User guide

122
CHAPTER 7
Using SIM 3
Figure 7.3. Delay Finder Settings dialog, including Amp Threshold levels
7.2.2. Coherence (Coh) Threshold
The coherence threshold is the level at which the analyzer accepts that the correlation of
the data at the two measurement points is sufficient to present that data as a trace within
the viewer at a particular frequency or subset of frequencies. Coherence is shown as a
percentage, and the higher this percentage is, the more likely the measurement at that
frequency is accurate.
7.2.2.1 Using the Coherence Trace
It is normal to see numerous peaks and dips within the red Coherence trace, especially when
using live program material as a measurement source and when measuring at a greater
distance from the loudspeakers. In areas of low coherence, the corresponding frequency
response trace (amplitude over frequency) is more likely to be missing or showing a sharp
and usually narrow dip.
NOTE: Low coherence often occurs because the measured signal is low
compared to ambient noise that is masking it (from other audio sources,
reflections, or when the measurement mic is located within the reverberant field). It
can also occur when the same signal arrives at the measurement mic from various
loudspeakers at slightly different times, delayed by the different distances between
these speakers and the mic.