Here's how Audyssey does it:
NOTE: this was an old setup, when I first fired up my THT in the middle of the room....
Here's the sub output with Audyssey OFF:
Here's the sub output with Audyssey ON:
Here's both together (relative levels):
You can see the rolloff of signal from the electronics below 10Hz in each of the images.
You can also see what Audyssey does is the following:
1. It places cuts at any peaks you may have in your response.
2. It doesn't make any adjustments below a certain frequency (about 20-22Hz in my case).
3. It then BOOSTS the ENTIRE signal to acheive equal loudness between the Audyssey 'ON' and Audyssey 'OFF' settings. It does so in this case by about 10dB....
This is SIGNIFICANT, it is 10x the power!!! In the frequency band where subwoofers do poorest!! I don't know of many subwoofers that have 10dB worth of headroom below 20Hz. This is why I needed a 2nd highpass (Reckhorn B-1) in addition to my amp's built in one, to keep those subsonics from generating sonic distortion....
I have shown these images to Chris Kyriakis of Audyssey, and he agrees, it is a glitch, but that most people (not listening anywhere near reference) may not notice the effect, and I agree. Unfortunately, any updates to the Audyssey algorithm will go unseen by us, as our receivers do not do firmware upgrades easily.....sucks.
But I put up with this 'boost' because of the EQ Audyssey provides my mains/center/surrounds, and sub. The difference between Audyssey on and off when a sound effect is panning across several channels is night and day....
So, be careful at first. set that 18Hz highpass to ON when you first listen, and if it is OFF, approach high volumes carefully.