ER: T T T T T T T
a
a
>" 16 Standard drill
rf
3
w
& 100
>
a
a
w 90
© 1
3 Hi
°
See Hy
iS 1
S ’
D 79 Y 1 ! 1 n n 1
31550 100 200 500 1,000 2,000 5,000 10,000
FREQUENCY, Hz
FIGURE 8. - One-third-octove-band noise levels of
standard ond prototype drills.
2

d
8

a
T

3
°
T

Steel chuck
ON i;

chuck

°

SOUND PRESSURE LEVEL
©
3°

1 i 1 1 L

3

|

FREQUENCY, Hz

FIGURE 9. - One-third-octave-band noise levels of pro-
totype drill with standard steel, standard

and Sonoston alloy rotation chucks.

50 100 200 500 1,000 2,000 — 5,000 10,000

pi

Table 7 summarizes
drilling noise as a function
of progressive drill modifi-
cation using 4-foot integral-
chisel-bit and 4-foot
detachable-bit rope-thread
steels and their constrained-
layer-damped versions.
Starting with an unmuffled
BBC-16 drill, noise levels
are listed for the same
drill with the BBC-17 inte-
gral muffler conversion, for
the BBC-17 drill with the
VIP-fiberglass add-on muf-
fler cover, and for the lat-
ter with an alloy rotation
chuck. These noise measure-
ments amplified and con-
firmed some of the previous
findings. The data in
table 7 show similar results
to those in table 6 for
operation at 90 psi under
the CAGI-PNEUROP test
conditions.

Exhaust and drill body
noise reduction obtained by
the use of the muffler cover
on the BBC-17 drill is shown
by the difference in noise
readings between columns 2
and 3 using the constrained-
layer-damped steels. For
the standard steels the
steel resonance noise pre-
dominates, and the overall

noise reduction is not as great. The effectiveness of the constrained-layer
treatment on reducing drill steel resonance noise is shown by the noise dif-
ferences between the standard and treated steels in column 3. Column 4 shows
that a further reduction of 1 db in operating noise is obtained by the alloy

rotation chuck.


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A definite increase in noise level due to higher operating air pressure
was observed for all drill and drill steel combinations tested at 90- and 70-
psi air pressure. Noise levels at 90-psi air pressure averaged 3 to 4 db
higher than those at 70 psi, as shown in table 9.

TABLE 9. - Drilling noise in granite, 90- and 70-psi air pressure,
CAGL-PNEUROP test code, 4-ft. standard and fully
constrained chisel-bit steels

Steel Noise level, db and (dbA)
90 psi 70 psi
4-ft standard chisel..... 118 (115) 116 (111)
4-ft constrained chisel.. 116 (112) 114 (108)

4-ft standard chisel...
4-ft constrained chisel..

114 (111) 113 (108)
110 (107) 107 (103)

BBC-17 with VIP- 4-ft standard chisel..... 113 (108) 111 (107)
fiberglass muffler cover

Dosis vais misses dieioaiates 4-ft constrained chisel.. 101 (98) 97 (94)
BBC-17 with VIP- Pee eWOe cess veeseea eee a 100 (97) 96 (92)

fiberglass muffler cover
and Sonoston chuck.

Performance Evaluation

Drilling performance tests were conducted to determine the effect of
drill modifications and steel damping treatments on drilling efficiency. The
tests were made with selected drill steels using the BBC-17 drill with and
without the VIP-fiberglass muffler cover. The results for the 45 drill and
drill steel combinations are compiled in table 10, which shows for both drills
the total drill steel weight, the measured penetration rate into the rock, and
the rate of rock removal for each drill steel-drill bit combination.

The data on drilling rates show evidence of measurable loss in drill per-
formance because of the use of the muffler cover and the constrained-layer
damping on the drill steels. In figures 11 and 12, granite removal rate is
plotted against total drill steel weight for the damped and undamped drill
steels listed in table 7, divided as to whether the muffler cover was or was
not employed, respectively. The scatter inherent in a test of this type is
understandably great, but the trend is evident and there is statistical evi-
dence that the muffler cover may reduce drilling rate 10 or 12 percent if a
very light drill steel is used. With long, heavy drill steel the cover prob-
ably has insignificant effect on drilling rate. The results showed that the
influence of drill steel weight on drilling rate is linear, and that there
would be a calculable loss of efficiency when a constrained-layer damper was
applied to a drill steel for noise suppression.


Lt

OUHALW

ow

TABLES --Continued

Comparison exhaust noise from BBC-17 drill.
Free-running-rock-drill noise levels..
Summary of drilling noise measurements.
Summary of indoor test site drilling noise levels..
Average drilling noise in granite
Drilling noise in granite, selected drill steels, CAGI-. PNEUROP

Hest codes vecccicis ie We 25 Keaiae 8 Se Risiernidnce Kewisienale 8a &
Drilling noise in granite, 90- and 70-psi air pressure
Penetration rates for selected drill steels.........0-



16

TABLE 7. - Average drilling noise in granite, selected drill

and steel combinations, CAGI-PNEUROP test code
90-psi air pressure

Drill and noise level, db and (dbA)

BBC-17 with BBC-17 with VIP-
Drill steel type BBC-16 BBC-17 |VIP-fiberglass | fiberglass muffler
muffler cover | cover and Sonoston
rotation chuck
4-ft chisel, standard. | 118 (115)/114 (111) 113 (108) 108 (107)
4-ft chisel, 1-3/8-
inch OD, full-length
steel tube, VIP
LETLER: cerccwae commons 116 (112)]110 (107) 101 (98) 100 (97)
4-ft rope, standard,

1-3/4-inch bit....... | 118 (114)]112 (109) 106 (102) 104 (102)
4-ft rope, 1-3/8-inch

OD, full-length steel

tube, VIP filler,

1-3/4-inch bit....... 116 (112)}110 (107) 101 (98) | 100__(97)

Noise levels for a selection of standard and damped drill steels are sum-
marized in table 8. These measurements were made with the modified BBC-17
with the VIP-fiberglass muffler cover to minimize the exhaust and drill body
noise. The data show that the 4-foot steels were the loudest; for them, maxi-
tum noise reduction is obtained by the addition of a full-length constrained-
layer damper. The fully damped 4-foot chisel-bit and rope-thread steels
operated at equal noise levels, whereas of the standard steels the rope-thread
type with the detachable bit is considerably quieter. Partial constrained-
layer dampers added to these steels were also very effective, reducing the
noise level to within 2 to 3 db of the full constrained-layer treatment. No
significant noise reduction resulted from the addition of full-length
constrained-layer dampers to
the 2-foot rope-thread
steels. This is attributed
to the fact that the detach-
able bit acts as an effec-
tive damper for the shorter
steels. Similarly, the
4-foot standard rope-thread
ern eet en steels are considerably
woe Xi quieter than the standard
Damped steel _| 4-foot integral-chisel-bit
steels.

