I do this manually when watching TV at night, and I’m sure others do too. Half the time the TV goes on I find myself compelled to turn the subwoofer up or down. I do it by ear, and it depends on the content, but usually within around a 6 dB range.
In the room I do most serious listening in I use a DEQ2496, a nice, cheap, Swiss army knife (or maybe spork) DSP with pro audio features. I’m only using it as a DAC and to EQ and delay for phase matching the subwoofer, although it is capable of other tricks. I have it set up to adjust the low response based on volume, and thought I would share the settings.
These settings use the dynamic equalizer (DEG) to track the volume, and progressively boost the low end by up to 10dB when things get quiet. There was nothing scientific about these settings. At first I kind of eyeballed the Fletcher-Munson curves, and then ended up using my ears and picking 10dB as a nice, round, safe number.
There are many caveats:
- There’s no compensation for changes in loudness as I move around the room.
- Settings depend on the relative volume of the equipment downstream, such as speaker sensitivity.
- Loudness is only approximated by the average recent dB. There’s a delay in changing the EQ when dynamics change.
- A song with soft and loud sections will have inconsistent EQ.
More generally, this is an impossible problem. Recordings of instruments played quietly do not sound like music played loudly but with the volume turned down. You could easily tell the difference between a piano played pianissimo and one played forte if the sound pressure levels were matched. The sound system has no idea what the original recorded volume was, or the intended distance to convey from the source. The process of recording and mastering sound already took its best guess at how to take loudness contours into account, and that step can’t be inverted.
Having said that, this compensation does seem to work in the sense of doing what I tried to make it to do. But I have found I usually prefer it off. I’m conceptually bothered when it is on, because I keep wondering how it might be coloring things at this moment, and I can only satisfy that itch by running over to the little DEQ2496 screen to see, which ruins my listening experience.