Acoustic Glossary


I : Sound and Vibration Terms, Definitions, Units, Measurements ...

Ia'' airborne sound insulation index - former name for weighted apparent sound reduction index, R'w

ICP ® Integrated Circuit Piezoelectric, see IEPE below.

IEC : International Electrotechnical Commission,

founded in 1906, the IEC is the world’s leading organization for the preparation and publication of International Standards for all electrical, electronic and related technologies. These are known collectively as "electrotechnology".

Throughout this Glossary we include IEC Definitions where appropriate

IEPE : Integrated Electronic Piezoelectric,

accelerometers with built-in electronics, also known as ICP : integrated circuit piezoelectric.

The built-in electronic preamplifier transforms the high impedance charge output of the sensor into a low impedance voltage signal that can be transmitted over longer distances.

This technique is widely used under other trade names: ICP ®, Deltatron ®, Piezotron ®, etc.

See also • piezoelectric

Imaginary (of a number or quantity), expressed in terms of the square root of a negative number (usually the square root of −1, represented by i or j ).

See also • real

Immission Level,

a descriptor for sound exposure, in decibels, representing the total sound energy incident on the ear over a specified period of time.


short duration noise(s), usually associated in acoustics with an object in motion hitting another object.

See also • various impulse definitions.

Impact Noise Rating

Impact Sound,

produced by the collision of two solid objects. Typical sources are footsteps, dropped objects, etc., on an interior surface (wall, floor, or ceiling) of a building.

See also • impact sound pressure level (Li)normalized impact sound pressure level (Ln & L'n)

Impact Testing

, a method of measuring the frequency response function of a structure by hitting it with a calibrated impact hammer and measuring the system's response.

The impact hammer has a transducer to measure the input force pulse while the response is typically measured using an accelerometer. The impact imparts a force pulse that excites the structure over a broad frequency range.

Impedance Definition IEC 801-25-13,

at a given frequency, quotient of a dynamic field quantity (such as force, sound pressure) by a kinematic field quantity (such as vibration velocity, particle velocity), or quotient of a voltage by a current.
Note 1 : the term impedance is generally applied to a linear system and to steady sinusoidal signals.
Note 2 : in the case of a transient, impedance as a function of frequency is the quotient of the respective Fourier or Laplace transforms.
Note 3 : an impedance is the quotient of two quantities the product of which has the dimensions of power or power per unit area.

See also • acoustic admittanceacoustic impedanceacoustic ohmacoustic reactanceacoustic resistanceadmittancecharacteristic acoustic impedancecharacteristic impedance of a mediumcomplex acoustic impedanceconjugate impedancesdriving point impedancespecific acoustic impedancespecific acoustic reactancespecific acoustic resistancespecific wall admittancespecific wall impedancetransfer impedancetransmission impedance and radiation

Impedance Tube under standing wave tube

Impulse, change in momentum


, in acoustics, events with a very short duration, see the formal definition and other terms in common use below.

Impulse Definition

IEC 801-24-26, time integral of a force over the time during which the force is applied.

Impulse Response,

the way a device responds to an impulse. For example, the reverberation of a room can also be thought of as its impulse response. A great deal of information about a device can be determined by it's impulse response.

The frequency response, phase response, and transient response are all tied to this specification.

Impulse Time Weighting

Impulse Weighted Average Sound Level (Lleq)

used in Germany as defined by DIN 45641, 3 dB exchange rate.

Impulsive Noise,

1 )  a single or multiple sound pressure peak(s) with a rise time less than 200 milliseconds or total duration less than 200 milliseconds.
2 )  or generally speaking, a noise which manifests itself as a succession of distinct pulses or transients.

See also • impulsive time weightings

Incident Sound

is received directly from the source, as distinguished from sound that is reflected from a surface.

See also • direct sound field

Incident Sound Wave

Incoherent Sources

, sound levels resulting from different sound sources as opposed to a coherent source.

Industrial Noise: UK Government Environmental Noise Regulations

Indicators • LAeq,16hLdayLevening.

Inertance Definition IEC 801-25-43,

at a frequency for which inertia is dominant, quotient of sound pressure by the resulting in-phase volume acceleration during sinusoidal motion, also known as acoustic mass.
Note : inertance has dimensions of mass divided by the square of area

Infrasound Definition IEC 801-21-03,

acoustic oscillation whose frequency is below the low-frequency limit of audible sound (about 16 Hz).

See also other oscillation terms

Initial Time Delay (ITD),

the gap in time between the arrival of direct sound and the first sound reflected from a surface of the room to the listener.

See also • decay and reverberation.

In Phase, two periodic waves reaching their maximums and going through zero at the same instant are said to be in phase.

