Help Topics - F

Fahrenheit scale

A range of temperatures over a 180-degree range expressed as 32 degrees at the freezing point and 212 at the boiling point for pure water at standard atmospheric pressure.

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Far field

The far field is a notional volume around a noise source such that the inverse square law applies to the dissipation of the energy. In this region the measured sound level reduces at a rate of 6 dB every time the distance from the source doubles. In the far field region the particle velocity of the molecules is in phase with the sound pressure.

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Fast response

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

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Fast weighting

The fast weighting is the same as the fast 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 value of 125 milli seconds and is a continuous function that is calculated all the time by the instrument.

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Filter cassette

A mechanical container designed to minimize the handling of the delicate filters used in sampling for particulates. Filter cassettes are available in various sizes such as 25 mm, 37 mm or 47 mm.

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Filters

Can be made from glass fiber (GF/A) or mixed cellulose ester (MCE). Used to collect the particulates in the air when performing sampling conventions. Usually used in 25 or 37 mm diameter sizes.

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Filter set

A device for separating the different components of a signal on the basis of their frequency. It allows components in one or more frequency bands to pass relatively un-attenuated while it attenuates components in other frequency bands.

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Flanking noise

Noise that is transmitted from one room into another by indirect means such as through sidewalls or floors rather than through the common partition that separates the rooms.

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Flow meter

A device used to verify the flow rate of a personal sampling pump used for sampling conventions to ensure stable and well-known conditions.

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Free field

Sound fields in which the effects of walls or other obstacles or boundaries on sound propagation are negligible. As a measurement point moves away from a source the noise level will drop according to the inverse square law.

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Frequency

The number of times a second that a sine wave of sound repeats itself, or a vibrating object, repeats itself. The more repeats per second then the higher the frequency is said to be. The unit of frequency is Hertz, Hz, numerically equal to the earlier unit cycles per second, cps.

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Frequency band

A range of frequency components in the audio bandwidth used to characterize different noise sources. Frequency bands can be expressed as whole octaves, third octaves, sixth octaves etc. For noise measurements to assess the affect on humans the step intervals for contiguous frequency bands are usually expressed on a logarithmic scale.

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