We include *IEC Definitions* where appropriate in this 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 field

Incident Sound Wave

Indicators : LAeq,16h • Lday • Levening.

See also aircraft noise • railway noise • road traffic noise

Note :

**Infrasound Definition** IEC 801-21-03, acoustic oscillation whose frequency is below the low-frequency limit of audible sound (about 16 Hz).

See also decay and reverberation.

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

International System of 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 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.

See also • sound pressure inverse square law.Inverse Proportionality.

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

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.

See also point source • line source • sound level calculations • sound pressure level and distance.

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IOA : Institute of Acoustics

Isotron ®

ISQ : International System of Quantities

ITD : initial time delay

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