Acoustics The science that studies the waves that are conducted
through matter due to the motion of the matter. Usually air is the
material that most people think of when it comes to acoustic waves.
But acoustic waves exist in all matter. Architectural acoustics, is
the study of acoustics when the air is contained in a room. Church
acoustics is a sub-division of architectural acoustics.
Sound (waves)
Pressure fluctuations in the air that are heard when an acoustic
wave passes by. They are usually caused by objects in the air that
quickly change position or a stream of air that quickly changes
position. Sound escapes away from the sound source as an expanding
spherical wave that travels at the speed of 1130 feet per second,
traveling about 1 1/8th of a foot each one thousandths of a second
(millisecond).
Sound Level The measure of the strength of sound. Units
are decibels (dB) and usually measured with a dB meter. The threshold
of quiet sound is zero dB and the onset of painful sound is 100
dB. Conversations are at 50 dB, whispers at 30 dB and shouting is
70 dB. When the sound strength of something doubles, it increases
by 3 dB, or halved, it drops by 3 dB.
Loudness
The apparent strength of the sound to the listener. A change in
1 dB is just barely noticed as a change in loudness. Something twice
as loud is actually 10 dB stronger, (10 times stronger). Something
half as loud is 10 dB weaker, (1/10th as strong).
Direct
Sound (direct signal)
The part of a sound wave that travels directly along the line of
sight path between the speaker or sound source and the listener.
The dry or actual sound.
Reflections
Sound waves that strike a surface and bounce off are reflected sounds.
They bounce off the wall, changing directions but keeping the same
angle off the wall as they had when they approached the wall.
Early
Reflections
Reflections that are heard within 1/20 of a second of the direct
sound are called early reflections. Early reflections cannot be
distinguished from direct signals, they merge with the direct sound
to form one composite sound. This combining effect can cause the
sound of the direct signal to change in tonal characteristics and
apparent direction.
Late
reflections (Echoes)
A distinct reflection that arrives at the listener later than 1/20th
of a second after the direct sound is heard. The listener can identify
from where an echo comes. An echo does not change the tonal characteristics
of the direct sound.
Flutter
Echo
This type of echo is most easily heard as one claps their hands
out in front of them, while standing in a hallway. The sound "zings"
and it's tone depends on how many times a second the reflection
passes by the listener's head. In a hall 8' wide, the clap will
expand out, hit the wall and return 143 times a second and the zing
will sound like a 143 Hz buzzy tone. Not a real sound, just a pseudo-tone.
Reverberation
For sound in a large room, reverberation begins at about 1/5 second
following the direct sound. It is due to the accumulation of many
reflections, compounding one upon the other, so much that the sound
no longer seems composed of echoes but rather just a sound of noise,
a din of chaos that has no discrete direction and no discrete timing.
Diffusion
Reflections off of a non flat surface that causes the sound wave
to become more quickly disorganized than if off a flat surface is
a diffusive surface. Diffusion decreases the time it takes for echoes
to become converted to reverberation. The beautiful gothic churches
of the old world have very diffusive or sound scattering surfaces.
That is part of the sonic beauty of those spaces.
Decay Rate
(RT-60)
The time (in seconds) it takes for reverberation to change from
very loud to imperceptibly quiet, a total sound level difference
of 60 dB. For a living room the RT-60 might be 1 second but in a
gym, it might be 4 seconds.
Absorption
The loss of sound energy that occurs when the sound wave strikes
a fibrous surface. The fibers provide acoustic friction for the
sound wave. The wave does not slow down due to the friction, it
keeps it's same speed but it does lose energy and get quieter.
NRC Rating
(Noise Reduction Coefficient) A rating for absorption. It gives
the % efficiency for a surface to absorb sound. If a surface is
30% absorptive, then only 70% of the incident sound is reflected
back into the room.
Noise
(Background Noise)
The unwanted, undesirable and usually interfering sounds present
in a listening space, typically due to an air conditioner or other
conversations.
Sibilance
A sibilant is a type of fricative or affricate consonant, made by
directing a jet of air through a narrow channel in the vocal tract
towards the sharp edge of the teeth. Sibilants are louder than their
non-sibilant counterparts, and most of their acoustic energy occurs
at higher frequencies than non-sibilant fricatives. [s] has the
most acoustic strength at around 8,000 Hz, but can reach as high
as 10,000 Hz. [?] has the bulk of its acoustic energy at around
4,000 Hz, but can extend up to around 8,000 Hz.
Articulation
The clarity of a sound, particularly a message conveyed by sound,
such that it can be easily and completely understood. A slurred
sound may be well heard but the message it carries may still not
be well understood, it is inarticulate. Also, a clear and distinct
sound may be drowned out by a nearby louder noise, rendering the
message not understandable, inarticulate. Echoes also cause articulation
problems. Articulation is most often measured in some form of a
desired signal to unwanted noise ratio.
Intelligibility
A measure of the clarity of sound based on the comprehension of
the message being conveyed by sound. A "cat, bat, tat, rat...
type of recognition test. The conversational version of Articulation.
Bright/Lively
The condition of sound in which there is an abundance of treble
range reflections giving the feeling of "brightness" or
"liveliness" to the sound. Sound in a tile bathroom or
kitchen is bright. Too much can seem harsh and irritating.
Dark/Dead
The condition of sound in a room when there is a lack of reflections
and a lack of reverberance. Too much can seem lack-luster and uneasy
feeling.
Psychoacoustics
The study and science of how the human comprehends and makes sense
out of the sounds they hear. The difference between an early reflection
and a late (echo) reflection, is an example of psychoacoustics.
The blending of the early reflections with the direct sound is another.
Frequency
(Hertz, Hz, cps)
A single sound pulse as from a fire cracker has sound energy but
no tone. Tones are sounds that come from voices or instruments which
have a repetitive pressure pulse characteristic. The number of repeat
times per second that a sound has is called it's frequency. It's
unit of measurement is cycles per second (cps) also called Hz (Hertz).
Similar to pitch in musical terms.
Sound
Spectrum
The sound level measured at different frequencies. Most tones are
composed of more than one frequency, a combination of frequencies,
as in a musical chord. The sound spectrum would measure the strength
of each frequency and display that graph as a plot of Sound Level
vs. Frequency, also known as a sound spectrum. The "color"
of sound is used as emphasis in the spectrum.
Sonic Color
The shift in emphasis of a complex sound within it's spectral range.
A neutral color is the preferred natural sound but sometimes sound
can have a warm color, an emphasis on lower frequencies or a cold
color, an emphasis on higher frequencies or a nasal color, an emphasis
on midrange frequencies.
Octave
Sound that exists within a limited frequency range, between a lower
set frequency and a set upper frequency. The difference between
the lower and upper frequency is specified to be equal to the lower
frequency. The octave sequence for the note "C" starts
at 31 Hz and continues thru 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1k Hz,
2k Hz, 4k Hz, 8k Hz and ends at 16k Hz. (k = thousand).
Voicing
The process of defining the desirable condition of sound in an acoustic
space. It integrates the direct, early and late reflections with
the reverberation, including a sense of timing and direction for
each into an appropriate and desirable acoustic condition for the
listener. It combines both the art and science of sound. It requires
an understanding of the purpose to be served by each acoustic space.
As an art form, it recognizes the aesthetic side of sound, the impression
that most people prefer to have of each particular type of sound
that exists in some particular place. As a science, it is based
on psychoacoustics.
