What is Coherence?

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 decimal value between 0 and 1 (or as a percentage from 0 -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 shown on the plot. 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. This looks like stalactites or "blood on the screen" on the coherence trace, which is where comb filtering gets its name.
• To fix, move your mic closer to your sound source, move your sound source closer to your mic, and/or dampen the room's reverberance or reflections. 

Coherence View Options

Display options for the coherence trace are found in the Coherence section of the Transfer Function Graph Settings Menu, accessible via Graph Settings > Transfer Function or the [Alt/Opt] + [T] hotkey.

Squared Coherence exaggerates differences in coherence across the spectrum. Non-linearities such as harmonic distortion are subtle compared to the causes of low coherence mentioned above, 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 allows the use to shrink the coherence trace to 1/4 of the graph (as opposed to 1/2) for a more compact view.

Coherence Blanking Threshold (%): 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 percentage within this menu. (Note: Coherence Blanking doesn't affect the data itself, only how it's displayed, so a captured a trace will include any data hidden by this threshold.)

Always Display MTW Coherence, as one might expect, will cause the coherence trace to correspond to a MTW FFT, even when the trace is set to a different FFT size. This setting is active by default. When deactivated, coherence will follow the globally-set FFT size, instead.

Checking Line Thickness Follows Foreground will match the coherence trace's thickness to the thickness of the top trace in the Z-order as set within the Transfer Function tab of the Graph Settings dialog. When unchecked, you can use the dropdown to change the thickness of the coherence trace independently, if desired.