Spider DSA User’s Manual
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Alternatively, the RMS trace can be stored using RPM as a variable. This method
is particularly useful in the automotive NVH applications. The picture below
shows how one of the filter outputs can be stored in RPM trace.
Figure 146. Store RPM based RMS traces.
Exponential and Linear Averaging
Linear averaging: Linear averaging uses a fixed time period to sum up the
historical power value of each filter and then takes the square-root to calculate the
averaged RMS value. The RMS trace update time is governed by the time period
of the averaging. For each time period of averaging, one RMS value per frequency
bin is produced.
Exponential averaging: Exponential averaging applies an exponential time
constant to the historical power values of each filter and takes the square-root of
the averaged power value. A time constant of 0.125 seconds is equivalent to “Fast”
averaging and 1.0 second is equivalent to “Slow” averaging of a sound level meter.
In exponential averaging, the RMS trace update time is independent of the time
constant.
Peak Hold averaging: Peak Hold retains the maximum value in each
frequency bin over the period of time since last “start” or “restart”. It is a one-time
extreme observation over the interval rather than an averaged property.
As discussed previously, each filter may have a different settling time of
approximately 5/BW seconds.
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