A test between two objects. For example, a test between two different microphones or preamps. For the test to be scientifically valid, levels should be matched, and the tests should be performed in a double-blind controlled environment. See double-blind.

In acoustics, the reduction in sound-pressure levels through the conversion of sound energy to heat captured within an acoustic attenuator. The opposite of reflection.

A measurement of sound energy reduction absorbed into or passed through a material. Measurement values range from 0 to 1 (which translates to 0% through 100%) and may vary with any particular material based on frequency and angle of incidence of the sound. This coefficient is typically referred to as sabin units. See sabin units.

A complex quotient obtained when sound pressure, averaged over a surface, is divided by the velocity of the volume through the surface.

Any material considered in terms of its acoustical attenuating properties. It is commonly a material designed to absorb sound.

As used in this book, the study of the physical and psychological properties of sound as relates to isolating construction techniques and attenuation of modal and non-modal activities within a particular space.

Audio Engineering Society.

Sound transmitted through the medium of air.

The quotient of air pressure difference (steady or alternating state) across a specimen, divided by the volume velocity (steady or alternating state) of airflow through the specimen.

The quotient of the specific airflow resistance of a homogeneous material divided by its thickness.

The measured percentage of Articulation Loss of Consonants by a listener. A % Alcons of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding; 10% and beyond is increasingly poor intelligibility; 15% is typically the maximum acceptable loss; beyond 15% is unintelligible.

The acoustic characteristics of a space with regard to reverberation. A room with a lot of reverb is said to be “live”; one without reverb is said to be “dead.” Note that the observation of degrees of ambiance is subjective at best, differing from person to person.

All-encompassing sound at a given place, usually a composite of sounds from many sources near and far.

The magnitude of an oscillating quantity, such as sound pressure. The peak amplitude is the maximum value.

An electrical signal whose frequency and level vary continuously in direct relation to the signal.

Without reverberation.

A room designed to suppress sound reflections. Typically used for acoustical measurements.

A resonance related to a room mode. An anti-node exists on either side of a node. Anti-nodes create an increase in amplitude, a reinforcement of a particular frequency or range of frequencies

A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.

A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded. An articulation index of 100% means that all speech can be understood and 0% means that no speech can be understood. Articulation index is calculated from the ⅓ octave band levels between 200Hz and 6300Hz center frequencies.

Reverberation generated to simulate that of concert halls, etc. This is added to a signal to make it sound more lifelike.

The sum of sound-pressure levels, in a particular frequency band, divided by the number of measured levels from different positions, times, or both.

Acoustical Society of America.

To alter the level of an acoustical signal through absorption, diffusion, electronic, or other means.

The range of sound frequencies heard by a human ear. The audible range typically spans from 20Hz to 20,000Hz.

The sensory area lying between the thresholds of hearing and pain.

Total room absorption in sabins, divided by total room surface area in consistent units of measurement.

Ten times the logarithm of the ratio of the space and time average of squared sound pressure to the squared reference sound pressure, the space average being taken over the total volume of the room, except for the regions of the room where the direct field of the source and the near field of the boundaries are of significance. Unit: decibel (dB).

The A-weighting curve is a wide bandpass filter centered at 2.5kHz, with ~20dB attenuation at 100Hz, and ~10dB attenuation at 20kHz. It exhibits heavily roll-off at the low end and a less aggressive roll-off on higher frequencies. This rating is the inverse of the 40-phon equal-loudness of the Fletcher-Munson Curve.

Room resonances associated pairs of parallel or non-parallel walls or ceiling/floor.

Existing noise within a space, from sources unrelated to a particular sound, or sounds, that are the object of interest.

The frequency range of a system. Generally specified by establishing an upper and lower frequency range, i.e., 20–20,000Hz plus or minus 4dB.

The range of audible frequencies below 250Hz.

A listening term that refers to excessive bass response.

A listening term, which generally refers to upper frequency energy.

A wideband spectrum consisting of frequency components, no one of which is individually dominant.

