ICP ® : Integrated Circuit Piezoelectric : see IEPE below.

IEC : International Electrotechnical Commission

IEPE : Integrated Electronic Piezoelectric : Accelerometers - with built-in electronics, also known as ICP or 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.

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.

Impact Sound : the 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.

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 instrumented 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.

Impulse : in acoustics, events with a very short duration, see the various definitions below.

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.

Incoherent Sources : Sound levels resulting from different sound sources as opposed to a Coherent Source.

Infrasound : sound 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.

Insertion Loss : the sound level reduction at a given location due to the insertion of a noise control device, expressed in decibels. The difference, in decibels, between the Sound Pressure Level before and after the effect of a sound-attenuating device.

Instantaneous : existing or measured at a particular instance, for example measurement of Instantaneous Particle Velocity as opposed to the
Effective (RMS - Root Mean Squared) or the Peak levels.

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 or commonly known as the Leq Meter.

Integration - 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.

An 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.

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

Inverse Square Law : when Sound Power radiates from a point source, the power is distributed over larger and larger spherical surfaces as the distance from the source increases.
Since the surface area of a sphere of radius r is A = 4·π·r^{2}, then the intensity I (power per unit area) of radiation at distance r is I = P/A = P/(4·π·r^{2})
Therefore the energy or intensity decreases (divided by 4) as the distance r is doubled. Measured in dB it decreases by 6 dB per doubling of distance.

Inverse Direct Law : measuring Sound Pressure Levels is more common in the practical world and the sound level decreases by 50% every time the distances is doubled.
However we know that intensity I ∝ p^{2} so the 6 dB rule still applies. Strictly speaking this is not Inverse Square but Inverse Proportional and is known as Inverse Direct Law.

Isotropic : is derived from Isotropy i.e. 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.

ITD : Initial Time Delay : the gap in time between the arrival of direct sound and the first sound reflected from a surface of the room to the listener.