Acoustic Terms Glossary
Absorb: Sound waves are converted into energy such as heat that is then captured by insulation.
Absorption: In acoustics, the changing of sound energy to heat.
Absorption coefficient: The fraction of sound energy that is absorbed at any surface. It has a value between 0 and 1 and varies with the frequency and angle of incidence of the sound.
Acoustic material: Any material considered in terms of its acoustical properties. Commonly and especially, a material designed to absorb sound.
Acoustics: The science of sound. It can also refer to the effect a given environment has on sound.
Airborne sound: Sound that arrives at the point of interest, such as one side of a partition, by propagation through air.
Ambience: The acoustic characteristics of a space with regard to reverberation. A room with a lot of reverb is said to be "live"; one without much reverb is said to be "dead".
Ambient noise: The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.
Anechoic: Without echo.
Anechoic chamber: A room designed to suppress internal sound reflections. Used for acoustical measurements.
Articulation: A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.
Artificial reverberation: Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., added to a signal to make it sound more lifelike.
Attenuate: To reduce the level of an electrical or acoustical signal. Reduction in sound level.
Audible frequency range: The range of sound frequencies normally heard by the human ear. The audible range spans from 20Hz to 20,000Hz
Average room absorption coefficient: Total room absorption in sabins or metric sabins, divided by total room surface area in consistent units of square feet or square meters.
Axial mode: The room resonances associated with each pair of parallel walls.
Background noise: Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structureborne, and instrument noise.
Baffle: A moveable barrier used to achieve separation of signals from different sources. The surface or board upon which a loudspeaker is mounted.
Bandwidth: The total frequency range of any system. Usually specified as something like: 20-20,000Hz plus or minus 3 dB.
Bass: The lower range of audible frequencies.
Boomy: Listening term, refers to an excessive bass response that has a peak(s) in it.
Bright: Listening term. Usually refers to too much upper frequency energy.
Broad band noise: Spectrum consisting of a large number of frequency components, none of which is individually dominant.
Coherence: Listening term. Refers to how well integrated the sound of the system is.
Coloration: Listening term. A visual analog. A "colored" sound characteristic adds something not in the original sound. The coloration may be euphonically pleasant, but it is not as accurate as the original signal.
Critical frequency: The frequency below which standing waves cause significant room modes.
Cross-talk: Unwanted breakthrough of one channel into another. Also refers to the distortion that occurs when some signal from a music source that you are not listening to leaks into the circuit of the source that you are listening to.
Damp: To cause a loss or dissipation of the oscillatory or vibrational energy of an electrical or mechanical system.
Decibel: The term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1.
Diffuse field: An environment in which the sound pressure level is the same at all locations and the flow of sound energy is equally probable in all directions.
Digital: A numerical representation of an analog signal. Pertaining to the application of digital techniques to common tasks.
Dipole: An open-back speaker that radiates sound equally front and rear. The front and rear waves are out of phase and cancellation will occur when the wavelengths are long enough to "wrap around". The answer is a large, wide baffle or to enclose the driver creating a monopole.
Distortion: Anything that alters the musical signal. There are many forms of distortion, some of which are more audible than others.
Divergence: The spreading of sound waves which, in a free field, causes sound pressure levels in the far field of a source to decrease with increasing distance from the source.
DSP: Digital Signal Processing. DSP can be used to create equalization, compression, etc. of a digital signal.
Echo: A delayed return of sound that is perceived by the ear as a discrete sound image.
Equalization: The process of adjusting the frequency response of a device or system to achieve a flat or other desired response.
Equalizer: A device for adjusting the frequency response of a device or system.
Extension: How extended a range of frequencies the device can reproduce accurately. Bass extension refers to how low a frequency tone will the system reproduce, high-frequency extension refers to how high in frequency will the system play.
Feedback, acoustic: Unwanted interaction between the output and input of an acoustical system, e.g., between the loudspeaker and the microphone of a system.
