Hearing protection and distance

[ace77]

I already have constant tinnitus from some bad choices in my life and I dont want to make it worse.

So I thought about this when I walked outside while my neighbor was target shooting with a pistol from about 300 ft away. First shot wasnt that loud but then they let off about 12 rapid fire shots and I was like damn thats pretty loud and covered my ears.
Assuming a 9mm makes 160db at 3 ft and due to the inverse square law sound level drops 6db every doubling of distance in an open field. 300 ft from the shooter would be 120 db, roughly 40 db of attenuation. What's considered a safe distance for a bystander with no hearing protection?


Also how safe is the best hearing protection for the gun range? Even with dual protection you only get the NRR rating of the higher value plus 5-8 additional db attenuation. The combined amount for 32 db ear plugs and 24 db ear muffs is only 38-40db. If you are firing a gun at 175 db, my math is that you are still being exposed to a deafening 135 db. Why is this not harmful?

 

[buckhorn_cortez]

If you are firing a gun at 175 db, my math is that you are still being exposed to a deafening 135 db

Not true. Decibels are logarithmic. You cannot simply add and subtract them. Addition is equal to multiplication and subtraction is division.

As an example, this is the NIOSH noise reduction calculation used for the noise reduction rating for hearing protection:


----------------------------f=8000------------------------
NRR = 107.9 dBC - 10 log Σ 10 0.1(LAf-APVf98) - 3dB
---------------------------- f=125-------------------------

Calculating the amount of energy reaching the ear with hearing protection is equally as complex. If it was 135dB your ears would hurt and ring after one shot.

I calculated a table at one time using a dB formula (which is on a different computer than the one I'm currently using) and the noise level reaching your ear from a 150 dB gunshot is about 51 dB if you have on 18 dB NRR hearing protection.

One of the best things hearing protection does for your ear is reduce the impulse noise which is what causes the most amount of hearing damage.

It is frequency dependent as well as noise volume dependent which is why you have to include an integration over a frequency range as shown in the NRR formula that integrates from 125 Hz to 8,000 Hz.

 

[ace77]

Yes decibels are measured on a logarithmic scale, but your calculations seems wrong. 150 db gunshot is not 51 db at the ear if you are wearing 18 NRR hearing protection. There is just no way. If that formula were true then they would not recommend doubling up on protection because 51 db is far below the 85 db safe zone.

I have always been told to subtract the NRR of hearing protection from the noise level to come up with an accurate estimate of DB at your ear.

I run a manufacturing company and work around loud equipment. I have talked to OSHA about hearing protection before and this is actually how OSHA told me to calculate NRR.
http://www.coopersafety.com/noisereduction.aspx

Noise Reduction Rating Explained

Noise Reduction Rating (NRR) is the measurment, in decibels, of how well a hearing protector reduces noise as specified by the Environmental Protection Agency. The higher the NRR number the greater the noise reduction. While wearing hearing protection your exposure to noise is equal to the total noise level minus the NRR of the hearing protectors in use. For example, if you were exposed to 80db of noise but were wearing earplugs with an NRR of 29, your actual noise exposure would only be 51dB.

Based on their simple formula, 150 db gunshot while wearing 18 db nrr hearing protection exposes your ears to 132 db. Furthermore I came across a chart from NIOSH that recommends the maximum number of gun shots you may do in a day before risking hearing damage because the decibels are so high even with protection.

Maybe I read it wrong but if I can find the NIOSH report I will post it. Decibels are very confusing.

 

[ace77]

Here is some information regarding this. The EPA, NIOSH and OSHA tell you to subtract the NRR from the noise level to determine db at your ear.

Conflicting Information about How to Use the NRR

Do you know how to use the NRR values that are required on the packaging of all hearing protection devices sold in the United States? If you do, you are one of a kind!

EPA Requirement: The only law (passed by the U.S. Congress) regulating how to use the NRR, is actually the EPA labeling regulation, dating back to 1981. This law directs the user to deduct the NRR from the noise level at hand, in order to get the noise level at the ear. That is pretty straightforward: Noise level [98 dB] minus Hearing Protector NRR [25 dB] = Noise at the Ear, 73 dB.

OSHA’s Field Manual: Although OSHA has no legislative authority, the agency has in its field manual directed their inspectors to use the following formula for how to apply the NRR.

