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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Rob McIntosh on February 13, 2013, 10:44:10 pm

Title: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Rob McIntosh on February 13, 2013, 10:44:10 pm
Hey Everyone,

     I am building my first Smaart rig and am looking for an affordable measurement microphone, I know a lot of people with the Behringer mic ($55) and they say its fine for the price point. I noticed however that Rational Acoustics makes their own mic for $85. I was hoping to find someone that has used them both and let me know if the extra $30 is worth it or not. Thanks everyone!
Title: Re: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Jay Barracato on February 14, 2013, 12:33:44 am
Hey Everyone,

     I am building my first Smaart rig and am looking for an affordable measurement microphone, I know a lot of people with the Behringer mic ($55) and they say its fine for the price point. I noticed however that Rational Acoustics makes their own mic for $85. I was hoping to find someone that has used them both and let me know if the extra $30 is worth it or not. Thanks everyone!

I would buy the RA mic given those choices. I trust the company. I am sure they have selected a mic that's performance is appropriate to its price point coupled with great customer service.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Ivan Beaver on February 14, 2013, 07:21:10 am
Hey Everyone,

     I am building my first Smaart rig and am looking for an affordable measurement microphone, I know a lot of people with the Behringer mic ($55) and they say its fine for the price point. I noticed however that Rational Acoustics makes their own mic for $85. I was hoping to find someone that has used them both and let me know if the extra $30 is worth it or not. Thanks everyone!
The 420 mic is a good mic for the price.  There is a bit of a "wiggle" up high, but it depends on what you are going to be doing with it.  That wiggle may or may not bother you.

The thing I have "heard" (I have no first hand knowledge-so take that comment how you will) is that the Behringers are very inconsistent   Which means you may get a good one or a bad one-who knows.  How would you know?

I have no idea how consistent the 420's are either-but have not heard any bad reports about them.  Of course there are lots more Behringer mics out there-so you will hear more.

I know this doesn't give you any real information.  But real measurement mics cost real money.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Doug Fowler on February 14, 2013, 01:17:59 pm
Hey Everyone,

     I am building my first Smaart rig and am looking for an affordable measurement microphone, I know a lot of people with the Behringer mic ($55) and they say its fine for the price point. I noticed however that Rational Acoustics makes their own mic for $85. I was hoping to find someone that has used them both and let me know if the extra $30 is worth it or not. Thanks everyone!

I have three.  Two are perfectly matched, the third is not.  Ivan is correct about HF wiggle.  It's easy to see when compared with a better quality mic.

It's a good starter mic, but don't trust it for precision work above 8KhZ. 

If you measure something above 8K that's 10 dB more than surrounding freqs, feel free to take some of that out but don't trust it for fine tuning there.  Addressing broad trends yes, fine tuning no. 

Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Art Welter on February 14, 2013, 08:24:58 pm
I have three.  Two are perfectly matched, the third is not.  Ivan is correct about HF wiggle.  It's easy to see when compared with a better quality mic.

It's a good starter mic, but don't trust it for precision work above 8KhZ. 

If you measure something above 8K that's 10 dB more than surrounding freqs, feel free to take some of that out but don't trust it for fine tuning there.  Addressing broad trends yes, fine tuning no.
I only have one RTA 420, bought used, and the HF "wiggle" is only about 1-2 dB hotter up high than one of my Bruel &Kjaer 4004 . The other 4004 has about the same deviation the other direction than the RTA 420.
The B&K are very expensive, and the original test sheets show them dead flat 20 Hz to 10K, rising 1 dB at around 40K.
At the other end of the spectrum, the RTA 420 is about 3 dB down in response at 20 Hz compared to the B&K.
I seldom bring out the B&Ks any more, the difference in response is so slight.

One thing to note is the RTA 420 is pin three hot, reverse polarity from current "normal" convention.

Art
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Grant Conklin on February 15, 2013, 02:14:18 am
I have 6 of these mics, but they go by a different name.   As you can see, the "wiggle" up around 12k varies from about 2 - 4 db.  These measurements were made by comparing to a G.R.A.S. 40am capsule. 



Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Bradford "BJ" James on February 15, 2013, 03:20:05 pm
I have 6 of these mics, but they go by a different name.   As you can see, the "wiggle" up around 12k varies from about 2 - 4 db.  These measurements were made by comparing to a G.R.A.S. 40am capsule.
Grant, are yours the Apex mic (220)? Just curious as I have a few of those too. Pin 3 hot eh?.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Art Welter on February 15, 2013, 03:35:43 pm
Pin 3 hot eh?.
Yes. I was hoping to just switch it internally, but it uses a PCB connection, so I just use a short  polarity flip cord.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Bradford "BJ" James on February 15, 2013, 04:32:20 pm
Yes. I was hoping to just switch it internally, but it uses a PCB connection, so I just use a short  polarity flip cord.
ditto
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Grant Conklin on February 15, 2013, 11:21:54 pm
Grant, are yours the Apex mic (220)? Just curious as I have a few of those too. Pin 3 hot eh?.

Yes, Apex 220. The DBX mic is the same as well.  I created weighting curves for each of my mics. 
http://www.apexelectronics.com/category/Microphones/Special_Applications/Test_Reference/product/Apex220/
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Arthur Skudra 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


Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Micah McFadden 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 (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.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Arthur Skudra 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 (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.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Mark McFarlane 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.
Title: Re: Thouhts on Rational Acoustics' RTA-420 Measurement Mic?
Post by: Arthur Skudra 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.