Rion NA-42, Instruction Manual

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98
Technical Reference
During noise level measurements, the level often fl uctuates drastically, which
would make it diffi cult to evaluate readings without some kind of averag-
ing. Sound level meters therefore provide the capability for index weighting
(index averaging) using the rms circuit. The parameters of this weighting
process are called the time weightings, determined by the time constant (see
next page).
Noise level meters usually have a FAST and SLOW setting for the time con-
stant. The time range that is considered for averaging is narrow in the FAST
setting and wide in the SLOW setting. In the FAST setting, the instantaneous
level has a larger bearing on the displayed value than in the slow setting. From
the point of view of the measurement objective, the FAST setting is more
suitable to situations with swiftly changing noise level, whereas the SLOW
setting yields a more broadly averaged picture.
The FAST setting is more commonly used, and sound level or sound pres-
sure level values given without other indication are usually made with FAST
characteristics.
The SLOW time constant setting is suitable for measuring the average of
noise with fairly constant levels. Aircraft noise and high-speed train noise is
usually transient noise with high fl uctuation, but here the SLOW setting is
used to determine the maximum level for each noise event.
The IMP (impulse) setting enables the meter to track noise bursts of very
short duration.
In the
L
peak
mode, no averaging is carried out, and the peak value of the
frequency-weighted sound pressure waveform is displayed.