing. Still, you may consider purchasing basic measurement equipment first before investing in the ASC SubTrap.

Young-Ho

bpape · « **Reply #12 on:** 17 Dec 2007, 02:21 am »

Actually, it's closer than you think. Measure your ears in a chair - average position is usually around 40-42" from the floor and that 70Hz peak is definitely noticible.

As for the height, it's no different than any other dimension in the room. You keep things away from the wall behind and beside speakers. Why would the wall under a sub be any different?

Bryan

Vinyl-Addict · « **Reply #13 on:** 17 Dec 2007, 02:26 am »

Quote from: bpape on 17 Dec 2007, 02:21 am

Actually, it's closer than you think. Measure your ears in a chair - average position is usually around 40-42" from the floor and that 70Hz peak is definitely noticible.

As for the height, it's no different than any other dimension in the room. You keep things away from the wall behind and beside speakers. Why would the wall under a sub be any different?

Bryan

youngho · « **Reply #14 on:** 17 Dec 2007, 02:33 am »

Whoops, you're right. 40". Sorry about the typo. However, 48" represents the NULL, not the peak, for the 70 Hz mode in an 8' room. It would represent the peak for the 140 Hz mode.

youngho · « **Reply #15 on:** 17 Dec 2007, 02:35 am »

In addition, speakers and subwoofers behave somewhat differently.

Young-Ho

bpape · « **Reply #16 on:** 17 Dec 2007, 03:14 am »

Agreed - but the principle is the same. Stay away from boundaries to avoid reinforcing modes. This is why you don't put a sub in a wall/wall corner - so why do we leave it against the 3rd boundary?

Measure an 8' tall room with the mic at 40" - you'll see the 70Hz peak.

Bryan

youngho · « **Reply #17 on:** 17 Dec 2007, 05:00 am »

The principle is somewhat different because speakers and subwoofers behave somewhat differently.

Most people DO put the subwoofer in the corner or near a floor/wall junction to maximize output. It's certainly cheaper than buying another subwoofer or two. Tom Nousaine and Floyd Toole suggest that corner placement is an excellent starting place for placing subwoofers. Then, if the listener experiences problems because of room dimensions or listener position, they can move the subwoofer, or they can add bass traps to reduce the detrimental effects of standing waves, just like the original poster is doing with products from your company in all four corners, or both. Particularly because he will be using Tri-Traps, I really think that the original poster should consider buying basic measuring equipment before spending money on the ASC SubTrap.

I do want to reiterate that the standing waves excited by a subwoofer depend on its position, and the modes perceived by a listener depend on his or her position. Ringing is another issue. If you're going to argue that the node of a mode actually is an antinode and should measure accordingly, then there's not much point in discussing the physics of standing waves further. I'll certainly grant that many people might experience a relative "suckout" at 70 Hz while seated in 8' tall rooms, due to the null or cancellation that occurs, but a peak? That is not consistent with physics. And even with the null, output from the other 2-5 channels might sufficiently address this null with a reasonable crossover between them and the sub.

However, please don't believe me just because I throw around a few technical terms and use the word "physics". Why not consult F. Alton Everest, who wrote the oft-quoted "The Master Handbook of Acoustics" (see page 284 of the 3rd edition) , or Floyd E Toole, who has been published in JAES? Toole's white paper on bass (see slides 138, 142, and 189), which I linked to earlier, is recommended by no less than Sigfried Linkwitz. Please read these sources for yourself, or if you've read them long ago, please take a few minutes to review them, as well as the physics of standing waves (http://en.wikipedia.org/wiki/Standing_wave).

In any case, I don't think there's much point in my posting much further in this thread since Everest, Toole, and Linkwitz know far more about room acoustics than I do. I'm not trying to be obnoxious here. I would like to provide the original poster with correct information.

To all, a good night, and happy listening,

Young-Ho

bpape · « **Reply #18 on:** 17 Dec 2007, 05:44 am »

Yes, I've read those papers and the books - some of them many times. Many of them deal with purely axial theory and relate in terms of purely rigid rooms (none are) and perfectly rectangular spaces (few are). Would it surprise you to know there have been tests done that show that with standard construction of 2 identical rooms, they can have different modal centers and intensities based on the distance OUTSIDE that room to the next rigid surface? Now, what's common in most rooms in that vein? How about an extra 10-12" air space to another hard boundary (like the floor above you in the joist cavity or the ceiling below you).

Placing a sub in a corner for max output is indeed a cheaper way to get more 'oomph' but almost always at the expense of the smoothness of the frequency response. The exception is when you use 4 subs, 1 in each corner, as shown in the Harman papers.

http://www.harman.com/wp/pdf/multsubs.pdf

That kind of defeats the idea of a cheaper way to get more output when you have to have 4 to smooth the response aberrations though. If you don't want to read it all (though it's fascinating work done by the author, Toole, and Kevin Voelks), the 3 best sub setups are:

1 sub at the midpoint of 2 opposing walls.
1 sub at the midpoint of each of the 4 walls
1 sub in each of the 4 corners.

None of the good solutions was for 1 or 2 subs in corners. Surprising since Toole in a different paper recommends starting in a corner purely to get the most output and coupling and then EQ'ing out the abberations (which will do zero for time and decay based issues).

If you look purely at the vertical height, then yes, there is a null - at 48". We're not at 48", we're at 40-42" so we're not in the null of the height dimension. We're actually at approx 41% of the dimension - a notoriously bad place to sit in any other dimension - but we do it all the time in the height dim. So, what do we do? We change the vertical sub position to avoid exciting those axial modes quite as much. Ideally, you'd put it at 1/2 of the height but that's not terribly practical.

A lot of this is pure theory and the only way to see what any room will do in the real world is to measure it. Every room is different.

Bryan

satfrat · « **Reply #19 on:** 17 Dec 2007, 11:47 am »

Looks like a good place to throw out for consideration what I've done in the way of sub placement. I made a subwoofer stand that has my ACI Force XL sitiing 28" off the floor and pretty much lined up just below my midrange driver of my right main Lorelei. The stand itself is a bass trap made from a Real Traps Soffit, 2" oak plaforms top/bottom and 2" corner trim all from Timberlands. Inside the 1.5" deep end caps of the Soffit, I placed 3" open cell memory foam, then compressed & glued the whole damn thing together. I have Audiopoints largest 2" cones at the base and Audiopoints 1.5" cones under the sub. Directly behind and about 6" away( it has to be movd to the ner in the summer to make way for an air conditioner), I have a 48"h Real Traps Soffit with .75" oak platforms on each end made up to be a plant stand. I feel this Soffit really tightens up the bass. I have my processor cut the bass at 80hz. To my ear, this sub isn't there as aren't my Lorelei's. It's just a seamless soundstage that's lingering in the air and behind the loudspeakers. I'd say one of the nicest attibutes of raising the sub like this is I don't only hear the bass, I feel it too and I so do love that feeling. It's a feeling oof involvement.

This sub stand was 1 of the smartest moves I've made to my system imho.

This was last summer's picture of an old Velodyne before I upgraded to my ACI Force XL. I have also relocated the 1 gallon Crown Royal momento (been dry 6.5 years) and the porcelain country cottages to the plant stand.

Cheers,
Robin