GregM,
Listening fatigue is usually associated with some type of distortion in the reproduced sound, especially in the midrange and treble. It can certainly result from loudspeakers with a "peaky" response in the midrange (lots of peaks and dips), which can add an irritating "edge" or "stridency" to mid and treble instruments or vocals. Horn-loaded loudspeakers are especially prone to this because of the distortion of the mid and upper spectrum that the horn introduces.
Amplifiers driven to near their limits may have dramatically rising distortion of several kinds (IM and THD--Intermodulation distortion and Total Harmonic Distortion) which can be fatiguing over relatively brief listening sessions. Using music or soundtracks as test material, the distortion has to rise to fairly gross levels--past 1% or more--before our ears start to complain. With test signals, it's possible to detect distortion at a fraction of 1%.
Badly EQ'd sound, whether on the source recording or in live performance, can also induce listening fatigue. As a matter of fact, I attended an off-broadway musical yesterday with a friend. The show featured three women with excellent voices singing in tight harmony, and the women's vocals were horribly EQ'd so they sounded really nasal and irritating. Adding to that, the sound operator was running the system at absurdly loud levels (the theater wasn't large, seating about 400). Certainly overly loud music that has gross response anomalies like this will induce listening fatigue really fast--and it did!
My friend and I lasted 40 minutes. I was getting a headache and she agreed it was way too loud so we snuck out during a blackout between songs. (There was no intermission, so we had to escape.) I'm thinking of sending the sound designer a letter of complaint. I've heard superb sound reinforcement in New York shows as well as really awful work and this show numbers among the worst I've heard.
I've also found that mood shifts, alcohol (other substances as well) lack of sleep and other influences may influence the onset and duration of listening fatigue.
During years of serving on the listening panel at the National Research Council in Ottawa, we generally found that about six 20-minute sessions (listening to four different speakers each session) spread over a work day were about the limit. That was because the sessions required intense concentration with the same sequence of music selections repeated each session.
Regards,
