My colleague, Russ Bowman, wrote a blog about “Sound Power vs Sound Pressure vs Sound Pressure Level”. He discussed the logarithmic equations around sound. I will be discussing what happens when you have more than one sound source, as often heard within manufacturing plants. Sounds can be added together to determine the overall sound level that you hear. This is very important when it comes to minimizing hearing loss.
In looking at a single source of sound, sound pressure is created by the loudness of a noise. The units are measured in Pascals. The lowest pressure perceived by human hearing is 0.00002 Pa, and we can use this value as a reference point. From sound pressures, we can arrive at a sound pressure level which is measured in decibel, dB. This correlation between sound pressures and sound pressure levels are calculated by Equation 1:
L = 20 * Log10 (P / Pref)
L – Sound Pressure Level, dB
P – Sound pressure, Pa
Pref – reference sound pressure, 0.00002 Pa
As an example, the sound pressure from a passenger car as heard from the roadside is 0.1 Pa. With Equation 1, we can get the following decibel level:
L = 20 * Log10 (0.1Pa/0.00002Pa) = 74 dB
Because human ears are sensitive to different frequencies, the sound pressure levels can be modified, or weighted, to indicate an effective loudness level for humans. This adjustment is done in two different ways; A-weighting and C-weighting. The C-weighting is for very loud noises with high peaks or sharp impacts like gunfire. The A-weighting is the most used value as the sound pressure levels are adjusted by the frequency level. For higher and lower frequencies, the change in the sound value is much greater than the mid-level frequencies that are within our hearing range. Sound measurements for safety are measured on an A-weighted scale. OSHA created a chart in the standard 29CFR-1910.95(a). It shows the noise levels over exposure times for an operator. To use the OSHA chart accurately, the total noise level in dBA should be calculated.
To determine the total sound level, we can add all the sound pressure levels together by Equation 2:
Equation 2:
Where L1, L2… represents the sound pressure level in dBA for each sound source.
As an example, a manufacturing plant had an operator using a machine that had four copper tubes to blow off a cutting operation (reference photo below).
The decibel level for a copper tube was measured at 98 dBA. The total amount of sound that the operator was exposed to was determined by Equation 2 with four values.
L = 10 * log10 (109.8 + 109.8 + 109.8 + 109.8)
L = 104 dBA
In looking at the OSHA chart above, the operator would only be allowed to operate the machine only a little over one hour without hearing protection. In this same example, we replaced the copper tubes with an EXAIR Super Air Nozzle, model 1100SS. The noise level for each nozzle is 74 dBA at 80 PSIG (5.5 bar). By replacing all four copper tubes with Super Air Nozzles, Equation 2 becomes:
L = 10 * log10 (107.4 + 107.4+ 107.4 + 107.4)
L = 80 dBA
The total sound level is now in accordance with OSHA regulations for the operator to work all 8 hours at the machine without hearing protection.
A commonly used acronym for hearing safety is NIHL, or Noise Induced Hearing Loss. To keep your operators safe and reduce NIHL, it is important to measure the total sound level in that area. As a protocol in safety, it is a requirement to use engineering standards before purchasing personal protective equipment or PPE. For the customer above, they followed that protocol with our Super Air Nozzles. If you need to reduce noise levels in your facility, EXAIR offers a large line of blow-off products that can meet the safety requirements. You can contact an Application Engineer for more information.
John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb
