10.2 FOLDING CARTON PACKING STATION NOISE

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In manufacturing folding cartons, such as those used for soft drink bottles,

the individual cartons are cut and stacked on a pallet (Salmon et al., 1975).

The cartons are held together for transfer by a nick or uncut part of the

carton. The individual cartons are separated by an air-driven chisel, which

breaks the nicks and frees the entire stack of cartons. When the operations

are completed, the stacks of cartons are packed in cases for shipment. A

schematic of the layout of the stripper and packer line is shown in Fig. 10-1.

The air hammer or chisel produces noise that has not been practical to

eliminate by system design. Because of this characteristic, the stripper was

required to wear hearing protection while working with the air chisel. The

cartons are transferred from a conveyor belt to a skid for shipment. The

packer is located about 4.27m(14 ft) fromthe air chisel. The purpose of this

noise control study was to develop a means for reduction of the noise

experienced by the packer at the end of the conveyor.

10.2.1 Analysis

The noise generated by the air chisel is broadband, with no significant peaks

in the frequency spectrum, as shown in Fig. 10-2 (Plunkett, 1955). The Aweighted

sound level at the packer’s location with no noise treatment is

95 dBA, which exceeds the OSHA limit for an 8-hour daily noise exposure.

A barrier would solve the noise control problem if the direct field were

found to be significant, compared with the reverberant field, as discussed

in Chapter 7.

The room constant R for the space without the barrier may be estimated

frommeasurements of the reverberation time Tr and the total surface

area of the space So. The number of absorption units a is defined by Eq.

(7-34):

a

So ј

55:26V

TrcSo

(10-1)

476 Chapter 10

Copyright © 2003 Marcel Dekker, Inc.

The average surface absorption coefficient __ may be found from Eq. (7-30):

1 _ __ ј expр_a=SoЮ (10-2)

The room constant may be determined from Eq. (7-13):

R ј

__So

1 _ __ ј р1 _ e_a=So ЮSo

e_a=So ј рea=So _ 1ЮSo (10-3)

The ceiling height of the room in which the system was located was

3.66m (12 ft) and the room constant was relatively large. For example, for

the 1000 Hz octave band, the room constant was approximately R ј 3360

m2 (36,170 ft2). The direct distance between the air chisel and the packer’s

ear was r ј 4:27m (14 ft), and the directivity factor for the chisel was

approximately Q ј 2. The contributions of the reverberant and the direct

sound fields may be found as follows for the 1000 Hz octave band:

Reverberant field: 4=R ј р4Ю=р3360Ю ј 0:001190m_2

Direct field: Q=4_r2 ј р2Ю=Ѕр4_Юр4:27Ю2_ ј 0:00873m_2

Case Studies in Noise Control 477

FIGURE 10-1 Air-hammer stripper and packer line layout.

Copyright © 2003 Marcel Dekker, Inc.

The contribution of the direct field is about eight times that of the

reverberant field, so a barrier would be effective in reducing the noise experienced

by the packer. If we combine Eq. (7-18) for the sound pressure level

without the barrier (Lop

) with Eq. (7-96) for the sound pressure level with the

barrier in place рLpЮ we obtain the sound pressure level reduction:

Lop

_ Lp ј _Lp ј 10 log10

4

R ю

Q

4_r2

4

Rb ю

Qрab ю atЮ

4_рA ю BЮ2

2

664

3

775

(10-4)

The room constant with the barrier in place and the room constant

without the barrier are practically the same, Rb _ R, and the transmission

478 Chapter 10

FIGURE 10-2 Sound pressure level spectrum for the air-hammer noise at the packer’s

location (1) before installation of the barrier, LA ј 95 dBA, and (2) after installation

of the barrier, LA ј 85 dBA.

Copyright © 2003 Marcel Dekker, Inc.

coefficient is generally negligible compared with the barrier coefficient,

at _ab. For a barrier coefficient of ab ј 0:02 at 1000Hz and

(AюBЮ ј 4:724m (15.5 ft), the anticipated reduction in sound pressure

level (in the 1000Hz octave band) by using a barrier is as follows:

_Lp ј 10 log10

4

3360 ю

2

р4_Юр4:267Ю2

4

3360 ю р2Юр0:02Ю

р4_Юр4:724Ю2

2

664

3

77

5 ј 10log10

0:00993

0:001333

_ _

_Lp ј 8:7 dB

Because the noise level reduction that is needed is about р95_90Ю ј 5dBA

or more, this magnitude of sound pressure level should be satisfactory.

10.2.2 Control Approach Chosen

A barrier wall was selected as the noise control measure in this case. The

wall was 3.048m (10 ft) long and 1.829m (6 ft) high. The air chisel was

located about 1.219m (4 ft) behind the barrier and about 1.143m (3 ft

9in) below the top of the barrier. The distance from the barrier to the

packer’s ear was about 3.035m (9 ft 11.5 in) from the barrier and about

0.305m (1 ft) below the top of the barrier.

The barrier was constructed of 1

4-inch (6.4mm) thick plywood attached

on both sides of a frame constructed of 2_4’s. The barrier was simple to

construct and was quite sturdy. No sound-absorbing materials were needed

on the plywood surface.

The sound level spectrum at the packer’s location with the barrier in

place is shown in Fig. 10-2. With no barrier, the A-weighted sound level was

95 dBA, and the sound level was 85 dBA with the barrier in place. The

overall sound pressure levels (measured on the C-scale) were 97 dB with

no barrier and 88 dB with the barrier in place. The addition of the barrier

reduced the sound level such that the packers did not need hearing protection.

10.2.3 Cost

The material and labor costs for the barrier were as follows. Five sheets of 1

4-

inch plywood, 4 ft _ 8 ft, had a total cost of $85.00. (Note: Cost values used

throughout this chapter are US dollars at year 2000.) A total of 60 ft of

2 _ 4’s were used, for a total cost of $20.00. The in-plant labor cost for

construction of the barrier was $205.00. Therefore, the total cost for the

barrier, which reduced the noise level by about 10 dBA, was $310.

Case Studies in Noise Control 479

Copyright © 2003 Marcel Dekker, Inc.

10.2.4 Pitfalls

In this installation, the room size was large and the sound radiated directly

from the air chisel to the packer’s ears was a significant portion of the total

noise. The barrier would not have given satisfactory results for an application

in which the room was small and the walls had very low surface absorption

coefficients (acoustically ‘‘hard’’ surfaces).

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