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Subwoofer Delay in a Two-Channel Setup.....


SWL

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I would hope that some pro-audio/live sound guys here would understand where I'm coming from.

Colter?? phillipbarret??? Confused

Subs are unpredictable beast. So often, what works well in one situation completely fails in another. Often these situations look surprisingly similar so why such divergent results?

My most succesfull use of subs is in a mono cluster, center placed on the floor. Obviously this is not always physically possible but when I can get away with it, this delivers the most consistent LF throughout the venue. My reasoning is, that the closer to a point source you can make the LF, the less cancellations you will cause between sources. I truly believe that the ear cannot really determine direction below 100Hz (I disagree with the 250Hz theory) so the loss of stereo is irrelevent. Most "pro" system subs are run mono, my only recollection of stereo subs was the Showco Prism system which used a frequency dependent filtering system to achieve a very even coverage.

If I move to left/right sub clusters & flown clusters I can get great results but they become very location dependent. The wavefronts in reinforcement are impressive, the ones in cancellation leave a huge hole. Moving a few feet changes everything so it's hard to deliver solid LF to every seat.

As for the time-alignment, in a typical PA rig the subs are physically ahead of the main system by virtue of floor placement against flown cabinets. I have never really been able to accurately measure this relationship as the conditions in the typical venue render measurement at those frequencies, well nigh useless. However, I have experimented with adding delay (very easily done while walking the room with aDolby Lake processor) and have certainly found that I can move the perceived LF power around a little using this technique. However, the effect is subtle at best & of course is highly frequency dependent, what works well at 40Hz can fall down at 80Hz as the wavelengths change. As usual with sound reinforcement, you go for the best compromise.

You have the advantage that you know where you want your subs to sound best plus you've found a delay setting that works for you. I think the Behringer DCX2496 crossover would be the best value for your needs. They run around $250 and are designed specifically for this. I would only use it on your subs, leaving the mid/hi outputs unused, plus you can use it to equalize some of the inevitable room anomolies out with a high degree of accuracy.

As for your question about speaker delay, you're only looking to delay the subs relative to the speakers. As long as the subs were "early" (which according to your previous set up they were) then you don't need to touch the mains. Delaying the mains at 10ms and the subs at 13ms is no different than zero timing the mains & running the subs at 3ms.

However, without starting a flame war, below 100Hz, a single mono sub source may solve all your needs for free. What you are hearing is the cancellation of your stereo subs at 10 feet & a combining at 5 feet, this may be caused by the room dimensions but is much more likely the subs working against each other. You are merely attempting to move that combining further out into the room but by doing so you are building a solution that is good at ONE frequency only, if 42Hz combines at 10ms (for example), 63Hz will not due to the significantly differing wavelengths. If you used a single LF source the problem would not be present in the 1st place.

Did I bore everyone completely rigid here?

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I thank SWL (whomever and where ever you are) for letting me know about this topic.

I am no phillipbarret nor colter; I am a rank amateur here. However, I offer the following based solely on my hearing, my main room, and my "rig".

Main room, roughly 15-17' x 40' x 14' (sloping ceiling). Combo of brick and wood walls, old leakiy french doors, bookcases and "stuff" galore. Main room includes kitchen, dining room, den, etc. Must be fairly weird sonically.

Someone from another av forum had suggested some large pro audio type subs to deal with the roughly 9000 cu ft room and "match"my Khorn fronts, LaScala rears, and Academy center (my pre and peri-Katrina rig). In late '07 the 2 dual 18" driver subs arrived. Each cab was 2 x 2 x 4 feet in size. Across the wall were: KH (small space), sub, audio rack, 26" TV, sub, (a second small space gap), and KH. I never even considerd subs before Katrina - "I have KHs, what more bass could you ask for". Boy was I wrong.

Once I was done fooling around with all of the surround settings, I noted large accoustic differences between running 1 or both subs. I had thought about trying the full range output 2 Ch idea, but never did it. Perhaps I should have tried it. Eventually I moved the left sub towards the middle of the room, only a bit behind the couch, along the left wall. The small furnace closet was behind it. Thus it sat in a small corner. Sound was a bit different, but still pleasing. I think I always have used an 80 Hz LFE preamp/processor bypass for the subs. My pre/pro has 2 sub out mono RCA jacks. I use 1 for each sub. My sub amps also have settings to dial in the cutoff point as desired.

