FrankenMauser: Speaking of which...
Where can I find the standards and procedures for establishing the NPR rating?
I was trying to find some information last week, but couldn't find anything online.
Most of that stuff stays buried in ISO standards; sometimes you can luck into finding copies of this stuff for free online. It does take some digging. One thing that you won't find much of is pertaining to sound quality; I think that generally speaking, a single dB rating across a broad spectrum is pointless. 6dB at 20Hz is nowhere near as noticeable as 6dB at 4kHz, which is right in the middle of our peak sensitivity. A weighting corrects for this, but it still doesn't quite get it right.
Here's some of them:
ISO 1999 (1990). Acoustics – Determination of occupational noise exposure and estimation of noise-induced hearing impairment.
ISO 11904-1 (2002). Acoustics – Determination of sound emission from sound sources placed close to the ear. Part 1: Technique using a microphone in the real ear (MIRE technique).
ISO 4869-1 (1990). Acoustics – Hearing protectors – Part 1: Subjective method for the measurement of sound attenuation.
ISO 4869-2 (1994). Acoustics – Hearing protectors – Part 2: Estimation of effective A-weighted sound pressure levels when hearing protectors are worn.
ISO TR 4869-3 (1989). Acoustics – Hearing protectors – Part 3: Simplified method for the measurement of insertion loss of ear-muff type protectors for quality inspection purposes.
ISO 4869-4 (1998). Acoustics – Hearing protectors – Part 4: Measurement of effective sound pressure levels for level-dependent sound restoration ear-muffs.
ISO/TR 3352 (1974). Acoustics – Assessment of noise with respect to its effect on the intelligibility of speech.