Sound Acoustics and Auditorium Terminology

Vocabulary terms and definitions covering the fundamentals of acoustics, sound measurement, reflection types, and auditorium design based on the lecture transcript.

Acoustics

The science that studies the waves that are conducted through matter due to the motion of the matter.

Architectural acoustics

The study of acoustics specifically when the air is contained within a room.

Church acoustics

A sub-division of architectural acoustics focused on church environments.

Sound (waves)

Pressure fluctuations in the air heard when an acoustic wave passes by, usually caused by objects or air streams quickly changing position.

Speed of Sound

The speed at which sound travels, measured at $1130\,\text{feet per second}$ or about $1\,1/8\text{th}$ of a foot per millisecond ($ms$).

Sound Level

The measure of the strength of sound, measured in decibels ($dB$) using a $dB$ meter.

Decibel Reference Levels ($dB$)

Threshold of quiet: $0\,dB$; whispers: $30\,dB$; conversations: $50\,dB$; shouting: $70\,dB$; onset of painful sound: $100\,dB$.

3 dB Rule

When the sound strength of something doubles, it increases by $3\,dB$; when halved, it drops by $3\,dB$.

Loudness

The apparent strength of sound to the listener; a $1\,dB$ change is barely noticed, while a $10\,dB$ increase makes a sound twice as loud ($10$ times stronger).

Direct Sound (direct signal)

The part of a sound wave that travels directly along the line of sight path between the source and the listener.

Reflections

Sound waves that strike a surface and bounce off, changing direction but maintaining the same angle off the wall as they approached.

Early Reflections

Reflections heard within $\frac{1}{20}$ of a second of the direct sound that merge with the direct signal to form one composite sound.

Late reflections (Echoes)

Distinct reflections that arrive at the listener later than $\frac{1}{20}\text{th}$ of a second after the direct sound.

Flutter Echo

A type of echo, often heard in hallways, that produces a "zinging" pseudo-tone based on how many times per second the reflection passes the listener.

Reverberation

The accumulation of many compounding reflections starting about $\frac{1}{5}$ second after the direct sound in large rooms, creating a "din of chaos" with no discrete direction.

Diffusion

Reflections off of non-flat surfaces that cause sound waves to become disorganized more quickly, decreasing the time it takes for echoes to convert to reverberation.

Decay

The dying out of sound, typically referring to the steady decline in the loudness of reverberation.

Decay Rate (RT-60)

The time in seconds it takes for reverberation to change from very loud to imperceptibly quiet (a total difference of $60\,dB$).

Absorption

The loss of sound energy occurring when a sound wave strikes a fibrous surface, providing acoustic friction that makes the wave quieter without slowing it down.

NRC Rating (Noise Reduction Coefficient)

A rating for absorption representing the percentage efficiency of a surface to absorb sound.

Noise (Background Noise)

Unwanted, undesirable, and interfering sounds present in a listening space.

Noise Floor

The strength of the background noise measured in $dB$.

Signal-to-Noise Ratio (S/N ratio)

The difference in sound level ($dB$) between the desired sound (direct signal) and the noise floor.

Articulation

The clarity of a sound or message such that it can be easily and completely understood.

Intelligibility

A measure of sound clarity based on the comprehension of a message, often tested through word recognition (e.g., "cat, bat, tat, rat").

Bright/Lively

A sound condition with an abundance of treble range reflections.

Dark/Dead

A sound condition characterized by a lack of reflections and reverberance.

Boomy

A condition where lower frequencies, especially in the male voice range, are excessively reverberant.

Psychoacoustics

The science studying how humans comprehend and make sense of the sounds they hear.

Audiology

The science and practice of amplifying or improving how well a person hears sound.

Frequency

The number of repeat times per second that a sound has, measured in cycles per second ($cps$) or Hertz ($Hz$).

Sound Spectrum

A graph or plot of Sound Level vs. Frequency measuring the strength of different frequencies in a tone.

Sonic Color

The shift in emphasis of a complex sound within its spectral range (e.g., warm, cold, or nasal).

Ultrasonics

Sound with a frequency range above human hearing, exceeding $20,000\,Hz$.

Infrasonics

Sound with a frequency range below human hearing, less than $20\,Hz$.

Octave

A frequency range where the upper frequency is double the lower frequency; for the note "C," it starts at $31\,Hz$ and ends at $16k\,Hz$.

Voicing

The process of integrating direct, early, and late reflections with reverberation to define the desirable acoustic condition of a space.

Acoustician

An acoustical engineer specifically trained and experienced in voicing rooms.

Sound Engineer

A professional trained in setting up microphones and speakers.

Acoustic Contractor

A professional trained in installing acoustic tiles and wall panels.

Indirect Sound

The approximately $99.83\%$ of sound emitted by a loud speaker that does not reach the listener's ears directly.

Acoustic Noise

Noise specifically created by the echo and reverberation of sound emitted from a loudspeaker.

Operating System Noise

Background noise resulting from building systems such as electricity, water, lighting ballasts, and HVAC units.

Onset Time Delay

The time between when the direct signal is heard and the reverberation begins; ideally about $\frac{1}{3}$ of a second for speech.