A space between the surfaces of walls, ceilings, floors, etc., where insulation is typically installed.

The specific normal acoustic impedance at a point in a plane wave in a free field.

A type of distortion that occurs when an amplifier is driven into an overload condition. A “clipped waveform” generally contains an excess of high-frequency energy.

A listening term that refers to how well integrated the sound of a system is.

A listening term that refers to adding to a sound, something that was not in the original sound. Coloration can be a factor of room characteristics, a signal imparted by the gear being utilized in the sound reproduction, or a combination of the two.

A distortion produced by combining an acoustical signal with a delayed replica of itself. The result is constructive and destructive interference, which results in a series of peaks and nulls introduced into the frequency response.

The portion of a sound wave in which molecules are pushed together, forming a region with higher-than-normal atmospheric pressure. Acoustic compression (within a room or space) can be caused by room modes, as well as non-modal activity.

Frequency components within a narrow bandwidth that mask a given tone. Critical band varies with different frequencies, but is commonly found between 1/6 and 1/3 octaves.

The distance from a sound source at which direct sound and reverberant sound are at the same level.

The frequency below the separating point where standing waves will cause significant room modes.

The frequency of a sound attenuator above which the normal incidence sound-absorption coefficient is most effective.

A weighting curve designed into filters for equipment measuring sound output levels. The C-curve is ″flat,″ with -3dB corners at 31.5Hz and 8kHz, respectively. It is intended to loosely correspond to how people perceive sound at higher volume levels.

The frequency of a wave measured in Hertz (Hz).

To cause a loss or dissipation of the oscillatory or vibrational energy of a signal or object. In the case of a membrane (i.e., a wall surface, the face of a panel trap, etc.), the placement of insulation touching the inner face of the membrane will create this effect.

See decibel.

See A-weighting

See C-weighting

As relates to airborne sound, the rate of decrease of the level of vibratory acceleration, velocity, or displacement, measured after the initial excitation has ceased.

dB, unit used to measure the amplitude of sound. A measure of sound intensity as a function of power ratio, with the difference in decibels between two sounds being given by dB=10 log10(P1/P2), where P1 and P2 are the power levels of the two sounds. The faintest audible sound, corresponding to a sound pressure of about 0.0002 dyne per sq. cm, is arbitrarily assigned a value of 0dB.

As used in this book, room surfaces or diaphragmatic sound attenuators (panel traps) that vibrate in response to sound.

A change in the direction of sound energy in the area of a boundary discontinuity, such as the edge of a reflective or absorptive surface.

An environment in which sound-pressure levels are the same throughout and the flow of energy is equally probable in all directions.

A converted numerical representation of an analog signal.

As used in this book, an undesired change in the waveform of a signal.

A testing procedure, as relates to recording equipment, designed to eliminate biased results, in which the identity of the equipment is concealed from both administrators and subjects until after the study is completed.

A reflected sound that arrives at the ear long enough after the original source to be perceived as a separate image.

Sound travels faster through a dense material than through the air. Thus, it is possible for a sound to reach a microphone through a building structure before reaching it through the air.

Energy-frequency curve.

Energy-frequency-time curve.

For all audible frequencies, a contour representing a constant amplitude.

Energy-time curve.

The extent of the range of frequencies that a device can reproduce accurately.

That part of the sound field in which the decrease in sound pressure is inverse in relation to the distance from the source. This corresponds to a reduction of 6dB for every doubling of distance. See inverse-square law.

Fast Fourier Transform.

Sound transmission class calculated in accordance with ASTM E 413.

Sound transmission loss measured in accordance with ASTM E336

The time it takes for flame to spread, measured in accordance with a variety of standards based on the use of the materials involved.

Transmission of sound, from the source to a receiving location, through a structural path other than the one anticipated.

Equal-loudness contours indicating sound-pressure levels (measured in decibels), over the range of audible frequencies, which are perceived as being of equal loudness (measured by Fletcher and Munson at Bell Labs in 1933).

