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Post by jstrouth on Oct 24, 2010 14:59:31 GMT -7
Hi, I'm in the design phase of a mancave listening room (HT too), that you can see I'm building for as optimum acoustics as possible - the 'Golden Trapagon', optimum average dimensional ratio, etc. The attached shows the basic design. Speaker positioning alternatives (using Vandersteen's method) shown also. The volume of the room is ~ 3350 cubic ft. I love movies, but my primary interest is music. I'm especially interested in IB to augment the rest of my already good system. My speakers (Ref 3a Grand Veena) don't plumb the depths as I'd like. - they're down 3db at about 36hz or so, and down over 14db at 20hz. The rest of the system is very good as well....BAT amps, Bryston DAC, VPI turntable, etc. As far as IB goes, I'm still learning, but the Tempest X2 15" seems like a good choice. I like the relatively low QTS, musical accuracy being paramount to me. Certainly open to better choices, though. I'm thinking - and asking for advice - on a stereo setup with Tempests per channel. First question, is that enough? I'm thinking about an 'outie' install, with the sub positioned outside the mains, approximately even with their front baffle to maintain (hopefully!) phase alignment. I'm considering a triangular 'teepee' sort of box (similar to the angled manifolds I've seen in the cult threads, but projecting into the room). That would seem to provide some benefits of array with strength to prevent flexing. At an approx 17" length per side to house the speaker (facing one front, one rear), and a 90 deg angle at the top, that would make the opening into the floor ~ 17x24 ... plenty large I'm guessing (??). Also, is it better to have both drivers mounted the same, or opposing. Again, I'm looking for best, most accurate performance. One of my quandries is how to interface an IB with the rest of the system. The BAT VK52-se preamp has no secondary output. I'm hesisitant to insert anything into the existing chain that might change the excellent sound. I don't know what the effects of splitting the preamp's output might be (but I presume that would mean I'd need a filter or crossover to the sub - I'd still run full range to the main speakers). I want to add great bass, not change anything about the existing sound, which is excellent. Any and all thoughts are welcome. That about gets me started. More and more questions later, I'm sure. Great 'cult'!!! Thanks. Attachments:
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Post by ThomasW on Oct 24, 2010 18:51:31 GMT -7
Hi,
If the goal is optimal performance use a crossover, let the IB take the workload off the 7" midwoofers. There's a significant increase in both definition and clarity when the mains are not run fullrange.
Take a look at the tube version of the Marchand XM44 unless you're aware of something better.
It's always best to mount the drivers in opposition.
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Post by jstrouth on Oct 25, 2010 5:57:29 GMT -7
Thanks for the quick replies.
Thomas, I get you on the crossover - agreed.
What about drivers and number of them? 2 Tempests per channel sufficient? If not, can 2 18" (which one) per channel do the job?
Is my idea about the triangular outie a sound one (forgive the pun)? One firing rearward that big a problem? Wouldn't that be pretty much like a dipole (I used to have electrostatics)? Would the 45 degree angle be of any consequence?
It is important to me to retain the phase alignment of the mains - thus having them about equal to the bottom of the front baffle. I don't want to lose the cohesive, one-speaker sound I have now. Would manifolds in the same position do that? If underfloor, how to 'optimize'?
Thanks again!
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Post by ThomasW on Oct 25, 2010 7:40:14 GMT -7
Wavelengths in the bassband where an IB sub operates are so long, driver placement on a differing faces of a manifold don't matter.
I'd use 4-15"s, mounted as pairs firing in opposition in 2 manifolds.
I wouldn't use outies built close to the mains, that has the potential to screw things up by placing a reflective surface (boundary) near the midwoofers. My suggestion would be to use a pair of underfloor manifolds, one mounted directly in front of or off to the side of each main speaker.
For reference, phase alignment with the mains can only be adjusted (and occur) at the crossover frequency. And at this point you don't have any equipment that provides phase adjustment. I don't know if Phil (Marchand) can design a plug-in phase card for the XM44. So you should ask....
Understand that accurate phase adjustment requires the use of an oscilloscope to measure the sub and mains...
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Post by jstrouth on Oct 25, 2010 8:26:53 GMT -7
To clarify, I've added the IB position next to the main placement alternatives. My idea is one firing forward, one rearward, mounted on a triangular 'teepee' sort of setup. The rear firing unit will fire toward the 'super chunk' corner bass trap as shown in the drawing, which I'd hope (??) would help with undesirable consequences. I appreciate all your comments. Attachments:
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Post by jstrouth on Oct 25, 2010 8:50:28 GMT -7
Thanks Thomas,
I see your point about interference with the mains. I'd wondered about that...hence my idea about the relatively low 'teepee'. You're probably right about going with manifolds. Placement the same as shown in my last post? or would slightly in front and to the side be better?
What would be optimum floor opening..I know that's discussed but I didn't really get the translation to a rectangular opening.
Do you think the DIYAudio Tempest is the best choice for me?
Face f1200 TS a good amp choice?
Am I making too big a deal about phase alignment?
Don't really have ocilloscope access or know what to do with it if I did...that''s over my head (as is much of this stuff!). But it would be nice to make sure everything was right. Is that straightforward for some electronics type?
thanks again
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Post by ThomasW on Oct 25, 2010 10:56:03 GMT -7
Since they're not labeled I don't know what the objects in the drawing are.
The Tempest-X is the best choice if you want a lower Q driver. I don't know whether or not Kevin is set up to offer reconing should something happen to a driver.
Info about determining floor opening size (including a chart) is discussed in FAQ #15
The Face amp is fine, it's a little fussy about driving low impedance loads running bridged or in stereo.
Some people are more sensitive to phase than others. I say listen to what you build, if you hear a problem we'll solve it after the fact.
Measuring phase is straight forward if you know how to operate an oscilloscope, and have a measurement mic/phantom power source/mic preamp. If these things are foreign to you then measuring phase will be a bit problematic.
FWIW there are freeware programs that turn a PC into an O'scope.
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Post by jstrouth on Oct 25, 2010 11:54:07 GMT -7
Its great to get the benefit of the vast experience of others! Amended drawing shows main speaker location alternatives (Vandersteen method), and potential location for IB manifold opening (16" x 12" seems plenty). Seems like putting forward and to the side makes sense? And help maintain phase? Also, the Marchand 44 crossover also has a delay module. Wouldn't that do the trick for phase adjustment? I THINK I want lower Q drivers, since I'm primarily focused on music and want accuracy. Am I correct in my thinking? Again, Thanks so much. Attachments:
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Post by chrisbee on Oct 25, 2010 12:50:50 GMT -7
The problem with underfloor manifolds is their tendency to strongly excite the fundamental room modes. Raising the drivers only excites the much weaker harmonic modes. This is one of the reasons why vertical (but probably not low horizontal) arrays are superior to compact manifolds.
According to your plans you have two, empty, asymmetric, cheese-wedged enclosures behind your side walls. Assuming ~16' total length x ~9' high x 2' base length that's about 144cu.ft each. This is enough for two X15s each side with a total Vas multiplier of 6.5 per side.
If you stuff these spaces with fibreglass you will fool the drivers into thinking the volumes are even larger. Using these volumes will have the advantage of allowing you to place the drivers at any point along the walls to match your speakers using stand-off, vertical, "outie" manifolds.
Going back to Thomas' conjecture that driver spacing is irrelevant to an IB subwoofer I did some simple tests:
Effect of 24dB/octave crossover at nominal 100dB and 90dB IB output settings at 80Hz. Actual measurements using REW sinewave generator and Galaxy CM-140 SPL meter set to 100dB range (C) Slow. System volume remained untouched between stepped frequency settings in REW. These are actual SPL readings confirming that the crossover slope is representative of the manufacturer's claims.
Designed to show that a subwoofer is far from silent above a nominal 80Hz crossover point. Nor are speakers silent below 80Hz.
Also, that commonly reproduced subwoofer wavelengths are not excessively long in relation to typical IB driver topology.
F________Hz______W/L______Loss______dB____dB
F1_______80______14'_______0dB______100___90
F2 ______160______7'_______24dB______86____77
F3_______240______4.7'_____36dB______63____55
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Post by jstrouth on Oct 25, 2010 13:26:43 GMT -7
Well, it seems our experts difffer. Chrisbee advocating wall array (not sure what 'stand off outie' is), Thomas opting for floor manifolds. How do I decide?? HELP!!
Why is it that floor manifold will excite primary modes so much, and wouldn't the room shape and dimensional ratios mitigate this substantially?
Chrisbee, I don't follow your Vas multiplier calc...and I'm sure its me not getting it.
I get: Vas of Tempest = 255L, 144 cubic feet of space between walls= 4077L, 4077/255 = 7.99. What am I doing wrong?
Chrisbee, I don't get the last half of your post at all - is there any advice to me intended there?
You really have to talk down to a newbie like me!
Thanks ... once again.
