Help Topics - I

IEC

The International Electrotechnical Commission based in Switzerland that publishes numerous standards specifying the performance of sound level meters, noise dosimeters and frequency filters.

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Impact insulation class

A single figure rating that compares the impact sound insulating capabilities of floor-ceiling assemblies to a reference contour.

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Impact sound

The sound produced by the collision of two or more solid objects. Typical sources are footsteps on a floor or on an interior surface. Other sources can include drop forges or other metalwork processes.

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Impulsive noise

Either a single pressure peak with a rise time of less than about 200 milli seconds (or total duration of less than 200 milli seconds) spaced by at least 200 milli seconds or a sound pressure occurring in a short interval of time.

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Impulse response

This is one of the standard responses in a sound level meter (or dosimeter).

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Impulse weighting

The impulse weighting is the same as the impulse response in a sound level meter and is part of the rms. circuit controlling the response of the meter to the variability of the instantaneous levels of the noise. It has a rise time value of 35 milli seconds and a decay time of 1500 milli seconds. The asymmetric nature of the impulse weighting means that any average sound level calculated based on samples of the instantaneous will be biased towards the higher noise events than will the average calculated from either the fast or slow response. It is a continuous function that is calculated all the time by the instrument.

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Indoor air

The breathing air inside a habitable structure or conveyance.

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Indoor air pollution

Chemical, physical, or biological contaminants in indoor air.

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Infrasonic

Sound frequencies that are below the normal limit of human audibility, typically less than about 20 Hz.

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Inner ear

The part of the hearing mechanism that transfers the mechanical vibrations into electrical impulses that are then sent to the brain to be interpreted as sounds. Also associated with the balance mechanism.

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Insulation

The ability of a material to keep out sound from a given source on the other side of the material. The performance of the material depends on its composition and density, the higher the density the better the material is as a sound insulation product.

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Integrated sound level

The average sound level over a period of time that represents the energy contained in the noise. The integrated level will be the same whether the fast or slow time weightings are used in the measuring instrument since these weightings are symmetrical for both rising and falling sound levels. Use of the impulse time weighting will produce a higher average level compared to the true energy level computed from fast or slow sampled levels.

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Integrating sound level meter

A measuring instrument that can calculate the time average noise level over a specified period of time in addition to measuring and displaying the simple instantaneous level. Sometimes called Leq meters when the exchange rate is fixed at Q = 3 dB. It is also possible to obtain the integrated sound level with the exchange rates of 4, 5 or 6 dB.

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Intensity

The sound energy flow through unit area (1 m2) in unit time (1 second).

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Inverse square law

A description of the acoustic wave behavior in which the mean-square pressure varies inversely with the square of the distance from the source. This behavior in free field situations where the sound pressure level decreases 6 dB with each doubling of distance from the source.

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IR

Infrared radiation beyond the limit of normal human sight. Sometimes used as a wireless communication method to transfer data from an instrument to a printer or to a computer.

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ISO

The International Standards Organization based in Switzerland that publishes numerous standards outlining how different measurements may be carried out.

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Isokenetic sampling

Sampling velocity equal to local gas velocity. Usually considered inside chimney stacks and other such locations.

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Isolation

The physical separation of one item from another to prevent the flow of energy through a structure. This can be for both the control of sound or vibration energy.

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