3

3
T
1

3
8
T

Undamped steel >

3
T

80

SOUND PRESSURE LEVEL, db

1 1 1 1 1 L
315 50. 100 200 500 1,000 2,000 5,000 10,000
FREQUENCY, Hz

FIGURE 10. - One-third-octave-band noise levels of pro-
totype drill with standard- and constrained-

layer-damped 4-ft integral-chisel-bit drill
steels.



22

in drilling performance, they cannot be avoided if noise reduction is the pri-
mary goal. These modifications have left the basic drill configuration, air-
hose requirements, controls, and operating methods of the drill unchanged, and
it appears that any further noise reduction would require enclosing the com-
plete drill and steel in an air bag or similar device. The exhaust noise with
these modifications is approximately 90 dbA, and a further reduction in
exhaust noise would not have a noticeable effect on the overall noise level.
Requirements that the drill cover must be rugged and durable, yet light in
weight and not bulky, limit the design and materials that can be used. Drill-
ing performance can be improved by the use of partial-length constrained-layer
dampers at an increase in noise level, and a possible compromise between
drilling performance and noise levels could be made here.


18

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13

Ti T ‘ |i! replaced in succession with
the new steel, manganese-
copper alloy, and Sonoston
Sonoston chuck chucks. This trend is
ia Tae observed most consistently
= in tests employing the drill
steel with constrained-layer
damping.

ORILL

Mn-Cu alloy chuck

Ww

New steel chuck Other efforts to uti-
lize damping alloys were not
- effective in reducing the
Worn steel chuck noise. Substitution of

Sonoston and manganese -
Steel stub in Damped 4-foot Undamped 4-foot copper air valves in the

rubber pad chisel-bit steel chisel-bit steel BBC-17 drill produced no
observable reduction in
overall noise under free-run,
simulated drilling on the
test pad, or when drilling
= in granite. A Sonoston
Mn-Cu alloy chuck | damping alloy back end for
| TT | this drill did not reduce
the noise from this area of
the drill. Mechanical
| | impacting tests on damping
alloy blocks confirmed the
assumptions of Miller (8)
] that this type of material

prootioet trite te tis) ; "
90 95 100 105 To 115 20 tS inadequate for piston

NOISE LEVEL, dbA construction.

FIGURE 7. - Variations in noise level obtained by chang- RESULTS WITH COMBINED
ing rotation chucks in a new and a worn MODIFICATIONS

BBC-17 drill. No drill case, exhaust hoses Laboratory Noise Tests
attached, 70-psi air pressure. Indoor)

The prototype drills
with the combination muffler-drill body covers, constrained-layer-damped
chisel-bit drill steel, and Sonoston alloy rotation chuck operated at a sound
pressure level of 102 db and 99 dbA, compared with a sound pressure level of
121 db and 115 dbA for the unmodified BBC-16 and chisel-bit steel. Table 6
summarizes the stepwise reduction in noise as various modifications were added
to the standard drill until it was converted to the prototype. The summary in
table 6 represents data acquired at the indoor test site over the period of
drill development.

NE

: eS}
hcg ys

Sonoston chuck

DRILL

New steel chuck

WORN

Worn steel chuck



by deadweight attachments to the drill. Drilling performance of steels with
equal bit diameters was evaluated on the basis of penetration rates; perform-
ance of steels of different bit diameters was evaluated on the basis of gran-
ite removal rates. The latter are proportional to the rate of energy expended
in removing the rock and offer a more suitable measure of drilling efficiency
because the differences in bit diameters are compensated for.

Special Noise Measurements

To measure the specific noise components of the drill, special methods
were devised. To isolate the noise from the drill mechanism alone, a short
stub of drill steel, with the exposed section below the collar enclosed in
thick-walled rubber tubing and sound-absorbent foam, was used to eliminate the
drill steel noise. The stub was inserted into a lubricated, close-fitting
hole in a large rubber block, and the drill was operated in the normal way for
brief periods. This eliminated virtually all the drill steel noise. Addition
of exhaust hoses eliminated the exhaust noise and permitted the drill machine
noise radiated from the case to be measured and analyzed.

To measure and analyze drill steel noise, the drill body was encased in
an absorber-barrier cover and the exhaust air was carried away by hoses.

Exhaust noise levels were for three drill configurations: (1) The drill
was operated free running without steel in a suspended position; (2) to deter-
mine the exhaust noise level under load, the drill was operated with the stub
on the rubber block so that the steel resonance noise was eliminated; and
(3) the exhaust noise level was calculated from the difference in noise levels
when drilling in granite without and with exhaust hoses.

DRILLING NOISE LEVELS

Table 2 summarizes representative noise levels observed in the laboratory
when drilling in granite with the unmodified Atlas-Copco BBC-16 and 17 drills
using standard 4-foot integral-chisel-bit and 4-foot detachable-bit rope-
thread steels. The data show a variation from a high sound pressure level of
121 db to a low of 112 db at the 70-psi operating pressure. Variation in the
corresponding A-weighted noise level was from 115 to 108 dbA. The noise lev-
els of the drill and steel combinations of table 1 are shown in 1/3-octave
bands in figure 1. From the curves in figure 1 it can be seen that the BBC-17
drill with the built-in muffler is considerably quieter than the BBC-16, and
that most of the noise reduction is at the lower frequencies. Also it is
apparent that the detachable-bit rope-thread steel is quieter than the
integral-chisel-bit steel, with most of the noise reduction at higher
frequencies.

