10.6 AIR-OPERATEDHOIST MOTOR

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There are many noise sources associated with discharge of shop air through

vents that cause noise problems in industry. One such situation is that of airoperated

hoist motors used in industrial materials-handling systems

(Salmon et al., 1975).

The sound pressure level spectrum at the operator’s location around

the air exhaust from a hoist motor is shown in Fig. 10-9. The A-weighted

sound level is 115 dBA, at which the allowable daily exposure time is only

15 min, according to the OSHA criteria.

492 Chapter 10

Copyright © 2003 Marcel Dekker, Inc.

The octave band sound pressure level increases as the frequency is

increased, which is characteristic of flow-induced noise at frequencies

below the peak frequency. The frequency fp at which the peak sound

power level occurs for turbulent air jet noise (Beranek, 1971) is given by

the following expression:

fpD

u ј 0:2 (10-11)

The quantity D is the diameter of the jet outlet and u is the exit velocity of

the fluid. For an air exhaust pipe with a diameter of D ј 12:7mm (0.500 in)

and an exit air velocity equal to the sonic velocity рc ј 347:2 m/s at 300K),

the frequency at which the peak acoustic energy is radiated from the exhaust

is as follows:

Case Studies in Noise Control 493

FIGURE 10-9 Sound pressure levels around the hoist: (1) before, LA ј 115 dBA,

and (2) after installation of the muffler on the air exhaust of the hoist motor,

LA ј 81 dBA.

Copyright © 2003 Marcel Dekker, Inc.

fp ј р0:2Юр347:2Ю

р0:0127Ю ј 5468 Hz

This frequency lies in the 4000Hz octave band, but it is near the edge of the

8000Hz octave band.

A muffler or silencer would provide a straightforward solution to the

noise control problem. The octave band sound level spectrumgiven in Table

10-1 would result in an A-weighted sound level of 85dBA. This spectrum is

obtained as follows. Since there are eight octave bands given in the data, we

may calculate the following parameter:

LpрOBЮюCFA ј 10log10Ѕр1=8Юр1085=10Ю_ ј 76dB

LpрOBЮ ј 76_CFA

The conversion factor for the A-scale weighting is given in Table 2-4.

An off-the-shelf muffler was selected as the noise control procedure in

this case, because the major source of noise was the exhaust air noise from

the air-operated hoist motor. The sound pressure level spectrum with the

muffler installed is given in Fig. 10-9. The A-weighted sound level with the

muffler in place was LA ј 81 dBA, which is well below the OSHA limit for

daily noise exposure.

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