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It makes no difference whether it is music or a finely-tuned machine, like a HD...a dB is a dB.
It does. A machine at 85 db will output continuous noise at that level affecting the ear in exactly the same frequencies for a continuous period - like a jack hammer on the hearing it is specifically working certain nerve bundles and hairs.
Music (especially classical) will have much more dynamic range - so you get a lot of soft passages mixed with louder ones that may reach 85 db only on the louder passages.
The key with hearing is exposure. Giving your ears a break is very important. That break can be either several soft classical passages or an album of laid back acoustic music played in the middle of a session or harder and louder rock.
Variety is important - 4 hours of continuous Metallica played loud is much worse than an eclectic mix with some balads thrown in.
Does your system provide a non-fatiguing listening experience? Do your ears ring afterward in a quiet/noiseless place? How far from the speakers is your listening position?
I'll go out on a limb and say that if your system is not very resolving and causes fatigue after less than an hour into the listening session, and you continue to listen - then yes this level is not good for your hearing (it's probably not good for your hearing even if a system is non-fatiguing - 'causing one to listen longer - but at least it's perceptively enjoyable).
I think it depends on the peaks, not the mean. I have read several articles on how to set up PA sound systems so the music is very loud but no one gets hearing damage. Read. Many things have been written in audiophyle mags on how the brain hears the average spl, but the ears hear the the peaks but tehy do not relay this info to the brain, so you can get fooled by music that is very uncompressed and has a high peak to mean ratio. Most pop/rock recordings are so compressed you won't have problem. I do have some home recorded rock music that is not compressed, and it will leave your ears ringing, even at levels that don't seem to loud.
What matters is the peaks that make up the average level. I suspect the levels you use are OK. I assume you know dB(C) is spectrally pretty flat, whereas dB(A) reflects auditory sensitivity.
I don't understand what 6550c may have read. Acoustic energy impinges on the eardrum, is passed through the middle ear mechanism to the inner ear where displacement along the basilar membrane cause neurons to fire, such firing runs up the auditory nerve, and is represented in the auditory cortex. This, of course, is a simplistic characterization of the process. IIRC, at lower levels than the auditory cortex nerve activity between the two ears is compared for timing information. The hair cells along the basilar membrane can be damaged by excessive noise exposure, and various substances.
There are all kinds of ways to justify excessive levels. Although peaks do matter, just limiting them is not a recipe for safe levels. Loud is loud. It is a matter of the total exposure: pressure level, length of time, frequency of exposure, and spectral content.
What I am saying is the BRAIN only THINKS it hears the mean sound pressure level. The peaks give you NO sensation (until you turn off the music - you go deaf without knowing it!!) . Pick up any pro sound book (and put away the ten dollar words) or numerous Audio Magazines- it is well documented. Why do you think you can goto a Van Halen concert and not come home stone deaf? The quys that set up concerts know this, or else they would be sued for damages. You can listen to 110 db and be okay as long as there are no 150db peaks. High peaks not only waste enery, but they also leave people deaf.
As a grad student in the early 60s I worked at a research center set up to study the effects of noise on man. The work was funded by NIH grants. As a post doc I worked on binaural processing. I have not worked in psychoacoustics for more than 50 years, nor have I read the literature (JASA, etc.). Perhaps the auditory system has evolved since then or acoustic tricks have been invented that are beyond my time.
Although I have not claimed expertise in this field for at least 40 years, IIRC the auditory system pretty much responds to the acoustic events incident upon it. There are limits to temporal resolution, but we perceive acoustic events more or less as they occur.
By "ten dollar words" do you mean basilar membrane and hair cell, the mechanical precursors to the neural firings that are transmitted to the auditory cortex and perceived as sound