10.5 AIR SCRAP HANDLING DUCT NOISE

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In a facility for the manufacture of corrugated boxes, the sheets of corrugated

paper were trimmed with circular-blade cutters. The side-trim scrap

was removed fromthe conveyor by an air jet. The trimwas passed through a

trim blower fan with extra thick blades to cut the strips of trim into smaller

pieces. The smaller pieces of scrap trimwere conveyed through 305-mm(12-

inch) diameter ducts to bins where the scrap was baled. The ductwork was

suspended from the ceiling at a level of about 3m (10 ft) from the floor and

passed across a 12m (40 ft) distance through a work room to the bins in the

baler room(Salmon et al., 1975). The duct cross section is shown in Fig. 10-

7.

The trim was moved through the duct by an air stream having a

nominal velocity of about 30m/s (100 ft/sec). The major source of noise

was the impact of the trim against the duct wall, especially at bends in the

duct. The sound level spectrum at the worker’s ear level (approximately

1.5m or 5ft from the duct) is shown in Fig. 10-8. The A-weighted sound

level with no acoustic treatment was 93 dBA, which exceeded the 8-hour

allowable noise exposure for OSHA compliance (90 dBA). In addition, the

noise level was such that it was difficult for workers to communicate in the

area under the duct.

10.5.1 Analysis

The speech interference level рLSILЮ may be determined from the average of

the octave band sound level readings in the 500 Hz, 1000 Hz, 2000 Hz, and

4000 Hz octave bands, as discussed in Sec. 6.4:

LSIL ј 1

4 р86 ю 86 ю 86 ю 85Ю ј 85:75 dB or LSIL ј 86 dB

488 Chapter 10

Copyright © 2003 Marcel Dekker, Inc.

According to the data given in Table 6-4, the expected voice level for faceto-

face communication for this value of SIL would be ‘‘shouting’’

(77 _ LSIL _ 91dB, women; 80 _ LSIL _ 99dB, men).

Because of the presence of other noise sources in the room, it would

probably be impractical to try to reduce the SIL to values such that conversation

in a ‘‘normal voice’’ could be carried out. Let us consider the

situation where the worker would communicate in a ‘‘loud voice.’’ The

SIL range for this condition is given in Table 6-4:

61 _ LSIL _ 71dB, women; 63 _ LSIL _ 77dB, men

The reduction in the SIL is as follows:

16 _ _LSIL _ 20 dB, women; 17 _ _LSIL _ 22dB, men

The midrange of the required SIL reduction is 18 _ _LSIL _ 20dB, or

about 19dB. If we consider the 1000Hz octave band only, the reduction

in sound pressure level required to achieve the SIL reduction of 19 dB would

also be approximately 19 dB.

Let us examine the feasibility of using an acoustic pipe wrapping to

reduce the noise radiated from the scrap-handling duct. Pipe wrappings

usually consist of a resilient layer of porous material (fiberglass, mineral

wool, etc.) placed directly on the pipe, and the resilient blanket is covered

Case Studies in Noise Control 489

FIGURE 10-7 Cross-section of the scrap-handling duct with the acoustic wrapping

in place.

Copyright © 2003 Marcel Dekker, Inc.

with an impervious jacket (plastic, galvanized steel, etc.). The following

empirical expression has been developed for the insertion loss (IL) for

pipe wrapping (Michelsen et al., 1980). The insertion loss is defined as the

difference between the sound pressure level before the noise control measure

has been applied and the sound pressure level after the noise control measure

has been applied:

IL ј

40

1 ю рDref=DЮ fЅlog10р f =foЮ_ _ 0:342g (10-9)

This expression is valid for frequencies greater than about twice fo, or for

f            2fo. The quantity D is the outside diameter of the pipe, and Dref is a

reference diameter, Dref ј 120 mm. The quantity f is the frequency, and the

frequency fo is given by the following expression:

490 Chapter 10

FIGURE 10-8 Sound pressure levels around the scrap duct: (1) before,

LA ј 93 dBA, and (2) after application of the duct wrapping, LA ј 72 dBA.

Copyright © 2003 Marcel Dekker, Inc.

fo ј

1

2_

_oc2

_shsh

" #1=2

(10-10)

The quantities _o and c are the density and sonic velocity, respectively, of

the air within the porous layer. The quantities _s and hs are the density and

thickness, respectively, of the solid jacket material. The quantity h is the

thickness of the resilient material over the pipe.

The thickness of the solid jacket hs may be estimated as follows. Let us

try a resilient material thickness, h ј 50:8mm (2 in). The density and speed

of sound for air at atmospheric pressure and 300K (278C or 808F) are _o ј 1:177 kg=m3 (0.0735 lbm=ft3) and c ј 347:2 m/s (1139 fps), respectively. For

the jacket material, let us try building paper having a density of _s ј 1120

kg=m3 (70 lbm=ft3). For an insertion loss of IL ј 19 dB at a frequency

f ј 1000Hz and a duct diameter of D ј 304:8mm (12 in), Eq. (10-9) may

be used to find the jacket thickness:

IL ј 19 dB ј р40Ю

1юр120=304:8Ю fЅlog10р f =foЮ__0:342g

f =fo ј 10:09 > 2

The frequency fo is as follows:

fo ј р1000Ю=р10:09Ю ј 99:08Hz

The required jacket thickness may be found from Eq. (10-10):

_shs ј р1:177Юр347:2Ю2

р4_2Юр99:08Ю2р0:0508Ю ј 7:21 kg=m2

hs ј р7:21Ю=р1120Ю ј 0:00643m ј 6:43mm р0:253 inЮ

The thickness of heavy building paper is approximately 3.2mm (1/8 in);

therefore, two layers of building paper could be used for the jacket over

the resilient material.

10.5.2 Control Approach Chosen

The noise control approach chosen for this application was to wrap the

ducts with 50mm (2 in) of mineral wool building insulation material to

provide the resilient and sound-absorbing layer. The jacket of the wrapping

was two impervious layers of heavy building paper (‘‘tar’’ paper), spirally

wrapped over the mineral wool with 50%overlap between layers.

The sound pressure level spectrumin the roomwith the pipe wrapping

applied is shown in Fig. 10-8. The A-weighted sound level was reduced to

Case Studies in Noise Control 491

Copyright © 2003 Marcel Dekker, Inc.

72 dBA, which is well within the OSHA limits for daily worker noise exposure.

The speech interference level with the wrapping in place is as follows:

LSIL ј 1

4 р66ю67ю65ю60Ю ј 64:5dB or LSIL ј 65 dB

This SIL value falls in the range for communication in a ‘‘loud voice’’ for

either men or women. If men were to carry on a face-to-face conversation

with a background SIL of 65dB, the expected separation of the two people

could be determined from Eq. (6-5),with K ј 60:

LSIL ј 65 ј 60_20 log10 r

r ј 10_5=20 ј 0:562m р22:1inЮ

The standard building materials selected for the pipe wrapping were

economical; however, special acoustic pipe coverings with leaded vinyl

sheeting could also have been used if higher attenuation or insertion loss

were required.

10.5.3 Cost

The material costs were relatively small. The cost for building paper to cover

two 40-ft sections of 12-in diameter duct was $20, and the cost of the mineral

wool was $70 for the job. In-house labor costs were approximately $240, so

the total cost for the noise reduction system was $330.

10.5.4 Pitfalls

The case study presented in this section illustrates that common inexpensive

building materials may be used to achieve a modest attenuation of sound in

some cases. For pipe wraps, it is important to eliminate any leakage at seams

of the wrap. In this case, the building paper was overlapped by 50% to

eliminate leakage and reduction of attenuation.

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