Well,
John quantified reference levels pretty well, and Buzz expressed one oppinion of those levels. What we haven't addressed is the key word here..."reference." Referenced to what?
There is a worthwhile article at Dolby's website which those interested should dig up regarding playback levels. As John points out, we are calibrating the actual sound level at the listening position. When this calibration is done with a common signal, in this case our -30dB pink noise test, different systems will play back the same recording at roughly the same level. This now allows the recording engineer to know what absolute level recorded signals will produce, not just relative levels as is the case with CD.
Our upcoming guest Greg Miller of Gold-Line would be a good person to bring this topic up to as they undoubtedly looked heavily into this issue in making their latest audio test DVD. The question this does bring up in my own mind is how this limits instantaneous dynamic peak levels within the recording process? There are also things to be considered in the test signal of pink noise and how that does and does not relate to possible dynamic levels at any given frequency.
To briefly address the recurring hearing loss preachers, such absolute sounding statements would preclude us from ever going to a live concert of nearly any type. Similarly, leaving your home in the morning opens you to a myriad of bodily harm, but life is quite booring when watched from inside. Actual damage to hearing is not only dependent on level and duration, but also frequency. Our brain and ears have some rather amazing capabilities to filter and compress sound which we are exposed to much in the same way our eye's iris adjusts to the amount of ambient light.
Finally, most systems will be straining in some form to cleanly reproduce these reference levels in the absolute sense. Remember your SPL meter generally can't distinguish between distorted or clean output, just the cumulative sound pressure level. A system which can accurately reproduce these levels compared to a very similar system which is limited will be percieved rather differently at these reference levels.
Mark Seaton
Sound Physics Labs, Inc.
ServoDrive