stacking decibal reductions


February 1, 2013, 10:20 PM
I've been told that the decibel ratings of multiple sets of ear protection don't stack, so even though my sonic defenders and ear muffs are 24 and 30 respectively, I'm not actually getting a decibel reduction of 54. Can anyone tell me approximately what I'm getting?

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February 1, 2013, 10:29 PM
I'm not sure exactly what the dB reduction is but it's a lot and it's not 54. Decibels are measured on a logarithmic scale so even going from a 22 dB to a 24 dB reduction is significant.

February 2, 2013, 07:50 PM
I'm not exactly sure either but, I worked in chemical plants and refineries for a few years and certain areas that created high noise required double hearing protection. Ear plugs with muffs and nothing to break the seal of the muffs like corded plugs or the arms of safety glasses. The difference wasn't as significant as I expected, but there must be at least some benefit or I doubt they would spend money on all those muffs that were free issue to anyone inside the gates.

I fixed active leaks on process equipment that was still running. Some leaks were 2000 psi steam screaming out of a valve or pipe. Nothing will help you from that! The pressure makes it hard to breathe around the leak and the screaming seems like it's coming from inside your head. Not to mention the static elecricity that builds up and shocks the crap out of you.

February 2, 2013, 08:48 PM
Here is guidance from OSHA in regards to worker protection.

When double hearing protection is used, add 5 dB to the highest NRR hearing protector.

February 2, 2013, 09:07 PM
What he said ^.
Power (noise) doubles or halves every 3dB, so a 3db difference is significant.
"... The decibel scale is a compressed scale, not a linear scale, such that a 3 dB drop in noise level (that is, 3 dB more attenuation in the earplug) reduces incoming noise by half. To jump from 30 to 33 dB, an earplug has to be so good that it blocks twice as much noise. Then, to go from 33 to 36, an earplug has to be so good that it blocks twice as much noise as 33. You can see quickly how you reach limits in terms of seal of the ear canal and materials."

February 5, 2013, 12:57 AM

33 db Howard Leight - by the box. Use by themselves when riding motorcycle, Army shooting. Use muffs on top when shooting indoors.

February 10, 2013, 08:25 AM
I was always told that at a certain point skull vibration transfers sound. Also as others have pointed out, db is a logarithmic scale. In most industrial applications double protection will only lower sound a few more db than single. Then you course you only get one set of ears. This is why I carry a handful of extra plugs to the range.

February 13, 2013, 09:58 PM
Another point: the reduction values on the package are approximate, and make certain assumptions about the frequency of the sounds being blocked. Something that cuts 30 db from a 400 Hz motor hum will not always take 30 db off of an impulsive sound like a gun shot.

In general, though, more is better. Stacking is better than that -- wear all the protection you can get away.

Anyone ever noticed that the shots are significantly louder if your mouth is open, especially at an indoor range? Try it.

Double Naught Spy
February 14, 2013, 08:37 AM
If you sequence, you are getting the 54 db reduction claimed, but only through the noise suppression. The problem is the amount of sound pressure transferred through bone conduction.

Sound doesn't care and doesn't know. If the sound pressure hits a 30 db barrier, the sound passing through is reduced by 30 db. When that sound then hits the next 24 db barrier, it will be reduced by 24 db. So sound that has been through a 30 db barrier doesn't know it isn't supposed to NOT be reduced by 24 db when going through the next 24 db barrier as claimed.

So if all works as planned, you will get the claimed reduction at the auditory canal entrance via the protection, but so much of the sound will bypass the auditory canal and be transferred through your body tissues. You have to shield the entire body to get the full shielding. So you don't get the actual 54 db reduction in sound with doubled protection, though you may find conversations very difficult.

February 16, 2013, 02:10 PM
You can't just add them up. It's a logarithmic scale, not linear. Pairing up two devices gives you roughly 3-5dB of additional attenuation. Popping on two 27dB devices will give you about 30-32dB total attenuation.

February 18, 2013, 12:06 AM
When I got out of engineering college in 1978, I worked the next three years designing audio gear for Boeing. The lead engineer I worked for was like me, building hi fi equipment at home. He subscribed to "DB" magazine, and I read his copy.
I started college with a slide rule and finished with a TI-51 calculator.
Part of the reason they worked in DB was because they did not have calculators.
Part of my job was to go through and modify all the ARINC specification [That controlled all planes, not just Boeing] to modernize the DB of amplitude and the output impedance of audio side tones. That is a boring task, but after a while, I was really ready to argue with anybody from any company about DB.

If you have a 15.8 to one attenuator and put in series with a 31.6 to one attenuator, then to calculate the attenuation, multiply those two numbers.
If you don't have a calculator, but you know those are 24 db and a 30 db attenuator, you simply add the numbers.
Together they are a 54 db attenuator.... That is 501 to one attenuation.

And don't let anyone confuse you about power DB vs Sound pressure level DB.
If you stay in the same system, a DB is a DB is DB.
The power DB is 10 log base 10 of the ratio.
The SPL DB is 20 log base 10 of the ratio
Power already has a squared term in there, so a square in the ratio amounts to a 2 in front of the log operation. That is explained in here:

Adding the two attenuations depends on them being in series. If the ear plugs are really inside the ear muffs it works.
But if the ear plugs were to be touching the ear muffs, then we would be getting some extra coupling and loosing some attenuation.

Jorg Nysgerrig
February 18, 2013, 03:54 PM
But if the ear plugs were to be touching the ear muffs, then we would be getting some extra coupling and loosing some attenuation.
That's an interesting point, particularly with some of the longer plugs out there.

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