I bought 2 single 18" driver Behringer 1800X subs last fall. They are old, but when stacked and placed next to the left sub, they are actually louder. Due to continuous post Katrina (de)struction, I have been unable to use them at the back of the room to even out the bass. The bass, perhaps due to cancellation, is very weak in the rear or kitchen area. I also guessed that I might end up with only 1 by the kitchen (sort of like the way Dr. Hsu recommends using his smaller sub at the back of large rooms) and then could use 1 in the bedroom, and thus have unequal sort of stereo subs (RSW-15 is in the bedroom now).

Why all of the subs? (I cannot assess the degree of any noise cancellation in my main room. Every month I have to rearrange the place, thus changing where "stuff" is too.) Despite what many write, I CAN tell where the subs are. Perhaps this is due to them handling frequencies above some certain low Hz point, or maybe some ?harmonics or rattling from the sub cabinets? Could also be from some higher frequncy rattling of adjacent brick a brack, chatchkies, etc.

Perhaps at an open air concert or in a venue above a certain size, given large numbers of bass drivers in a couple or more clusters, the aural experience or wavefronts act differently than in smaller spaces like rooms at home.

Just my 2 cents

I would also like to learn more about differences as to when the tweeter, squaker, and woofer/horn sounds reach a desired spot, eg-the couch. This stuff was mentioned in a prior post, but I did not understand it. I thought sound travelled through air at the same speed - regardless of frequency. Are these timings different becase of the distance the air pressure "wave" has to travel from the diaphragm location? If so, the mid and tweeter are only a foot or so (?1-2ms) different; no idea how to calcluate that in the folden horn woofer.

:)

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I haven't been to a live show in quite awhile until this past weekend. I was reminded yet again what I'm trying to recreate in my living room. An audiophile, or a purist listener I am not......but rather prefer the sound of hi-fi or the likes of a live band playing in my living room.

I've learned quite a bit over the last couple of years about what kind of listener I am. Started out with the whole HT thing and that came and went. Not getting any younger....I've settled into a zone where I have my system dialed in to where I can listen in the mid 90db range and enjoy the dynamics as if it was much louder without damaging my hearing. Although, brief 110+ db listening sessions are not uncommon from time to time. [;)]

A good live PA system is so impressive when done right. Even normal people, unlike us, are so moved at a live show playing their favor music. Even my wife last weekend while we were listening to Beatles band at a local festival....."I can feel it hitting my chest". How cool is that? There's a lot of two-channel guys that don't beleive in using subs....and that's cool....but when you're after the sound I described above I beleive they are a "must" but should be used sparingly. It seems like live shows these days are much more sophisticated technically speaking......the live show I was at last weekend.....I wanted to go home and get my recliner. I could have sat there all day.

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I would also like to learn more about differences as to when the tweeter, squaker, and woofer/horn sounds reach a desired spot, eg-the couch. This stuff was mentioned in a prior post, but I did not understand it. I thought sound travelled through air at the same speed - regardless of frequency. Are these timings different becase of the distance the air pressure "wave" has to travel from the diaphragm location? If so, the mid and tweeter are only a foot or so (?1-2ms) different; no idea how to calcluate that in the folden horn woofer.

The speed of sound in air varies slightly due to humidity & pressure differences. But you are correct, the timing differences you are refering to are related to the physical location of the drivers. This can be addressed by either mechanical or electronic alignment. As you say, mechanical is difficult where horn loaded lows are concerned, made more problematic by the fact that cone drivers produce different frequencies at different parts of the cone, the larger the driver, the more pronounced these effects can be.

Back when I toured with Noah we would physically align the cabinets using the trusted "by ear" method to achieve the "best" results. Nowadays it's so much easier and more accurate to align using electronics & an analyzer (of course you can use an analyzer to do mechanical alignment too but room resonances are going to make it hard at the low end). Although, you could argue that the mechanical method is better because it uses less electronic degredation in the signal path.

