The only time I always double up is when shooting indoors, otherwise I just use my electronic Peltoirs. Which lets me hear any commands by the range officer.
I also double up if I have the misfortune of getting stuck next to a big magnum rifle with a muzzlebrake on the range. No point in developing a flinch under those circumstances.
I never wear them while hunting because you’ll sweat like a pig in a good fitting pair of muffs with even the lightest exertion. I just except the minor amount of damage a couple of hunting shots a year doles out.
A good fitting pair of muffs or properly inserted foamies are all you really need to protect your hearing. Doubling up is more for just comfort than hearing protection.
The way you actually hear something is that ganglia (small hairs in your inner ear) vibrate from the result of moving air hitting them, transmitting a message to your brain. When the ganglion are assaulted by an extremely loud stimulus (e.g. crash, jet engine, gun shot), they get "pushed down" a little. They never "stand back up" to their full extension. People lose their hearing as they get older as a result of continued assaults upon the ganglion, pushing them down more and more each time. This results in hearing loss, the ganglion are no longer able to effectively transmit a given message. The loss of frequency (pitch) recognition can be segments of the full spectrum, all hertz between low and high.
Facts on noise levels:
1. Decibels measure sound pressure and are logarithmic. That means that only a 3db increase almost doubles sound pressure, a 6db increase quadruples sound pressure, etc.
2. Gradual hearing loss may occur after prolonged exposure to 90 decibels or above.
3. Exposure to 100 decibels for more than 15 minutes can cause hearing loss.
4. Exposure to 110 decibels for more than a minute can cause permanent hearing loss.
5. At 140 dBA noise causes immediate injury to almost any unprotected ear.
6. There is also the more extreme ‘acoustic trauma’, which is an immediate loss of hearing after a sudden, exceptionally loud noise such as an explosion.
Comparative noise levels and length of time exposed to cause permanent damage
Jet engine taking off 140 dB Instant damage
Thunder/Ambulance siren 119 dB 3 minutes Hammer drill 113 dB 15 minutes
Chain saw/Earphones/Concert 110 dB 30 minutes
Bull Dozer 105 dB 1 hour
Tractor/Power tools 96 dB 4 hour
Hairdryer/lawnmower 90 dB 8 hours
Noise levels of firearms:
.22 caliber rifle 130dB
.223, 55GR. Commercial load 18" barrel 155.5dB
.243 in 22" barrel 155.9dB
.30-30 in 20" barrel 156.0dB.
7mm Magnum in 20" barrel 157.5dB. .308 in 24" barrel 156.2dB. .30-06 in 24" barrel 158.5dB. In 18" barrel 163.2dB.
.375 18" barrel with muzzle brake 170 dB.
.410 Bore 28" barrel 150dB. 26" barrel 150.25dB. 18" barrel 156.30dB.
20 Gauge 28" barrel 152.50dB. 22" barrel 154.75dB.
12 Gauge 28" barrel 151.50dB. 26" barrel 156.10dB. 18" barrel 161.50dB.
.25 ACP 155.0 dB.
.32 LONG 152.4 dB.
.32 ACP 153.5 dB.
.380 157.7 dB.
9mm 159.8 dB.
.38 S&W 153.5 dB.
.38 Spl 156.3 dB.
.357 Magnum 164.3 dB.
.41 Magnum 163.2 dB.
.44 Spl 155.9 dB.
.45 ACP 157.0 dB.
.45 COLT 154.7 dB.
Properly fitted earplugs or muffs reduce noise 15 to 30 dB. The better earplugs and muffs are approximately equal in sound reductions, although earplugs are better for low frequency noise and earmuffs for high frequency noise.
Using muffs and plugs together: Take the higher of the two and add 5 dB. 30 plug with 20 muff gives an effective NRR of 35.
If you are shooting by yourself, with plugs and muffs on, you get to shoot up to a thousand rounds without damage (louder ammo/gun and the allowable drops by a factor of 5). Shoot with other people and you have to add all the rounds shot cumulatively (10 people shoot 100 rounds and everybody's done for the day; toss a handcannon or 30 cal rifle in and you're back down to 200 rounds cumulative). If you shoot on an indoor range then all the rounds fired while you are on the range go into your total. So you can see that it doesn't take very long on a range to have a thousand rounds popped off around you.
Don't forget about bone conduction of concusive sound waves. The mastoid bone will transmit the sound vibrations directly to your inner ear where the cochela and the hearing nerves resides. Constant exposure to this kind of concusive sound wave, (e.g. 50 BMG, industrial heavy machinery) will result in the degradation of your hearing quality. Even with ear muffs, bone conduction is a big factor in hearing.