What is Coherence?

Created by Jake Bedard, Modified on Thu, 13 Jun 2024 at 02:18 PM by Jake Bedard

What is Coherence?

In Smaart, coherence is the “red line” at the top of the magnitude plot, but what is it?

Coherence uses the averaging buffer to show how stable/consistent your transfer function data is between measurements. You can think of it as a measure of data quality given as a value between 0 and 1 (or between 0% and 100%).

Coherence is always calculated for all transfer function measurements that use temporal averaging, but only the coherence trace for the topmost magnitude trace is plotted. If your coherence trace isn't showing, use the [C] hotkey to ensure it is turned on.

What Causes a Drop in Coherence?

Here are the 3 most common causes of bad coherence:

1. Measurement Delay is not set

  • To fix, check that the measurement delay is set (if not, top end coherence measurement will be low). This can appear as if the coherence is stair-stepping down as frequency rises.

2. Poor Signal to Noise Ratio.

  • To fix, check the strength of your signals. Slowly bring up your measurement level(s). If the coherence gets better with the raising level, it means that the measurement signal is overcoming environmental noise. When the coherence trace no longer improves, you’re there!

3. Poor Direct-to-Reverb Ratio.

  • Reflections and reverberant energy can also cause low coherence - looks like stalactites on the coherence trace.
  • To fix, move your mic closer to your sound source, move your sound source closer to mic, and/or dampen the room's reverberance or reflections. 

Coherence View Options

Squared coherence exaggerates differences in coherence across the spectrum. Non-linearities such as harmonic distortion are subtle compared to the previously mentioned causes of low coherence, so squaring the values makes them more visible. This view can also be used to measure the impact of distortion in electronics/system when testing maximum output level as well as M-Noise. This view can be togged via checkbox in the Graph Settings fly-up menu.

1/4 height coherence allows the use to shrink the coherence trace to 1/4 of the graph for a more compact view.

Coherence Blanking: If coherence is below a certain level within a particular frequency range, Smaart obscures the data in that range in a process called Coherence blanking. The threshold for which Smaart does this can be toggled via the grey slider on the right side of the magnitude chart. It can also be entered as a percent under Coh blanking threshold (%) in the Graph Settings menu.


Display options for the coherence trace are found in the Coherence section 

of the Transfer Function Graph Settings Menu (Graph Settings > Transfer Function).

In Smaart v9.4 and newer, this section of Graph Settings includes a checkbox for "Always display MTW Coherence", which is checked by default. This setting has been moved from the Transfer Function Measurement Settings menu where it was also checked by default and labelled simply as "Use MTW Coh".

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