**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. ..

**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.

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 : various Impulse definitions.

**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 Sound Insulation** see
Normalized Impact Sound Pressure Level, Ln & L'n and also
Impact Sound Pressure Level, Li

**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.

**Impedance** IEC Definition, 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 Impedance.

**Impedance Tube** a method for measuring absorption coefficients by means of standing waves in a tube.

**Impulse** IEC Definition, time integral of a force over the time during which the force is applied

**Impulse** change in Momentum.

**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.

See also : Impulsive Time Weightings

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

See also : Direct Sound Field

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

**Infrasound** IEC Definition, 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.

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** 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 the measurement of instantaneous sound pressure as opposed to the effective (RMS - root mean squared) or the Peak levels.

**Instantaneous Acoustic Pressure : p(t)** IEC Definition, pressure at a particular instant in time and at a particular point in an acoustic field, minus the ambient pressure

Instantaneous Intensity

Instantaneous Kinetic Sound Energy Density

Instantaneous Particle Acceleration

Instantaneous Particle Displacement

Instantaneous Particle Velocity

Instantaneous Potential Sound Energy Density

Instantaneous Sound Energy Density

Instantaneous Sound Pressure

**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 - 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.

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.

**Intensity** under Sound Intensity.

International Electrotechnical Commission under IEC

International System of Units under SI units

International Standards Organisation under ISO

International System of Quantities : ISQ

**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.

See also : Point Source

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