Hi, Sorry for the late reply. It is one of the chapters of interest for me and I found very educational and interesting. That said I still have a lot of questions. I was hoping I might be able to estimate how many sub-woofers I would need by making some measurements but it seems trial and error are a large part of figuring this out. This is difficult of course because without ordering the sub-woofers I can't determine experimentally what the optimal placement and number of sub-woofers required. Suffice to say I would like to get away with 2 but will 2 be sufficient or will there be a tangible benifet in 4. The issue is cost of course and having unused stock or not enough.
I assume you have the new edition, has this chapter been rewritten to make it clearer, perhaps with new information or is it basically the same?
Greg.
Not at all!
Hopefully by now in your reading, you're coming to realization that
everything matters.
You can predict what influences a room will have on your sound, but there's too many variables to predict a specific subwoofers performance.
"At resonance frequencies, the incoming sound is delayed by multiples of half wavelengths, summation occurs, and standing waves (room modes) are created - a pattern of higher and lower sound pressure levels within the room. Seat to seat variations in bass are an issue to be dealt with." - Ch.8 'Sound Reproduction' 3rd Edition
A subwoofer is a pressure source. The placement of that sub in relation to other subs, the room boundaries, and the listening positions, determines its ability to constructively or destructively drive those standing waves.
You'll need to start with predicting the room modes with the 'back of the envelope' calculations (p203). In the second edition, you'll see a diagram with suggested placement of two subwoofers on one wall, in Ch. 13 p221. That is an example of one way you can drive standing waves/room modes by using your pressure source (subwoofer) to drive or cancel out an offending frequency, without DSP.
Because loudspeakers are minimum phase devices, a steady state frequency response measurement will suffice for determining the phase of modal frequencies at the listening position. If a predicted mode (which is often a bit higher than measured, due to flex of boundaries by the inclusion of doors and windows) is measuring as a null, that indicates the mic location is out of phase at that frequency. You would then refer back to your predictions as to whether that mode is a height, width, or length mode and position the subs accordingly to drive it (null) or cancel it out(peak). DSP
may be needed: Frequency = center of peak/null, Q= width in Hz of peak/null, Gain= dB that needs to added/subtracted to bring that frequency to the relative level.
After you've done what you can with placement and are ready to apply DSP, start with the lowest offending modes, and work your way up. You'll see with correct DSP filters, higher frequency peaks/nulls (that are unrelated to predicted modes) start to subside on their own. Some subs offer a few programmable DSP filters, like my SVS PC12's, but they only have two filters each. My open living room has a lot more modes than two/four filters can solve so I've had to incorporate a minidsp 2x4. But hopefully you've seen enough response graphs in this book that you understand that 'flat' is not the goal.
If you take the time to write all of that out, the trial and error will mostly subside. But it is a fair amount of work. Acousticians get paid a lot for a reason!
The 3rd Edition is thoroughly revised, and 'Ch.8 Below the Transition Frequency' is a bit easier to grasp than the former Ch.13 of the 2nd edition. I had to read the previous quite few times to get my head around it. But if you take the time to apply this knowledge, the process becomes more intuitive.