Fiber glass insulation: An energy-efficient glass fiber insulation used to ensure the best thermal and noise control performance available.
Fidelity: As applied to sound quality, the faithfulness to the original.
Flame spread: A measure of the time it takes for flame to spread. Compared to red oak, whose Flame Spread Index (FSI) is 100 in accordance with ASTM E 84. The infill of IAC sound absorptive metal panels has an FSI of 20.
Flutter: A repetitive echo set up by parallel reflecting surfaces.
Frequency: The measure of the rapidity of alterations of a periodic signal, expressed in cycles per second or Hz.
Frequency response: The changes in the sensitivity of a circuit ,device, or room with frequency.
Fusion zone: All reflections arriving at the observer's ear within 20 to 40 msec of the direct sound are integrated, or fused together, with a resulting apparent increase in level.
Grain: Listening term. A sonic analog of the grain seen in photos. A sort of "grittiness" added to the sound.
Hard room: A room in which the surfaces have very low values of sound absorption and are therefore highly reflective.
Hearing sensitivity: The human ear is less sensitive at low frequencies than in the midrange.
Turn your volume knob down and notice how the bass seems to "disappear". To hear low bass requires an adequate SPL level. To hear 25Hz requires a much higher SPL level than to hear 250Hz.
Helmholtz resonator: A reactive, tuned, sound absorber. A bottle is such a resonator. They can employ a perforated cover or slats over a cavity.
Hertz: the unit of frequency, abbreviated Hz. The same as cycles per second.
Imaging: Listening term. A good stereo system can provide a stereo image that has width, depth and height. The best imaging systems will define a nearly holographic re-creation of the original sound.
Impact insulation class, IIC: A single-number rating derived from measured values of normalized impact sound pressure levels I accordance with Annex A1 of Test Method E 492. It provides an estimate of the impact sound insulating performance of a floor-ceiling assembly.
Impedance: The opposition to the flow of electric or acoustic energy measured in ohms.
Impulse: A very short, transient, electric or acoustic signal.
Isolate: A dampening mechanism made a part of the assembly or system, which reduces structureborne vibrations from passing through the structure.
Linear: A device or circuit with a linear characteristic means that a signal passing through it is not distorted.
Live end dead end: An acoustical treatment plan for rooms in which one end is highly absorbent and the other end reflective and diffusive.
Masking: The amount (or the process) by which the threshold of audibility for one sound is raised by the presence of another (masking) sound.
Mass law: An approximation that describes the Sound Transmission Loss (TL) of a limp, flexible barrier in terms of mass density and frequency. For each doubling of the weight or frequency of a partition, mass law predicts a 6 dB increase in TL.
Metric sabin: [L2] the unit of measure of sound absorption in the metre-kilogram-second system of units.
Midrange: A speaker, (driver), used to reproduce the middle range of frequencies. A midrange is combined with a woofer for low frequencies and a tweeter for high frequencies to form a complete, full-range system.
Millisecond: One thousandth of a second, abbreviated ms or msec.
Modal resonance: See mode.
Mode: A room resonance. Axial modes are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest at low frequencies and for small rooms.
Monophonic: Single-channel sound.
Monopole: Any speaker that encloses the back-wave of the speaker device even though part of this back-wave may be released via a port or duct. The primary radiation at most frequencies will be from the driver front. If the driver is not enclosed it becomes a dipole.
Muddy: Listening term. A sound that is poorly defined, sloppy or vague. For example, a "muddy" bass is often boomy with all the notes tending to run together.
Muting: To greatly decrease the volume level. Many receivers and pre-amplifiers have a muting control which allows the volume level to be cut way down without changing the master volume control. Great for when the phone rings.
Near field: Locations close to the sound source between the source and the far field. The near field is typically characterized by large sound pressure level variations with small changes in measurement position from the source.
