Audyssey Pro EQ Listening System - Part 1
In our living room/home theater layout I run an acoustic suspension Infinity Intermezzo 1.2 1", 850-watt sub in the front corner of our living room. The sub sits just behind the equipment rack that will hold our 56" Samsung LED DLP video display. Using Infinity's R.A.B.O.S. CD measurement system, in-room response of the 1.2 measures -1 dB @ 20 Hz with ±1.5dB frequency response up to 80Hz. Back at the center-of-couch listening position the 1.2 can hit 108dB without breaking a sweat.
Listening area; left surrounds at top left above fan blade. Listening sofa at bottome left. Three Beta 10 LCRs and Infinity Intermezzo 1.2 tucked behind equipment rack in corner. On right, close-up of Intermezzo 1.2 in front corner behind equipment rack.
A second sub, an acoustic suspension Infinity CSW-10 was one of my designs and originally part of the Infinity Beta series. The CSW-10 is a 25 lb. magnet structure, 10", 650-watter, also with an on-board R.A.B.O.S. single-band parametric equalizer. The CSW-10 is only 6' behind our sofa but 8' away from the two rear walls because a 90°- bend, two-section staircase to our downstairs is built into that corner. That leaves the CSW-10 working much harder then the Intermezzo 1.2 because the smaller 10-incher doesn't have the luxury of free sidewall or corner bass augmentation. (And yes, for those who may be wondering, the still available Revel SP12 is essentially the same subwoofer.)
If I calibrate the Infinity CSW-10's R.A.B.O.S. parametric in it's open floor position six feet behind our sofa, the sub will chart down to -3dB @ 27Hz and -6dB @ 24Hz, (at 105dB and ±1.5dB to 80 Hz). These are exactly the lower roll-off points and max SPL I had measured in Harman's large 4 pi anechoic chamber when I was designing it. (In the chamber the frequency response was much flatter at ±0.5dB.)
Now plug the two subs together by using a Y-connector from the receiver's subwoofer output and bingo, 6dB more headroom than with a single subwoofer and smoother in-room response, right? Well, not so fast. Once I plugged the two subs together using the Y-connector to the Denon AVR-3806's single subwoofer output I re-plotted the two subwoofers' frequency response to see how the two subs reacted together at the listening position. Horrors! The interaction of the two subs together raised the 3dB down point to 33Hz and the frequency response variation envelope expanded to ±2.5dB up to 80Hz.
The reason for this problem was that the Infinity subs only have 0° or -180° phase controls. And the CSW-10's position, back 75% into our room would require its compatible (at the listening position) phase setting ( relative to the Intermezzo 1.2) somewhere in between the 0° and 180° choices. When trying to use the Denon AVR-3806's internal MultEQ XT along with the Infinity subs' 1/20th octave frequency dial-in capability the phase differential problem could not be resolved with enough accuracy. Thus, to apply a sufficiently accurate frequency-phase data package for each sub, as measured from the listening position, requires two completely separate packages. (The Denon AVR-3806 can't measure the subs separately as it has only one dedicated and assigned subwoofer channel.)
This interaction of two different, but very capable subwoofers points out just how absolutely intertwined, because of wavelength, half-wavelength and every portion of wavelength in between both the room and asymmetrically placed subwoofers become. At the listening position each mono, electrically correlated bass note will arrive in a slightly differing positive or negative phase, ready for a fight. Remember the rest of Forest Gump's "Life is like a box of chocolates" line?
The upfront corner mounted Intermezzo 1.2, for instance, would generally be assumed to be at 0° starting phase because of its position in the front system corner. This sub is almost in line physically and therefore should be pretty close phase-wise with the front 5.25" two-way L/C/R-s. Plus the L/C/Rs' close proximity to the corner walls augment their low bass performance. Thus when read by the mic at the primary listening position the Beta10s might appear to be able to go down past 80Hz (-6dB) with ease. My thinking then was that the often tough 80Hz subwoofer-to-satellites splice should be a pretty easy blend for either the Audyssey MultEQ XT or MultEQ Pro programs.
The problem comes when the second CSW-10 is added into the mix. If you're talking about any room over 2000 cubic feet of enclosed volume, the potential ~6dB extra SPL headroom gained with a second subwoofer will always be needed to get to realistic SPL on low bass program peaks. The larger your room and the lower you want to go into the first 20Hz to 40Hz bass octave the more headroom (subwoofers) will be required. The Robinson and Dadson equal loudness curves below illustrate this.
To attain a rather loud 85dB at 1kHz you'll see that to match the subs to the satellites you need 27dB of subwoofer gain at 31.5Hz! Since the auto set-up routines for Dolby Digital EX and DTS Neo 6 already boost sub levels +10dB we still need 17dB of below-80Hz gain just to get to "equal loudness". That's one reason why two subs is always so much better a solution than a single larger or more powerful sub. And since we're talking about achieving the smoothest, widest bandwidth possible over the largest acoustic bubble listening area I would consider two high quality/high acoustic output capability subwoofers the absolute minimum to wring out the best bass performance from Audyssey's Sound Equalizer and most listening environments.
Listening Position Seats Matter: Keep Listener heads in the clear
The Audyssey technologies are computer programs which must depend on the skill of the System Designer and the Electronics Tech to chose proper equipment and install, position and calibrate the room/components as a unified system. Even the seats are part of the system so do yourself and your clients a favor and stay away from the high backed models that block the ears' ability to hear direct surround information.