The reduction in noise in changing from the BBC-16 drill with the chisel-
bit steel to the BBC-17 with the rope-thread steel is representative of what
can be achieved by judicious selection of available standard equipment. While
the reduction in noise is significant, the lowest level is still extremely
high and the drill and steel combination that produced it may not be practical


protective device or system, including personal devices, which the Secretary
or his authorized representative finds to be hazardous or to cause a hazard to
the miners in such mine."' Hence, the act is considered to be restrictive in
the possible use of ear muffs and plugs to provide ear protection, and empha-
sis is placed on the reduction of operational noise to acceptable levels.

As part of the Bureau of Mines research program on coal mine noise prob-
lems under the act, the Rolla Metallurgy Research Center was assigned the task
of modifying the rock drill to reduce its noise level to acceptable limits, as
shown in table 1. The approach chosen to produce the prototype quiet drill
was to start with existing drills and make such modifications and component
substitutions as would be possible in a short period of time. Use of noise-
suppressing materials, damping alloys, and innovative approaches and methods
was to be stressed in fabrication of external covers and muffling devices and
in modifying the internal drill mechanism.

TABLE 1. - Permissible noise exposure under the
Walsh-Healey Public Contracts Act!

Sound level, | Duration per day, |} Sound level, | Duration per day,
dbA hours dbA hours
90 8 102 1-1/2
92. 6 105 1
95 4 110 1/2
97 3 215 1/4
100 2: >115 0

When the daily noise exposure is composed of 2 or more periods
of different levels, their combined effect is to be used,
rather than the individual effect of each. If the sum of the
fractions Cl1/Tl + C2/T2 + ... + Cn/Tn exceeds unity, then the
mixed exposure is considered to exceed the limit value. (Cn
indicates the total time of exposure at a specific noise
level, and Tn indicates the total time of exposure permitted
at that level.)

Functional performance requirements and operator acceptance were major
considerations in planning and making the modifications. From a functional
standpoint, the prototype drill should operate without serious loss of drill-
ing efficiency; it should neither overheat nor ice up to a point of reduced
efficiency or complete stoppage. It should be rugged and durable enough to
take the stresses of normal use without breakage and special maintenance.
From an operator's standpoint, the added weight of modifications should be
kept to a minimum. Projecting mufflers and other devices that could catch or
break, or be broken off intentionally, should be avoided, there should be no
interference with operating controls, and the modified drill must withstand
the same severe treatment as the standard drill.


NOISE REDUCTION OF A PNEUMATIC ROCK DRILL
by

Aarne Visnapuu! and James W. Jensen!

ABSTRACT

Experimental modifications have been made by the Bureau of Mines on stan-
dard pneumatic rock drills to reduce the noise of the air exhaust, drill steel
resonance noise, and noise radiated by the drill body. A close-fitting case
and muffler around the drill body, consisting of a metallic honeycomb skeleton
filled with viscoelastic absorber on the inside and a durable outer shell,
provide both exhaust and drill body noise muffling and absorption. Drill
steel resonance noise is reduced by a constrained-layer treatment consisting
of a tubular metal cover bonded to the ouside of the rod by a viscoelastic
filler. Damping alloy components have been developed to reduce metallic
resonance noise. Combined, these modifications have reduced the drilling
noise level in granite from 115 dbA to 97 dbA. Data are presented on the
individual and combined effects of these modifications on drilling noise and
performance.

INTRODUCTION

The pneumatic rock drill is one of the most severe noise sources in the
mining industry. A 1971 environmental noise survey in 21 coal mines by the
Bureau of Mines revealed that pneumatic rock drilling for the installation of
roof bolts exposed individuals to the highest sound levels observed, a range
of 104 to 118 dbA (Ze 3 The pneumatic drill is used worldwide and in hard
rock mining, comparable noise levels are common.

The Federal Coal Mine Health and Safety Act of 1969, Public Law 91-173,
states in section 206 that "on and after the operative date of this title the
standards on noise prescribed under the Walsh-Healey Public Contracts Act...
shall be applicable to each coal mine and each operator of such mine shall
comply with them."' The act further stipulates that "in meeting such standards
under this (noise) section, the operator shall not require the use of any

*Research physicist.

2Underlined numbers in parentheses refer to items in the list of references at
the end of this report.

5Noise levels given in dbA are measured on a weighted scale that simulates
response of the human ear.


14

TABLE 6. - Summary of indoor test site drilling noise levels, singular
and combined drill modifications, average of three drilling
tests in granite, 70-psi air pressure

Test | Sound level | Drill Muffler cover Rotation chuck Drill steel
db dbA (chisel)
1 121.3 | 115.0 | BBC-16 Standard.
2 116.5 | 112.0 | BBC-16 Damped.
3) 117.3 | 113.5 | BBC-17 Standard.
4 110.2 | 106.5 | BBC-17 Damped.
5 117.5 | 111.0 | BBC-17 Standard.
6 104.2 99.5 | BBC-17 Damped.
7 115.2 | 109.0 | BBC-17 Standard.
8 102.7 98.5 | BBC-17 Damped.
g 115.2 | 111.0 | BBC-17 Standard.
10 104.7 | 102.0 | BBC-17 Damped.
11 114.2 | 109.0 | BBC-17 Standard.
12 102:.3. 99.0 | BBC-17 Damped.

The effectiveness of selected singular and combined modifications in
reducing drilling noise, listed in table 6, is further illustrated in fig-
ures 8, 9, and 10, where the 1/3-octave-band noise spectra are compared.
Figure 8 compares the noise spectrum of the original standard drill and
chisel-bit steel (test 1) with the final prototype (test 8). The effect on
the noise spectrum in changing from the standard steel rotation chuck (test 6)

o the Sonoston alloy rotation chuck (test 8) is demonstrated in figure 9.
‘lost of the noise reduction appears to be above 400 Hz in this case. Fig-
ure 10 shows noise reduction resulting from the use of a constrained-layer-
damped chisel-bit steel (test 8) in place of the standard chisel-bit steel
(test 7). The decrease of noise is most pronounced at the higher
frequencies.

