FWIW, below roughly 300-500 Hz, your room dominates the frequency response of your sub. Imagine your room in 1 dimension -- it's like a guitar strings natural vibrational modes. No matter where you stick your stimulus, you *will* stimulate the fundamental and harmonics of the string, and depending on where your ear is, you'll hear very, very different levels of the fundamental and harmonics. At the center of the string, you'll hear very strong fundamental. Near the nut, you'll hear a bunch of stuff. Except a room is 3-dimensional, and you get these 'modes' in all 3 dimensions.
With only 1 sub, it's quite impossible to get consistent frequency response through the whole room. You may notice that when you stand, or sit, or move around, the bass response changes dramatically. This has nothing to do with your sub. It does have something to do with the placement of the sub. Imagine the guitar string analogy: if you pluck the string at dead center, you get a nice pure tone. This is NOT what you want in a speaker -- you don't want to hear the room ring, you want to hear the 'pluck' (i.e. your speaker). Also, if you place it at 1/4 or 3/4, you hear a pretty pure mixture of the fundamental+2nd harmonic. When you pluck near either end of the string, you hear much less of the natural string tones, and more 'pluck'. Extend this idea a bit, and pluck in multiple places at the same time, you can get relatively consistent sound throughout the string. So, in order to get truly great bass response through a large listening area you *must* use multiple subs, and room EQ. If you only have 1 listening spot (or 2 very close together), you can use a single sub, but you still need EQ. How many subwoofers is enough? In principal you need quite a few. In reality, 3-4 tends to get the job done, if care is taken in the setup and EQ. Just as room EQ is mandatory below 300-500 Hz, it's utterly hopeless above 500 Hz. As the room goes from acting like a resonance chamber (below this critical band), to a transmission line/waveguide, it's totally hopeless to do room EQ. There is no possible way that I know of, with a stereo or 7.1 system, you can EQ a wide listening area at, say 1kHz -- 1 kHz is 1 foot wavelength, which means that you can at best EQ about a 1/2 foot diameter sphere. At 20 kHz this goes to a sphere less than 1/2 inch! As for the difference between IIR and FIR, a little background may be helpful. IIR filters are generally based on the analog prototype filters, like butterworth, chebychev, etc. They are always minimum phase filters, which means that the big impulse in the frequency response comes first. FIR filters are usually thought of as *symmetric* FIR filters, which are *linear* phase, constand delay. That means the waveform is minimally distorted, and it makes it easy to add filters together and manage the phase response. FIR filters can happily be truncated butterworth or chebychev minimum phase filters -- you just make them non-symmetrical. This is easy to do. However -- to do a proper EQ of a speaker, the speaker has a 2nd or 4th order rolloff -- and to EQ that properly for a crossover, you need to use an IIR prototype filter. If you do it with an FIR filter, you'll end up making a truncated IIR filter without even knowing it, because you need to compensate for the IIR-ness of the speaker itself. Another interesting tidbit: studies have been done determining the audiblility of minimum phase, vs. linear phase filters. IIRC, neither is minimal audibility. In order to make the minimally audible filter, you need something close to IIR, but not quite totally minimum phase. But I may be remembering this one wrong. I can't put my finger on the paper right now. For information on the subwoofer placement and EQ question, this is a great paper that describes the issues and runs simulations: http://www.aes.org/e-lib/browse.cfm?elib=11355. You can also check out "Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms" by floyd Toole, chapter 4.3 "Domestic rooms and controls rooms". -Caleb -- ccrome2 Caleb Crome Sr. Hardware Engineer Logitech SMBU (i.e. the Squeezebox people) <B>The future is here. It's just not widely distributed yet.</B> <I>-William Gibson</I> ------------------------------------------------------------------------ ccrome2's Profile: http://forums.slimdevices.com/member.php?userid=18023 View this thread: http://forums.slimdevices.com/showthread.php?t=67022 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