Noise: Interference of an electrical or acoustical nature. Random noise is a desirable signal used in acoustical measurements. Pink noise is random noise whose spectrum falls at 3 dB per octave: it is useful for use with sound analyzers with constant percentage bandwidths. Unwanted, bothersome, or distracting sound.
Noise isolation class, NIC: A single-number rating calculated in accordance with Classification E 413 using measured values of noise reduction. It provides an estimate of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.
Noise reduction (NR): The difference in sound pressure level between any two points along the path of sound propagation. As an example, noise reduction is the term used to describe the difference in sound pressure levels between the inside and outside of an enclosure.
Noise reduction coefficient (NRC): The arithmetic average, to the nearest multiple of .05, of the sound absorption coefficients in the 1/3 octave bands centered at 250Hz, 500Hz, 1000Hz, and 2000Hz..
Null: A low or minimum point on a graph. A minimum pressure region in a room.
Octave: An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. Each octave you add on the bottom requires that your speakers move four times as much air!
Octave bands: Frequency ranges in which the upper limit of each band is twice the lower limit. Octave bands are identified by their geometric mean frequency, or center frequency.
One-third octave bands: Frequency ranges where each octave is divided into one-third octaves with the upper frequency limit being 2* (1.26) times the lower frequency. Identified by the geometric mean frequency of each band.
Passive absorber: A sound absorber that dissipates sound energy as heat.
Peak sound pressure level: LPK ten times the common logarithm of the square of the ratio of the largest absolute value of the instantaneous sound pressure in a stated frequency band during a specified time interval to the reference sound pressure of 20 micro pascals.
Pink noise: Noise with a continuous frequency spectrum and with equal power per constant percentage bandwidth. For example, equal power is any one-third octave band.
Receiving room: In architectural acoustical measurements, the room in which the sound transmitted from the source room is measured.
Reflection: For large surfaces compared to the wavelength of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equaling the angle of reflection.
Reflection-phase grating: A diffuser of sound energy using the principle of the diffraction grating.
Refraction: The bending of sound waves traveling through layered media with different sound velocities.
Resistance: The quality of electrical or acoustical circuits that results in dissipation of energy through heat.
Resonance: A natural periodicity, or the reinforcement associated with this periodicity.
Resonant frequency: Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate. A stretched piano string, when plucked, will vibrate for a while at a certain fundamental frequency. Plucked again, it will again vibrate at that same frequency. This is its natural or resonant frequency. While this is the basis of musical instruments, it is undesirable in music-reproducing instruments like audio equipment.
Response: See frequency response.
Reverberant sound field: The sound in an enclosed or partially enclosed space that has been reflected repeatedly or continuously from the boundaries.
Reverberation: The persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped; by extension, in some contexts, the sound that so persists.
Reverberation room: A room so designed that the reverberant sound field closely approximates a diffuse sound field, both in the steady state when the sound source is on, and during the decay after the source of sound has stopped.
Reverberation time: The tailing off of a sound in an enclosure because of multiple reflections from the boundaries.
RFZ: Reflection-free zone.
Room mode: The normal modes of vibration of an enclosed space. See mode.
RT60: Formula for calculating reverberation time.
Sabin: Unit of measure of sound absorption in the inch-pound system.
Sabine: The originator of the Sabine reverberation equation.
Self-extinguishing: A material which will not support combustion when external source of flame is removed.
Self-noise: Extraneous non-acoustical signals, generated or induced in a measurement system.
Signal-to-noise (SN) ratio: The range or distance between the noise floor (the noise level of the equipment itself) and the music signal.
Sine wave: A periodic wave related to simple harmonic motion.
Slap back: A discrete reflection from a nearby surface.
Smoke developed: Measure of smoke density developed by a material when compared with red oak, which has a smoke density index (SDI) of 100 in accordance with ASTM E 84. The infill of IAC sound absorptive metal panels has an SDI of 20.
Soft room: Room with highly sound absorptive surfaces.
Sone: The unit of measurement for subjective loudness.
