**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".

We include the *IEC Definitions* where appropriate in this *Acoustic Glossary*

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

See also real

See also impact noise rating • impact sound • impact testing

**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 admittance • acoustic impedance • acoustic ohm • acoustic reactance • acoustic resistance • admittance • characteristic acoustic impedance • characteristic impedance of a medium • complex acoustic impedance • conjugate impedances • driving point impedance • specific acoustic impedance • specific acoustic reactance • specific acoustic resistance • specific wall admittance • specific wall impedance • transfer impedance • transmission impedance and radiation

Impedance Tube

**Impulse** in acoustics refers to events of short duration.

**Impulse Definition** (IEC 801-24-26) time integral of a force over the time during which the force is applied.

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

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

See also direct sound

Incident Sound Wave

Indicators : LAeq • Lday • Levening.

See also aircraft noise • railway noise • road traffic noise

**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.

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).

**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.

INR : impact noise rating

**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.

Instantaneous Kinetic Sound Energy Density

Instantaneous Sound Intensity

**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.

Instantaneous Potential Sound Energy Density

Instantaneous Sound Energy Density

Instantaneous Sound Pressure

Instantaneous Speech Power

Institute of Acoustics

*Integrated Circuit Piezoelectric and Integrated Electronic Piezoelectric* under IEPE.

**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 differentiationIntelligibility

Intensity

**International Electrotechnical Commission (IEC)** 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".

We include *IEC Definitions* where appropriate in this glossary

International System of Units (SI units)

International Standards Organisation

**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.

**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 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.

Therefore sound intensity (Watts/m²) levels decrease by a factor of 4 each time the distance from the source is doubled, in decibels this is 10·log (4) = 6 dB* for power quantities.

Sound pressure level measurements are more common and they decrease by a factor of 2 every time the distances is doubled, in decibels this is 20·log (2) = 6 dB* for root power quantities. Strictly speaking this is not *inverse square* but inverse proportionality, also known as inverse distance law.

See also point source • line source • sound level calculations.

IOA : Institute of Acoustics

Isotron ®

ISQ : International System of Quantities

ITD : initial time delay