Subwoofer Placement and Bass traps

jermmd · « **Reply #20 on:** 4 Jan 2005, 11:55 pm »

I'm not convinced of the need for two subs. My single Maestro sub integrates perfectly with my RM30's, I can't localize the deep bass, and there's more than enough power. I do have a deep dip in decibals around 40hz but it's otherwise pretty flat to the lowest test track that I have (20hz). I'm not certain that a second sub would correct this dip.

Joe M.

youngho · « **Reply #21 on:** 5 Jan 2005, 01:07 am »

What are the dimensions of your room, and where is your listening position? What does "pretty flat" mean? +/- how many decibels disregarding the dip? +/- how many decibels including the dip? Do you think that the dip is due to a odd-order or even-order axial mode?

Did you look at the paper? A second subwoofer, if both are properly positioned, will most likely correct this dip if you're dealing with an odd-order mode. Or, if you position the second sub on the other side of the node, about 14 feet away from your current sub, it should theoretically correct it through mode cancellation.

Young-Ho

denverdoc · « **Reply #22 on:** 5 Jan 2005, 03:10 am »

Youngho,

Waves are indeed confusing and have to be mathematically defined as a set of partial differential equations whose solution depends on boundary/initial conditions. The Schroedinger equation is a good example.

The velocity of zero-- analogous to the ends of a string-- condition, at the room corners defines the fundamental of course and these axial modes in all 3D along with their even order harmonics are very heavily supported when they also coincide with a pressure maxima (sound source). When a pressure source is introduced midway along the string/pipe you are right to say it is more complicated than arbitrarily adding a room boundary. Now we have high pressure and non zero velocity: to go back to the string analogy, seems like this would correspond to point of where it is plucked. So lets ask the guitar players to answer this. Not talking fret work which would be like moving one wall inward. Just the effect at strumming at various points w/o respect to location over sound board.

If anything, a potential criticism of the half wall placement is an increase in tangential mode reinforcement. But these are already fairly well down on the scale. If i had to summarize my thoughts, inchoate as they may be at this time, any time you add energy to a room, it will take time to decay--would you rather have the decay be of one flavor (frequency) or attempt to spread it out, so while the Joules are still there, at least less monotonic. Then of course you can try like mad to transduce it into heat with bass traps.

Now, if you're into my thinking on the matter, one should first always determine the frequencies that like a pendulum will tend to elude dissapitave efforts and hence reduce the amplitude before they are unleashed into such a reinforcing environment. This then reduces the need to first, in a very inefficient process, transduce electrical into mechanical energy and back again finally ending up as heat. Like inject a little beforehand knowledge into the equation. This mitigates against entropic effects, or at least i have been been led to believe. Can't answer whether the interplay between thermodynamics and info science has been worked out, but looms large in our future.

BTW I was one of the first purchasers of Dr, Hsu's early swoof's back in 1993-4, made and makes a good product, and have utmost respect for his idas. But with a nine foot ceiling i'm not losing sleep over nodes outside the passband. With two LA's of a total of 20 6.5" woofers facing me right in the eye, methinks this is the least of my worries.

Certainly not slamming traps--they are ugly and generally way overpriced, for the real ones that is, but do have function: now I do have an opinion re these little microwedges: definitely fall under you get what you pay for or LESS, maybe MUCH LESS. If nothing else try sourcing some OEM products of whatever density and sculpt.

Right now I am trying to find the right closed cell, density of 6-8 lbs/ft^3 from which to make baffles, but along the way have discovered there is an injurious markup on open cell acoustic foam!
J

jermmd · « **Reply #23 on:** 5 Jan 2005, 03:18 am »

Youngho,

I have seen the paper but I'm not sure how to apply it to my room. My set up is a finished basement with a very irregular shape-basically opening into another room to the rear and left of the listening position. I am using only acoustic foam for room treatments and have another sub I could use for experimentation purposes. It is a Hsu VTF-2 and not nearly the equal of the Maestro but perhaps it will serve for these purposes. I really don't know for sure why I have a 40hz dip but it is surely a room problem.

