The Short Answer: No
The processing will not compress and lower the volume as the event goes on. But there are plenty of other things that could contribute to that perception.
Tops vs. Subs: The Power Imbalance
People tend to want 6 to 12dB more SPL at subwoofer frequencies than at tweeter or midrange frequencies. Every octave lower in frequency requires 4x the power to produce the same SPL.
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, your subwoofers will begin limiting before your tops do. The usual result is that the subs limit for self-preservation while the tops can continue to get louder — and the perception is that the subs lose impact.
Thermal Compression
As power passes through a voice coil, its temperature 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 through the coil is reduced.
The safe extent of thermal compression tends to be below 2dB, so the limiting is set to prevent temperature increases that would cause more than 2dB of thermal compression.
Other Factors That Create the Impression of Volume Loss
- Increased people in the room, who soak up sound
- Adrenaline — you're less sensitive when you're excited
- Increased demand on the electrical system from ACs, refrigerators, lights, or ice machines
- Temporary threshold shift — your ears get used to the increased stimulation and become numb to it
How to Verify What's Really Happening
Use a Z-weighted SPL measurement. Z-weighting measures SPL from 8Hz and above — you need a Z-weighted meter in order to measure the output of subwoofers accurately.
