Specifications

CHAPTER ONE
SOUND
Because “good” sound quality is the goal of any
meeting facility sound system, it is helpful to be familiar
with some general aspects of sound: how it is produced,
transmitted, and received. In addition, it is also useful to
describe or classify sound according to its acoustic behavior.
Sound is produced by vibrating objects. These include
musical instruments, loudspeakers, and, of course, human
“vocal cords”. The mechanical vibrations of these objects
move the air which is immediately adjacent to them,
alternately “pushing” and “pulling” the air from its resting
state. Each back-and-forth vibration produces a
corresponding pressure increase (compression) and
pressure decrease (rarefaction) in the air. A complete
pressure change, or “cycle”, occurs when the air pressure
goes from rest (or “normal”), to maximum, to minimum,
and back to rest again. These cyclic pressure changes
travel outward from the vibrating object, forming a pattern
called a sound wave. A sound wave is a series of pressure
changes (cycles) moving through the air.
A sound wave can be described by its frequency and
by its amplitude. The frequency of a sound wave is the rate
at which the pressure changes occur. It is measured in
Hertz (Hz), where 1 Hz is equal to 1 cycle-per-second. The
range of human hearing extends from a low of about 20 Hz
to a high of about 20,000 Hz. In practice, a sound source
usually produces many frequencies simultaneously.
The amplitude of a sound wave refers to the
magnitude (strength) of the pressure changes and
determines the “loudness” of the sound. Amplitude, or
sound pressure level (SPL), is measured in decibels (dB)
and ranges from 0 dB SPL (the threshold of hearing), to
above 120 dB SPL (the threshold of pain). The level of
conversational speech is about 65 dB SPL. A change of 1
dB is about the smallest SPL difference that the human
ear can detect, while 3 dB is a noticeable step, and an
increase of 10 dB is perceived as a “doubling” of loudness.
Another characteristic, related to frequency, is
wavelength. The wavelength of a sound wave is the
physical distance from the start of one cycle to the start of
the next cycle, as the wave moves through the air. Since
each cycle is the same, the distance from any point in one
cycle to the same point in the next cycle is also one
wavelength: for example, the distance from one maximum
pressure point to the next maximum pressure point.
Wavelength is related to frequency by the speed of sound.
The speed of sound is the velocity at which a sound
wave travels. The speed of sound is constant and in air is
equal to about 1130 feet per second. It does not change
with frequency or wavelength, but it is related in the
following way: the frequency of a sound, multiplied by its
wavelength, always equals the speed of sound. Thus, the
higher the frequency of sound, the shorter the wavelength,
and the lower the frequency, the longer the wavelength.
A 20 to 20,000 Hz frequency range corresponds to a
maximum wavelength of about 55 feet. at 20 Hz to a
minimum wavelength of about one-half inch at 20,000 Hz.
This large range of wavelengths is responsible for many
acoustic effects, both desirable and undesirable.
MEETING FACILITIES
Audio Systems Guide for
5
Elements of a sound wave
Fundamental frequencies of sound sources