Rational Acoustics Measurement Mic curiosity...

[Ryan Gallagher]

Does anyone recognize this full spectrum type of transient "bursting" as a particular issue.

We recently tried to better quantify the subjective experience of dialog intelligibility problems in our admittedly extremely challenging room. As a result we decided to leave one of the measurement mics set up at FOH so the operator could kinda keep an eye on things or quantify subjective impressions film to film (especially when complaints are occurring).

This mic we left at FOH worked as expected for several days/screenings, interface being powered down between sessions, then yesterday started exhibiting this wide band bursting behavior, struggling to maintain even response sensitivity between bursts. Same behavior with or without computer connected, and across all 8 inputs.

Swapping mics fixed it, but those are not cheap mics! Could our Octa-Capture interface be having +48v problems that is killing the attached mic slowly? How else would such a measurement mic die, if it is really a dead mic?

[Marcel Birgelen]

I'm not sure what the deal is with those calibrated microphones, but we've seen several of those go bust over the years. While every normal microphone lasts a lifetime, those things seem to be made from eggshells or something similar.

Did you keep the Roland interface and thus also the microphone powered 24/7?

[Jay Wyatt]

It looked like your input was clipping and then blowing up the RTA data. But maybe it was the "bursting" causing clipping not the program material. Try lowering your pre-amp gains on the Octacap and see if it persists.

[Ryan Gallagher]

It looked like your input was clipping and then blowing up the RTA data. But maybe it was the "bursting" causing clipping not the program material. Try lowering your pre-amp gains on the Octacap and see if it persists.

Thanks. So the "sensitivity" settings on the Octa preamps didn't seem to track like you would expect. It was indeed clipping the inputs there, but turning it down did not solve any transients, they just maybe didn't have as much amplitude. Most definitely not the program material... I assume those kind of RTA signals would have been audible in the room. There was no other RTA input than the one measurement mic (fully calibrated and within a usable gain range hours before).

And again, another mic swapped in with same pre-amp gains had no issues.

I'm not sure what the deal is with those calibrated microphones, but we've seen several of those go bust over the years. While every normal microphone lasts a lifetime, those things seem to be made from eggshells or something similar.

Did you keep the Roland interface and thus also the microphone powered 24/7?

It had only been set up for maybe 4 days, but was powering down between shifts at the Octa. (it also has a self-powerdown timer of 4 hours i believe).

Octa-Capture is 15 year old hardware... so it would not shock me if something was on the way out. But it turning out to follow the mic was not what I was expecting.

[Ryan Gallagher]

I say all that, but I'm pretty new with Smaart. What pre-amp gain/sensitivity should you set against a 94db SPL 1K tone calibrator? I say mic was calibrated... but the pre-amp gain was indeed more of a guess.

I think there is some math involved for expected room volume and leaving ample headroom? I think I just put the pre-amp meter in the middle when there was tone and did the calibration using that.

[Marcel Birgelen]

Do you know the sensitivity of your mic? It should be in there somewhere.

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[Marco Giustini]

Bad solder joint in an XLR?

They can be tricky to diagnose if they're marginal. Changing the microphone might have helped as you moved the cable and the XLR plugs. What microphone is that?

Can you record and play that captured sound? I feel you'll be hearing lots of static noise.

[Ryan Gallagher]

Bad solder joint in an XLR?

They can be tricky to diagnose if they're marginal. Changing the microphone might have helped as you moved the cable and the XLR plugs. What microphone is that?

Can you record and play that captured sound? I feel you'll be hearing lots of static noise.

I did also try an alternate cable with that mic, same result. But I'll record something with it next week when I circle back. I'm on vacation for a week now. Try with a more strategic pre-amp gain setting etc. The fact it still did it in a quiet room after the film kinda rules signal clipping out though. I can also just plug it into the desk, giving it an alternate 48v phantom supply, and see if it's still popping.

[Marco Giustini]

I agree, it doesn't look like it's about to clip. What microphone is that?

[Marcel Birgelen]

Does Rational Acoustics sell anything else than the RTA-420? I think the RTA-420 is still pretty affordable. We have two of those Earthworm or Earthworks mics... I remember they were not entirely cheap for a microphone...

Stupid question: You haven't inadvertently zoomed in the Smaart frequency plot to a narrow frequency band? Hard to see on the recording and I'm also not using Smaart every day.

[Marco Giustini]

ah, I should have guessed the model from the title ?

[Jay Wyatt]

Does Rational Acoustics sell anything else than the RTA-420? I think the RTA-420 is still pretty affordable. We have two of those Earthworm or Earthworks mics... I remember they were not entirely cheap for a microphone...

Stupid question: You haven't inadvertently zoomed in the Smaart frequency plot to a narrow frequency band? Hard to see on the recording and I'm also not using Smaart every day.

I'd highly recommend the iSemcon 7150. Very durable, accurate, small diaphragm, and high SPL capabilities (145dB vs 110 on the RTA420). It is spendy but far more digestible of a price than an earthworks. It is the standard issue service mic for field techs at my company.

[Stefan Scholz]

Regarding the microphones. Do you remember the training seminar? Or at least in mine.

The actual difference in this application between a low cost microphone (25$) and a qualified measurement ($ 500+) microphone is minute.
Place both in the same spot in the room under analysis, and your reading in the main spectrum is within a dB, with littler higher deviation at high and low end, which is not interesting for intelligibility.
Move both (or single one) by a foot, and you're up to 5 ... 8 ... 10 dB off - in the same room.

Only real difference. The expensive mike has been tested before shipping, to record the individual response graph. So you can be assured, it works "out of the box". And there is another difference. Drop it, likely the hot glued "eggshell" of the cheap one disintegrates. The expensive one eventually survives without any damage.
Lesson: Even the cheapest electret condensor capsules do an adequate job.
And yes, the mikes may differ in sensitivity. High priced ones can offer a low sensitivity, enabling a large dynamic range to be covered, with good results at really high SPL levels.

In your case, check with another interface, check integrity of cables and plugs. Check with the calibrator at both SPL setting on different inputs. A 10 dB jump must be visible. Even try swapping input channels on the octa core. Connect a signal generator to the input. Octa Capture is the inface which accepts the highest input levels in microphone sensitivity, compared to others in the same field. That is, why it was recommended for the purpose.

The microphones recommended to me were ISEMcon EMX 7150. Screenshot 2026-07-29 122228.jpg

[Phil Ranucci]

Where is the mic located in the room? Any chance it got damp/wet?

[Ryan Gallagher]

Where is the mic located in the room? Any chance it got damp/wet?

Other than relative humidity no, it is on a gooseneck arm mounted to the wall just in front of FOH. Admittedly not a great measurement location for real tuning, but just to have another tool in the bag (that can be referenced for live music too)

I’ll circle back when back at the venue after vacation.