You would be a candidate for the cardiod/directional subwoofer trick though, now that's an interesting path. I don't want to become a bore here (too late probably) so if you're interested in knowing more, let me know in a follow up posting.

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......the live show I was at last weekend.....I wanted to go home and get my recliner. I could have sat there all day.

Did you tell the sound guy that? It would have made his day. Most of the time we hear complaints:

1) "it's too loud" closely followed by...

2) "crank it up dude" then...

3) "do you know what all these knobs do?" and finally...

4) "I mix sound in my church, how many watts do you have here?"

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Did you tell the sound guy that? It would have made his day

Yes, I did. It was in a huge tent.....aprox. 150 to 200 feet in diameter about 40 feet tall?

Although very reflective with lots of echoes going on in the back......up front in the "sweet spot" was awesome. All EV speakers.

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OK, here goes. Sorry Amy I'm taking up a lot of forum space here. I think I'll break this up into 4 posts so hang on to your horses for a mo'

Using the Meyer MAPP prediction software, I built a 20' long, 12' high room and placed in it, various combinations of Meyer 700HP, front loaded, double 18s. The type of sub is not critical to the measurements, these are merely used to illustrate the points. All measurments are done at 63Hz with all room treatments turned OFF.

1) Single subwoofer on the floor. Notice the smooth, almost omnidirectional coverage, red is loudest, graduating down to blue for quietest.

1xstacked.png

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Now for the party tricks, or how to create directional LF with super-even coverage!

4) 2 subs stacked on the floor. The left (front) sub is placed 2 feet behind the right (rear) sub. The signal polarity is reversed to the front (sub is running "out of phase") and the front sub is delayed by 3.75ms (2 feet), combining it theoretically with the rear.

I've never tried this at home (don't have subs) but the approach would seem to be very workable in a larger listening room. Obviously the spacing is not critical, the time alignment just has to match. If you want the pattern to face the other way, time align the rear sub not the front.

Picture1-15.png

edit : for clarity

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Wow, a 1000 words is indeed worth a picture.

That seems to be in keeping with the idea of cancellation, etc. However, if the summation does occur where we lie as couch potatoes or while eating dinner it might be nice indeed, even if the bass (or other frequencies) get cancelled out elsewhere-bad for parties.

I take it you would disagree with the following scenario (full range 2CH stereo):

turntable / radio / CD player, L/R out to variable gain tube amp (allows some control of volume), "Y" split output signal - for each L or R run 1 interconnect to stereo amp (again maybe with gain control to control volume, like my MC240s) and run KHs/LSs or Heresys full range in true stereo; the other interconnect to sub amp (1 per side) and use sub amp controller to x out frequencies above some point or other, say 60 (lg KH spkr) to 120 Hz (for smaller, say Hersesy or Icon series spkr). This technique would allow for relatively clean signal path to the spkrs and allow full range sound reproduction. Probs: where to set cutoff for subs, to avoid duplicating main spkr abilities and the afore mentioned prob of sound cancellation.

Mr P B, (noob Q) is sound cancellation just a problem in low freq, or is it a real and true problem acrss the 20-20,000 Hz range? I ask this, because I don't remember anyone complaining about cancellation when running regular spkrs full range - but without any subs.

Also, with older recordings (vinyl, reel to reel, 1940s to around ?mid 1960s), the tracks were laid down in an era before subs and with low power output tube amps. Therefore, most end users (folks at home) were using spkrs akin to the heresy. Thus, I would guess that the audio engineers of the day laid down the record or RTR tracks with that in mind. I have always been told to listen to my older records in plain old stereo, without subs, because of this. (But Barry White and that voice demand maximal bass). What are the thoughts of forum members in this regard?

Mr P B / Colter, do you think my ability to tell directionality is due to sub cabinet vibration or adjacent brick a brack vibrating at higher than bass frequencies?

Also please tell us about your thought on the cardiod or directional sub trick. :)

In our local audio club, we have a few speaker and tube amp builders, along with Mr Gray (of the Power Company). One of the guys (hangs out on the Bottlehead amp forum, but not here) also builds single driver, near full range units. He also built a 15" driver 3 1/2 ft long by 24" wide cylinder sub. It is beyond awesome in his L shaped main room. In fact that 1 sub in his room is even a contender for "umph" compared to my dualie 18"s. Mind you, our rooms are completely different and neither space nor adding an addl sub is an issue for me-quite unlike the limitations most folks have. He also uses bass shakers mounted onto his chairs/couch. The overall bass effect is incredable. Maybe the bass shakers of some type or other (plus amps, etc) will be my late 09 or winter 2010 project.

