OFF: Earplugs revisited

[Ken Alexander]

> They stocked several flavours [of earplugs], the main
> difference being in the decibel rating, which was between about 18 and 29.
>
> Does anyone know what frequency this is measured at?  (I know that the
> attenuation is not flat for foam earplugs, i.e. they cut out more sound at
> higher frequencies.)

It is computed by a mysterious formula, possibly detailed in ANSI S3.19-1974.
You can have fun trying to deduce it from the frequency distributions
listed on two different types that I happen to have here:

type 1: foam cylinder.  computed noise reduction rating: 29 dB

freq in Hz       125   250   500   1000  2000  3150  4000  6300  8000
dB attenuation   37.4  40.9  44.8  43.8  36.3  41.9  42.6  46.1  47.3

type 2: wrapped cotton bullets.  computed NRR: 26 dB

freq in Hz       125   250   500   1000  2000  3150  4000  6300  8000
dB attenuation   25.4  26.6  29.5  31.9  35.3  37.9  37.8  39.1  41.8

> So what rating do folks use?  The various brands on
> offer seemed to indicate that the maximum rating was 30.

I think that means "common maximum", not "mathematical absolute maximum".
The kind I sleep with, green foam bullets that I don't have a freq breakdown
for, boast a 31 rating.  To add an additional wrinkle, something used to be
different about the computation, because the NRR on a ten-year-old sheet
from a pack of type #1s above gives the same freq breakdown but a 35 NRR!