A distinctive ringing sound caused by echoes bouncing back and forth between hard parallel surfaces following a percussive sound.

The application of the Fourier transform to a signal to determine its spectrum.

Any environment in which a sound wave may travel in all directions without obstructions or reflections.

As used in this book, the number of waves (pressure peaks) that travel a distance in one second measured in Hertz (Hz).

As used in this book, the frequency within the human hearing range, roughly 20Hz to 20kHz.

Frequency-time curve.

The lowest frequency of a complex tone.

A term evolving from misinterpretations of the Haas effect. It implies that all sounds arriving (at the human ear) within an interval of time (ranging from about 20 to 50ms) are fused into a single sound. Simplistic at best, totally wrong at worst, this does not take into account that within these time limits there are several levels of clearly distinguishable characteristics, including spaciousness, timbre change, image shift, multiple sound images, and, at large delays, echoes that can color the sounds to create a separateness between them.

A series of minute, parallel lines used to change the direction of a sound wave.

A diffuser of sound energy using the principle of diffraction grating.

A psychoacoustic effect, also referred to as precedence effect. If a listener hears two identical sounds (i.e., identical soundwaves of the same intensity) from two separate sources, then the source that is closer to the listener appears to be the only source of the sound. The level of the delayed signal may be up to 10dB higher than the direct signal before disturbing the localization effect.

A room in which the surfaces are very reflective.

Tones at frequencies that are multiples of a fundamental tone. These are further characterized as either “even-order” or “odd-order” harmonics. Even-order harmonics (second, fourth, sixth, etc.) are even octaves above the fundamental tone. Odd-order harmonics (thirds, fifths, sevenths, etc.) are simple multiples of the fundamental, which may or may not produce pleasant overtones.

A measurement unit of frequency in cycles per second.

A tuned sound absorber.

The effect created by a stereo system that provides a three-dimensional re-creation of the sound recorded.

As regards acoustics, a short signal containing a “spike” input that sharply rises from zero and as abruptly decays back to zero.

The response of a system to a short signal containing a “spike” input that sharply rises from zero and as abruptly decays back to zero. Impulse response measured in a room will indicate any series of reflections (from walls, etc.) of the direct response.

When two periodic waves of the same frequency synchronize, they are said to be “in phase.” Signals in phase boost one another in amplitude.

The span of time between the arrival of a direct sound and the first reflection of that sound.

The combining of two or more signals in either a constructive or destructive manner.

Under far/free field conditions, the effect that mandates a sound-pressure level decrease of 6dB for each (and every) doubling of the distance.

To reduce sound vibrations from passing through a structure or assembly.

Journal of the Acoustical Society of America.

Journal of the Audio Engineering Society.

See Hass effect

See Live End Dead End.

An acoustical treatment plan for control rooms in which the front end is highly absorbent and the rear of the room is reflective and diffusive.

The amount (or the process) by which the threshold of audibility for one sound is raised by the presence of another (masking) sound.

Simply, the greater mass a sound wave has to move to create vibration transfer, the greater the reduction of noise energy. Mass law dictates that for every doubling of mass, a 6dB increase in sound isolation will occur.

Relating to sound waves in an enclosure, the average distance traveled between successive reflections, generally measured in relation to reverberation time (RT60).

One thousandth of a second.

A resonance based on a room’s dimension, coinciding with the length of a particular frequency. See axial mode, tangential mode, and oblique mode.

See mode.

National Association of Broadcasters.

The sound field located between the source and the far field. The near field exists under optimal conditions at distances less than four times the largest sound source dimension.

The opposite of an anti-node. When standing waves occur, there are positions in space relative to the wave, called nodes, at which there is no movement at all. The wave interferes with itself to create this instance of opposition (e.g., a wave reflecting off a wall and back into its own path). Nodes are spaced one-half wavelength apart. Nodes cause a decrease in amplitude or (if perfectly out of phase) a full cancellation of the signal.

