The Musician's Room: Why Many Engineers Monitor Relatively Quietly
Why don't recording engineers crank up the monitors and "let it rip" all the time?
Let's talk about the frequency response of the human ear. The ear is designed in such a way that as loudness decreases, the ear begins to be more sensitive to midrange and less to bass and treble. At a very low level, a graph of the response curve looks something like a bell's cross-section, which allows us to hear very quiet whispers without lots of rumble and hiss. As the amplitude goes up, the curve becomes more shallow until it reaches it's "flattest" (most accurate) response around at about 85db of loudness. The easiest reference to 85db is the sound of an electric razor while shaving. This loudness level is the best range for audio monitoring, as the ear isn't being fooled by up-turned or down-turned curves.
Note that decibels are on the left and "phones" are in the center.
From 85db up, the response curve of the ear begins to invert, with bass and treble response becoming more acute than midrange. Above 110db, the ear enters third harmonic distortion and makes things sound even brighter than they are. To cap it off, psycho-acoustic level compression is pretty active up there as well. Many 1970s recordings were made and monitored at extremely high volume, and thus ended up have their EQ adjusted to have reduced bass and treble to prevent the "ice-pick-in-the-ear" effect. (Remember the slogan on the James Gang 'Rides Again' album: "Made loud to be played loud"?) If you lived through the '70s (and can remember them... Is it possible?) you'll remember that neither master volume controls nor higher-gain, lower-wattage guitar amps were readily available until late in that decade... So here's the main reason to monitor at a lower level: A product mixed at too high a level may not sound very good at home listening levels.
Another practical reason that many engineers don't just crank it and leave it is that they may be managing their exposure to high-level noise. You can just manage 85db daily for eight hours without damage (imagine an electric razor in your face for eight hours, every day!). As the level goes up, the time duration for which you can listen without damage decreases dramatically. Many engineers pop up the level enough to get an idea where things are and then duck back down to protect their ears.
In order to get an idea of what 85db feels like, you might invest the $39.95 in a basic Radio Shack sound-level meter. Many consider them the industry-standard, inexpensive sound pressure level meter.
So, think about it before you say, "Crank it up, man!"