AbsorptionThe changing of energy to heat which reduces the amount of energy which can be reflected.
Airborne soundSound that arrives at the point of interest, by propagation through air.
AmbienceThe 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 noiseThe composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.
AmplitudeThe instantaneous magnitude of an oscillating quantity such as sound pressure. The peak amplitude is the maximum value.
Anechoic chamberA room designed to suppress internal sound reflections. Used for acoustical measurements.
Audible frequency rangeThe range of sound frequencies normally heard by the human ear. The audible range spans from 20Hz to 20,000Hz.
Background noiseNoise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structure-borne, and instrument noise.
BaffleA free hanging sound absorbing unit used for reduction of reverberation and noise levels.
BassThe lower range of audible frequencies.
BeatsPeriodic fluctuations that are heard when sounds of slightly different frequencies are superimposed.
Broad band noiseSpectrum consisting of a large number of frequency components, none of which is individually dominant.
CompressibilityThe amount (%) of material thickness reduction that occurs when the proper external load is applied to a material.
Creep RelaxationA transient stress strain condition in which the strain increases concurrently with the decay of stress.
DampTo cause a loss or dissipation of vibrational energy in solid media and structures.
dB(A)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.
dB(B)A sound-level meter reading with a B-weighting network simulating the human-ear response at a loudness level of 70 phons.
dB(C)A sound-level meter reading with no weighting network in the circuit, i.e., flat. The reference level is 20 uPa.
DecadeTen times any quantity or frequency range. The range of the human ear is about 3 decades.
Decay rateFor 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.
DensityThe relative compactness of a material. Density is the mass of a material per unit volume.
DiffractionA 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 fieldAn 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.
DuctilityThe ability of the material to deform before it fractures.
Elastic ModulusSee Young's Elastic Modulus.
Electrical conductivityThe ability of a material to conduct an electrical current.
EuphonicAs a descriptive audio term, usually refers to a coloration or inaccuracy that none-the-less may be sonically pleasing.
Feedback, acousticUnwanted interaction between the output and input of an acoustical system, e.g., between the loudspeaker and the microphone of a system.
Ferrous MetalA metal that contains iron. Carbon steels are common ferrous metals
Free fieldAn 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.
FrequencyAll 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.
FundamentalThe lowest frequency of a note in a complex wave form or chord.
Fusion zoneAll 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.
GrainA sonic analog of the grain seen in photos. A sort of "grittiness" added to the sound.
HarmonicsAlso 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.
ImpedanceThe opposition to the flow of electric or acoustic energy measured in ohms.
ImpulseA very short, transient, electric or acoustic signal.
Impulse responseSound pressure versus time measurement showing how a device or room responds to an impulse.
In phaseTwo periodic waves reaching peaks and going through zero at the same instant are said to be "in phase."
IntensityAcoustic 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.
InterferenceThe 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.
IsolateA dampening mechanism made a part of the assembly or system, which reduces structure borne vibrations from passing through the structure.
Load/CompressionThickness reduction that occurs with a given gasket material at various known applied external loads, thereby establishing a load/compression curve.
LogarithmAn exponent of 10 in the common logarithms to the base 10. For example, 10 to the exponent 2=100; the log of 100=2.
LoudnessA subjective term for the sensation of the magnitude of sound. The subjective response to a sound level.
Mass lawAn 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 fieldLocations 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.
NoiseInterference of an electrical or acoustical nature. Unwanted, bothersome, or distracting sound.
Non-ferrousA metal that does not contain iron. Aluminum, copper, and zinc are nonferrous metals.
OctaveAn 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 bandsFrequency 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.
OvertoneA component of a complex tone having a frequency higher than the fundamental.
PhasePhase 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 shiftThe time or angular difference between two signals.
PhonThe unit of loudness level of a tone.
Pink noiseNoise with a continuous frequency spectrum and with equal power per constant percentage bandwidth. For example, equal power is any one-third octave band.
PitchA subjective term for the perceived frequency of a tone.
Pressure zoneAs 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 toneA tone with no harmonics. All energy is concentrated at a single frequency.
RarefactionThe portion of a sound wave in which molecules are spread apart, forming a region with lower-than-normal atmospheric pressure.
ReflectionFor 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.
RefractionThe bending of sound waves traveling through layered media with different sound velocities.
ResonanceA natural periodicity, or the reinforcement associated with this periodicity.
Resonant frequencyAny 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.
ReverberationThe persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped.
Reverberation timeThe tailing off of a sound in an enclosure because of multiple reflections from the boundaries.
RwThe weighted sound reduction index (excludes the effects of flanking transmission)
R'wThe apparent weighted sound reduction index (includes the effects of flanking transmission)
Sine waveA periodic wave related to simple harmonic motion.
SoneThe unit of measurement for subjective loudness.
SoundSound 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