Laboratory Noise Tests (Outdoor)

In addition to the drilling noise and performance evaluation conducted
concurrently with the prototype development, a final, comprehensive noise and
performance evaluation of the modified drills and drill steels was made in
comparison with standard drills and drill steels. Drilling noise was measured
outdoors in conformance with the CAGI-PNEUROP test code for pneumatic rock
drills. All the tests were conducted at an operating air pressure of 90 psi
and an applied thrust of 290 pounds, which was determined to give maximum
penetration rate at the 90-psi pressure. Forty drill and drill steel combina-
tions were tested, each twice, on a random schedule, for a total of 400 noise
measurements.


analysis instrumentation. Granite blocks, approximately 3 feet on a side,
were used as the drilling medium. The outdoor site was a level, cleared area
approximately 100 feet from the warehouse. The granite block was set in the
ground with the upper surface flush with the ground. Compressed air was pro-
vided from a remote compressor, and a ballast tank was installed at the test
site to aid in pressure regulation.

The sound laboratory was instrumented with a portable sound level survey
meter, a sound and vibration analyzer with graphic level recorder, sound level
calibrators, an impact noise analyzer, a real-time spectrum analyzer, a digi-
tal integrator with X-Y plotter and dual-trace display oscilloscope, labora-
tory and portable tape recorders, and other accessories.

Drilling Noise Measurements

Most of the noise measurements involved in developing the prototype drill
were taken at the indoor drilling site and at only 70-psi air pressure due to
limitations in compressor capacity. The drill was operated in a vertical posi-
tion into a granite block resting on the floor. The operator was positioned
on a wooden platform above the rock and behind the drill, which was located
over a cutout section on one side of the platform. The microphone pickup was
located on the clear side of the platform and normally was positioned 1 meter
from the geometric center of the drill. Normally, the front or exhaust port
side of the drill faced the microphone, but the drill could be rotated so that
any portion of the body pointed toward the microphone. The drills had two
exhaust ports that directed the airblast to the sides, so the air did not blow
directly on the microphone. The microphone could also be positioned above the
drill. Each test was tape-recorded, and simultaneously a linear real-time
spectral analysis was performed on the incoming signal. After each test the
stored real-time spectrum was plotted on an X-Y recorder to provide the ini-
tial analysis of the test. Further analysis, such as 1/3-octave band, was
made from the tape. The same procedure was used for vibration analysis by
accelerometers.

Drilling noise measurements at the outdoor location were conducted at
90-psi air pressure according to the CAGI-PNEUROP® test code for pneumatic
rock drills (5). The noise levels were measured at a distance of 1 meter in
four directions horizontally from the geometric center of the drill and also
at a position 1 meter above the geometric center of the drill. Data analysis
was the same as for the indoor tests, except that no real-time spectral anal-
ysis was made during the test.

Drill Performance Measurements

The effects of the various drill and drill steel modifications on drill
performance were evaluated on the basis of penetration rates in granite. Pene-
tration rates were determined from vertical drilling distances for fixed time
periods under specific thrust and operating pressure. The thrust was supplied

*Compressed Air and Gas Institute-European Committee of Manufacturers of Com-
pressed Air Equipment.


20

TABLE 10. - Penetration rates for selected drill steels, BBC-17
and BBC-17 with VIP-fiberglass muffler cover
290-1lb thrust, 90-psi air pressure,

Gray Georgia granite

Constraining Total steel Penetration rate, Granite removal rate,
treatment, and bit wt, in/min in® /min
number from 1b BBC-17 BBC-17, BBC-17 BBC-17,
table 8 muffler cover muffler cover
4-FT CHISEL-BIT STEEL, 1-5/8-INCH-DIAMETER BIT
1 922 13.7 12.6 28.5 26.1
2 10.7 11.0 9.7 22.8 20.2
3 11.3; 12.0 Tw 25.0 22.9
4 10.3 NA 11.5 NA 23.8
2 2:9, 8.6 766 17.8 15.8
6 13.4 Gad. 8.7 18.9 18.1
4-FT ROPE-THREAD STEEL, 1-3/4-INCH-DIAMETER 4-POINT BIT
7 11.6 10.5 9.7 2552 23.6
8 1952: 9.6 9.2 23.1 22.1
9i 15).2 NA 7.4 NA 17.9
10 15.6 7.8 6.7 18.8 16.1
11 14.5 8.0 7.8 19.3 18.8
12 13.5 8.4 7.7 20.3 18.4
2-FT ROPE-THREAD STEEL, 1-3/4-INCH-DIAMETER 4-POINT BIT
13 7.5 NA 10.2 NA 24.6
14 9.5 11.1 10.1 26.7 24.3
2S 9.6 a2. 8.9 22.1 21.4
16 8.5 NA 8.9 NA 21.4
4-FT ROPE-THREAD STEEL, 2-1/8-INCH-DIAMETER 4-POINT BIT
7 12.5 5.3 6.3 18.9 2265
8 14.1 5.7 5.7 20.4 20.4
9) 16,1 NA 4.7 NA 16.8
10 16.5 4.9 4.9 17.4 17.4
11 15.4 Dad. 5.6 18.3 19.9
12 14.4 5.3 5.3 18.9 18.9
2-FT ROPE-THREAD STEEL, 2-1/8-INCH-DIAMETER 4-POINT BIT
13 8.4 NA 6.8 NA 24.0
14 10.4 Fal 6.1 25%.1 2156
15 107.5, Sod 5.4 19.4 19.3
16 9.4 NA 5.8 NA 20.6
NA Not available.
DISCUSSION

Two versions of the prototype muffler cover, installed on the BBC-17, are
pictured in figures 3 and 4. -Figure 3 shows the 9-pound aluminum cover, and
the 6.5-pound fiberglass cover is pictured in figure 4. Both versions of the
prototype drill present a clean and smooth appearance, although both are some-
what bulkier than the standard drill. There are no appendages to interfere

ith normal use of the drill or to break off. The cover does not interfere
vith the operating controls or with the air and water couplings at the top of


pwr... |

RI »_

Bureau of Mines Report of Investigations/1975

Noise Reduction
of a Pneumatic Rock Drill

UNITED STATES DEPARTMENT OF THE INTERIOR



NOISE LEVEL, db

ig T T T T T

100 4-ft integrol~chisel-bit steel | a

90]|

80

60

FREQUENCY, kHz
FIGURE 2. - Noise spectrum of standard 4-ft drill steels.

106 dbA) and was muted, as indicated by the absence of sharp transverse reso-
nance peaks in the lower curve of figure 2.

Noise radiated by the drill body was observed to be considerably lower,
ranging from 101 to 105 db (96 to 100 dbA).