Sound: Sound is vibrational disturbance, exciting hearing mechanisms, transmitted in a predictable manner determined by the medium through which it propagates. To be audible the disturbance must fall within the frequency range 20Hz to 20,000Hz.
Sound absorption: (1) The process of dissipating sound energy. (2) The property possessed by materials, objects and structures such as rooms of absorbing sound energy. (3) A: [L2]; metric sabin---in a specified frequency band, the measure of the magnitude of the absorptive property of a material, an object, or a structure such as a room.
Sound absorption coefficient, *: [dimensionless]; metric sabin/m* ---of a surface, in a specified frequency band, the measure of the absorptive property of a material as approximated by the method of Test Method C423. Ideally, the fraction of the randomly incident sound power absorbed or otherwise not reflected.
Sound attenuation: The reduction of the intensity of sound as it travels from the source to a receiving location. Sound absorption is often involved as, for instance, in a lined duct. Spherical spreading and scattering are other attenuation mechanisms.
Sound energy density, D: [ML-1T-2]; J/m---the quotient obtained when the sound energy in a region is divided by the volume of the region. The sound energy density at a point is the limit of that quotient as the volume that contains the point approaches zero.
Sound energy, E: [ML2T-2]; J-energy added to an elastic medium by the presence of sound, consisting of potential energy in the form of deviations from static pressure and of kinetic energy in the form of particle velocity.
Sound insulation: The capacity of a structure to prevent sound from reaching a receiving location. Sound energy is not necessarily absorbed; impedance mismatch, or reflection back toward the source, is often the principal mechanism.
Sound intensity, I: [MT-3]; W/m2 the quotient obtained when the average rate of energy flow in a specified direction and sense is divided by the area, perpendicular to that direction, through or toward which it flows. The intensity at a point is the limit of that quotient as the area that includes the point approaches zero.
Sound isolation: The degree of acoustical separation between two locations, especially adjacent rooms.
Sound level: Of airborne sound, a sound pressure level obtained using a signal to which a standard frequency-weighting has been applied.
Sound power level, Lp: Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.
Sound power, W: [ML2T-3]; W---in a specified frequency band, the rate at which acoustic energy is radiated from a source. In general, the rate of flow of sound energy, whether from a source, through an area, or into an absorber.
Sound pressure level (SPL): Given in decibels (dB) is an expression of loudness or volume. A 10 dB increase in SPL represents a doubling in volume. Live orchestral music reaches brief peaks in the 105 dB range and live rock easily goes over 120 dB.
Sound pressure, p: [ML-1T-2]; Pa-a fluctuating pressure superimposed on the static pressure by the presence of sound. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous sound pressure or peak sound pressure, but the unqualified term means root-mean-square sound pressure. In air, the static pressure is barometric pressure.
Sound receiver: One or more observation points at which sound is evaluated or measured. The effect of sound on an individual receiver is usually evaluated by measurements near the ear or close to the body.
Sound spectrograph: An instrument that displays the time, level, and frequency of a signal.
Sound transmission class, STC: A single-number rating, calculated in accordance with Classification E 413 using values of sound transmission loss. It provides an estimate of the performance of a partition in certain common sound insulation problems. A single number rating that indicates the sound transmission loss of a partition or ceiling system between adjacent closed rooms, abbreviated STC.
Sound transmission coefficient, r: [dimensionless]---of a partition, in a specified frequency band, the fraction of the airborne sound power incident on the partition that is transmitted by the partition and radiated on the other side.
Sound transmission loss, TL: Of a partition, in a specified frequency band, ten times the common logarithm of the ratio of the airborne sound power incident on the partition to the sound power transmitted by the partition and radiated on the other side. The quantity so obtained is expressed in decibels. The reduction in sound level when sound passes through a partition or ceiling system.
Sound waves: Sound waves can be thought of like the waves in water. Frequency determines the length of the waves; amplitude or volume determines the height of the waves. At 20Hz, the wavelength is 56 feet long! These long waves give bass its penetrating ability, (why you can hear car boomers blocks away).
