This quote is from an article in Soundcheck Magazine called How Loud is Too Loud. I am not sure how many techs would agree with Curt Taipale but he does have an interesting take on how to mix sound levels due to the Equal Loudness Contours. In essence he says a mix should be established at 85 dB (average level I assume) and that this mix will then provide an excellent basis for either boosted or reduced master levels. Sorry if this is too technical for some of you to find interesting. (I also liked how he adjusted the EQ on softer ballads to compensate for ELC. Interesting.)
The Equal Loudness Contours reveal the average human hearing sensitivity at frequencies in the human hearing range over various listening volumes. They were determined something like this: The researchers would play a reference tone at, for example, 1 kHz, at a designated sound volume. They would then play a different frequency, adjust the volume and ask the listener to indicate when the new frequency sounded like it was at the same volume as the original frequency. They continued this process through several frequencies to come up with a "contour" of human hearing sensitivity at that one reference sound volume, say at 80 dB SPL. Next they would change to a reference volume of 85 dB, and repeat the process.
They averaged this data over several listeners to come up with their published Equal Loudness Contours. Part of what they discovered is that human hearing is most sensitive to sounds at around 3 kHz. At very soft listening levels, our hearing is least sensitive to very low frequency sounds, and slightly less sensitive to high frequency sounds. This is why there is a "Loudness" switch on your home stereo. You'll notice that when you switch it on, the sound gets a huge boost in the low frequencies, and also gets at little brighter. This is really just a special equalizer circuit tailored to counteract our hearing "deficiencies" at soft listening levels. At very loud listening levels, the contours start to flatten out, so the theory is that you would switch the Loudness circuit off when listening at elevated volumes. I know, I've turned it up loud with the loudness switch still turned on too.
Now, I figure that God didn't really build a deficiency into our hearing. I'm not really sure why it's that way. Maybe someday I will. But I do know that, when I'm trying to mix a "big" song at a soft volume, it helps the authority factor if, for example, I boost the bass guitar a little more than usual. Maybe I'll boost the kick drum slightly, or the low end piano mic. I would not go for the house system equalizer and offset it for this curve. That would cause more grief in other areas than it would help in this. But subtle adjustments to the mix of various instruments, or maybe a slight lift in the low frequency EQ on those channels, can provide a significant improvement to the sound. It won't solve everything. Louder will still sound bigger. But it may be the better compromise. If your sound system includes subwoofers, it can prove even easier to give your music that "authority".
Recording engineers have known about the equal loudness contours for years. Studies show that the average home stereo listening volume is 85 dB SPL. So the smart engineer will try to keep his mixing volume at around that volume. He will check it at very, very soft volumes, and he will turn it up loud and check it there as well. If the producer wants to hear it really loud, the really smart engineer will show the producer where the volume control is and leave the room. If this policy of mixing at 85 dB is followed, when you play it softer it should still sound fine. When it's played really loud, it should sound huge, bigger than life. If, on the other hand, the engineer mixes the songs at loud listening volumes, and then tries to listen to them at a soft volume, the resulting sound will not have the same punch. This isn't because of hearing fatigue. You'll still hear the same lackluster mix tomorrow after your ears have rested. Mixing at loud volumes alters both the EQ decisions as well as mix choices that the engineer makes. Those are based on what he's hearing at the time.