INR • Impact Noise Rating

Insertion Loss

is the difference, in decibels, between the sound pressure level before and after a sound-attenuating device.

Instantaneous Acoustic Pressure Definition IEC 802-01-03,

pressure at a particular instant in time and at a particular point in an acoustic field, minus the ambient pressure.

See also instantaneous sound pressure

Instantaneous level

, existing or measured at a particular instance, for example the measurement of instantaneous sound pressure as opposed to the effective (root mean squared) or the peak levels.

Instantaneous sound Intensity
Instantaneous Kinetic Sound Energy Density

Instantaneous Particle Acceleration Definition IEC 801-21-32,

the time derivative of instantaneous particle velocity.

See also • particle acceleration

Instantaneous Particle Displacement Definition IEC 801-21-25,

in an elastic medium, vector whose extremity is the position of the particle at a given instant, and whose origin is at the equilibrium position of the particle.

See also • particle displacement

Instantaneous Particle Velocity Definition IEC 801-21-28,

derivative with respect to time, of the instantaneous particle displacement.

See also • particle velocitypeak particle velocity

Instantaneous Potential Sound Energy Density

Instantaneous Sound Energy Density
Instantaneous Sound Pressure

Instantaneous Speech Power

Institute of Acoustics

Integer an exact (whole) number, no fractions or decimals points. For example 1, 2, 5, -5, 0 but not 1.25, 3/4. 0.75

Integrated Circuit Piezoelectric and Integrated Electronic Piezoelectric under IEPE.

Integrating (of an instrument),

indicating the mean value or total sum of a measured quantity.

Integrating Sound Level Meter, more correctly known as the Integrating-averaging Sound Level Meter and commonly known as the Leq Meter

Integration in mathematics, an integral assigns numbers to functions in a way that can describe displacement, area, volume and other concepts that arise by combining data.

Integration in vibration,

will convert an acceleration signal into a velocity signal, or a velocity signal into a displacement signal. For this reason, an accelerometer is the transducer of choice because velocity and displacement can be so easily derived from its output.

A vibration integrator is basically a low-pass filter with 6 dB or 12 dB per octave attenuation. Analogue integrators are only accurate over a discrete frequency range.

See also • differentiation


an electrical frequency filter used to convert a vibratory acceleration signal to one whose amplitude is proportional to velocity or displacement.

Intelligibility under speech intelligibility

Intensity under sound intensity.

Interferencesound wave interferencespeech interference

International Electrotechnical Commission under IEC
International System of Units under SI units
International Standards Organisation under ISO

International System of Quantities : ISQ Definition IEC 112-02-01,

system of quantities based on the seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.
Note 1 : the International System of Quantities is published in the International Standard ISO/IEC 80000, Quantities and units.
Note 2 : the International System of Units (SI) is based on the ISQ.

Interval see logarithmic frequency interval definition.

Inverse Distance Law (1/r)

, sound pressure (amplitude) falls inversely proportional to the distance 1/r from the sound source, where r is the distance from the noise source.

For example at 16 metres from a point sound source the sound pressure level will be reduced by 20 log (16) = 24 dB, relative to the level at 1 metre, to check this example type 20 log (16) into Google.

Inverse Proportionality under proportionality.

Inverse Square Law (1/r²)

, when sound radiates from a point source in midair, the sound energy is spread over a larger and larger spherical surface area as the distance from the source increases.

Since the surface area of a sphere is 4·π·r², then the surface area increases by a factor of 4 each time r (the distance from the source) is doubled

Therefore the sound intensity (Watts/m²) decreases by a factor of 4 each time the distance from the source is doubled. In decibels this is 10·log (4) = 6 dB *.

Measuring sound pressure levels is more common in the practical world and the sound pressure decreases by a factor of 2 every time the distances is doubled. In decibels this is 20·log (2) = 6 dB *. However we know that sound intensity ∝ p² so the 'general' 6 dB rule still applies. Strictly speaking this is not inverse square but inverse proportionality, also known as inverse distance law.

* to check the above examples type 10 log (4) and 20 log (2) into Google.

See also • point sourceline sourcesound level calculations article

IOA : Institute of Acoustics

ISO : International Standards Organisation,

creates Standards that provide requirements, specifications, guidelines or characteristics that can be used consistently to ensure that materials, products, processes and services are fit for their purpose.

Isotron ® trade name for IEPE : integrated electronic piezoelectric.

Isotropic, is derived from Isotropy i.e. an object or substance which is uniform in all directions. In acoustics it is sometimes used to describe noise sources like loudspeakers arrange in a dodecahedron format to give uniform sound output levels in all directions.

ISQ • International System of Quantities

ITD • initial time delay
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