Editing Earplug (section)

==== Double protection ====
The Canadian standard requires the use of two passive protective measures simultaneously at noise levels above 105 dBA. But the noise reduction does not increase very much. It is recommended to estimate the attenuation by adding 5 dB to the highest attenuation of one of the two HPDs. This recommendation does not take into account individual differences at all, and may lead to error.<ref>{{cite book |last1=Behar |first1=Alberto |last2=E.H. Berger, I.B. Bhunnoo |title=Z94.2-14. Hearing protection devices — Performance, selection, care and use |date=2014 |publisher=Canadian Standards Association (CSA Group) |location=Toronto, Ontario, Canada |isbn=978-1-77139-417-8 |pages=27–28 |edition=7th |url=https://www.csagroup.org/store/product/2701515/ |access-date=25 October 2023 |language=en |chapter=9.7 Double protection}}</ref>

Using both ear muffs (whether passive or active) and earplugs simultaneously results in maximum protection, but the efficacy of such combined protection relative to preventing permanent ear damage is inconclusive, with evidence indicating that a combined noise reduction ratio (NRR) of only 36&nbsp;dB (C-weighted) is the maximum possible using ear muffs and earplugs simultaneously, equating to only a 36 - 7 = 29&nbsp;dB(A) protection.<ref name=":0" /> Some high-end, passive, custom-molded earplugs also have a mechanical filter inserted into the center of the earmolded plug, with a small opening facing to the outside; this design permits being able to hear range commands at a gun range, for example, while still having full rating impulse noise protection.

Such custom molded earplugs with low pass filter and mechanical valve typically have a +85&nbsp;dB(A) mechanical clamp, in addition to having a [[Low-pass filter|lowpass filter]] response, thereby providing typically 30-31&nbsp;dB attenuation to loud impulse noises, with only a 21&nbsp;dB reduction under low noise conditions across the human voice audible frequency range (300–4000&nbsp;Hz) (thereby providing low attenuation between shots being fired), to permit hearing range commands.  Similar functions are also available in standardized earplugs that are not custom molded.<ref>{{Cite web|url=http://www.earplugstore.com/health-enterprises-acu-life-shooters-impact-ear-plugs.html|title=Health Enterprises ACU-LIFE Shooter's Ear Plugs (Sonic Valve II)|website=www.earplugstore.com|access-date=2017-03-03}}</ref>{{failed verification|date=June 2022}}