Acoustic Glossary

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

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.

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

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

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.

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 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

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

Industrial Noise and Port Noise (UK Government Environmental Noise Regulations)
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

See also • acoustic mass

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.

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 and static 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 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 velocity • peak particle velocity

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

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

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

Interference • sound wave interference • speech interference

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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.

Inverse Proportionality.

Inverse Square Law (1/r²) : sound radiates spherically from a point source and since the surface area of a sphere is 4 π r²  the surface area increases by a factor of 4 each time the radius r (distance from the source) is doubled.
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.

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

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

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.

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 arranged in a dodecahedron format to give uniform sound output levels in all directions.