10.3 METAL CUT-OFF SAW NOISE

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One of the common problems in industrial settings is that of protecting

workers from the effects of noise generated by machines that the worker

must guide or monitor directly. One example is a cut-off saw used on metal

shapes (Handley, 1973). The noise generated by the sawing operation originates

from two primary sources: (a) the saw blade and (b) the workpiece

being sawed.

The metal cut-off saw in this case study was actuated downward into

the workpiece by a lever attached to the hinged and counterbalanced saw

and motor. It was necessary that the worker observe the cutting operation.

In addition to the visual clues, vibration and opposing forces transmitted to

the worker through the lever arm furnished feedback on the cutting operation.

The problem was to reduce the noise that the worker received in front

of the saw, with little interference with the workflow, visibility, or with

operation of the lever arm.

10.3.1 Analysis

Because of the constraints placed on the situation, an enclosure with clear,

transparent front doors was considered as a solution to the noise problem.

The operator’s location would be directly in front of the doors, so the

important noise path would be from the saw through the doors to the

operator.

The change in sound pressure level due to the insertion of the enclosure

would be approximately the same as the difference in sound power level

radiated from the saw without an enclosure and the sound power level

radiated from the enclosure with the saw inside. The sound power level

difference is given by Eq. (7-85), expressed in ‘‘level’’ form:

_Lp ј Lop

_ Lp _ 10 log10рW=WoutЮ ј 10 log10 1 ю

_Sj_j

_Sjatj

_ _

(10-5)

Let us consider the 500 Hz octave band and determine the thickness

needed for a sound pressure level reduction of 12 dB. Because the primary

480 Chapter 10

Copyright © 2003 Marcel Dekker, Inc.

element of interest in the preliminary analysis is noise transmitted through

the door, let us consider this element only. The surface absorption coefficient

for Plexigas (_ ј 0:05 at 500 Hz) may be found in Table 7-4:

_Lp ј 10 dB ј 10log10 1 ю

0:05

at

_ _

р0:05=atЮ ј 10_1 ј 9:00

The required sound transmission coefficient for the doors is as follows:

at ј р0:05Ю=р9:00Ю ј 0:00556

The corresponding transmission loss is found from the following:

TL ј 10log10р1=atЮ ј 10 log10р1=0:00556Ю ј 22:6dB

The surface mass of the Plexiglas MS ј _wh, may be found from Eq.

(4-171):

TL ј 22:6dB ј 20log10MS ю20 log10р500HzЮ_47:3

20log10MS ј 22:6ю47:3_54:0 ј 15:9dB

MS ј 1015:9=20 ј 6:24 kg=m2

Using the density of Plexiglas from Appendix C, _w ј 1150 kg=m3, the

following estimate of the thickness h of the doors is obtained:

h ј р6:24Ю=р1150Ю ј 0:00542m ј 5:42mm р0:214 inЮ

Based on the preliminary analysis, an enclosure with transparent doors

having a thickness on the order of 1

4 inch (6.4mm) would solve the noise

control problem satisfactorily.

10.3.2 Control Approach Chosen

The enclosure chosen in this case is illustrated in Fig. 10-3. Workpieces were

fed into a slot in one side of the enclosure and exited through another slot on

the opposite side. Flaps of leaded vinyl covered the slot openings to reduce

noise transmitted through the slots. The front of the enclosure was closed by

two doors constructed of 1

4-inch thick clear plastic (polymethylmethacrylate,

PlexiglasTM or LexanTM). The plastic allowed the operator to see clearly the

piece being cut. The doors closed with a gap having a width slightly greater

than the width of the control lever. Each door had a flat strip of leaded vinyl

approximately 76mm (3 in) wide to close the gap. The saw operating lever

pushed aside the flaps only in the place where it protruded through the door

opening.

The sound pressure level spectrum at the worker position before and

after the enclosure was installed is shown in Fig. 10-4. Before the enclosure

Case Studies in Noise Control 481

Copyright © 2003 Marcel Dekker, Inc.

was applied, the A-weighted sound level at the operator’s location was

97 dB, whereas the sound level was 84 dBA after the enclosure was installed.

10.3.3 Cost

The cost of a commercially available acoustic enclosure, as described in the

previous section, was approximately $5000.

10.3.4 Pitfalls

As is the case with any acoustic enclosure, it is important to seal openings as

effectively as possible. The slots through which material was introduced and

withdrawn and the opening through which the operating lever protruded

were sealed with leaded vinyl strips in this case.

One possible improvement of the system would be to offset the operating

lever of the saw such that the operator’s head is not directly in front of

the gap between the two doors. In addition to moving the region of noise

leakage further from the operator’s ears, the operator’s view of the workpiece

would be improved.

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