How to use the Coherence curve effectively in your project.
The coherence curve is maybe the most underrated metric in Smaart. Everybody, or at the least the once having studied the manual and / or hav been to a Smaart seminar know what is, but is the coherence curve used enough?
Let’s break down what coherence is. The word “coherence” comes from the Latin “cohaerentia”, which means “a sticking together” or “consistency.” It’s from “co-” (together) + “haerere” (to stick). Originally it referred to things physically sticking together, and later it was applied metaphorically to ideas, speech, and writing.
Metaphorically speaking this basically what we strive for. Expectancy and outcome. If these match 100% we have a maximum coherence reading. Understanding a coherence focussed approach can be used in understanding to match the customers expectancy with the project result. So he will match your invoice 100% with a respective payment. Topp notch coherence achieved. That is measurable as well. You see in your personal or companies balance sheet.
Back to acoustics and Smaart.
So coherence is a comparison of at least two sets of data. The reference and the measurement channel. The essence of dual channel FFT. At each of the channels the complex audio waveform is transformed from time domain to frequency domain. That way we can see the energy content (magnitude) per frequency.
The reference channel is our “clean” undisturbed signal. The microphone which pics up our measurement signal does not only hear the loudspeaker. Besides the audio from the loudspeaker, there are tons of other signals hitting that little membrane. Unrelated noise from the ventilation system, traffic on the street, the cleaning ladies having a casual chat next to the microphone – all that affects the quality of our measurement. Besides these unreacted signals we get related signal. The reflections of the room bouncing the audio of the loudspeaker around and sending it to the microphone.
In this article I won’t go into all the very clever math and trickery that goes on inside Smaart so it can find the original, or better said direct signal in all the noise. If you want to dive deeper into that you can read this article – https://support.rationalacoustics.com/support/solutions/articles/150000183871-what-is-coherence-
So how can we use the coherence trace in a PA setup?
We can make use of the coherence trace very early in our rigging scenario. The coherence curve is not judging whether the sub system / single loudspeaker or array is already perfectly EQ’d etc.
100% coherence would be the best, which is only achievable at very close range or an anechoic chamber. Reflections degrade the coherence. Place a measurement mic at the furthest seat yo while your rig goes up. Aim your speaker system to this point until you hit the maximum possible coherence. It should read something like 70% or better. Should you find it impossible to a come to an acceptable result, you need to check these points:
- 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. - 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! - 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.
Still bad? Most likely you are dealing with Poor Direct-to-Reverb Ratio. If re-aming the speaker does not help, it means that the reflections are to strong, so you need to consider doing something to the room acoustics and dampen the reflections.
Not possible? If really bad you might want to discuss with production whether these seats should be blocked. That is a very tough call and a last resort option, but at least it shows that you care and make informed designs.
You can also discuss heling the bad signal back in the bleachers by installing a delay system. So you can also use coherence reading to find out whether you need a delay system and where to place it. You can also immediately identify misalignment delay times.
The coherence trace is an invaluable tool to all find out about problems and limitations early in the installation phase of the sound system. One could as well call it the QC curve.
Related article: Understanding Coherence in Real-Time Mode
Tip: press “C” to show / hide the coherence trace.
Smaart is excellent on its own, but most users find they get much more out of it after some structured training. That’s where our seminars come in. At TZ Audio we run practical seminars, both online and in-venue. We offer seminar-only or full “all you need packages” including software & hardware. It’s simply the fastest way to become comfortable and confident with the measuring a sound system.
If you’re in Norway, Sweden, Denmark or Iceland – or elsewhere – we offer is online seminars and traveling to Norway is a valid option too of course. We’re here if you have any questions about the software or upcoming seminars.
Thanks for reading!