No. 548 March 2012 

Bit Isolator Reduces Drilling Noise in Underground Coal Mines 

Objective 

To develop an engineering noise control that reduces roof 

bolting machine (RBM) operators’ exposure to noise in 

underground coal mines, thereby decreasing the risk of noiseinduced hearing loss (NIHL) in RBM operators and other 

workers in close proximity to RBMs. 

Background 

NIHL is one of the most widespread occupational illnesses 

in the United States. Studies conducted by the National Institute 

for Occupational Safety and Health (NIOSH) indicate that 70%–

90% of miners have NIHL significant enough to be classified as 

a hearing disability. Mine Safety and Health Administration 

(MSHA) studies indicate that RBM operators are among the 

miners who are most frequently overexposed to noise. To 

reduce the noise exposures of RBM operators, noise controls 

must be developed. 

NIOSH research has shown that the drilling process itself, 

rather than the hydraulics and electric motors, is the major 

contributor to the noise generated during the roof bolting 

operation. NIOSH field studies have shown that the drilling 

portion of the roof bolting cycle is the primary source of an 

RBM operator’s noise exposure. Figure 1 shows results from a 

beamforming analysis that NIOSH conducted during laboratory 

replication of RBM operation. Beamforming is a technique that 

uses a large array of microphones to locate noise sources. The 

contour plot in Figure 1 shows that the top of the drill steel is 

the dominant source of drilling noise, and the chuck is a 

secondary contributor to drilling noise (both indicated by the 

light yellow color bands). The noise radiated from the drill steel 

is a result of vibration generated at the bit-rock interface that is

transmitted from the bit to the drill steel. Because the drill steel 

generates more noise than the other noise sources and the 

operator works close to it, the drill steel is the primary source of 

the operator’s noise exposure. 

Figure 1. NIOSH’s beamforming analysis showing dominant 

noise sources at the top of the drill steel and the drill chuck. 

Highest

Noise

Lowest

Noise

top of 

drill steel

chuck

Technical Approach 

The preferred first step in reducing operator noise exposure 

is the application of engineering controls. Engineering controls 

address noise at the source, reducing the sound levels and 

preventing overexposure of all personnel in the vicinity, not just 

the roof bolter operator. The NIOSH Office of Mine Safety and 

Health Research (OMSHR), in collaboration with Corry Rubber 

Corporation and Kennametal Inc., has developed a drill bit 

isolator to reduce drill-steel-radiated noise when drilling with 1-

3/8" diameter drill bits. 

The bit isolator is a noise control that reduces sound levels 

generated during drilling and can reduce the operator’s risk of 

NIHL. The bit isolator (Figure 2) consists of two steel 

cylindrical tubes with a rubber layer between them. Hex-shaped 

drill steel couplers attached to the inner and outer cylinders 

allow the bit isolator to be connected in-line with the drill steel. 

The rubber layer isolates drill bit vibration from the drill steel, 

thereby reducing the noise radiated from the drill steel and the 
chuck. Consequently, the RBM operator is exposed to less 

noise. 

 

Figure 2. Drill bit isolator. 

outer 

cyl.

drill 

steel

drill 

steel

inner 

cyl.

rubber 

layer

With the bit isolator installed, the drill steel is used in the 

same manner as a standard hex drill steel. Figure 3 shows the bit 

isolator installed in a drill steel. A short piece of hex drill steel is 

installed between the isolator and bit seat. Any standard drill bit 

can then be used to drill 1-3/8" diameter holes. The bit isolator 

may be used for deeper holes by stacking an additional drill 

steel under the first piece of drill steel using a coupling. 

Figure 3. Drill bit isolator installed in a drill steel. 

bit

isolator

bit

seat

bit

short piece of

hex drill steel

drill 

steel

Results of Laboratory and Field Tests 

In order to evaluate the effectiveness of the bit isolator,

NIOSH OMSHR performed laboratory and field tests. The

laboratory tests were conducted in OMSHR’s hemi-anechoic

chamber using an RBM and a rock test fixture. Tests were

performed using Barre granite, which has demonstrated

repeatable results during previous laboratory drilling tests.

Sound level measurements were collected at the operator

location. The bit isolator was also tested at various underground

coal mines, where worker noise exposure and sound level data

were collected. 

The results from these laboratory and field tests show that

the bit isolator significantly reduces noise generated by an RBM

without increasing roof bolting cycle time. During laboratory

testing, a 7 dB sound level reduction was observed at the

operator location. The results from the in-mine testing show that

using the bit isolator during drilling reduces sound levels by 4 to

7 dB. Noise exposure measurements underground demonstrated 

a 36% reduction in accumulated noise dose. This difference 

between a standard drill steel (solid line) and the bit isolator 

(dashed line) during an eight-hour work shift is illustrated in 

Figure 4. 

Figure 4. Operator noise exposure 

comparison shows reduction from 100% to 64%. 

Cumulative Dose (%)

Time (hours)

Summary 

Addressing the dominant noise source with engineering 

noise controls is the best method to protect workers from noise 

exposure. The bit isolator is an effective and easy-to-use noise 

control that reduces worker exposure to noise during RBM

operation. Laboratory and field testing have shown this control 

to be practical and effective in protecting workers from noise 

exposure in underground coal mining. 

For More Information 

For more information on the bit isolator for reducing noise 

exposure of RBM operators, contact Lynn Alcorn 

(LAlcorn@cdc.gov or 412-386-6732), Dave Yantek 

(DYantek@cdc.gov or 412-386-4498), Amanda Azman 

(AAzman@cdc.gov or 412-386-6731), or the Health 

Communications Coordinator (OMSHR@cdc.gov), NIOSH 

Office of Mine Safety and Health Research, P.O. Box 18070, 

Pittsburgh, PA 15236-0070. 

To receive NIOSH documents or for more information 

about occupational safety and health topics, contact: 1-800-

CDC-INFO (1-800-232-4636), 1-888-232-6348 (TTY), email: cdcinfo@cdc.gov, or visit the NIOSH Web site at 

http://www.cdc.gov/niosh

Mention of any company name or product does not 

constitute endorsement by the National Institute for 

Occupational Safety and Health.

DHHS (NIOSH) Publication No. 2012–124

coupler (silver) (black) (red) coupler