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OK, let's not try to describe everything about the NRR in this paragraph. Suffice it to say, it is highly misunderstood and has not been the single number answer to hearing protection labelling that was envisioned back in the 1970's. It has been studied and shown that the NRR is not only not understood by most people, but is not even able to convey to the wearer much useful information. But what I really want to say is that Frequency Attenuation curves are also "misused" as a determinant of earplug effectiveness. Most earplugs have a difficult time reducing low frequencies. That has much to do with the structure of the ear and the nature of low frequencies being less "directional" in nature. The lows just seem to slip around any gaps. Their wavelength makes it hard to stop them in their tracks. So, when people look at a chart and say....these don't block the lows much, and I need more of a blocker. Well, they may be correct...but so much has been left unsaid. The source of the music (or noise) which is to be blocked, and the proximity to the person, and the intensity (SPL) and the duration have much to do with the true amount of attenuation needed in the lows. Music isn't typically overloaded in the low frequencies - it makes the ensemble sound muddy (upward spread of masking). The ear is more adept at handling low frequency amplification without discomfort and the lows tend to dissipate radially and that also helps to "get away" from the lows.
In order to reduce your risk of over-exposure, you want to reduce the average exposure over time and in some extreme cases, the maximum SPL at any given time. The overall loudness over time can best be understood by a food analogy. The SPL (loudness) can be thought of as the calories. The flavor of the food is best understood as the shape of the attenuation curve across frequency. Really sweet might mean too many highs. Too many saturated fats are too much low frequency. So, while changing the flavor (removing some sugar / highs) can impact the overall SPL, certainly reducing ANY part of the mixture will have an overall effect on the SPL (total calories). The frequencies all have a different amount of contribution to the total SPL. An earplug with a low NRR may simply mean that it was penalized based on the "outdated" formula....certainly one that did not predict either flat attenuation earplugs, or flat response earplugs (Earasers). Certainly a flat response earplug will have even more protection at 3KHz than in the low frequencies and therefore more protection at the peak ear canal resonance than flat attenuation earplugs.
The most important point though, is that you need to reduce the overall SPL for the whole duration. Frequency, Amplitude, and Time of exposure all contribute to hearing damage. The frequencies have to audible to concern us (the ultra high and low frequencies not so much) and if you study the Fletcher Munson loudness growth contours you will see that the ear is more sensitive to loud sounds at 3KHz. So, as musicians we have to accept the fact that occasionally a sugar substitute will be worth the tradeoff. Earasers are so much more comfortable than other earplugs that we dominate the "time of exposure." Being more willing to wear the earplugs for the duration is more important than any other component. We like to think that Earasers might compare to a sugar substitute, but before the end of your first gig, you won't even taste the difference and certainly your caloric intake will be under control! :)