MODIFICATIONS AND INNOVATIONS

It is apparent that successful sound reduction can be realized only if
the noise contribution from each source is reduced to a level considerably
below that of the final total level, because of the way sound levels combine
(10). Ultimately even the drill body noise must be reduced to produce a pro-
totype drill that is "quiet" in operation.

Exhaust

A study to compare the merits of expansion and reactive types of mufflers
was made to determine which should be incorporated into the prototype. The
expansion muffler was based on the work of Barth (1), modified for adaptation
to the BBC-17 drill. The reactive muffler was of the Pi type® made according
to the design data of Chester, DeWoody, and Miller (4). The test results,
summarized in table 3, showed that the reactive type was more effective in
reducing the exhaust noise from this drill, apparently because the shape of
the drill did not permit construction of an expansion chamber cavity as long

SA Pi-type reactive muffler consists of two expansion chambers with a connect-
ing pipe.


120 T T

ir)
a 110
os)
+ 105
w
>
iy
4 100
w
«
5
2 95
wo
wi
a
a 90
2 BBC-16, 4-ft chisel bit
S85 —— BBC-I7, 4-11 chisel bit 4
ro) ---=-- BBC-16, 4-{t ropethread bit
n BBC-I7, 4=ft rope-threod bit

80 iN. LA =

er
- | l ! \ l !
31.5 50 100 200 500 1000 2000 5000 10,000
FREQUENCY, Hz
FIGURE 1. - One-third-octave-band noise levels for standard drills and steels.

in actual use. It is evident that noise levels of these typical drill and
steel combinations need further reduction.

TABLE 2. - Drilling noise levels in granite, standard BBC-16
and BBC-17 drills, 4-ft integral chisel bit and

4-ft detachable-bit rope-thread steels, indoor
drilling site, 70-psi air pressure

Drill Steel Noise level
db dbA
BBC-16.. 121 115
BBC-17.. LL? 112.5
BBC-16.. 116 113.5
BBC-17.. 12: 108

Exhaust noise of the BBC-16 ranged from 116 to 120 db (112 to 114 dbA).
The corresponding noise levels for the BBC-17 were 108 to 112 db (107 to
110 dbA), confirming that the integral muffler conversion is effective in
reducing the exhaust noise level of the BBC-16 drill as advertised.

The integral-chisel-bit steels produced a sound pressure level of 115 to
117 db (112 to 113 dbA). Tones at the frequencies of transverse vibrations of
the steel dominated the sound frequency spectrum, as shown in the narrow-band,
‘onstant-bandwidth plot of the noise spectrum in figure 2. The noise of the
detachable-bit rope-thread steels was not as intense at 107 to 109 db (105 to


An Atlas-Copco BBC-16* feed-leg drill was selected for development of the
prototype drill. This drill can be converted to a muffled version, the BBC-17,
by substitution of a patented silencing exhaust cylinder for the standard
exhaust cylinder. This releases the exhaust air through a ring of small holes
into an expansion chamber, which according to the manufacturer reduces the
exhaust noise by 75 percent. The drill weighs 59 pounds and has a piston
diameter of 2.75 inches and a stroke of 2.125 inches. At an air pressure of
85 psi, the BBC-16 is rated at 2,300 impacts per minute, and the BBC-17 at
2,200.

This research and other earlier studies (2, 8) show that drilling noise
is the sum of three major noise sources: Exhaust, drill steel resonance, and
drill machinery noise. Exhaust noise is normally the most severe and is pro-
duced by the high-velocity cyclic release of spent compressed air to the
atmosphere. Mixing of the high-velocity air with the relatively still atmos-
phere gives rise to broadband random noise, and the cyclic nature gives it the
characteristic staccato sound. Drill steel noise results from the transverse
and longitudinal resonance vibrations excited by the piston impacts on the
steel shank. Drill machinery noise is produced by the metallic resonance
excited by the piston striking the steel, and the moving and impacting parts
inside the drill, and is then radiated to the surroundings by the drill body.
It is the least offensive of the three noises and becomes apparent only when
the exhaust noise and drill steel noise have been reduced to low levels.

Exhaust noise can be reduced two ways: By hoses to carry the expelled
air away from the drilling site, and by muffling. Hoses are effective and
have been used with more or less stationary equipment. In mining operations
hand-held and feed-leg drills require freedom of movement in limited space,
making weight and mobility critical factors. Design of noise reduction into
the drill itself is most desirable. Mufflers, to be used effectively, should
reduce exhaust noise by 20 db or more and should be lightweight, compact,
durable, and not prone to freezing during operations. Mufflers of expansion
(1) and reactive (4) types have been investigated for use on pneumatic drills.
In this work the reactive type was selected for prototype development because
it provided better noise reduction and design flexibility. Using the crite-
rion that the method of suppressing the drill steel resonance noise should be
an integral part of the steel, constrained-layer damping was selected as
offering the best method for reducing noise from this source. For reducing
drill body noise, two innovations were investigated. One was the use of a
combination muffler cover, and the other was substitution of energy-absorbing-
alloy components where possible to damp impact shock waves.

EXPERIMENTAL WORK

Experimental Facilities

Experimental drilling was conducted at two locations. The indoor site
was in a large warehouse building that also housed the laboratory and noise

4Reference to specific trade names does not imply endorsement by the Bureau of
Mines.


