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__I__| J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z**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.

**Fast response**

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

**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.

**Flanking noise**

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

**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.

**Frequency**

The number of times a second that a sine wave of sound or vibration 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.

**Frequency band**

A range of frequency components used to characterize different noise, vibration or cyclic sources. Frequency bands can be expressed as whole octaves, third octaves, sixth octaves or linearly spaced components. To assess the affect of noise on humans the step intervals for contiguous frequency bands are usually expressed on a logarithmic scale.