Definitions, Terms, Units and Parameters

I

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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

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

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 the various Impulse definitions.

Impact Noise Rating : INR

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 - Normalised Impact Sound Pressure Level : Ln and L'n
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 Tube

: a method for measuring Absorption Coefficients by means of standing waves in a tube.

Impulse

: a sound with a very short duration, see the various

impulsive

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 Time Weighting

Impulse Weighted Average Sound Level : Lleq

: used in Germany as defined by DIN 45641 : 3 dB Exchange Rate.

Impulsive Noise

:
♦ a single sound pressure peak with a rise time less than 200 milliseconds or total duration less than 200 milliseconds.
♦ multiple sound pressure peaks with rise times less than 200 milliseconds or total duration less than 200 milliseconds
spaced at least by 200 millisecond pauses.
♦ a sharp sound pressure peak occurring in a short interval of time.

Infrasound

: sound at frequencies below the audible range, that is, below about 16 - 20 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.

In Phase

: two periodic waves reaching their maximums and going through zero at the same instant are said to be in phase.

INR : Impact Noise Rating

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 Sound Pressure

: is the change in ambient pressure caused by a sound wave at an instant in time.

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
Integrated Electronic Piezoelectric

: under IEPE.

Integrating-averaging Sound Level Meter

: a sound level meter that accumulates the total Sound Energy over a measurement period and calculates the energy or Leq - Equivalent Sound Level

Integration

: is the mathematical operation that is the inverse of differentiation. In vibration analysis, integration will convert an acceleration signal into a velocity signal, or a velocity signal into a displacement signal. Integration can be done with excellent accuracy with an analogue integrator in the Time Domain or can be done digitally in the Frequency Domain. For this reason, an accelerometer is the transducer of choice because velocity and displacement can be so easily derived from its output. An analogue integrator is actually a low-pass filter with 6 dB of attenuation per Octave. This is true of an analogue integrator only above its low cut-off. And since the low cut-off cannot be zero, analogue integrators have low-frequency limits, usually either 1 or 10 Hz.

Integrator

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

Intensity

: Sound Intensity is the product of Sound Pressure and Sound Particle Velocity.

International System of Units

: under SI units

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·r², then the intensity I (power per unit area) of radiation at distance r is
I = P/A = P/(4·π·r²)

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 ∝ p² so the 6 dB rule still applies. Strictly speaking this is not Inverse Square but Inverse Proportional and is known as Inverse Direct Law.

IOA

: Institute of Acoustics : UK

ISO

: International Standards Organisation.

Isotron ®

: trade name for IEPE - Integrated Electronic Piezoelectric.

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.

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