So your sweet guitar tone sounded like crap when you got to the concert. Or maybe band practice was awesome... but things sounded really bad at the gig. Why does my stereo sound so bad when I turn it up really loud? Or down really quiet?
The answer: it's your ears.
No, I'm not saying "it's a personal problem." You are normal. But, your ears have a built-in equalizer... and the EQ settings are different depending on how loud the noise is. Scientists have studied this, and the result is called the "Equal Loudness Curve:"
If you're like me, you're wondering, "huh?" Each line is an "equal loudness line." The bottom line is the threshold of hearing — stuff we can just barely hear. The top line is dangerously painful sound levels. In the middle (around 1000 Hz, about a high C) we hear sounds the easiest. In comparison, the bass sounds have to work harder and move the air more harshly. Likewise with some of the higher frequencies.
But even more interesting is that the curves are shaped differently depending on how loud it is. This means that if you change the volume, the balance between hi/lo/mid will change.
Ok, time for a science experiment. Get your MP3 player or iPod and sit down with a "large" song (like Steppenwolf's "Born to be Wild"), and do this:
- Turn it down until you can barely hear it. This is the bottom line on the chart. What do you hear? You can hear mostly the cymbols. A little bit of guitar and vocals. Probably can't hear the bass guitar or any other drums.
- Turn it up a little so that you can hear it clearly, but still quiet (about 20%). What do you hear? You can probably hear the vocals, guitars, keyboards, cymbals, snare, some of the drums all clearly. Hardly any bass guitar... or tone.
- Now turn it up so that it sounds "full" (probably about 50%). You can probably hear everything clearly... but the stuff in the middle (like the cymbals) are starting to get annoyingly loud.
- Turn it up more... so that it's "a little loud." Chances are, this is the first time you heard a decent bass guitar sound. Notice how all the other volumes have changed dramatically... but not the bass?
- Turn it up really, really loud. Now, you probably can't stand it because the drums, guitar, and keyboards are overdriven.
This has a couple implications::
- Loudness is subjective. That's why the developed the A-weighting for decibel meters, because this reflects the way our ears work. In contrast, the C-weighting has a flat frequency response and is useful for scientific measurements.
- You have to change your EQ settings when you change the volume. You can't set the tone knobs in your room and then turn it up for the gig. It will sound different.
- Get some kind of media player with an equalizer (5 or more bands, e.g. WinAmp or Amarok).
- Set all your EQ's to 0.
- Set the volume to about 50% and get a feel for how it sounds (even if it's not good).
- Now set the volume to be about half as loud. Change the EQ settings until it sounds about the same as it did at 50%. If you succeed, chances are that your EQ looks like the chart above. You had to push up the bass... but the rest didn't need much adjustment.
- Now set it even lower and try again. You probably had to turn down the mids and trebles to make it sound like the bass was louder. (Kind of like the curve above.)
- Now turn it up pretty loud (75%) and see what you have to do. For me, it ended up looking a little like a line that gently sloped downward toward the high frequencies.
 Source: http://en.wikipedia.org/wiki/Equal-loudness_contour
 If you can, set all your EQ settings to 0.
 See http://en.wikipedia.org/wiki/A-weighting
 See also the last post, which discussed loudness.