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I take it you would disagree with the following scenario (full range 2CH stereo):

Not all, I assume by "variable gain tube amp" you mean a preamp? My only concern would be loading down the outputs of the preamp. As for the crossover frequency, your ears should be able to tell you that. Subs stop really being subs above 100Hz, for home I would look at a 12db/octave crossover at around 60Hz which should blend in nicely with Heritage speakers run full range. Live I tend towards an Linkus R at 24db/octave but then I'm also high pass filtering the mains.

Mr P B, (noob Q) is sound cancellation just a problem in low freq, or is it a real and true problem acrss the 20-20,000 Hz range?

Happens at all frequencies but the wavelengths become so short that the time differences become minimalized. At 40Hz the wavefronts are at nearly 30 feet, by 1KHz they drop to around a foot and by 12KHz it's an inch or so. As you can see, mechanical time alignment becomes much trickier at higher frequencies.

I have always been told to listen to my older records in plain old stereo, without subs, because of this.

Old wives tale. Back in the days of vinyl, low end had to be controlled and mono'd in order for the cutter heads to create a groove that could be played back without the stylus jumping out of the groove (or the cutter burying itself in the acetate). There's plenty of low information on older records, either intentionally or unitentionally. If the subs make you happy leave them on, Bonham's 24" kick has all kinds of wacky stuff down there.

Mr P B / Colter, do you think my ability to tell directionality is due to sub cabinet vibration or adjacent brick a brack vibrating at higher than bass frequencies?

Also please tell us about your thought on the cardiod or directional sub trick. :)

Your hearing most of the directionality at higher frequencies. Put a 40Hz tone in there, eliminate the rattles (generally not possible of course) then try to locate. As for the cardiod subs; the MAPP predictions are not making this up, they really work if you have the room, processing & the $ to spend double the money (2 subs with 2 amps). I've set up big walls of subs in this fashion & watched stage hands walking in front & behind scratching their heads, it's that dramatic. Obviously room artifacts will come into play pretty significantly in the home but your area is pretty huge.

In fact that 1 sub in his room is even a contender for "umph" compared to my dualie 18"s.

Becuase his one sub is propagating per my 1st & 2nd predictions & yours are cancelling per the 3rd. It really is that simple. Further proof below.

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Once again, WOW

In my case front and right side (long) walls are brick and lots of bookcases etc, floor is uneven / undulating stone with a bit of carpeting, high ceiling is sloped with thick wooden beams and sort of a weird cardboard/thin plywood between the beams (c.1969), lt side is a mess-french doors, bookcases, furnace closet etc, rear is the kitchen with cabinets, appliances, etc. No idea how to model that. However, as stated, your ears will figure it out. As per placing subs, with babies this big, placement is not easy. In my case, I simply plunked them down in convenient spaces and hoped for the best. Seems fine on the den carpet and couch but whole rear of the place seems like a dead zone - likely cancellation I guess.

Also, I never messed with horizontal vs vertical placement or rear or wall aiming of the drivers. I know some subbs fire towards the floor or closest wall. I had a major issue with cages for the Madison Amp 18" drivers being to weak for their weight. Thus I was afraid to reorient them off vertical. For no specific reason I have the RT sub facing the couch and the mid left sub faces towards the front and short brick wall and TV, about 45 degrees off from the couch and about 30 degrees off from the carpet center in the den.

I will have to look up the cardiod stuff later on. :))

BTW are you thinking of coming to Indy in 2 weeks. Be cool to meet.

Your friend NOLA in the Big Easy - my feet are dry, the crawfish are boiling, Jazz in the street - life is good.

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This is the beautifully even coverage achieved by placing your low end in a corner, imagine that! The yellow lines are canceling nodes, there should be one but I probably had a sub an inch or so off in placement. In a perfect world it would be an infinitly narrow plane exactly between the speakers.

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