If noise measurements are made with the dB(A) scale, the following formula applies. (Noise level [98 dB] minus NRR divided by 2 [25 dB minus 7 dB divided by 2] = Noise level at ear, 89dB).
If the noise level measurements are made with the dB(C) scale, this formula applies. (Noise level [98 dB] minus NRR divided by two [25 dB divided by 2] = Noise level at ear, 85.5 dB).
Although OSHA has only enforcement authority, their interpretation has held up in court. OSHA only recommends that the 50% safety factor is applied. Realize that if a company runs an effective hearing conservation program, they are not likely to be challenged. An effective hearing conservation program can be defined as a program that assures that no new permanent and noise induced hearing losses occur among its employees.
NIOSH, Criteria for a Recommended Standard, Occupational Noise Exposure, June 1999: NIOSH recommends that Subject Fit data in accordance with ANSI S12.6-1997 be used. (To our knowledge no U.S. manufacturer has made "Subject fit" test data available). NIOSH recommends the following de-rating of hearing protector NRR’s, if subject fit data is not available.

Earmuffs, Subtract 25% from the manufacturer’s labeled NRR
Formable earplugs, Subtract 50% from the manufacturer’s labeled NRR
All other earplugs, Subtract 70% from the manufacturer’s labeled NRR
The above de-ratings apply only when the noise measurement was made with a dB(C) scale. When only a dB(A) scale measurement is available, the de-rated NRR’s should be reduced by seven dB. Observe that earmuffs require the lowest de-rating.

OSHA Web Site, January 1999: OSHA recommends that manufacturers include a secondary label for NRR-SF. The SF (Subject Fit) refers to a NRR value achieved with the new ANSI S12.6-1997, using naive subjects. Naive subjects are defined as individuals having no previous experience in the use of hearing protection. (Authors comment: The method assures lower NRR’s, but we are no longer testing hearing protectors, we are testing test subjects. Better products will not necessarily get higher NRR’s. How about testing the safety record of new cars by having drivers without license drive a test course, and the car without dents gets the highest rating). The naive subjects are part of the ANSI standard, why OSHA bears no blame for this situation.

_________________________________________-

More information from 3M.

http://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSufSevTsZxtUPx_eO8MSevUqevTSevTSevTSeSSSSSS--

An Example of Reducing the NRR
8-hour TWA noise exposure: 93 dBA
NRR of hearing protectors: 29 dB
Subtract 7 dB from the NRR: 29 dB - 7 dB = 22 dB
Divide by 2: 22 ÷ 2 = 11 dB
Subtract 11 dB from the
8-hour TWA noise exposure: 93 dBA - 11 dB = 82 dB
Decide if 82 dB (known as the “Protected Exposure”)
is below the PEL for noise

 

[ace77]

According to NIOSH, you need to be very careful about noise exposure even with hearing protection. Peak noise level at the ear while shooting can very easily be over 120 db with double protection.

In 2002, NIOSH proposed a simplified formula to reduce the risk
of exposure to impulse noise in terms of the number of gunshot
impulses to which a person can be exposed per day [NIOSH 2002]:
N = 10((140 – PI )/10)
where N is the number of gunshot exposures permitted, and
PI is the peak impulse level in dB under hearing protection. PI
is determined by subtracting the noise attenuation for hearing
protection from the peak noise exposure level for a gunfire
impulse.
Figure 6 shows the number of gunshot exposures permitted on the
basis of peak noise levels under hearing protection. For example, if
the peak noise level under hearing protection is 120 dB, applying
this formula yields N=100 gunshots. The NIOSH proposed
formula is a conservative estimate and does not take into account
the duration of the impulse, its spectral content, or its energy.

http://www.cdc.gov/niosh/hhe/reports/pdfs/2011-0069-3140.pdf

I calculated a table at one time using a dB formula (which is on a different computer than the one I'm currently using) and the noise level reaching your ear from a 150 dB gunshot is about 51 dB if you have on 18 dB NRR hearing protection.

buckhorn, I'm not sure where you found that information, but according to OSHA, NIOSH and the EPA; 150 db minus 18 db NRR puts 132 db at your ear. Also, remember the NRR is the optimal average of attenuation achieved in a laboratory setting. That is why OSHA recommends you should derate the NRR for real world attenuation.

Always choose hearing protection with the highest NRR and double up. Even then you are subjected to a permissible exposure limit, limiting the amount of shots you may do in one day without risk of hearing damage as this chart from NIOSH shows.