1. So, this wouldn't apply to headphones.
2. Who only listens at 70dB?
With headphones the ambient tone would be lower, so the THD threshold would be a little lower... perhaps around .5dB. Pro grade digital equalizers aren't that expensive and they don't add anywhere near 1dB of distortion. Most modern electronics perform more in the range of .1%. THD which is well below audibility. But remember the headphones or speakers themselves add distortion at levels considerably higher than electronics. It's not likely you'd even hear the threshold unless your heaphones or speakers were remarkably clean.
70dB is a pretty typical medium loud listening volume. People sometimes listen louder, but not that much louder. Comfortable normal listening level is between 45 and 60dB, depending on how compressed the music is.
Just to clarify this, I believe bigshot's quoting RMS (i.e., the typical average) listening levels in the 45--70dB SPL range while riverlethe seems to think that this figure is low. I think the confusion stems from the forum's discussions on target
maximum SPL people desire from their headphone+amp combinations to be capable or reaching,
but certainly not for extended listening, and the typical number quoted for that is 110dB (ish) SPL. (Here, SPL = Sound Pressure Level and is given in dB referenced to the typical human threshold of hearing around 20
microPascals). There is a big, big, big difference between peak sound pressure levels and the typical sound pressure levels. If a recording is particularly dynamic, the peak SPLs could be +20 dB or more than the rest of the recording. One of my favorite examples of this is in King Crimson's "Lizard":
At the beginning of the piece is "Prince Rupert Awakes," a part of which is shown in the figure. Jon Anderson provides guest vocals, which are pretty quiet. For example, from t = 5 seconds to 10 seconds (highlighted in green), the RMS level is 0.0057 of full scale, which is -44.8 dB. The peak level during this section is 0.0258, or -31.8 dB.
After Jon finishes his verse, Andy McCulloch's drums come pounding in (see the bit highlighted in red), which is a huge dynamic change. Here, the RMS levels during this section are 0.0537 (-25.4 dB) while the peaks of the drums reach 0.3573 (-8.9 dB) (and even exceed it in the blue part immediately after the red!). Overall, the recording ranges from RMS levels of -44.8 dB to a peak value of -1.9 dB.
That's a 42.9 dB range in the recording between the RMS value of the nominally quite part to the loudest drum strike. If you were listening to the piece on your system, you'd probably have typical volume parts around 60dB SPL or so (like the red bit in the figure above), which puts the quite parts around 40dB SPL. Meanwhile, your system will stlil have to reproduce the peak transients that would be at 83dB SPL.
Perhaps you're like me, and you really like the piece and want to hear Jon Anderson (Yes!) sing with Fripp and friends, so you turn the thing up until the quiet bit is medium-loud (say 60 dB SPL), then the typical parts of the music are a loud 80 dB SPL and the peak transients are over 100 dB SPL.
So, hopefully this little example helps to illustrate the difference between the average (RMS) listening loudness versus the peak transient loudness levels. To sum it up, bigshot's number of 45--60dB SPL is certainly in the ball park of listening levels. Meanwhile, the 110 dB SPL levels that seem to get quoted all over these message boards are for
Peak SPL levels and one certainly should never listen to their music at 110 dB RMS SPL levels under any circumstances (unless they hate their hearing!). There can be a big difference between the RMS loudness of a piece of music and the peak level. Conversely, it is also possible that the RMS and peak loudesses are very close to one another ( equality holds in the case of a square wave, or metallica's
"death magnetic" \end{snark})
Cheers