Instruction manual

114
Technical Notes
RMS Detection Circuit
The sound pressure level meter uses rms detection. The effective value E (rms) is defined
by the following equation.
The voltage e which changes over time is raised to the power of 2, and integration for the
time interval T is performed. The result is divided by T and the square root is extracted.
The circuit configuration for performing the above mathematical operation looks as fol-
lows.
The NA-27 performs all above processing functions in the digital domain.
For measurement and evaluation of the rms detection circuit, a signal with known crest
factor is used. The crest factor is defined as crest value divided by effective value. If the
signal type is known, the crest factor can be calculated precisely.
The rms detection error of the NA-27 is about ± 0.2 dB for an input signal with a crest
factor of 3.
During sound pressure level measurements, the level often fluctuates drastically, which
would make it difficult to evaluate readings if some kind of averaging is not applied.
Sound pressure level meters therefore provide the capability for index weighting (index
averaging) using the rms circuit. The parameters of this weighting process are called the
dynamic characteristics, determined by the time weighting (see page 116).
Sound pressure level meters usually have a "Fast" and "Slow" setting for the time weight-
ing. 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 bear-
ing on the displayed value than in the "Slow" setting. From the point of view of the mea-
surement objective, the "Fast" setting is more suitable to situations with swiftly changing
sound pressure level, whereas the "Slow" setting yields a more broadly averaged picture.
The "Fast" setting is more commonly used, and sound pressure level or sound pressure
level values given without other indication are usually made with "Fast" characteristics.