Materials & Applications



The changing of energy to heat which reduces the amount of energy which can be reflected.

Airborne sound

Sound that arrives at the point of interest, by propagation through air.


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.


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


Without echo.

Anechoic chamber

A room designed to suppress internal sound reflections. Used for acoustical measurements.

Audible frequency range

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

Background noise

Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structure-borne, and instrument noise.


A free hanging sound absorbing unit used for reduction of reverberation and noise levels.


The lower range of audible frequencies.


Periodic fluctuations that are heard when sounds of slightly different frequencies are superimposed.

Broad band noise

Spectrum consisting of a large number of frequency components, none of which is individually dominant.


The amount (%) of material thickness reduction that occurs when the proper external load is applied to a material.

Creep Relaxation

A transient stress strain condition in which the strain increases concurrently with the decay of stress.


To cause a loss or dissipation of vibrational energy in solid media and structures.


The frequency response curve which resembles the normal frequency hearing curve for most people. A sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons.


A sound-level meter reading with a B-weighting network simulating the human-ear response at a loudness level of 70 phons.


A sound-level meter reading with no weighting network in the circuit, i.e., flat. The reference level is 20 uPa.


Ten times any quantity or frequency range. The range of the human ear is about 3 decades.

Decay rate

For airborne sound, the rate of decrease of vibratory acceleration, velocity, or displacement level after the excitation has stopped.

Decibel (dB)

The standard measure of loudness. dB is a ratio of two quantities on a logarithmic scale.


The relative compactness of a material. Density is the mass of a material per unit volume.


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

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.


The ability of the material to deform before it fractures.

Elastic Modulus

See Young's Elastic Modulus.

Electrical conductivity

The ability of a material to conduct an electrical current.


As a descriptive audio term, usually refers to a coloration or inaccuracy that none-the-less may be sonically pleasing.

Feedback, acoustic

Unwanted interaction between the output and input of an acoustical system, e.g., between the loudspeaker and the microphone of a system.

Ferrous Metal

A metal that contains iron. Carbon steels are common ferrous metals

Free field

An environment in which a sound wave may propagate in all directions without obstructions or reflections. Anechoic rooms can produce such an environment under controlled conditions.


All sounds can be described by their frequency or their mix of frequencies, and can be measured on a scale in units of Hertz (Hz), expressing the number of cycles per second.


Frequency-time curve.


The lowest frequency of a note in a complex wave form or chord.

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 and a pleasant change of character. This is the Haas effect.


A sonic analog of the grain seen in photos. A sort of "grittiness" added to the sound.


Also called overtones, these are vibrations at frequencies that are multiples of the fundamentals. Harmonics extend without limit beyond the audible range. They are characterized as even-order and odd-order harmonics. A second-order harmonic is two times the frequency of the fundamental; a third order is three times the fundamental; a fourth order is four times the fundamental; and so forth. Each even-order harmonic second, fourth, sixth, etc.-is one octave or multiples of one octave higher than the fundamental; these even-order overtones are therefore musically related to the fundamental. Odd-order harmonics, on the other hand third, fifth, seventh, and up-create a series of notes that are not related to any octave overtones and therefore may have an unpleasant sound. Audio systems that emphasize odd-order harmonics tend to have a harsh, hard quality.


The opposition to the flow of electric or acoustic energy measured in ohms.


A very short, transient, electric or acoustic signal.

Impulse response

Sound pressure versus time measurement showing how a device or room responds to an impulse.

In phase

Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase."


Acoustic intensity is sound energy flux per unit area. The average rate of sound energy transmitted through a unit area normal to the direction of sound transmission.


The combining of two or more signals results in an interaction called interference. This may be constructive or destructive. Another use of the term is to refer to undesired signals.


A dampening mechanism made a part of the assembly or system, which reduces structure borne vibrations from passing through the structure.


Thickness reduction that occurs with a given gasket material at various known applied external loads, thereby establishing a load/compression curve.


An exponent of 10 in the common logarithms to the base 10. For example, 10 to the exponent 2=100; the log of 100=2.


A subjective term for the sensation of the magnitude of sound. The subjective response to a sound level.

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.

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.


Interference of an electrical or acoustical nature. Unwanted, bothersome, or distracting sound.


A metal that does not contain iron. Aluminum, copper, and zinc are nonferrous metals.


An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. For example, each octave you add on the bottom requires that a speaker to 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.


A component of a complex tone having a frequency higher than the fundamental.


Phase is the measure of progression of a periodic wave. Phase identifies the position at any instant which a periodic wave occupies in its cycle. It can also be discribed as the time relationship between two signals.

Phase shift

The time or angular difference between two signals.


The unit of loudness level of a tone.

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.


A subjective term for the perceived frequency of a tone.

Pressure zone

As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.

Pure tone

A tone with no harmonics. All energy is concentrated at a single frequency.


The portion of a sound wave in which molecules are spread apart, forming a region with lower-than-normal atmospheric pressure.


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.


The bending of sound waves traveling through layered media with different sound velocities.


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. This is its natural or resonant frequency.


The persistence of sound in an enclosed or partially enclosed space 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.


The weighted sound reduction index (excludes the effects of flanking transmission)


The apparent weighted sound reduction index (includes the effects of flanking transmission)

Sine wave

A periodic wave related to simple harmonic motion.


The unit of measurement for subjective loudness.


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 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

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

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

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

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 na area, or into na 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.

Sound pressure, p

Pa - 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 Reduction Index, R

The airborne sound insulation of a material in a particular frequency range, expressed as a decibel value

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 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.


The distribution of the energy of a signal with frequency.

Spherical divergence

Sound diverges spherically from a point source in free space.

Structureborne noise

Generation and propagation of time-dependent motions and forces in solid materials which result in unwanted radiated sound.

Temperature Resistance

A material's ability to withstand defined temperatures.

Tensile Strength

The maximum stress applied in the plane of the material (not perpendicular to its surface) while stretching a specimen to rupture.

Thermal Conductivity

The ability of a material to conduct heat.

Thermal expansion

The tendency of a material to increase in size as it increases in temperature.

Threshold of feeling

The sound pressure level that makes the ears tickle, located about 120 dB above the threshold of hearing.

Threshold of hearing

The lowest level sound that can be perceived by the human auditory system. This is close to the standard reference level of sound pressure, 20uPA.


The unwanted but acceptable deviation from a specified dimension.


A tone results in an auditory sensation of pitch.


The higher frequencies of the audible spectrum.

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.


Colloquial equivalent of sound level.


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.


Adjustment of sound-level meter response to achieve a desired measurement.

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.

Young's Elastic Modulus

The ratio of tensile or compressive stress to corresponding strain below the proportional limit of the material.