If you're mixing audio, you need to know about the Fletcher-Munsen curves, sometimes known as the smile curves - and if you're mastering then you really need to know about them. Luckily though, you don't need to know very much !
You can get the full technical low-down from the Wikipedia article, but in a nutshell:
Louder music sounds bassier and toppier than quiet music
Put another way, quieter music sounds thinner and duller. It's not really, it just sounds that way to our ears - they just work that way.
Maybe it's an evolutionary thing, to protect us from predators, and pay more attention to sounds that are closer and potentially more dangerous - maybe not. But it's the reason you used to see a "loudness" control on amplifiers - the idea was it added bass and treble to compensate for lower listening levels. It's one of the reasons for the loudness wars, and it's very important to bear in mind when you're making mix decisions.
The reason is - if you turn something up, it will sound a little clearer and fuller - slightly better, in most people's opinion. Now, this only works if you have plenty of headroom, which is where the loudness wars have gone so horribly wrong, but it means that if you want a fair comparison of two pieces of audio, they need to be level-matched.
It's no good comparing that last mix with the one from earlier in the evening unless you've matched the levels first, because if it's half a dB louder, it'll automatically sound better, even if it's not.
And if you do as I suggest and listen to your mixes at a variety of different levels, you'll always be tempted to add bottom and top if you're listening quietly. Don't do that...
So, level-matching is important. Luckily for us, matching levels of similar mixes is usually straightforward. Get yourself a copy of an LUFS loudness meter and measure the LUFS loudness of two mixes at the same point in the song, and set the levels to match them - then do the comparison.
Take care with things that sound more different, though - what you hear is more important than measurements, and one of the key ways we judge loudness is by listening to the vocal level. So if you're trying to compare two different tunes, or very different mixes of the same tune, make sure the vocals sound a similar level - if they don't, take the loudness meter readings with a pinch of salt.
Loudness-matching (or better, balancing) is a real art, and a key skill in mastering, but just knowing it's necessary and bearing it in mind when you're comparing different pieces of audio will always help you make better, more objective decisions.
Level-match when you listen, always !
Update - Since writing this post I've made a YouTube video demonstrating the importance of matching levels, and how to do it - to check it out, click here.
