Most machines and many hand operations produce noise, which is an almost unavoidable byproduct of manufacturing. Noise is unwanted sound and sound is a pulsating form of energy.
The cone of a speaker is made to vibrate by electrical impulses sent from your home or automobile sound system. Surrounding air molecules vibrate and transmit this sound energy in all directions at a speed of about 342m/s. In turn, these vibrating air molecules strike your ear drum causing it to vibrate and transmit nerve impulses to the brain. Finally, the brain interprets the impulses as sound or noise.
Sound is Made of Two Parts:
There are two basic components to sound: the frequency or Hz (Hertz), which the ear hears as pitch and the intensity or loudness. The noise from a compressed air hose is a high-pitched noise, while the noise from a truck motor is low-pitched.
Frequency of Sounds:
The human ear can hear frequencies ranging from 20 to 20,000Hz with some loss at the upper end occurring with aging. Hertz are vibrations per second. Sound with frequency above 20,000Hz is called ultrasonic and below 20Hz are called subsonic (or infrasonic).
The frequency range of the human voice during normal conversation is usually between 250 and 3000Hz with a maximum around 8000Hz.
The frequency range of a piano is from about 22Hz up to about 5600Hz with middle C at 250Hz.
Most factory noise has a frequency range from 600 to 10,000Hz.
Noise in the range of 800 to 5000Hz appears to be more damaging to hearing than lower frequencies. Loss of hearing in this range is serious because it hinders conversation and communication as well as our ability to hear danger signals, e.g., alarms, automobile horns, etc.
Intensity of Sound:
The human ear can hear an enormous range of loudness or intensity of sound. Sound intensity is measured in units called decibels (dB).
The dB scale can be confusing to use and understand. It is a logarithmic scale based on multiplication rather than addition. For example, a noise of 90dB is 10 times as loud as 80dB and 100 times as loud as 70dB!
This 'shorthand' decibel scale is used because of the tremendous range of power between loud and soft noises. Small changes in decibel levels mean large changes in noise intensity levels. Note that an increase of 3dB doubles the loudness of a noise.
Some typical noise levels close to the source of noise are:
softest noise that can be heard (hearing threshold)
OHS studies indicate the following facts about the effects of noise on hearing:
1. Apparently there is no hazard from exposures, no matter how prolonged, to noise levels up to 80dBA.
2. A noise level of 85 to 95dBA will be harmful to the hearing of some people.
3. Prolonged exposure to noise levels of 100 to 120dBA will damage the hearing of most people.
4. Noise levels over 120dBA may do permanent damage to most persons even after short exposure. Noise levels over 120dBA produce immediate discomfort. Noise levels over 130 dBA produce pain.
The A-scale is an averaged decibel reading, measured by instruments to account for the fact that human hearing is more sensitive to the middle frequencies rather than the upper and lower ones. It is a measure of "loudness" rather than intensity.
Temporary exposure to high noise levels tends to produce temporary hearing loss called "temporary threshold shift". This "numbness" of hearing will usually wear off at some time after the exposure ends.
For example, exposure to 100dB noise for 10 minutes generally reduces one's hearing sensitivity (causes a threshold shift of) by 15dB for about a half an hour.
Exposure to 100dB for 100 minutes would cause a 30dB loss in hearing for about 3 hours.
Long term exposure to high noise levels (over a period of years) produces permanent hearing loss! The condition is irreversible.
Several other adverse health effects have been associated with high noise exposure including high blood pressure, digestive problems, and simple annoyance, although these are not well documented.
Under the OHSA, section 144 of the regulations for industrial establishments (1991), 90dBA is the maximum allowable noise level to which a worker may be safely exposed for up to 8 hours per day. Higher noise levels are only allowed for shorter periods of time as shown in the following table:
The regulations state that 'measures shall be taken to reduce the sound level below ninety decibels; and where such measures are not practicable, reduce the duration of exposure of workers to that shown in the table, or the worker shall wear hearing protection suitable to reduce the exposure to below the appropriate level in left column.'
It is further required that, where noise levels exceed 90dB, signs shall be posted indicating that hearing protection is mandatory. In addition, the permitted daily exposure time will be posted.
Like other industrial hazards, noise can be controlled at three locations: at the source, along the path, and at the worker. Control at the source is the preferred control measure while control at the worker should be the last measure.
-design/modification-- of machinery may be practicable. Some suggestions follow:
1. Reduce speed of moving parts (e.g., fans, impellers), and of flow velocities in air, gas, or liquid circulation systems (e.g., increase diameter of air lines).
2. Reduce vibrations by balancing rotating parts and secure parts with extra framing, and by adding sound absorbing covers.
3. Reduce impact force of striking parts with shock absorbing mountings.
4. Reduce frictional resistance by lubrication and alignment.
-isolate-- by enclosing the machine or the operator:
1. Use vibration dampening materials which absorb sound and convert it to frictional heat, e.g., rubber pads, flexible caulking, and sheet metal composites.
2. Build walls from heavy materials (brick and mortar) to reflect low frequency noise and cover them with sound absorbing materials.
Control Along the Path:
Sound barriers, acoustic ceilings, and epoxy floor coatings reduce workplace noise to some extent. Separation is effective since air molecules absorb sound. Doubling the distance from a noise source reduces the noise exposure by a factor of four (6 decibels).
Control at the Worker:
Most commonly, hearing protection is used for noise control at the worker. This ppe was discussed in Unit 5-Control Measures.
Other methods include administrative controls such as job rotation out of high noise areas.
A Noise Abatement Plan:
Companies with high noise levels should establish a long term plan for noise reduction. Some of the aspects of a plan are listed:
1. Noise surveys should be conducted and a floor plan posted showing high noise areas.
2. Workers should have "audiometric testing" by a physician at least annually to detect and check for any occupational hearing loss.