Acoustic Glossary
Sound and Vibration Terms and Parameters
I
ICP : Integrated Circuit Piezoelectric : see IEPE below.
IEC : International Electrotechnical Commission : .
IEPE : Integrated Electronic Piezoelectric : accelerometers - with built-in electronics to enable the use of 'long' cables. Also known as ICP or Integrated Circuit Piezoelectric
Immission Level : a descriptor for noise exposure, in decibels, representing the total sound energy incident on the ear over a specified period of time.
Impact : a noise with a short duration, usually associated in acoustics with an object in motion hitting another object. See also the various Impulse definitions.
Impact Noise Rating : INR : a single number rating on the effectiveness of a floor construction.
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 : a sound with a very short duration, see Impulsive and 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 : a standard time weighting applied by a sound level meter. In this mode the meter responds very quickly (35 ms) to an increase in noise level but responds slowly (1.5 s) when the impulse stops - a hang over from the days you had to watch the meter needle and the human eye was not fast enough to 'catch' the maximum level - also known as Impulse Response and Impulsive Time Constant - see also exponential averaging.
Impulse Weighted Average Sound Level : Lleq : used in Germany as defined by DIN 45641 : 3 dB exchange rate.
Impulsive Noise :
A:- a single sound pressure peak with a rise time less than 200 milliseconds or total duration less than 200 milliseconds.
B:- 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.
C:- 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 - see also decay : reverberation.
In Phase : two periodic waves reaching peaks 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 - see also Sound Pressure.
Integrated Electronic Piezoelectric : 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 logarithmic average
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 : is the product of the sound pressure and the acoustic particle velocity. For more details see Sound Intensity
International System of Units : under SI units
Inverse Square Law : in a free-field [no reflecting surfaces] the sound level decreases by 6 dB for every doubling of distance from the noise. This is because, left to it's own devices, sound radiates uniformly in all directions - spherically.
The surface area of a sphere = 4πr2 so if you double the radius (distance from the source) the surface area increases 4 times and the sound pressure is spread over the enlarged surface, i.e. reduced by 6dB.
IOA : Institute of Acoustics : UK
ISO : International Standards Organization.
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
Other Glossary Terms
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