Mining haul trucks comprise the majority of the equipment used in underground limestone mining operations and are known to emit high levels of noise. A previous study conducted by the National Institute for Occupational Safety and Health (NIOSH) indicates that 70-90% of all miners have a noise-induced hearing loss (NIHL) great enough to be classified as a hearing disability by retirement age. These results demonstrate the public health need to protect the hearing of workers in the mining industry, including haul truck drivers. Cab enclosures present an opportunity to isolate the haul truck operator from both truck and other noise in the mining environment. A total of 25 haul truck cabs were studied. They were divided into three style (treatment) categories determined by soundproofing features and technology for noise reduction: old-, new- and retrofitted-style. This study examines the contribution of cab acoustics, operator performance and maintenance to noise reduction for each cab style. Dosimeters were used to measure eight-hour time weighted average sound pressure levels (TWA8 SPLs) inside and outside the cabs. The main objective was to determine the noise levels inside of the three types of cabs (with different acoustical treatments) and determine if the noise levels were significantly different. Adherence to the Mine Safety and Health Administration (MSHA) permis- Introduction This study investigates haul truck cab noise in underground limestone mines that employ nearly 2,000 workers at 117 mines across the United States. In this industry, hazardous noise is present from drilling, blasting, rock crushing operations and the predominance of large and noisy equipment. Continued exposure of miners to high noise levels can cause damage to the inner ear. The result of this damage is a permanent shift in the hearing threshold, known as a noise-induced hearing loss (NIHL). A NIHL makes it difficult to hear and understand everyday speech and is irreversible. Of special interest is the haul truck (Fig. 1) because it comprises of the largest class of equipment used in the underground limestone industry. With these trucks and most diesel-powered equipment, the engine is generally a major source of noise. Engine noise may emanate from the exhaust, the intake and the cooling fan. Other significant noise sources include the transmission, drive train and hydraulic system. Noise from these sources reaches the ear via several paths, both directly, by airborne paths and indirectly, by reflections from various surfaces. In addition, sound in the form of vibrations may travel along or through structures (Daniel et al., 1981). An approach to eliminate or control noise at its source, engineering controls, is through the use of mufflers, gaskets and control of reflected noise. Another way to lower noise levels is to identify, isolate and treat the many paths along which noise travels with barriers, absorbers and dampers. Control of haul truck cab noise is important because haul truck operators spend a majority of their time inside the cab. Most mine policies require haul truck operators to remain inside their cab throughout the entire shift except for restroom use, attendance at safety meetings, during maintenance and sometimes during lunch breaks. Therefore, it is typical for operators to spend almost the entire shift (eight to 10 hours) inside the haul truck cab. According to Daniel et al., "Cab enclosures generally sible exposure limit (PEL) of 90 dB TWA8 (with a 90 dB threshold) was used as the main indicator of overall noise reduction achieved. Dosimetery results indicated that all but two samples measured outside of the cab exceeded the MSHA PEL. However, only 2% of the samples measured inside of the cabs exceeded the PEL, but samples could still be reduced much further. Descriptive and comparative statistics indicate that noise levels inside the new-style cabs are significantly lower than the other two cab styles. Also, data suggest that there is no difference in noise exposures when comparing the old-style to retrofitted cab styles. Operator influence (opening doors and windows) was a significant factor for increasing noise exposure. This paper demonstrates that properly designed cabs can achieve major noise reductions, but noise levels could still be reduced much further below the MSHA PEL. New-style cabs, equipped with modern noise-reduction treatments, exhibit much lower noise exposures than the other two cabs styles, and the effectiveness of the current noise-reduction treatments for retrofitted cabs is questionable. Haul truck driver observations indicate that improved noise exposure reduction training is needed. Finally, specific targets for future noise reduction research are suggested that will further contribute to the prevention of hearing loss for haul truck operators.