The abbreviation for Noise Reduction Coefficient. A specification used to indicate the effectiveness of acoustic absorption materials. It is arrived at by averaging the sabin absorption coefficients of a material in the octave bands between 125Hz to 4kHz. The greater the number, the more sound absorbed.

A minimum pressure region in a room or space.

A room mode involving all six surfaces, i.e., the four walls, ceiling, and floor.

A doubling or halving of frequency.

A sound attenuator that dissipates sound energy as heat through absorption.

Phase-frequency curve.

An occurrence when two signals of the same frequency are out of phase with each other, resulting in a net reduction in the overall level of the combined signal. If two identical signals are 180 degrees out of phase, they will completely cancel one another if combined.

A unit of perceived loudness, a subjective measure of the strength (not intensity) of a sound. At a frequency of 1kHz, 1 phon is defined to be equal to 1dB of sound-pressure level above the nominal threshold of hearing.

Random noise produced with equal energy per octave.

A large (typically), absorbent-lined duct through which conditioned air is routed. Plenums are generally used as a supply (or return) connection for multiple branch ducts.

The study of the effect of the human auditory system and brain as it relates to acoustics.

A tone without harmonics.

A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform distribution of energy.

The portion of a sound wave in which molecules are spread apart, an expansion of the sound pressure field. The opposite of compression.

The opposition to the flow of electricity posed by capacitors and inductors.

A sound absorber, such as the Helmholtz resonator, which involves the effects of mass and compliance as well as resistance.

As relates to acoustical measurements, the space in which sound transmitted from a source room is measured.

As relates to sound, a return of residual sounds after striking a surface within a room or space. Reflection in low frequencies does not truly exist in the sense of ray tracing. Higher frequencies will display off a surface, although the strongest signal will be returned based on the theory that the angle of reflection equals the angle of incidence

A sound diffuser using the principle of diffraction grating.

Resonance is the tendency of a mechanical or electrical system to vibrate or oscillate at a certain frequency when excited by an external source, and to keep oscillating after the source is removed.

The frequency at which resonance occurs.

The remainder of sound that exists in a room after the source of the sound has stopped.

Reflection-free zone.

The normal modes of vibration of an enclosed space. See mode.

Reverberation time measured as the time required for a reduction of sound pressure levels by 60dB.

Wallace Sabine, the originator of the Sabine reverberation equation.

A unit of acoustic absorption equivalent to the absorption by one square foot of a surface that absorbs all incident sound.

The property possessed by materials, objects, and structures, such as rooms of absorbing sound energy, measured in sabin units.

The absorptive properties of a material, in a specified frequency band, measured as outlined in ASTM C423; again, typically measured in sabin units.

Ten times the logarithm of the ratio of the time-mean-square pressure of a sound, in a stated frequency band or with a stated frequency weighting measured as a decibel (dB).

A single number rating, calculated in accordance with ASTM E 413, using values of sound transmission loss. STC ratings are calculated based on the frequency range of human speech.

A measurement of the reduction in sound level when sound passes through a partition, floor, or ceiling assembly.

A low-frequency resonance condition, within an enclosed space, in which sound waves traveling in one direction interact with those traveling in the opposite direction, resulting in a stable condition exhibiting a series of localized peaks and nulls.

Transmission of sound through structural members in a building.

See RT60.

A room mode produced by reflections off four of the six surfaces of the room.

Time-delay spectrometry.

Time, energy, frequency.

The resonant frequency of a tuned sound attenuator.

The frequency range that exists and is higher than the nominal frequency range of human hearing, measures as hertz (Hz).

Oscillation of a parameter that defines the motion of a mechanical system.

A reduction in the ability of a structural system to transmit vibration in response to mechanical excitation, attained through the use of a resilient coupling or any other manner of decoupling of separated structural assemblies.

A measurement unit of electrical or acoustical power.

The distance a sound wave travels to complete one cycle. The wavelength of any frequency is found (in the simplest sense) by dividing the speed of sound by the frequency.

Noise with a continuous frequency spectrum and with equal power per unit of bandwidth.

Random noise with equal energy per frequency.