TABLE 8. - Drilling noise in granite, selected drill steels, CAGI-PNEUROP

test code, BBC-17 drill with VIP-fiberglass muffler cover,

90-psi_ air pressure

7

Campaign A Campaign B
No. Constraining treatment Bit diam,|Noise level,|Bit diam,|Noise level,
inches |db and (dbA)| inches |db and (dbA)
and type and type
4-FT CHISEL-BIT STEEL
Es (NOWC asin seacaverinaey scovecaraigce ore soaraue 1-5/8, 113 (108) NAp NAp
chisel.
2 |1-3/8-inch OD, 13-inch long] ...do....] 104 (101) NAp NAp
steel sec., clamped on
top, rubber filler.
3 |Two 1-3/8-inch OD, 12-inch veAG. o> -| 105 (101) NAp NAP
long steel sec. on top and
bottom, VIP filler.
4 |One 1-3/8-inch OD, 12-inch ..do....] 106 (104) NAp NAp
long steel sec. on top,
VIP filler.
5 |1-3/8-inch OD, full length +-do....] 101 (98) NAp NAp
steel tube, VIP filler.
6 |1-3/8-inch OD, full length wedO. 16] LOL (98) NAp NAp
steel tube, Flexane 30
filler,
4-FT ROPE-THREAD STEEL
7 \Noneseg ts8e,ee S259 5ON se eeis.2 1-3/4, 106 (102) |2-1/8, 108 (104)
a 4 point. 4 point.
8 |1-3/8-inch OD, 13-inch long | ...do....] 104 (100) .-do....] 103 (100)
steel sec., clamped on
top, rubber filler.
9 }1-1/2-inch OD, full length -.do..../ 100 (97) vesdos 102 (98)
steel tube, VIP filler.
10 |1-3/8-inch OD, full length «.do....| 100 (98) --do....] 102 (98)
steel tube, VIP filler.
11 |1-1/4-inch OD, full length --do....| 102 (99) endOre 102 (98)
steel tube, VIP filler.
12 |1-1/2-inch OD, full length «-do....} 102 (98) sados...] 102 (98)
Al tube, VIP filler.
2-FT ROPE-THREAD STEEL
WS Neoreete apterteiemtetelase ote ceniaeeiaterara 1-3/4, 103. (99) |2-1/8, 105 (101)
4 point. 4 point.
14 |1-3/8-inch OD, full length «dG 102 (99) --do....{ 103 (100)
steel tube, VIP filler.
15 |1-3/8-inch OD, full length sedoO.se.+| 103 (99) «+-do....] 104 (100)
steel tube, Soundcoat
filler.
16 |1-1/2-inch OD, full length «.do 104 (100) «-do....] 104 (101)
Al tube, VIP filler.
NAp Not applicable.


10.

23

REFERENCES

Barth, M. G. A Practical Approach to the Exhaust Silencing of a Pneu-
matic Rock Drill. M.S. Thesis, University of Missouri--Rolla, 1972,
159 pp.

Beiers, J. L. A Study of Noise Sources in Pneumatic Rock Drills. J.
Sound Vib., v. 3, 1966, pp. 166-194.

Birchon, D. H. Metals To Reduce Noise and Vibration. The Engineer,
Aug. 5, 1966, pp. 207-209.

Chester, J. W., R. T. DeWoody, and W. C. Miller. Noise From Pneumatic
Rock Drills. Shape and Exit Noise of an Exhaust Muffler. BuMines
RI 6450, 1964, 12 pp.

Compressed Air and Gas Institute. CAGI-PNEUROP Test Code for the Measure-
ment of Sound From Pneumatic Equipment. Compressed Air and Gas Insti-

tute, New York, Ist ed., 1969, 20 pp.

Jensen, J. W., and D. F. Walsh. Manganese-Copper Damping Alloys.
BuMines Bull. 624, 1965, 54 pp.

Lamonica, J. A., R. L. Mundell, and T. L. Muldoon. Noise in Underground
Coal Mines. BuMines RI 7550, 1971, 11 pp.

Miller, W. C. Noise From Pneumatic Rock Drills. Measurement and Signif-
icance. BuMines RI 6165, 1963, 30 pp.

Paone, J., D. Madson, and W. E. Bruce. Drillability Studies--Laboratory
Percussive Drilling. BuMines RI 7300, 1969, 22 pp.

Peterson, A. P. G., and E. E. Gross, Jr. Handbook of Noise Measurements.
General Radio Co., West Concord, Mass., 6th ed., 1967, p. 209.

INT.“BU.OF MINES,PGH.,PA. 20615


CONTENTS

Abstract
Introduction.....
Experimental work..
Experimental facilities.
Drilling noise medstvenents
Drill performance measurements..
Special noise measurements..
Drilling noise levels.........
Modifications and innovations.
Exhaust.....
Drill steel.
Drill body. .
Results with combined modifications. .
Laboratory noise tests (indoor)
Laboratory noise tests (outdoor).
Performance evaluation. .
Discussion......
Conclusions...
References....

ILLUSTRATIONS
1. One-third-octave-band noise levels for standard drills and steels...
2. Noise spectrum of standard 4-ft drill steels..... ee ceccceesens eoeees
3. Plastic-aluminum-outer-surface muffler cover on Atlas-Copco
BBC=17) Grill..o.. o0.o00.0000 0009010 © 08 ajesnssie;sisiernisie'9 Pn Foseoies ai) eseieaaraieis «

4. Fiberglass- outers leur ace muffler cover on Atiasccopeo BBC - 17 drill..
5. Installation of plastic-aluminum-outer-surface muffler cover

on Atlas-Copco BBC-17 drill........ oe cence ecesercese Pee eeceeeene .
6. Detachable-bit rope-thread and iategral-chisel-bit drill steels

with constrained-layer damping COVErS.....ccesccscccccssscecceeres
7. Variations in noise level obtained by changing rotation chucks

in a new and a worn BBC-17 drill........... Scere nee ereereresns

8. One-third-octave-band noise levels oF standard and prototype drills,
9. One-third-octave-band noise levels of prototype drill with standard
steel, standard and Sonoston alloy rotation chucks........ Preiser @
10. One-third-octave-band noise levels of prototype drill with standard-
and constrained-layer-damped 4-ft integral-chisel-bit drill steels
11. Dependence of granite removal rate on total drill steel weight,
BBC-17 drill with VIP fiberglass muffler cover.
12. Dependence of granite removal rate on total drill steel weight,
BECS17. MELT ccs eissrsew ow emaiereies sewieniae aia sieeve 4s a .

TABLES

1. Permissible noise expsoure under the Walsh-Healey Public Contracts
Actars saseek tees Ses. o 4. So Ais swig aS Se, 9i0.18,518 GS) ere-a:ccnyere exe sieve: .

E
ay
lo

NNUOP POW He

10

ilu

13
15)

15

16


10

Figure 3 shows the muffler cover made with the plastic aluminum outer
surface, and figure 4 shows the muffler cover with the fiberglass outer sur-
face. Figure 5 shows the aluminum-putty-surface muffler cover being attached

to the drill.

Noise levels under free-running conditions with these two muffler covers
are compared with levels with the bare BBC-16 and 17 drills in table 4.

