Coherence: Your Trust Meter in Smaart Transfer Function Measurements
When tuning a sound system with Smaart, the Transfer Function mode reveals critical insights through traces like magnitude and phase. But the most vital one? The coherence trace, often called the “trust meter” or reliability indicator.
Coherence measures how closely the measurement signal (what your microphone captures) matches the reference signal (the direct input from the console). Expressed as a percentage, high coherence (near 100%) means the data is stable and trustworthy. Low coherence signals that noise, reflections, or misalignment are contaminating your measurement, making the magnitude trace unreliable.
What Causes Low Coherence?
Several common issues can drag coherence down:
- Room Reflections: In reverberant spaces, echoes arrive delayed at the mic. The software sees multiple versions of the signal, reducing correlation, especially if the mic is far from the speakers where reverberant energy dominates over direct sound.
- Poor Signal-to-Noise Ratio (SNR): Background noise from crowds, HVAC, or stage activity overwhelms your pink noise test signal. Aim for at least 10-20 dB above the noise floor for solid coherence.
- Time Misalignment: If the reference and measurement signals aren’t properly delayed, high frequencies suffer most (due to shorter wavelengths). This creates comb filtering and drops coherence rapidly above certain frequencies.
Other factors like distortion, multiple overlapping sources (e.g., both sides of a stereo PA active), or even floor bounces can contribute.
The Rule of Thumb: Trust Only High Coherence Data
A practical guideline: If coherence dips below about 80-90% (many users blank data below 85% for critical work), especially in high frequencies, ignore the magnitude trace in those regions. You’re likely seeing noise or artifacts, not the true system response.
Instead, fix the root cause first:
- Use Smaart’s Delay Finder to align signals precisely.
- Increase test signal level or average more frames.
- Move the mic closer to the source for better direct-to-reverberant ratio.
- Measure one subsystem at a time to avoid interference.
Smaart even has coherence blanking to visually obscure unreliable data, reminding you not to EQ based on garbage.
In the end, coherence keeps you honest. It prevents chasing phantom EQ fixes on noisy data, ensuring your tuning decisions are based on reality. Next time you’re staring at a wiggly magnitude trace, glance up at that coherence line, it’s telling you whether to trust what you see.
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!