Page 249 - Advances In Productive, Safe, and Responsible Coal Mining

228 Advances in Productive, Safe, and Responsible Coal Mining

generated by the interaction of ripped coal with different parts of the shearer and the
armored face conveyor, the water spray noise sources, and electric and hydraulic noise
sources. This was the second step in validating numerical modeling results for the cut-
ting drum under realistic operating conditions.
Sound levels were recorded using personal dosimeters at the following locations:
[1] on the longwall shearer approximately 2m away from the headgate drum, and [2]
on the longwall shearer approximately 2m away from the tailgate drum. These dosim-
eters were mounted on magnetic stands installed on the shearer, thus keeping them at a
fixed distance from the cutting drums. Additionally, sound pressure data were
recorded at the headgate drum operator location using an MSHA-permissible audio
recorder at a 44.1-kHz sampling rate, while the shearer was cutting coal from tailgate
to headgate. Explosive hazard restrictions prevent the use of sound level meters or
more sophisticated instrumentation at the face, which limits the level of detail that
can be measured. Comparison of dosimeter data collected in the mine during the
use of standard cutting drums, and dosimeter data collected in the mine during the
use of modified cutting drums, showed a noise reduction of approximately 2.6dB
(A) at the operator location during an 8-h period [22].
Sound pressure time data were recorded using an MSHA-permissible audio
recorder. These data were then converted into the frequency domain using a fast Fou-
rier transform (FFT) algorithm to obtain the sound pressure spectrum. Fig. 12.9 shows
the unweighted and A-weighted one-third octave band spectrum of the data recorded
at the collaborating mine while operating with the standard (baseline) drums as well as
the spectrum of the data recorded while operating with the modified cutting drums.
The spectra show a reduction throughout the frequency range. This broadband reduc-
tion is a major accomplishment that was not attainable with any other noise control
options. In terms of overall sound pressure level, a reduction of approximately
3dB is observed using the unweighted spectra; however, when these spectra are
A-weighted, the overall sound pressure level reduction is approximately 6dB.

110 100
95
100 90
85
Linear SPL (dB) 80 A-Weighted SPL (dBA) 80
90
75
70
70
65
60
60
55
50 50
125 250 500 1000 2000 4000 8000 Total 125 250 500 1000 2000 4000 8000 Total
Frequency band (Hz) Frequency band (Hz)
(A) (B)
Fig. 12.9 Sound pressure spectra of the standard (baseline) drum in gray bars and the modified
drum in yellow bars, computed from audio recordings at the shearer operator location
during normal operation. (A) Linear spectra. (B) A-weighted spectra.