Specifications

Sound is transmitted through some “medium”. Air is
the typical medium, but sound can also be transmitted
through solid or liquid materials. Generally, a sound wave
will move in a straight line unless it is absorbed or reflected
by physical surfaces or objects in its path. However, the
sound wave will be affected only if the surface is large
compared to the wavelength of the sound. If the surface is
small (compared to the wavelength) the sound will proceed
as if the object were not there. High frequencies/ short
wavelengths can be reflected or absorbed by small
surfaces, but low frequencies/long wavelengths can only
be reflected or absorbed by very massive surfaces or objects.
Once a
sound has been
produced and
transmitted,
it can be re-
ceived by the
ear and, of
course, by a
microphone. In
the ear, the ar-
riving pressure
changes “push
and pull” on the
eardrum. The
resulting motion
of the eardrum
is converted by
the inner ear into nerve signals that are ultimately
perceived by the brain as “sound”. In a microphone, the
pressure changes act on a diaphragm. The resulting
diaphragm motion is converted into an electrical signal
that is sent to the sound system. The resultant sound
picked up is a combination of all pressure changes
occurring at the eardrum or microphone diaphragm.
Sound can be classified by its acoustic behavior. An
example is “direct” sound versus “ambient” (or indirect)
sound. Direct sound travels from the sound source to the
listener in a straight line (the shortest path). Indirect
sound is reflected by one or more surfaces before
reaching the listener (a longer path). Since sound travels
at a constant speed, it takes a longer time for the indirect
sound to arrive, and is delayed relative to the direct
sound. There are several kinds of indirect sound,
depending on the room acoustics.
Echo occurs when an indirect sound is delayed long
enough (by a distant reflecting surface) to be heard by the
listener as a distinct repetition of the direct sound. If indirect
sound is reflected many times from different surfaces it
becomes “diffuse” or non-directional. This is called
reverberation, and it is responsible for our auditory
perception of the size of a room. Reverberant sound is a
major component of ambient sound, which may include
other non-directional sounds, such as wind noise or
building vibrations. A certain amount of reverberant sound
is desirable to add “depth” and “fullness” to the sound,
but an excess tends to make the sound “muddy”,
“hollow”, and unintelligible.
One additional form of indirect sound is known as a
standing wave. This may occur when the wavelength of a
sound is the same distance as some major dimension of a
room, such as the distance between two opposite walls.
If both surfaces are acoustically reflective, the frequency
corresponding to that wavelength will be amplified, by
addition of the incoming and outgoing waves. This results
in a strong, stationary wave pattern between the two
surfaces. A standing wave happens primarily with low
frequencies, which have long wavelengths and are not
easily absorbed.
An additional property of direct sound is that it
becomes weaker as it travels away from the sound source,
at a rate governed by the inverse-square law. For example,
when the distance increases by a factor of two (doubles),
the sound level decreases by a factor of four (the square of
two). This corresponds to a drop of 6 dB in sound pressure
level (SPL), which is a substantial decrease. In contrast,
ambient sound, such as noise and reverberation, is at a
constant level everywhere in the space. Therefore, at a
given distance from a sound source, a listener (or a
microphone) will pick up a certain proportion of direct
sound to ambient sound. As the distance increases, the
direct sound level decreases while the ambient sound level
stays the same. A properly designed sound system can
increase the amount of direct sound reaching the listener
without increasing the ambient sound significantly.
Audio Systems Guide for
MEETING FACILITIES
6
Direct sound decreases with distance and is eventually
no louder than ambient sound.
Sound pressure level of typical sound sources