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Author Topic: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?  (Read 4489 times)

Arthur Skudra

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Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
« Reply #10 on: February 18, 2013, 05:37:12 pm »

Here's my six RTA-420's and one Apex 220 that I had Cross Spectrum Labs measure for me for mic compensation curves.  RTA420-3 obviously has some issues! ;D  Mic compensation curves are handy if you buy cheap mics and want some reasonable validity in your measurements.  The RTA-420, Apex 220, and the DBX RTA mic all share the same form factor and likely come from the same factory in Asia, and having measured many of these mics in my Smaart classes, many exhibit a similar high frequency boost you see here, and they are all wired pin 3 hot.  And no, you can't modify them to pin 2 hot without having to replace the internal circuit board.


Here's a link to Cross Spectrum Labs measurement services, no personal connection here, just a happy customer:  http://www.cross-spectrum.com/


The Behringer ECM mic is a crap shoot, some of the ones I have measured are flat enough to rival test mics worth hundreds of dollars, and the rest have a similar response to the typical RTA-420.  And oddly enough, a few Behringers I have measured are wired pin 2 hot, the rest pin 3 hot!


If you don't want to spend money getting your cheapie test mics measured for compensation curves, you can do it yourself with a broadband pink noise source and a known flat test mic (I use an Earthworks M30) by simply doing a transfer function between the "flat" mic and the mic under test.  Not perfect by any means, but close enough to get you in the ballpark.

Keep in mind that if you compensate the response of a cheap test mic with a really wild HF response, there are unintended consequences off axis, so it's best to spend your money finding a mic that is reasonably flat to begin with.  My RTA420-3 mic is only used when there's a risk that the mic may get knocked over or run over by a lift!  :D


« Last Edit: February 18, 2013, 05:49:14 pm by Arthur Skudra »
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Micah McFadden

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Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
« Reply #11 on: February 21, 2013, 04:58:40 am »

Cross Spectrum does sell the ECM8000 with a calibration file:  http://cross-spectrum.com/measurement/calibrated_behringer.html
It does cost a bit more at $90, but it does eliminate any worry of receiving an inconsistent one.
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Arthur Skudra

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Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
« Reply #12 on: February 21, 2013, 01:38:40 pm »

Cross Spectrum does sell the ECM8000 with a calibration file:  http://cross-spectrum.com/measurement/calibrated_behringer.html
It does cost a bit more at $90, but it does eliminate any worry of receiving an inconsistent one.
You can buy test mics with calibration files from the Rational Store as well, including the RTA-420's and the iSEMcons.  I know that Ray Rayburn offers this service with the mics he sells:  http://www.testmic.com/  Earthworks now sells their test mics with the calibration curves at no extra cost.
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Mark McFarlane

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Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
« Reply #13 on: March 18, 2013, 11:54:38 pm »

Here's my six RTA-420's and one Apex 220 that I had Cross Spectrum Labs measure for me for mic compensation curves.  ...

Arthur. I'm curious what your test setup was. Not hardware, but physical stuff.  Done outdoors 1m from a 2-way speaker...?  I'd like to do a similar test of a few mics I have.
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Mark McFarlane
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Arthur Skudra

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Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
« Reply #14 on: March 19, 2013, 07:01:40 pm »

Arthur. I'm curious what your test setup was. Not hardware, but physical stuff.  Done outdoors 1m from a 2-way speaker...?  I'd like to do a similar test of a few mics I have.
Hey Mark, I didn't make the mic measurements, Cross Spectrum labs did.  There are some posts in this thread that go into further detail on how they measure microphones:
http://www.hometheatershack.com/forums/spl-meters-mics-calibration-sound-cards/15951-cross-spectrum-microphone-calibration-service-usa.html
You might want to take a look at the white paper on measuring microphones at the Earthworks website as well.

If you want to do it yourself, there is a "quick and dirty" method of using a broadband noise source, and using a known very flat high end measurement microphone for your reference channel (don't take it on the road, keep it locked up in an air conditioned office), and the mic under test plugged into the measurement channel, and the difference between the two (transfer function) would be your quick and dirty mic response curve.  Have the tips of the mics beside each other, set the time delay to "0", and watch the phase curve to get your physical alignment right.  The big weakness here is that with the two mics close to each other, you're going to get reflections, and thus some cancellation.  Not perfect, but gets you pretty close, and you can quickly determine whether a mic is really pooched or not.
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