Thanks, I was just thinking about for some reason how when I have actually built the sub we spent so long discussing, how I would get the 'rumble' effect from an Earthquake DVD as the 25Hz and 35Hz tones are all the rumble actually are on the DVD although in the cinema you never heard them as they activated the control box. I just had a sudden burst of inspiration.
Basically, if theres a 25Hz tone, the rumble generator is activated and its loudness varied by the voltage. If theres a 35Hz tone, the main soundtrack is redirected to the sub, again varied by the voltage. If both are on however, then the main soundtrack and the rumble generator are turned on, but the volume of the main soundtrack is increased by up to 8dB to immerse you in the sound. This was originally called step gain by MCA.
Basically its going to be a hugh pile of bandpass filters, opticouplers as they switch faster than relays and are silent and somehow a preamplifier varied by the voltage across it when both tones are so to get the step gain.
I am going to be building the enclosure for the sub in a few weeks as I have 6 weeks off school at the moment.