TABLE 4. - Free-running-rock-drill noise levels, standard drills

without and with muffler covers, indoor test site,

70-psi air pressure

Drill Muffler cover Noise level
db dbA

120 | 115

112 | 109
Urethane rubber-aluminum putty. ‘i 96 92.5

Syntactic polyurethane-fiberglass.. 96 92

Drill Steel

The constrained-layer damping system developed in this research consists
of a thin-walled tubular metal cover bonded to the drill steel by viscoelastic

FIGURE 5. - Installation of plastic-cluminum-outer-
surface muffler cover on Atlas-Copco

BBC-17 drill.

material that adheres well
to both. This makes the
noise suppression treatment
into an integral part of the
drill steel, not removable
or detachable.” The bonding
material must be strong
enough to withstand the vio-
lent vibrations in drilling,
yet compliant enough to
absorb the steel vibrations
without transmitting them to
the outer cover.

Constrained-layer -
damped steels were prepared
by slipping the metal tube
over the drill steel,

7Jensen, J. W., and
A. Visnapuu (assigned to
U.S. Department of the
Interior). Constrained
Layer Damper and Noise
Suppressor for a Rock
Drill Steel. U.S.
Pat. 3,842,942, Oct. 22,
1974.


12

Drill Bod

Two methods were developed to suppress the drill body noise. One was the
combination muffler cover that encased most of the drill body, and the other
was energy-absorbing alloy components substituted where possible to damp
impact shock waves.

Alloys composed of manganese and copper are noted for their ability to
damp vibration, and in mechanical strength they are comparable to mild steel
(6). The energy they absorb is converted to heat, and they lose their damping
capacity above 125° C. Miller (8) substituted a manganese-copper piston in a
drill and reduced the noise, but the piston expanded on heating and jammed,
and also lost its damping capacity.

In this investigation the use of damping alloy substitute components such
as the rotation chuck, air valve, back end, and piston was further explored.

The rotation chuck proved to be the most effective noise suppressor of
the damping-alloy substitute components. Its function is to transmit rotation
to the drill steel, and it encases the volume in which the piston strikes the
end of the steel. Stresses on the chuck are not as severe as on the percus-
sion train and other components in the drill, and substitution of a damping
alloy can be tolerated. It was postulated that the damping alloy surrounding
the piston-steel impact point would absorb much of the shock and noise trans-
mitted to the drill case through the chuck.

Two experimental rotation chucks, one of Sonoston and the other of
binary alloy (73 Mn-27 Cu), were cast and fabricated for the BBC-16 and 17
drills. The Sonoston alloy (3) was developed in England and consists (in
percent) of 54 Mn, 37 Cu, 4.25 Al, 3 Fe, and 1.5 Ni. The ingot was machined
roughly to the dimensions of the chuck and then was heattreated 4 hours at
450° C before final machining. The cast binary-alloy ingot was hot-forged to
3-inch-diameter rod from which the chuck was rough machined. Final aging was
for 2 hours at 450° C before finish machining.

The effectiveness of the alloy chucks in reducing the drilling noise was
compared in a series of drilling tests using each of the two alloy chucks, a
new steel chuck, and a worn steel chuck in two BBC-17 drills; one drill was
new, and the other was moderately worn. The drills were provided with exhaust
hoses, but no covers were used on the drill bodies. They were operated at an
air pressure of 70 psi and a constant thrust of 200 pounds. Standard and
constrained-layer-damped steels and the steel stub were employed in the tests.
Tests were made on a random schedule to give 3 tests in each of the 24 com-
binations of variables. The results are summarized in table 5, where the
average sound pressure levels and A-scale noise levels for each experimental
drilling condition are given. In figure 7 the 24 average A-scale values of
noise levels in table 5 are plotted as bar graphs, arranged in two sets thet
group the observations for the new and used drills. Each bar is shaded to
show the level observed for each of the three drill steels used. It is evi-

‘nt that the worn chuck is noisiest in all cases, and the prevailing trend in
neral is for the noise level to decrease stepwise as the worn chuck is


This publication has been cataloged as follows:

Visnapuu, Aarne

Noise reduction of a pneumatic rock drill, by Aarne

Visnapuu and James W. Jensen. [Washington] U.S. Bureau
of Mines [1975]

23 ps illus., tables. (U.S. Bureau of Mines. Report of investi=
gations 8082)
Includes bibliography.

1. Rock drills. 2. Noise control. I. U.S. Bureau of Mines. II.
Jensen, James W., jt. auth. II]. Title. (Series)

TN23.U7 ~— no. 8082» 622.06173

U.S. Dept. of the Int. Library



21

the drill, and it can be readily removed for servicing the drill. For these
reasons operator acceptance of the prototype drill should be favorable.

Constrained-layer treatment was demonstrated to be an effective method
for reducing drill steel noise. This method does not alter the integrity of
the steel rod and would not in any way weaken the rod and cause premature
failure. Conversely, it could prolong steel life by reducing fatigue damage
from transverse resonance vibrations and steel flexing. For the 4-foot, and
presumably for longer, drill steels, constrained-layer dampers can play an
essential role in noise reduction. This is especially true for the integral-
chisel-bit steels, which without damping treatment operate at extremely high
noise levels. In comparison, the standard rope-thread steel is 6 db quieter
because the detachable bit acts as a damper for steel resonance, but it also
operates at the quietest level with a full constrained-layer damper. With
partial constrained-layer dampers, both types of steel operate at noise levels
that are only 2 to 3 db higher than those of the fully treated steels. No
significant reduction in noise was realized for the 2-foot detachable-bit
rope-thread steels by the use of full-length constrained-layer damping treat-
ment. The low level of steel resonance noise in this case is attributed to
the detachable nature of the bit, which acts as an effective damper for the
short rod. Since the drilling rate is dependent inversely on the steel
weight, the use of partial covers or short detachable-bit steels where prac-
tical appears to offer a method of obtaining significant reduction in noise
level with minimal loss of drilling efficiency.

Comparison of noise levels between tests at 70 and 90 psi (table 8)
indicates that the 20-psi increase in air pressure causes an increase of about
4 dbA in noise level. Bureau of Mines data on the relationship between pene-
tration rate and operating air pressure (9) show that in hard-rock drilling
the increase in air pressure from 70 to 90 psi increases the rate by approxi-
mately 50 percent. For the prototype drill the 20-psi reduction caused the
noise level to fall from 97 to 92 dbA. Under Walsh-Healey limits, an operator
could work twice as long with air at 70 psi (6 rather than 3 hours) and be in
compliance, and possibly could drill more than twice as much footage as in
3 hours at 90 psi.

