I had asked about this a year or so ago (under a different pseudonym).
One would need a Real Time Analyzer (RTA) to most accurately predict the effects on us humans. As the expert stated, frequency, amplitude and time (decay, harmonics, etc.) are all necessary to quantify how a sound ... um ... sounds.
Unfortunately this would also require the drive to be by itself in an anechoic chamber (acoustically dead enviroment), and the measurements should be taken at different angles relative to the drive (at a proper distance). You *could* take a single measurement, but other sounds from say the bottom of the drive could mask (soften) the piercing quality of a certain sound.
Other factors to consider since our drives typically live in a dirty environment:
- Resonance from the drive's enclosure is a factor. This could manifest itself as vibration, a "hum" or both.
- Metal / plastic within the enclosure
- Noise from other sources (fans, etc.) can mask the drive's noise.
I have been impressed with RTA software available for computers. I'm seeing some for PDAs that offer 1/12th-octave RTA. I thought 1/3 octave measurements were cool! ( I would think 1/3 octave measurements would be sufficient for our purposes )
All of this could be compared to the "statistical human ear" to try and define "annoying" or loud. But we're all different, so "you're mileage may vary" applies here.
[ Personally I find the noise fluorescent lights annoying, but I can also hear dog whistles while most people cannot. So acoustic experts try to define "normal" and we go with it. ]
( I should stop now, I could talk about acoustics all day. )