Soundstage: A listening term that refers to the placement of a stereo image in a fashion that replicates the original performance. A realistic soundstage has proportional width, depth and height.
Source room: In architectural acoustical measurements, the room that contains the noise source or sources.
Speaker level: Taken from the speaker terminals. This signal has already been amplified.
Specific airflow resistance, r: [ML-2T-1]. Mks rayl (Pa-s/m)---the product of the airflow resistance of a specimen and its area. This is equivalent to the quotient of the air pressure difference across the specimen divided by the linear velocity, measured outside the specimen, of airflow through the specimen.
Spectral balance: Balance across the entire frequency spectrum of the audio range.
Spectrum: the distribution of the energy of a signal with frequency.
Spectrum analyzer: An instrument for measuring, and usually recording, the spectrum of a signal.
Specular reflections: Mirrorlike reflections of sound (angle of incidence equals angle of reflection) from a flat surface. Reflections that do not spread out.
Speech intelligibility: A measure of sound clarity that indicates the ease of understanding speech. It is a complex function of psychoacoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment.
Speech transmission index, STI: A single number that indicates the effect of a transmission system on speech intelligibility.
Spherical divergence: Sound diverges spherically from a point source in free space.
Splaying: Walls are splayed when they are constructed somewhat "off square," i.e., a few degrees from the normal rectilinear form.
Standing wave: A resonance condition in an enclosed space in which sound waves traveling in one direction interact with those traveling in the opposite direction, resulting in a stable condition.
Steady-state: A condition devoid of transient effects.
Stereo: From the Greek meaning solid. The purpose of stereo is not to give you separate right and left channels, but to provide the illusion of a three-dimensional, holographic image between the speakers.
Structureborne noise: Generation and propagation of time-dependent motions and forces in solid materials which result in unwanted radiated sound.
Stud: An upright 2 x 4 or 2 x 6 wall framing member.
Subwoofer: A speaker designed exclusively for low-frequency reproduction. A true subwoofer should be able to at least reach into the bottom octave (20-40Hz). There are many "subwoofers" on the market that would be more accurately termed "woofers".
Superposition: Many sound waves may transverse the same point in space, the air molecules responding to the vector sum of the demands of the different waves.
T60: See RT60.
Tangential mode: A room mode produced by reflections off four of the six surfaces of the room.
Timbral: Refers to the overall frequency balance of a system. In a perfect world, all systems would have complete tonal neutrality. With current technology, this ideal is approached but not met. Listening to many equally "good" speakers will reveal that some sound warmer than others, some sound brighter etc. In a surround sound system it is important that all speakers have a close timbral match for the highest degree of sonic realism.
Timbre: The quality of a sound that distinguishes it from other sounds of the same pitch and volume. The distinctive tone of an instrument or a singing voice.
Transients: Instantaneous changes in dynamics, producing steep wave fronts.
Transparency: Listening term. An analog that can be best "pictured" in photography. The more "transparent" the sound, the clearer the auditory picture.
Treble: The higher frequencies of the audible spectrum.
Tuning frequency: The helmholtz resonant frequency of a box. Also refers to the resonant frequency of other types of systems.
Vibration isolation: A reduction, attained by the use of a resilient coupling, in the capacity of a system to vibrate in response to mechanical excitation.
Volume: Colloquial equivalent of sound level.
Warmth: A listening term. The opposite of cool or cold. In terms of frequency, generally considered the range from approx. 150Hz-400Hz. A system with the "proper" warmth will sound natural within this range.
Wavelength: The distance the sound wave travels to complete one cycle. The distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelenth of any frequency may be found by dividing the speed of sound by the frequency. (speed of sound at sea level is 331.4 meters/second or 1087.42 feet/second).
White noise (ANS): Noise with a continuous frequency spectrum and with equal power per unit bandwidth. For example, equal power in any band of 100-Hz width.