Q: I took out my subs and as the night went on I heard it hit less and less does that mean that it will compress and lower in volume? It def wasn‚'t hitting as hard as the beginning.
The short answer to your question is no. The processing will not compress and lower the volume as the event goes on. But, like so many other things in sound systems and acoustics, there's never only a short answer... There are plenty of other things that could contribute to that perception.
The relative output of tops versus subs. If you start out with the blend of subwoofer and tops levels to your liking at lower volumes, and then increase the volume as the night goes on, it may be that your subwoofers will begin limiting before your tops do. People tend to want 6 to 12dB more SPL at subwoofer frequencies than at tweeter or midrange frequencies. Sensitivities remaining equal, every octave lower in frequency requires 4x the power to produce the same SPL. In other words, it takes 4x more power to produce 110dB at 60Hz than it does to produce 110dB at 120Hz. If it took 100 Watts to produce 110dB at 120Hz, it would take 400 Watts to produce 110dB at 60Hz and it would take 1600 Watts to produce 110dB at 30Hz.
Add to that the fact that people want or expect 6 to 12dB more SPL at subwoofer frequencies, which requires doubling the power for each 3dB, and you get to a demand for 12,800W to achieve 122dB at 30Hz in our hypothetical simulation. The usual result is that the subwoofers tend to limit themselves for self-preservation purposes long before the tops need to, and the perception is that the subs lose impact, when in fact they don't, it's just that the tops have an easier time achieving higher SPLs.
The next factor is called "thermal compression". As power is passed through a voice coil, the temperature of the coil rises. The rise in temperature causes a rise in resistance/impedance. With that increase in resistance, even though the amplifier is putting out the same voltage, the total power passing through the coil is reduced. The resulting output is therefore also reduced. The safe extent of thermal compression tends to be below 2dB, so the limiting is set to prevent temperature increases that would result in more than 2dB of thermal compression.
Another thing that could create the impression of a lowering of level is the increase in people, who tend to soak up sound.
A third factor is adrenaline. You're less sensitive when you're excited.
A fourth factor could be the increased demand on the electrical system in the building, from ACs, refrigerators, lights, or ice machines.
And finally, the most universal effect that causes a perceived reduction of level is temporary threshold shift. Your ears just get used to the increased stimulation and become numb to it.
You can verify all these effects using a Z-weighted SPL measurement. (For clarification, A-weighting measures SPL essentially above 1000Hz, C-weighting measures SPL above 100Hz, and Z (zero) weighting measures SPL from 8Hz and above. In other words, you need a Z-weighted meter in order to measure the output of subwoofers.)