DC Feedback: Can noise levels inside the cab cause hearing loss?

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This month's installment of DC Feedback deals with noise levels inside locomotive cabs.

 

It has been well established that continuous exposure to high noise levels will result in hearing loss. Noise-induced hearing loss is caused by one-time exposure to extremely loud sound or by exposure to sounds at high decibels over months or years. It occurs when the delicate hair cells in the inner ear that translate sound waves into nerve impulses are damaged.

If you think that you are an exception and noise does not bother you, there is a good chance that you may have experienced hearing loss. To understand noise-induced hearing loss we should look at the way sound is measured.

The intensity of loudness of noise is measured in decibels (dB). The decibel is a dimensionless unit based on the logarithm of the ratio of a measured quantity to a reference quantity. It is a measurement of the amount of sound energy and is related to sound pressure. Because it is a logarithmic scale, an increase of one bel produces a doubling of sound intensity. In other words, a noise level of 90 dB is two times as intense as a noise level of 80 dB and four times as intense as a noise level of 70 dB. If the dB reading is followed by an (A), then it is an indicator that the dosimeter unit that recorded the sound was set with a parameter known as the A-scale.

The legal standard under Occupational Safety and Health Act (OSHA) is 90 dB(A) exposure over an eight-hour work day. When noise levels at or above that limit are reached, certain provisions for noise reduction or mitigation through hearing conservation must occur. For example, if the time-weighted average noise level in your workplace was twice the allowable level for an eight hour shift, the permissible exposure would be limited to four hours, or half the period.

To give you a basis for understanding the relative "loudness" of sound pressures in decibels (dB) the table below provides sound levels in decibels for familiar environments.

The perception of sound is also relative. Some sounds can be purposely irritating (sirens or some of the newer locomotive horns) while others, even at higher decibels, may be less so. Sound becomes uncomfortable at 120 dB, the threshold of discomfort is 130 dB, and that of pain is 140 dB. It is believed that prolonged daily exposure to noise levels higher than 90 dB will cause hearing loss.

So, how loud is it?

Sound levels inside the cab of a locomotive depend on a large number of variables. Among those variables are: the type of locomotive; position of windows and doors; horn placement and air pressure to the horn; radio volume control; air exhaust characteristics; audible warning devices; proximity to reflective surfaces; use of the dynamic brake; use of equipment such as heater or air conditioning fans; general age and condition of the locomotive; and the nature of the run with its associated demands for power, dynamic brake, and horn use.

According to actual measurements, the sound level in cabs has reached a peak as high as 145 dB(A). It is assumed this would be in situations where the horn was in use and reflective surfaces permitted the sound to enter through open cab windows (some locomotive horns have been measured at over 116 dB(A) 100 feet in front of the locomotive). Measurements inside the cab also indicate levels of sound over a 100 dB(A) for extended periods of time.

Time of exposure is a significant factor in the development of noise-induced hearing loss. For OSHA and Federal Railroad Administration (FRA) purposes those time periods are based on a time-weighted average (TWA). OSHA and FRA's allowable exposure is 90 dB(A) for 8 hours. Since railroad employees can be exposed for up to 12 hours under the House of Service Act, FRA looks at 87 dB(A) as the limit.

Since 1992, FRA has conducted measurements of 350 locomotives. The FRA data is not a true random sample of locomotives or locomotive operating conditions; therefore, appropriate caution should be used when characterizing the findings. Approximately 16% of the 350 total measurements of locomotives exceeded 87 dB TWA. For 73 locomotive engineers (who were on lead locomotives where the horn was used) 38% of the measurements were found to exceed 87 dB.

Recent recommendations from the National Institute for Occupational Safety and Health (NIOSH) suggest a standard of 85 dB(A) in locomotive cabs is a better target to decrease the risk for hearing loss when exposed for an 8-hour period. Nearly all railroads have a hearing conservation program where hearing protection is offered. It is a good practice to use hearing protection when in a locomotive cab, especially older locomotives where conditions of open windows and long runs create exposure levels that will cause noise induced hearing loss.

This chart also available as PDF for easier viewing.

 

2000 Brotherhood of Locomotive Engineers