Glossary of Acoustic Terms
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.
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.
An acoustical engineer who is trained and experienced in voicing rooms.
One formally educated, experienced in the science and practice of acoustics.
Someone trained and experienced in installing acoustic tiles and wall panels.
Someone, not formally trained, experienced in providing acoustical services.
Someone, not formally trained, who prepares blueprints for acoustic projects.
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.
The condition of sound in a room when the lower frequencies, particularly the male voice range is excessively reverberant.
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.
The science and practice of amplifying or otherwise improving how well a person hears sound.
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.
The dying out of sound. Usually referring to the steady decline in the loudness of the reverberation.
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.
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.
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 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.
see Late Reflections, 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.
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 whose frequency range is below that of human hearing, below 20 Hz.
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.
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.
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).
Noise (Background Noise)
The unwanted, undesirable and usually interfering sounds present in a listening space, typically due to an air conditioner or other conversations.
The strength of the background noise, measured in dB. It is difficult to understand what is being said in a room with a high noise floor.
(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.
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).
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.
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.
see Early Reflections, Late Reflections
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.
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.
Signal-to-Noise Ratio (S/N ratio)
The difference in sound level (dB) between the desired sound and the noise floor.
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.
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).
One who envisions and directs the way sound plays out of a stage.
Someone trained in setting up microphones and speakers.
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.
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.
Sound whose frequency range is above that of human hearing, above 20,000 Hz.
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.