I recently did a little research on hearing protection and some of the information that I came upon was interesting. As hearing protection is something that each of us uses on a regular basis, I thought some of the information might be pertinent.
Sensorineural hearing occurs when there is some neurological damage to the inner ear. It is most commonly exhibited when the tiny cilia on the delicate hairs on our inner ear become damaged. These hairs cannot repair themselves, which is why prolonged and/or exposure to noisy environments is permanent. Tinnitus occurs when these hairs are only partially damaged and are constantly stimulated as a result. This causes a ringing or roaring sound, which can be temporary or permanent.
Decibels are the unitless, logarithmic measure of sound magnitude relative to a reference level. 0dB is the reference and is the smallest sound that a human ear can perceive.
Examples of SPL in dB
SPL (db) Type of sound
0dB Weakest sound
30dB Whisper
60dB Normal conversation
90dB Lawnmower
105dB Bulldozer
110dB Chainsaw
120dB Ambulance siren
140dB Jet engine @ takeoff
165dB 12 gauge shotgun
180dB Rocket Launch
Because decibels are on a logarithmic scale, they cannot be directly summed.
70dB + 70dB != 140dB
70dB + 70dB = 73dB
Every 3dB increase is a double in magnitude. Every time the distance from a sound source is doubled the magnitude decreases by 6dB.
Thus the Noise Reduction Ratio (NRR) cannot be summed if using earplugs simultaneously with earmuffs. For example, a pair of ear plugs with NRR of 22dB and a pair of earmuffs with NRR of 25dB does not net a total noise reduction of 47dB. Rather:
P = 10*log10(Sum 10^(Li/10))
For L1 = 22dB and L2 = 25dB
P = 10*log10(10^(22/10) + 10^(25/10))
P = 26.76dB
Since most people wouldn’t want to pull out a calculator to figure net noise reduction, this chart is pretty useful in simplifying the calculation. Simply find the difference between the two measurements and add the right column to the higher measurement.
Difference in numbers Add to Higher Number
0-1dB 3dB
2-3dB 2dB
4-9dB 1dB
10+dB 0dB
Thus,
P = 22dB + 25dB => 25dB + 2dB ~ 27dB
Hearing loss occurs at amplitudes greater than 85dB. OSHA requires that a worker receive less than 8 hours per day of exposure at this level. For each 3dB (double) increase in magnitude, the time of exposure is cut in half.
(Ex. 100dB is only allowed 15 minutes of exposure.)
In a series of over 20 studies performed from the 1970s through 1992 over 80 industries and 2600 employees, the a comparison was made between labeled NRR and actual NRR. Ear plug NRRs were found to be only 6 to 52% accurate, with an average of 25%. This means the typical ear plug that is rated at 20dB actually only provides a reduction of 5dB! Ear muffs were found to be 33 to 74% accurate, with an average of 60%. Thus it was recommended that comparing NRRs between products of less than 3dB difference with without any practical significance. In fact, when OSHA compares products, their NRRs are automatically cut in half.
Theoretically, an earplug type hearing protection device with a NRR of 22dB should be adequate for environments up to 107dB to achieve 85dB, but this is far from the truth. It’s actually only suitable for environments up to around 90dB for 8 hours or less.
Sensorineural hearing occurs when there is some neurological damage to the inner ear. It is most commonly exhibited when the tiny cilia on the delicate hairs on our inner ear become damaged. These hairs cannot repair themselves, which is why prolonged and/or exposure to noisy environments is permanent. Tinnitus occurs when these hairs are only partially damaged and are constantly stimulated as a result. This causes a ringing or roaring sound, which can be temporary or permanent.
Decibels are the unitless, logarithmic measure of sound magnitude relative to a reference level. 0dB is the reference and is the smallest sound that a human ear can perceive.
Examples of SPL in dB
SPL (db) Type of sound
0dB Weakest sound
30dB Whisper
60dB Normal conversation
90dB Lawnmower
105dB Bulldozer
110dB Chainsaw
120dB Ambulance siren
140dB Jet engine @ takeoff
165dB 12 gauge shotgun
180dB Rocket Launch
Because decibels are on a logarithmic scale, they cannot be directly summed.
70dB + 70dB != 140dB
70dB + 70dB = 73dB
Every 3dB increase is a double in magnitude. Every time the distance from a sound source is doubled the magnitude decreases by 6dB.
Thus the Noise Reduction Ratio (NRR) cannot be summed if using earplugs simultaneously with earmuffs. For example, a pair of ear plugs with NRR of 22dB and a pair of earmuffs with NRR of 25dB does not net a total noise reduction of 47dB. Rather:
P = 10*log10(Sum 10^(Li/10))
For L1 = 22dB and L2 = 25dB
P = 10*log10(10^(22/10) + 10^(25/10))
P = 26.76dB
Since most people wouldn’t want to pull out a calculator to figure net noise reduction, this chart is pretty useful in simplifying the calculation. Simply find the difference between the two measurements and add the right column to the higher measurement.
Difference in numbers Add to Higher Number
0-1dB 3dB
2-3dB 2dB
4-9dB 1dB
10+dB 0dB
Thus,
P = 22dB + 25dB => 25dB + 2dB ~ 27dB
Hearing loss occurs at amplitudes greater than 85dB. OSHA requires that a worker receive less than 8 hours per day of exposure at this level. For each 3dB (double) increase in magnitude, the time of exposure is cut in half.
(Ex. 100dB is only allowed 15 minutes of exposure.)
In a series of over 20 studies performed from the 1970s through 1992 over 80 industries and 2600 employees, the a comparison was made between labeled NRR and actual NRR. Ear plug NRRs were found to be only 6 to 52% accurate, with an average of 25%. This means the typical ear plug that is rated at 20dB actually only provides a reduction of 5dB! Ear muffs were found to be 33 to 74% accurate, with an average of 60%. Thus it was recommended that comparing NRRs between products of less than 3dB difference with without any practical significance. In fact, when OSHA compares products, their NRRs are automatically cut in half.
Theoretically, an earplug type hearing protection device with a NRR of 22dB should be adequate for environments up to 107dB to achieve 85dB, but this is far from the truth. It’s actually only suitable for environments up to around 90dB for 8 hours or less.