The average 2-db noise reduction obtainable with the damping alloy rota-
tion chucks under all drilling conditions where the exhaust noise is not a
factor is of enough significance to merit serious consideration in industrial
drill production.

CONCLUSIONS

The noise levels of the common hand-held or air-leg pneumatic rock drills
operating at an air pressure of 90 psi can be reduced from 115 dbA to 97 dbA
by modifications that do not change the basic configuration or utility of the
drill. Drill exhaust and mechanical noise radiated by the drill body were
reduced by a combination muffler cover that increased the drill weight by only
10 percent. Drill steel resonance noise was reduced by full or partial
constrained-layer dampers around the steel and by use of damping alloy rota-
tion chucks. Although these drill and steel modifications result in some loss


Report of Investigations 8082

Noise Reduction
of a Pneumatic Rock Drill

By Aarne Visnapuu and James W. Jensen

Rolla Metallurgy Research Center, Rolla, Mo.

UNITED STATES DEPARTMENT OF THE INTERIOR
Kent Frizzell, Acting Secretary
Jack W. Carlson, Assistant Secretary—Energy and Minerals

BUREAU OF MINES
Thomas V. Falkie, Director


as in the original design, whereas with the reactive muffler the shape of the
cavity was not a factor as long as essential volume was provided. Based on
these test results, the reactive-type muffler was selected for the prototype.

TABLE 3. - Comparison exhaust noise from BBC-17 drill operated on
rubber pads with resonant chamber and Pi-type muffler

under same experimental cover, 70-psi air pressure

Muffler type under cover Noise level

db dbA

No muffler or cover... 112 110
Resonant chamber.... dae eves 109 99)
Pi-type teactives ceiws wi aeves i ses 98 95:

A muffler cover was designed in which the exhaust from the drill vents
first into an open air space between the drill body and cover and then to the
atmosphere. It was decided to start with the BBC-17 drill to take advantage
of the lower exhaust noise level of this machine. Advantages of adding this
type of muffler cover to the BBC-17 are (1) the outer surface can be made to
conform to the general shape of the drill to avoid excessive bulk and protru-
sions, (2) the design maximizes the air volume of the muffler, (3) contact of
cold expelled air with heated parts of the drill body would tend to counteract
freezing of the exhaust and heating of the drill, (4) the muffler cover would
act as an absorber for the drill body noise, and (5) it could be removed
ceadily for drill servicing.

The cover enclosing the reactive muffler must meet a number of stringent
requirements. It should be a good barrier, rigid and light weight, with the
surfaces hard and durable to withstand rigors of rough usage. It should con-
form well to the mating surface of the drill to avoid noise sources from small
air leaks and should be thermally conductive to help prevent icing on the
inside.

Numerous materials and methods were tested to find the combination with
the desired properties, and two covers were constructed according to the fol-
lowing design. A sheet of 1/2-inch-thick, flexible aluminum honeycomb with a
foil thickness of 0.0019 inch was cut and shaped around the drill to which
were attached clay buildups of the resonance chambers for the reactive muf-
flers. The honeycomb cells were then filled with viscoelastic material,
either a liquid rubber, DevconFlexane 80, or a pour-in-place syntactic poly-
urethane foam, Emerson and Cuming Eccofoam VIP. The outer surface of one
cover was coated with commercial plastic aluminum, reinforced by expanded alu-
minum metal sheet. The other cover was coated with fiberglass built up to
four reinforced layers. The first cover weighed 9.5 pounds, whereas the lat-
ter weighed 6 pounds. Mating surfaces of the two cover halves, and of the
cover to the drill, were formed during pouring of the liquid filler, and made
very tight joints when the cover halves were clamped together. Two exhaust
tubes, 10 inches long and 5/8 inch ID, were installed at the lower front end
of the case to complete the reactive muffler.


1L

centering the steel in the
tube, and then filling the
space between the tube with
liquid viscoelastic filler.
After the filler cured, part
of the covering around the
collar and shank was removed.
Tubes with wall thicknesses
from 0.049 to 0.065 inch and
outside diameters of 1-1/4
to 1-1/2 inches were bonded
to 7/8-inch-diameter drill
steels. Two viscoelastic
fillers were selected, a
two-part-mix urethane rub-
ber, and a syntactic poly-
urethane foam. Three
finished constrained-lLayer-
damped drill steels are
shown in figure 6. Repre-
sentative noise reduction
resulting from constrained-
layer treatment of chisel
bit steels can be obtained

: ~~ from table 5 by comparin;
FIGURE 6. - Detachable-bit rope-thread and integral- the data 4a cae wal 3,

chisel-bit drill steels with constrained- and 2 and 4, respectively.
layer damping covers.
TABLE 5. - Summary of drilling noise measurements, four rotation chucks
in_a new and a worn BBC-17 drill, no drill case cover
exhaust hoses attached, 70-psi air pressure

Average observed noise level, db
BBC-17 Drilling and (dbA) with indicated
drill Drill steel medium rotation chuck
Steel Steel | Mn-Cu | Sonoston
(worn) | (new) alloy alloy
Worn...... .|4-ft steel chisel bit.|Granite...| 116.5 115.0 113.8 113.2
(111.3) | (109.3) ](107.5){ (108.5)
NeW. esse eee dias HAdORSs ON sie Ris, eevee|oeedOseeee| 117.3 113.2 114.7 112.7
(111.7) | (109.7) |(108.8)| (107.3)
Worn...... -|4-ft damped chisel bit]...do..... 106.3 104.8 102.5 101.8
(102.2) |(101.5) | (99.3) (98.3)
New. ..-eeee . oedOveeee 104.8 104.0 103.0 101.5
(100.2) | (99.8)] (98.5) (9757)
Worteees vie Tool Stubsisiey cecvesis » Rubber pad] 100.5 97.2 98.2 97.7
(97.7) | (97.5) | (95.7) (95.2)
NeW. ecescerlevecs dOs ses aie e ee eseee oeedO..s-.} 102.0 99.4 99.0 97.7
(99.5) | (97.5) | (97.2) (95.7)