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230 Advances in Productive, Safe, and Responsible Coal Mining

media because of its high compressive strength. Past research at NIOSH showed that
drilling into higher-compressive strength materials generates more noise than drilling
into lower-compressive strength materials [24]. Therefore, granite would provide the
worst-case scenario for noise emission. Also, a low rotation speed of 200rpm and a
low thrust of 2121lbs. were used during testing. Previous NIOSH research showed
that when drilling into hard materials, lower rotation speeds should be used [25].
The lower thrust was used, so a longer drill time could be obtained.
Fig. 12.11 shows acoustic maps obtained from beamforming measurements for
2500–6300Hz one-third octave bands. Each figure shows that as the penetration depth
increases, the noise source near the top of the drill steel remains in nearly the same
location. The source near the bottom of the drill steel travels with the drill head. These
results seem to indicate that the mechanisms of noise radiation are independent of dril-
ling depth [26].
Fig. 12.12 shows the one-third octave band spectrum at the position of the roof-
bolting machine operator’s ear. The overall sound level at the operator’s position
was 99.7dB(A). The frequency content of the noise radiated toward the operator is
dominated by the 1250–8000-Hz frequency bands.

12.4.2.2 Potential noise control concepts
To reduce the sound level at the operator ear location while drilling, noise controls
must target the noise generated at the drill bit and rock interface as well as the drill
steel and drill chuck interface. In addition, the controls must address the mid- to
high-frequency components of the drilling noise. To reduce the radiated sound at both
bit and chuck interface, isolation techniques were used.

Force isolation
A bit isolator was developed to reduce the noise radiated at the bit and rock interface
[27]. A chuck isolator was also developed to reduce the noise radiated at the drill steel
bit and drill chuck interface [28]. Fig. 12.13 shows the first prototypes built to prove
the concept of bit and chuck isolators. Both isolators were designed for noise and

90 90 91 89

80 80 81 79
(A) (B) (C) (D)

Fig. 12.11 Beamforming results for a 1.2-m hex drill steel at drilling depths of: (A) 0m (start of
hole), (B) 0.15m, (C) 0.30m, and (D) 0.45m.