R&D Portfolio - Research Goal 1.3 : Achieve a better understanding of the combined effects of continuous and impulsive noise exposures
It is unclear how to accurately measure and evaluate noise exposure for combinations of continuous and impulsive noise. There is a schism in the research community about the differences between impulsive noise and continuous-type noise in their effects on hearing. This schism is reflected in how noise exposures are measured. One thought supports the idea that impulsive noises below 140 dB sound pressure level (SPL) can be integrated with continuous noise to determine the noise burden on the ear of the combined signals. The ANSI standard for dosimeters calls for the ability to integrate sound levels below 140 dB SPL. Present OSHA and MSHA regulations follow this reasoning and call for all noises below 140 dB SPL to be integrated to determine the time-weighted average noise exposure. OSHA and MSHA have different regulations regarding exposure limits.,  OSHA regulations state that impulses should not exceed 140 dB SPL, whereas MSHA regulations state that mine operators must assure that no miner is exposed at any time to sound levels exceeding 115 dB(A).
The other thought supports the idea that impulsive noises above 100 dB SPL are more hazardous to hearing than continuous noises of the same level and that the combination of impulsive noise with continuous noise is more hazardous than either of the noises alone.
HLR program researchers found that impulsive noise of the same energy and frequency spectrum as a continuous noise caused more than 20 dB more hearing loss in chinchillas. In 1972, NIOSH recommended that exposure criteria should address the potential increased contribution to hearing loss from impulsive noise by suggesting that 5 dB be added to the measured dose if impulsive noises were present. At that time, however, research was needed to see if the effects of impulsive noise were the same on human hearing as on the hearing of chinchillas.
The automotive industry involves a wide variety of manufacturing processes that produce continuous noise, impulsive noise, or both. From 1988 through 1990, the HLR program conducted a retrospective study of the effects of continuous and impulsive noise on the hearing of automobile workers at the Ford Motor Company, Woodhaven Stamping Plant in Woodhaven, Michigan. The plant was chosen because:
- It had both stamping and assembly processes with near-identical background noise levels when integrated over the entire workday.
- Manufacturing processes had not changed since the plant went on line in 1970.
- Noise levels had not changed since the plant went on line in 1970.
- While the number of workers had been reduced over time due to attrition and layoffs, most had been there more than 15 years.
- Audiometric records were available for all workers beginning with pre-employment audiograms dating back to 1966 for some employees.
The study was limited to 140 workers: 70 from the stamping side and 70 from the assembly side. Area noise measurements showed no change from those taken in the late 1980s. Work-station noise measurements were taken and overall the exposures were equivalent for workers from both sides of the plant. However, workers on the stamping side were exposed to a low-frequency impact noise once every seven seconds. Sound exposure levels measured with a dosimeter which integrated impulses up to 140 dB and captured both 3- and 5-dB exchange rate data exhibited no difference as a consequence of the A-weighting and slow-response time constant of the dosimeter.
Each of the workers was given a confirmation audiogram by a contractor under HLR program supervision in 1989, and the audiometric records were transcribed back to the pre-employment audiograms for each subject. In addition, each worker in the study completed a related history form via a telephone interview as part of the process. Analysis of the audiometric data revealed that while the workers exposed to impulsive noise had an onset of noise-induced hearing loss sooner– within five years – than those exposed to continuous noise, the latter developed the same amount of hearing loss as the impulsive-noise-exposed workers seven to ten years later. Thus, the combination of continuous and impulsive noise was more hazardous than continuous noise alone even though both had the same measured exposure levels.
The study also revealed that the Ford hearing conservation program documented, but did not prevent, hearing loss. One of the limitations in the Ford program was the way in which relevant information was processed. Audiograms were administered and then the results were sent in batches of 100 to a central data base to see if a threshold shift had occurred. If there had been a shift, a letter was sent to the worker’s home to report back to the medical unit for a retest within three weeks, which many did not do. No feedback was provided to workers about their hearing beyond the statement that there had been no change since the prior year except for those who had a threshold shift. All workers were issued the same HPD, a very uncomfortable one that was finally pulled from the market in 1998. Additionally, noise measurements were taken less than once a decade and were tracked by building post number, not by work station where the noise levels could be 10 to 15 dB higher; leaving workers unknowingly under-protected given the hearing protector they were provided.
 Occupational Safety and Health Administration . CPL 2–2.35A–29 CFR 1910.95(b)(1), Guidelines for noise enforcement; Appendix A. Washington DC: U.S. Department of Labor, Occupational Safety and Health Administration, OSHA Directive No. CPL 2–2.35A (December 19, 1983).
 MSHA . Dept. of Labor, Mine Safety and Health Administration 30 CFR part 62 Section 130 Permissible Exposure Level (September 13, 1999).
 Dunn DE, Davis RR, Merry CJ, Franks JR.  Hearing loss in the chinchilla from impact and continuous noise exposure. J Acoust Soc Am.; 90(4 Pt 1):1979-85.