I : Sound and Vibration Terms, Definitions, Units, Measurements ...
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".
Throughout this Glossary we include IEC Definitions where appropriate
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.
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 ).
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.
acoustic admittance •
acoustic impedance •
acoustic ohm •
acoustic reactance •
acoustic resistance •
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
change in momentum
Impulse in acoustics, events with a very short duration, see the formal definition and other terms in common use below.
Impulse Definition IEC 801-24-26, time integral of a force over the time during which the force is applied.
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
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
impulsive time weightings
Incident Sound is received directly from the source, as distinguished from sound that is reflected from a surface.
direct sound field
Incident Sound Wave
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, also known as acoustic mass
Note : inertance
has dimensions of mass
divided by the square of area
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.
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)
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
Instantaneous Sound Intensity
Instantaneous Kinetic Sound Energy Density
Instantaneous Particle Acceleration Definition IEC 801-21-32,
the time derivative of instantaneous particle velocity
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.
Instantaneous Particle Velocity Definition IEC 801-21-28,
derivative with respect to time, of the instantaneous particle displacement
particle velocity •
peak particle velocity
Instantaneous Potential Sound Energy Density
Instantaneous Sound Energy Density
Instantaneous Sound Pressure
Instantaneous Speech Power
Institute of Acoustics
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.
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.
Integrator an electrical frequency filter used to convert a vibratory acceleration signal to one whose amplitude is proportional to velocity or displacement.
sound wave interference
International Electrotechnical Commission
International System of 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 noise 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 Square Law (1/r²)
when sound radiates from a point source
in midair, the sound energy
is spread over a larger and larger spherical surface area as the distance from the source increases.
Since the surface area of a sphere is 4·π·r², then the surface area increases by a factor of 4 each time r (the distance from the source) is doubled
Therefore the sound intensity
(Watts/m²) decreases by a factor of 4
each time the distance from the source is doubled. In decibels this is 10·log (4) = 6 dB *.
Measuring sound pressure levels is more common in the practical world and the sound pressure decreases by a factor of 2 every time the distances is doubled. In decibels this is 20·log (2) = 6 dB *. However we know that sound intensity ∝ p² so the 'general' 6 dB rule still applies. 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 article
IOA : Institute of Acoustics
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.
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)
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