Noise in Industry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 608

31.2 Sound and Noise

Because almost all aspects of hearing conservation and noise abatement in industry rely upon accurate
quantification and evaluation of the noise itself, a basic understanding of sound parameters and sound
measurement is needed before delving into other noise issues.

31.2.1 Basic Parameters
Sound is a disturbance in a medium (in industry, most commonly air or a conductive structure such as a
plant floor) that has mass and elasticity. For example, an industrial metal-forming process wherein a
hydraulic ram impacts a plate of sheet metal with great force causes the plate to oscillate or vibrate.
Because the plate is coupled to the air medium, it produces a pressure wave that consists of alternating
compressions (above ambient air pressure) and rarefactions (below ambient pressure) of air molecules,
the frequency ( f ) of which is the number of above below ambient pressure cycles per second, or hertz

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(Hz). The reciprocal of frequency, 1 f, is the period of the waveform. The waveform propagates

outward from the plate as long as it continues to vibrate, and the disturbance in air pressure that
occurs in relation to ambient air pressure is heard as sound. The linear distance traversed by the
sound wave in one complete cycle of vibration is the wavelength. As shown in Equation (31.1), wave-
length (l in m or ft) depends on the sound frequency ( f in Hz) and velocity (c in m

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sec or ft sec; in

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air at 68 8 F and pressure of 1 atm, 344 m sec or 1127 ft sec) in the medium. The speed of sound increases

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about 1.1 ft sec for each increase of 1 8 F.

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l ¼ c = f (31 : 1)

Noise can be loosely defined as a subset of sound; that is, noise is sound that is undesirable or offensive in
some aspect. However, the distinction is largely situation-and listener-specific, as perhaps best stated in
the old adage “one person's music is another's noise.”
Unlike some common ergonomics-related stressors such as repetitive motions or awkward lifting
maneuvers, noise is a physical stimulus that is readily measurable and quantifiable using transducers
(microphones) and instrumentation (sound level meters) that are commonly available. Aural exposure
to noise, and the damage potential therefrom, is a function of the total energy transmitted to the ear. In
other words, the energy is equivalent to the product of the noise intensity and duration of the exposure.
Several metrics that relate to the energy of the noise exposure have been developed, most with an eye
toward expressing the exposures that occur in industrial or community settings. These metrics are
covered later in this chapter. But first, the most basic unit of measurement must be understood,
namely the decibel.

31.2.2 Physical Quantification: Sound Levels and the Decibel Scale
The unit of decibel,or1 10 of a bel, is the most common metric applied to the quantification of noise

amplitude. The decibel, hereafter abbreviated as dB, is a measure of level, defined as the logarithm of the
ratio of a quantity to a reference quantity of the same type. In acoustics, it is applied to sound level, of
which there are three types.
Sound power level is the most basic quantity, is typically expressed in dB, and is defined as

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Sound power level in dB ¼ 10 log 10 P w 1 = P w r (31 : 2)

where P w 1 is the acoustic power of the sound in Watts, or other power unit, and P w r is the acoustic power
of a reference sound in Watts, usually taken to be the acoustic power at hearing threshold for a young,
healthy ear at the frequency of maximum sensitivity, or the quantity 10 2 12 W.