OccuSafe Industrial Hygiene Consulting

Noise is unwanted sound. Technically, it is sound pressure and includes a range of frequencies that can be detected by the human ear. There are a number of terms that are used in evaluating industrial noise and implementing  controls.
 
The sound pressure level (SPL) is the pressure level of a sound, measured in decibels (dB). It is a unit used to measure the intensity of a sound or the power by comparing it with a given level on a logarithmic scale.
 
The frequency of sound or pitch is the rate at which complete cycles of high and low pressure are produced by a source. The unit of frequency is cycles per second which is also known as hertz (Hz)
 
A-weighted decibel (dBA) is an expression of the relative loudness of sounds as perceived by the human ear. A-weighting gives more value to frequencies in the middle of human hearing and less value to frequencies at the edges as compared to a flat audio measurement. A-weighting is the standard for determining hearing damage and noise pollution and is the most common weighting system. 
 
1910.95 is the OSHA hearing conservation regulation for general industry while 1926.5 is the standard for construction. The Action Level is 85 dBA for 8 hours of noise exposure and if exceeded, requires the implementation of a hearing conservation program. The Permissible Exposure Limit is 90 dBA for 8 hours of exposure. It is the limit that should not be exceeded.
 
Noise-Dose is the percentage of allowable exposure incurred during the monitoring period. It is a linear scale as opposed to a logarithmic scale for decibels. 85 dBA for 8 hours is equivalent to a 50 percent dose and 90 dBA for 8 hours is equivalent to a 100 percent dose.
 
Other governmental and scientific organizations have different limits. For example, the National Institute of Occupational Safety and Health (NIOSH) and the American Conference of Governmental Industrial Hygienists (ACGIH) each have a limit of 85 dBA for eight hours of exposure.
 
Impulse noise is instantaneous such as banging, hammering, gunfire, and explosions. It can damage hearing even when the overall noise exposure is below noise exposure limit. OSHA has set a limit of 140 dBA and can be measured by most noise dosimeters.
 
A Sound Level Meter (SLM) is the basic measuring instrument. It measures the sound pressure levels in decibels which is proportional to the intensity. It can identify noisy equipment or areas of high noise where employee hearing may be affected.
 
Noise dosimeters are sound level meters that measure sound pressure levels over time. They can integrate these readings and  provide a cumulative noise-exposure reading for a given period of time, such as an 8-hour period. They can be worn by an employee during the shift and the results can be compared with occupational noise standards.
 
Octave Band Analyzers. Some SLMs are equipped with octave band analyzers which are important in noise abatement. They can measure the frequencies within an octave band and can determine sources of noise in machinery such as gear teeth, fan blades etc.
 
The Noise Reduction Rating (NRR) requires manufacturers to identify the noise reduction capability of all hearing protectors. It is a laboratory‐derived numerical estimate in decibels (dB) of the attenuation achieved by the hearing protector. Ratings for muffs or earplugs can range from 0 to 33 decibels. Actual protection provided by these devices depends upon how well and often they are worn.
 
Employers must provide audiometric testing for their employees who are exposed at the Action Level. Elements of the hearing testing include baseline audiograms, annual audiograms, training, and follow-up evaluation. The purpose of the testing is to determine if the hearing conservation program is preventing hearing loss. A licensed or certified audiologist, otolaryngologist, or other physician must be responsible for the program although professionals and trained technicians may conduct the testing.
 
There are many other terms used in noise control. The ones listed should provide a basic vocabulary needed to evaluate and control noise.