Can a sub be placed upside down?

No. But you'll get better results with the subs spread to smooth room modes.

The more locations employed the smoother the result.

You seem to have missed the point.

As Toole says:

"
Separating the reproduction of low frequencies is a good way to address the room resonance problems in a manner that is independent of the number of reproduction channels.
"

Do you want to do it the easiest way or the best way? BTW, the importance of the distance from the sub to listener is highly overblown. You're dealing with 14 to 40 foot wavelengths so you can't hear pathway differentials with subs the way you can with mains.

No, they won't. That would only happen if said subs were outdoors, or in a perfectly anechoic chamber. In a room of average dimension response is defined more by the relative distances between the subs, boundaries and listening position than the relative positions of the subs and listener alone. The presence of walls and a ceiling changes everything, and the more sub locations the better the result. But hey, don't believe me, I'm just an acoustical engineer.

Ahh...an acoustical engineer you are. hmm. The Force is strong with this one. Your light sabre, tuned properly is it? lol
Edited by Bond 007 - 12/1/12 at 2:19am

Not at all. I was just pointing out that you were using simple math to predict a result that's only accurate when there is one reflective surface to consider, the floor. Each additional boundary added introduces another variable, so while it is possible to mathematically predict the best placement it's not an easy calculation. Rules of thumb as posted by a number of sources, Geddes, Toole and Harman amongst others, are as good a starting point as any.

One doesn't have to be an acoustical engineer to know that this is totally incorrect. There is much you're not aware of, but since you've decided you'd rather argue than learn I'll just bow out of this thread.

A quick Google search shows there are several schools offering degrees in acoustics.

Penn State for one, has an multiple graduate degrees available.
http://www.acs.psu.edu/

Dude, you've managed to alienate one of the finest sources of information on this forum. Check out his website: http://www.billfitzmaurice.com/

And he is absolutely correct... you have much to learn, and a lot of what you say is wrong, especially what I bolded above. A little humility goes a long way.

Craig

Don't do it Yoda. You're better than that and I have abandonment issues. Just don't reply to his posts.

FYI, folks here like to discuss sound as if Fluid Dynamics and Thermodynamics, don't apply to acoustics,. Regarding some conversations, it's as if they don't exist at all. Makes for irrational conversations. Good luck.

Perhaps the issue is one of practicality. If you lose 20 percent of the energy at the boundary, that's a decibel, which will affect in-room behavior only slightly. Likely inaudibly. Certainly boundary losses are not sufficient to eliminate the peaks and nulls the everybody has in home-sized rooms.

I think what you're missing is how those reflections can interact with one another, ie constructive and destructive interference.

Some subs come with continuous (0-180) phase controls so you can adjust accordingly. You can also do the real time analyzer game and find a program that works with your microphone. Then of course, there's Anti-Mode, 8033S II which allows one to work with many subs in stereo while working in compliment to XT32.

Warning, doing what I suggest above costs money, time and one ends up with a boatload of additional cables.

I don't think anyone is really debating that. As Bill mentioned though:

Of course, if you don't want to follow those guidelines, nobody is going to put a gun to your head and force you to.

It's not about a "diffuse sound", but modal averaging to smooth the frequency response. In any case, it sounds like you've got your mind made up, and Bill already answered your original question. Good luck!

bass will never sound like an echo chamber. If bass was being absorbed significantly, you would not have peaks. While 75 dB plus 75 dB = 78 dB, 75 dB plus even onlty 70 dB equals only 76 dB. Not enough of a variance to even get concerned about. Yet pelple have 20 dB nulls and 10 dB peaks in the bass in real rooms.

If you poke around on these threads you'll find folks with very deep nulls. I mean way over 10 dB deep. Given the reality that you're never talking about just one "reflection" with bass, it'd by highly unlikely to find a place where all the reflections actually cancelled to 0 dB at some frequency. "significant" absorption or loss within the room would eliminate the very problems that people struggle with in real rooms: peaks and nulls. The fact that we can hear and measure peaks and nulls in any room indicates that they are indeed not significantly absorbed or passed through walls, floor, etc.

But really it's the chaos of the modes that occur in the bass that is indeed hard to model in real world rooms. But on the other hand it has been demonstrated pretty effectively to be most simply dealt wih by using multiple sub positions. I mean you could try to absorb all the reflections to keep the cancellations and peaks from occurring, but if you just want to go down to 30 Hz (just over a 37 foot wavelength) to be fully effective each absorber at each reflection point (actually all of every wall plus the ceiling, needs a quarter wavelength, or just over 9 feet, of absorption. Noplace left for equipment and seating in many rooms.

It's just more effective in the real world to "fix" the peaks and nulls by sort-of averaging them out and because our ears aren't using bass to identify direction or reflectiveness or anything else, in part just because our heads are too narrow versus the wavelengths involved, it's no great sin to let go of thoughts of everything being "in phase". The place where phase matters in home audio systems is the crossover range from mains/surrounds to the sub(s), because for everything to "add up" correctly they all have to be in the right relative phase. But our speakers and our subs all introduce phase delays that differ at different frequencies, so it becomes again a mantter of trial and error, preferably with measurements involved, to identify the phase change (read sub channel delay) that causes everything to splice together appropriately. But the measured and experienced level and phase of bass energy in a room, whether it comes from a sub, main speaker or somebody whacking on a bass drum, is dominated (really, dominated, that is not overstating the case at all) by the room modes,, and not by the native frequency content of the sound source.

When you place a subwoofer in a room, the room becomes the biggest factor in the reproduction of the bass. Period. Direct wave theory is useless in a room. The wavelengths are so long at bass frequencies that small distance discrepancies are immaterial. It is the modal response of the room that totally dominates. If you get an SPL meter and play a single frequency tone and walk around the room, you can measure the peaks and nulls throughout the room. These peaks and nulls are caused by constructive/destructive interference from the REFLECTED waves off the room boundaries combining, sometimes in-phase, sometimes out-of-phase. The in-phase combinations will add SPL; the out-of-phase combinations will decrease SPL.

With multiple subwoofers placed separately around the room, the peaks and nulls appear at different points in the room and they tend to cancel each other, yielding smoother response.

Here is the baseline response of one of my subwoofers measured outdoors with the mic placed on the ground one meter away from the side of the sub:



Bass frequencies propagate omnidirectionally, so that would be a measurement of the "direct wave" from the subwoofer.

Take that same subwoofer and place it in my room, and this is the response at the listening position:



Place it somewhere else and you get this:



Put 3 of them in the room, all daisy chained off one Subwoofer output, all placed randomly, at different distances to the LP, and EQ'd with Audyssey XT32, and you get this:



The *point* that Bill was trying to make to you was that, when you stack 2 of the subs, you forego an opportunity to spread them out and improve the smoothness of the response in your room. The "direct wave" is inconsequential when you're dealing with bass frequencies. The reflected sound has it's way with the direct sound and produces the final response you *HEAR*, which is the only response that matters. You can blather on about modeling and direct waves and thermodynamics all you want, but this is the only important aspect of optimizing multiple subwoofers.

With that, I will do what Bill did, and bow out of this thread.

You're welcome.

Craig
Edited by craig john - 12/4/12 at 3:07pm

a very informative post which certainly deserves a thank you .