1.0 OBJECTIVE: DETERMINING THE NOISE LEVEL
2.0 EQUIPMENT: i) Sound level meter
A sound source radiates power and these results in a sound pressure. In this sense, the sound power is the cause and the sound pressure is the effect. The sound pressure is what we hear. Too high a pressure and prolong exposure will cause hearing damage and stress. Noise is any unwanted sound.
In ergonomics study – sound level is to be measured in dB(A). This is due to the fact that the A – filter is adapted to the physiology of the human ear. Table 1 shows the effect of noise on man.
Occupational Safety and Health Administration (OSHA) has established permissible noise exposure which depends on the duration of exposure shown in Table 2. The exposure to any sound level above 80dB causes the listener to incur a partial dose. If the total daily exposure consist of several partial exposures to different noise levels, then the several partial dose are added to obtain a combined exposure using the following formula:
D= 100(C1/T1 + C2/T2 +…………… + Cn/Tn) ≤ 100
Where: D = Noise dose
C = Time spent at specified noise level (hours)
T = Time permitted at specified noise level (hours) (refer Table 2)
The noise dose is then converted to an 8 hour weighted average (TWA) sound level using the formula:
TWA = 16.61 log (D/100) + 90
The TWA is the sound level that would produce a given noise dose if a person is exposure to that sound level continuously over an 8 hour workday. Thus a worker may be working in a safe environment based on the permissible noise exposure (Table 2) but the cumulative effect of this exposure based on TWA may proof otherwise.
A noise dose of 50% (TWA = 85 dBA) is the action level and 100% (90 dBA) is the permissible exposure level.
4.0 EXPERIMENT GUIDE
This experiment consists of the measurement of sound level in 3 different environments. In sound level measurement, as a precautionary step, windscreen should always be used when measurements are made outdoor or in very dusty environment, to ensure validity of measurement obtained, two aspects have to be considered;
i. background noise must be low enough
ii. Sound level meter should be held at arm length or mounted on a tripod (to minimize
disturbances of the sound field due to the operator’s body)
a) Set the Range Selector switch to the appropriate measuring range for the
b) Set the Detector Response switch as follows;
“Fast” : For relatively steady sound levels
“Slow” : For relatively fluctuating sound levels
c) Select “Man” mode
d) Switch the Power switch to “ON”
e) Note the displayed value
f) Press the Reset button
g) Measure the same sound source for the second time
h) Turn the Power switch “OFF”
i) Go through step (a) to (e) again for 2 other sound sources.