Subwoofer placement?

If your crossover is 100 Hz or lower and you can tell where the subwoofer is located, then something else is going on like buzzing or rattling from a nearby window. As for the "best" place, the only way to know for sure is to measure the low frequency response as you experiment using suitable software.

--Ethan

While Ethan's right about measurement yielding a true optimum location, if you give up a little on the optimization, you can get close without it.

The classic method is the "sub crawl" where you put the sub in your seat and crawl around to find where the bass is best, then trade places.

Next is the "modal analysis" method where you lay out the room locations for nodes of the first few room modes, then select a location that minimizes energy transfer to the room at the problem frequency. That's the kind of thing you've done here - front wall at the 20% or 40% points misses the three lowest modes (nodes at corner, 50% and 33%)

That said, Ethan's also right about localization below 100Hz. My sub's in the same location, and it's not localized at all, so I suggest feeding it some bass to see if it has a rattle or buzz. I'd use a frequency generator like that in REW.
http://www.hometheatershack.com/roomeq/

If you can "hear" 20Hz, you're likely hearing harmonics...

Have fun,
Frank

Them are fighting words if posted in the DIY speaker section........

IME that is a generally true statement, but mostly due to the poor THD performance of virtually every commercial subwoofer.

Reality is that most bass instruments including huge pipe organs and the like put out more harmonics than fundamentals for fundamental frequencies below 100 Hz. If you look at the Fletcher Munson curves you find that the threshold of hearing at 20 Hz is about 75 dB SPL. Bottom line the music is likely to mask the fundamental, your ears are sorta recalcitrant to perceive it, and most subwoofers aren't a heck of a lot of help. On balance, a system that pumps 110 dB @ 20 Hz can be a real treat, and probably with more different program material than you expected.

yes, as long as the operator understands the results will not necessarily match the calculators due to complex acoustical impedance of the boundaries (versus that of a concrete bunker).

in a typical studded wall, there may be modal behavior exhibited in conjunction with the adjacent rooms as well..

Yes, and this is often overlooked. It's also counter intuitive because triangle and square waves etc have the most energy at the fundamental with less and less at higher frequencies. But many musical instruments have resonating bodies that don't support the lowest fundamental. For example, my cello has a fundamental resonance around 95 Hz, where the lowest note on the C string is 65.4 Hz.

The attached image from my Audio Expert book shows an FFT (spectrum) of my Fender Bass playing a low A note at 55 Hz. You can see that the 2nd and 3rd harmonics at 110 Hz and 165 Hz are louder than the fundamental.

--Ethan

This can work because the ear/brain tends to perceive the fundamental when only the second harmonic is present.

http://en.wikipedia.org/wiki/Missing_fundamental

Mike, if you can hear 20Hz, more power to you, but I will wager that the sensation isn't aural if it's a pure tone through a good sub. Also Sprach Zarathustra came on a cable music channel one night, and I recognized it from the first note. I did not hear that note...

Arny, yes, THD, because in my mind, "20Hz" is a pure tone, not an instrument playing that fundamental with associated harmonics and overtones. Thus the reference to REW for its frequency generator. Can't blame a sub for reproducing harmonics in the input.

Local, that's what I'm counting on, because the complexity is a result of energy removed from the reflected wave. My room modes are very benign, the exact opposite of many folks here. I long wondered why, then studied low frequency absorption and realized my room is full of diaphragmatic absorbers - windows/glass door, sheet rock on studs, paneling on standoffs - and has a double doorway and corner cabinets that spread the (100) resonance over a wide frequency range, or perhaps forms a huge (if unoptimized) Helmholtz absorber. In this case, adjacent rooms diffuse modal behavior, reducing audibility of modes in the listening room. I can't speak to the rest of the house, but bass does follow you out of the room.

The fundamental point was that perhaps "best" isn't required, just "good enough," and if you have a forgiving room, it may not be that hard to find a combination you find acceptable, even if it's not the absolute best possible result. The 80/20 rule applies.

Have fun,
Frank
Edited by fbov - 1/11/13 at 3:02pm

In fairness to your instrument, where you pluck has a huge effect on the harmonic mix that results. Vibrating string is a classic problem in Physics, where the result is highly dependent on the "initial conditions," namely how you pluck the string. Closer to the center reduces harmonics, closer to one end favors harmonics.

Any chance you pluck the string about 1/6 of the way up? That would be where the pick-up is located... gee, that location favors 3rd harmonic, too!

Have fun,
Frank

Of course, though in this case the fundamental is present, just softer than the first few harmonics.

--Ethan

I used to think that too, until I got my killer SVS sub with twin 12s. Now I can hear 16 Hz when the level is loud enough.

Absolutely. I usually pluck over the pickup, because I rest my thumb on the edge of the pickup when I play. Plucking near the center of the string favors the fundamental. I just wanted to show that the fundamental isn't always the loudest frequency. Especially for the lowest note on a piano.

--Ethan

John Atkinson from stereophile posted a fft of his P bass playing a low E, showing fundamental about 10 dB below second harmonic. There was a long and interesting thread at talkbass showing waterfalls of lots of different basses, and until you got really exotic (man made materials, very long scale and it helps to have an extra "arm" connecting the headstock to the body in parallel to the neck), nothing had near as much energy at 40 Hz as at 80 Hz when playing a low E. Very useful info to have when you're trying to figure out what you need to appropriately amplify the instrument.

IIRC, a full sized upright (maybe even a 3/4) has much more fundamental at lower frequencies, with the fundamental at least equalling the second harmonic.

I can't say I've seen a study on it, but it stands to reason (I think) that the harder you play, the lower the relative contribution of the fundamental to overall tone (most basses are brighter when ya whack em harder) although that might all be well above H2 for lower notes . . .

Yes, and the same goes for every other acoustic sound source.

--Ethan