Joe M.

denverdoc · « **Reply #24 on:** 5 Jan 2005, 04:27 am »

Joe,

My advice is this: if possible at least drive them with the same total gain measured by some mike at your seat, and make sure using a d battery that they are wired in phase! Then experiment with placement--after all this is about tuning your room to your tastes--many people like huge humps that flex the windowpanes! My goal is o find a visually inoffensive, low cost solution to reproducing 14 to 60 Hz at 120 dB or above, which on command will blow out the windows, while precisely, and with finesse/articulation repro some really good jazz CD/s at venue volumes.
J
" and you know two subs are better than one"
J. Mitchell

youngho · « **Reply #25 on:** 5 Jan 2005, 04:41 am »

denverdoc: er...i think i lost you there. i think you're making a case for equalization, which i completely agree with. nevertheless, addressing resonance issues with absorption will still benefit acoustics by addressing the problems of decay that you mention. rather than tube traps, i might consider the mondo traps from realtraps. they're quite nice looking, certainly better than the tube traps. the helmholtz resonator seems as though it would be a great, if narrow-band, solution, too.

can you imagine, though, having a pair of infinite baffle subwoofers, one located in the middle of the floor and the other in the middle of the ceiling? holy cow!!!

jermmd: you're probably right that a second sub wouldn't benefit you in this scenario. however, it seems to me that a 40 Hz dip would imply that you're sitting in a node for a 40 Hz resonance--are there two parallel and opposite walls in the basement that are, say, 14 feet apart? and are you sitting in the middle, between them? if so, that problem might be solvable.

alternatively, it seems to me that the harman paper implies that you could theoretically measure the distance from the subwoofer to your listening position, then place a second subwoofer exactly on the other side of your listening position at the same distance away. again, this assumes that you're sitting in a node.

but i may be mixed up, too. incidentally, if you decide to get rid of the hsu, please let me know! i'm trying to help a friend set up a budget system.

young-ho

denverdoc · « **Reply #26 on:** 5 Jan 2005, 05:30 am »

Youngho,
points taken and a tough negotiater you are -if anyone w/in 50 mi of denver loan me some tubers, i will submit to the site FR curves before and after!
j

ctviggen · « **Reply #27 on:** 5 Jan 2005, 01:37 pm »

Hey Joe,

My realtraps are on stands. I bought two Mondo traps and can bring them over so you can see what, if anything, they can do. Actually, I could bring all six of my traps over, as they are all on stands. Alternatively, you could bring your HSU here, and I could put my SVS one one side of the room and your HSU on the other side. I'm trying to buy one of Rupesh's largers, but I don't think this deal is ever going to go down.

ctviggen · « **Reply #28 on:** 5 Jan 2005, 01:48 pm »

Oh yeah, Joe,

I'd like to come over and see if I could localize your sub. I can easily localize mine, even after messing around with crossover and amplitude and phase (although I really need help to do phase properly). If I moved it to the side wall, it might be harder to localize, but I'm waiting for another sub to have stereo subs (I really can't put one or both on the side walls, so I'd rather have stereo subs to spread the sound and minimize localization).

jermmd · « **Reply #29 on:** 5 Jan 2005, 03:01 pm »

Bob,

Come over anytime. Night is OK. Give me a call. I would love to see what impact the real traps have on my system. You're welcome to try my Hsu sub as I don't need it any longer. At some point I'm going to sell all my extra equipment. I'm just too lazy to get started.

Joe M.

ctviggen · « **Reply #30 on:** 5 Jan 2005, 03:17 pm »

I hear you! I have two satellite receivers that have lying around doing nothing. I need to get on the ball and sell them. I did sell my Linn CC and Linn speaker cables, though. This week would be hard (well, darn near impossible with the snow/ice/rain we're to get), but I'll PM you about next week.

denverdoc · « **Reply #31 on:** 5 Jan 2005, 03:42 pm »

Guys, if possible post any measurements here so we have data to consider and not just a lot of hot air--my own included!
J

denverdoc · « **Reply #32 on:** 5 Jan 2005, 03:52 pm »

Gentlemen, please post any measurements made so we have some real data for consideration and not just hot air--my own included. I will likely have my IB drivers installed and running within a couple of weeks as well. Given a Vd of 13Liters (as in 3 plus gallons of displacement), I expect 120 dB to be accessible down into the teens--all for a little wood and 800 USD in drivers. To think I almost bought SVS B-4 or the like! Great subs but the performance/cost ratio can't even approach IB's, nor are they likely as tuneful and articulate--this goes for pretty much any box system, and not an attack on SVS per se.
J

jermmd · « **Reply #33 on:** 5 Jan 2005, 04:09 pm »

George Zybar is coming over one of these days with his Tact. We'll take measurements then and post them.

Joe M.

ctviggen · « **Reply #34 on:** 5 Jan 2005, 04:21 pm »

We should coordinate, if possible, to perform measurements with one/two subs, no sub, and with/without traps. Maybe next weekend (of Jan. 15-16)? I should have my ETF program operating by then, and should be able to post some graphs.