e. Noise. Noise, normal or abnormal, is simply defined as unwanted sound,
whether it is a pure tone, a complex of tones, or unwanted speech or music. For actual
complex sounds, the acoustical character is usually applied to sounds which contain a
large number of separate frequency components that extend over a wide range of
frequencies, coming from animate or inanimate objects, and may or may not convey
meaning or information. Then again, noise may be a single sharp, squeaky, or piercing
sound of short or long duration.
f. Steady Noise. Steady noise is a periodic or random variation in atmospheric
pressure at audible frequencies. It may be continuous, intermittent, or fluctuating with
the sound level varying over a wide range.
g. Spectrum of Sound. Decibels are measured in the A scale or dB(A). This
term refers to the pattern of the distribution of energy or sound pressure present at
different points or areas along the scale of audible (capable of being heard) frequencies.
It is common practice to analyze a single complex sound by means of octave bands, an
octave being a range of frequencies where the upper is twice the lower. This result is
called an octave band analysis of the sound.
h. Impulse or Impact Noise. Noise characterized by a sharp rise in intensity
followed by a rapid decline in intensity, such as that produced by gunfire. It cannot be
measured accurately with an ordinary sound level meter.
i. Audiogram. It is measured as dB(P). An audiogram is a graphic record of
an individual's hearing sensitivity in each ear for each of a number of pure tone or sound
test frequencies. The measure of hearing sensitivity is the individual's threshold of
hearing, which is the lowest intensity of a given tone that he hears. (This job function is
usually performed by a different specialist).
j. Audiometer. This is the electronic device that produces acoustic stimuli of a
known frequency and the intensity for the measurement of hearing or stated another
way; it is an instrument that detects response to sound stimuli by changes in the
electroencephalogram. (This job function is usually performed by a different specialist).
k. Hearing Level of an Ear at a Specified Frequency. This is the amount, in
decibels, by which the threshold of audibility for that ear exceeds, or is less than, the
standard audiometric threshold. This threshold of audibility varies among individuals.
l. Decibel. The decibel (dB or db) is a term borrowed from electrical
engineering, and represents a relative quantity. When used to express sound pressure
levels, it represents a level compared to a reference level. This reference level is
usually a sound pressure of 20 micropascals (uPa) and is usually referred to as "0
decibels" or "0 dB." Practically speaking, this beginning or starting point on the relative
scale of noise levels is about the level of the weakest sound that can be heard by a