In the frequency range lower than the real time frequency range, only a part of the time data in the time window that is newly processed under FFT can be processed as the same data as the previous time data. Average processing performed by overlapping time windows in this way is referred to as overlap processing.

The extent of overlapping depends on the arithmetic operation processing capacity of the FFT analyzer and the frequency range (or the length of the time window to be imported). When the frequency range is about 1kHz or less, most of the new time data cannot be imported for average processing in the initial setting (Max overlap), so that an overlap rate must be set. In the normal Hanning window setting, overlap of around 50% is appropriate.

Two average processing rate setting methods are available, an average count and an average time. If the rate is set using average time, averaging can be performed sufficiently without having to give much consideration to the overlap rate.