If you cut at 120Hz, I bet they'll change. IMHO, no more than 85Hz, and according to Dolby (and me, I think that's a bit low) no more than 55Hz.
but maybe I'm reading you wrong--your baffle construction would be of interest. I would love to bring a couple of CDs and an LP or two, to see what your setup is about.
The idea is to avoid using any electronics or even crossovers in the largest part of the audio band at the speaker level. As an analog system, everything you add to the signal path (even the direction, length and type of cabling used) makes a number of differences, some desired, some NOT. A crossover is a nightmare, because they alter the phase of the signal differently at different frequencies. Attaining the highest resolution in reproduction is only possible with the simplest possible system. A high quality analog source, a simple SET tube amp (mine has only eight solder joints in the signal path), a single driver for the entire audio spectrum would be ideal, but room-filling full range drivers are still a problem. They do not exist, so compromises are made.
I have actually used about four hundred dollars worth of copper wire, winding coils for this set up. I have several pairs of coils that I have tried. As you know, outside of enormous electrostatic systems which are incredibly inefficient, there is no such thing as a full range driver. One of my original goals was to build a single driver, point source speaker system, two for stereo, obviously. After trying several horn designs, some of them published on the internet, which I copied or bought plans for, (Ron's Horns (which, google) are excellent, BTW), I decided that the free breathing, open back designs were the only types of designs which were capable of the clarity I was looking for.
I have used Fostex, Tang Band, Lowther, JBL, Electro Voice coaxial, Altec coaxial and Visaton drivers, all of which are mostly considered the best of the best for this approach. The efficiency of the Altec drivers is hard to beat, but they are not smooth in response above two thousand Hertz and the low frequency drivers are extremely low Q. Same story for the EV. The Fostex are probably second best, very efficient, ultra flat, but not much on the low end, so my low frequency drivers would have to be crossed over at a higher frequency, and the Fostex drivers do not take much power and have relatively short excursion. They belong in horns.
The Visaton B200 (eight inch full range) are the single best, flattest, adequately efficient, extremely musical drivers I have yet to hear, even better than the Lowthers which cost three times as much. Unfortunately they also roll off (though gradually, smoothly, predictably at six dB per octave) at about one hundred Hertz, in open air.
So my goal, with the present state of technology, is still unattainable. I can not have a single point source full range driver, without using a horn. If you know much about horns, you understand that they must be huge to work well. There are compromise, high compression horns which are reduced in size and are capable of fairly flat response, but at the high cost of loss of waveform linearity. It is impossible to balance the degree of compression, evenly across the audio spectrum on opposing sides of the driver.
I have also tried other approaches. The most successful was an open baffle design where the driver sits above a Hemholtz chamber to enhance low frequency response. The resonator was fed merely by a slot exposed to the extreme pressure directly at the edge of the driver, kind of like blowing across a bottle top. While this could NEVER be accepted in the open air camp, being just another compromise, it still allowed the driver to operate, "unrestrictedly," in open air. I still have one of these in my garage, but I honestly have not put power to it in about five years. I would call that project back-burnered, but I have not completely given up the concept.
I have given up on horns, BTW, at least until I have enough space to do it RIGHT.
The most musical compromise that I have yet heard is to use a pair of large drivers to re-enforce the low frequencies, with a simple coil to roll them off near the frequency the "full rangers" peter out. Fortunately, the ability of human hearing to recognise phase differences is largely to do with a difference in percentage of the waveform for each frequency along with the distance from the sound source. In other words, at high frequencies which have very small wavelengths, a very slight physical offset between two drivers reproducing the same frequency simultaneously is audible and disturbing. At the lowest frequencies, which have long wavelengths the physical offset between drivers can be several inches at one hundred hertz before it becomes audible to us deaf creatures. So, using a secondary system to enhance low frequencies, closely positioned to the full range driver, is not as nasty of a compromise as a high compression horn would be and miles ahead of using a complex, steep cut-off crossover network.
BTW I also have used a single JBL driver, extremely efficient, very flat, amazing high frequency response for a fifteen inch driver, but unfortunately, quite directional above about one thousand Hertz and a tweeter would still be necessary (using a simple capacitor for a crossover). This means that the sweet spot will be even smaller. Not good.
Also, if I had not specified, this is not my surround system. This is my two channel stereo I am talking about. As far as Dolby's recommendations, I assume you are referring to movie soundtrack reproduction. I am still using a 5.1 system for movies. If you were talking about crossing over the LFE channel at a certain frequency, I have balanced mine with my fronts using an external EQ, pink noise and an RTA. I think the crossover setting on my LFE channel (which itself is bi-amped and equalised, quite complicated) is around sixty something Hertz. That entire system is so electronic that I could hardly call it a hi-res system. It's awesome for movie soundtracks, though!