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Post by ThomasW on Oct 25, 2010 17:40:09 GMT -7
Not to worry, a lot of the stuff he posts confuses me too....  The goal is to excite room modes, and excite as many and as much as possible (just ask Tom Nousaine (see his article in Vol 0 of the new publication Linear Audio for more on this) .....  |
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Post by chrisbee on Oct 25, 2010 22:46:29 GMT -7
en.wikipedia.org/wiki/Room_modesMy suggestion was for two, wall-mounted, "outie" manifolds. i.e. Literally hanging from the side walls. Since you have your speakers well away from the far wall such an arrangement makes more sense (to me) than making holes in the floor. Why follow the crowd when your main aim is SQ? Since the reaction forces will be easily resisted by the wall structure you could have the drivers two high and directly facing you. The front baffles of the manifolds could even be angled inwards to face the hot seat if desired. Some sub owners suggest that having the cones firing forwards provides more attack and kick. They routinely remove bottom plates and point the base and drivers of down-firing subs directly at the listener. My idea is easily tested for efficacy using REW and a pair of conventional subs on any suitable supports.
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Post by ThomasW on Oct 26, 2010 6:40:49 GMT -7
Ok now I too am confused......
You start off with statements about driver placement and room modes, then segue to posting data about the roll-off rates of a crossover. I'm a bit slow, so how about you connecting the dots for me?
As far as Wikipedia is concerned I'd certainly never use it as a definitive source of information.
'Kick/attack/punch' whatever people want to call it, is a function of 'overtones/harmonics' and frequencies much higher those than a subwoofer's passband.
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Post by chrisbee on Oct 26, 2010 9:54:37 GMT -7
Being able to see the whites of the dust cap's eyes is likely to offer marginally greater upper bass exposure for the listener. More so than allowing the upper bass to meander out of a manifold like smoke from a chimney on a still day. The chart was just a reminder that frequencies up 300Hz are still at very audible levels when listening to music at realistic levels with an IB. Assuming a fairly standard 80Hz 24/dB/Octave crossover. I played 80Hz at 100dB and measured the effect of stepping up in frequency without altering anything else. Then did the same for 90dB @ 80Hz. A gentler crossover slope than 24dB/octave or a higher crossover point would have produced even higher frequencies and therefore even shorter wavelengths. I was merely trying to correct your assertion that the wavelengths emanating from an IB are always too long to affect driver location or orientation. I don't agree with this. Where the IB builder is seeking the highest possible SQ one should at least look at all the potential alternatives. One may criticise Wikipedia but it always remains completely open to correction by somebody more expert. Nobody seems to have done so and therefore the content must match most reader's opinions. |
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Post by ThomasW on Oct 28, 2010 6:32:00 GMT -7
I don't have the time to type up a proper response to Chris' post as we're packing and moving into a vacation home in the Colo Mtns
The quick and dirty answer is simple sine waves aren't an adequate test method for drawing the conclusions about driver placement.
I'll post more in a few days when I have more time and a fast internet connection
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Post by jman on Oct 28, 2010 13:04:44 GMT -7
Sine waves are great for seeing what sine waves would sound like ;D
Real music/movie sound is better simulated using pink noise and quick sweeps.
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Post by chrisbee on Oct 28, 2010 13:34:11 GMT -7
I was going to mention this but it seemed a little like the mice playing while the cat was away. ;D On the other hand... one would assume that it is even easier to recognise source location and boxy colouration using pink noise than from sinewaves. But we won't go there. |
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Post by maxserg on Oct 28, 2010 20:37:12 GMT -7
Point source even in the verry low frequencies?  |
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Post by ThomasW on Oct 29, 2010 8:29:14 GMT -7
Chris' idea isn't workable for a variety of reasons.
First, as already mentioned music is made up of complex patterns of frequencies not sine waves. So sine wave testing isn't an accurate representation of what's heard when music is being played.
Second, yes sound is heard below the filter's hinge point when listening to the roll-off. Things is as one side of the filter is rolling -off the other side is getting louder.
Third, and probably most important is a psycho-acoustic characteristic of human hearing. When listening to identical frequencies one louder than the other, the brain will always focus on the loudest source. So one will hear higher frequencies coming from the main speakers rather than the attenuating high frequencies from the sub.
Regarding the idea that there's some audible benefit to a sub where the drivers are 'visible' rather than manifold mounted. The only scenario I can imagine where this would be true is one where the mains were woefully small, and a hole exists in the frequency response between the sub and mains.
It's best to have main speakers with adequate output a full octave below the crossover point. This situation minimizes any potential 'hole' in the frequency response between the mains and the sub.
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