Audio measurement gear

[DanB]

I'm using dual EK-23133-C36 mics. Apparently air turbulence around the input port is the limiting factor on the noise floor, and is somewhat unavoidable (or has been optimized).
The RME ADI 2 pro has line inputs.

 

[DanB]

And before you ask, yes, our Knowles hearing aid mic is battery powered externally and impedance matched.

 

[SIY]

You might look at Scott Wurcer's Linear Audio articles on interface electronics for the electret capsule.

As a measurement mic, the Knowles has some significant limitations. First off, its frequency response isn't close to flat. It starts its treble rolloff at 2kHz and is already 6dB down by 10kHz. Second, it's not quiet- the A-weighted noise is 26dB SPL, which is typical for small diaphragms like that. Third, the rated output impedance is 4k ohm, which is pretty high and will require signal conditioning very close to the capsule. So the sound card is unlikely to be the dynamic range limitation. Distortion info doesn't seem to be easily available but I'd guess that it's not inspiring.

If I were you, I'd be focusing on getting a better mic.

 

[DanB]

Thanks SIY. Very Interesting. I guess I just started using mics that are used in Etymotic otoacoustic emission hardware (Knowles EK mics), and went from there. I should add that these mics will be used for measuring sound in the ear canal, and thus need to be small (IEM size), or perform well when coupled into a small cavity via a probe tube.
AFAIK, most of the EIN for the knowles EK mics is in the low frequencies (<500Hz), and much of this is vibration noise (suppressed in a dual-mode arrangement). Above 500Hz the EIN is 10dB SPL.

But you're right, I should have a look at alternative mic technology, and I'll have a read of Scott's articles.

 

[DanB]

I attach a paper from Mead Killion (Chief Engineer at Etymotic back in the day) on microphone tech.

 

[DanB]

Perhaps I should move this discussion over to a new thread, as it was originally about DAC and ADC hardware, not mics etc...

 

[Blumlein 88]

With 4000 ohm output impedance thermal noise will be at something like a theoretical max of -118 dbV. And that isn't considering the 26 dbSPL self noise. That is with no gain. Gain will amplify the noise floor and self noise as well. So I don't think anything will get you to your goals with that microphone.

 

[SIY]

Thanks SIY. Very Interesting. I guess I just started using mics that are used in Etymotic otoacoustic emission hardware (Knowles EK mics), and went from there. I should add that these mics will be used for measuring sound in the ear canal, and thus need to be small (IEM size), or perform well when coupled into a small cavity via a probe tube.
AFAIK, most of the EIN for the knowles EK mics is in the low frequencies (<500Hz), and much of this is vibration noise (suppressed in a dual-mode arrangement). Above 500Hz the EIN is 10dB SPL.

Understood. And I had a similar problem several decades ago setting up a quiet infrared photoacoustic cell. It's not easy, and I ended up using an acoustic coupler from the tiny hole/cavity for the sample to the diaphragm of a 1 cm condensor mic. Your problem may be tougher because you can't start drilling holes where you need them in someone's head.

Or maybe you can. If so, please don't tell me.

 

[DanB]

I should add that these mics are capable of -17dB SPL noise floor (at 4kHz), with zero pre-amp gain. They are routinely used in hearing research, in Etymotic low-noise systems: https://www.etymotic.com/auditory-research/microphones/er-10b.html

 

[Blumlein 88]

I should add that these mics are capable of -17dB SPL noise floor (at 4kHz), with zero pre-amp gain. They are routinely used in hearing research, in Etymotic low-noise systems: https://www.etymotic.com/auditory-research/microphones/er-10b.html

Some of their specs are odd. The one in the link above shows -17 db SPL at 1 khz at 1hz bandwidth. That actually wouldn't be very good. Its self noise would be much higher the way the spec is normally done. Maybe something like 26 db SPL instead of -17 db. Maybe that is a misprint or I'm missing something. Noise would usually be over 20 khz compared to the level of 1 khz at 94 db SPL. I notice they leave blank the SNR spot too. Normally you by convention subtract equivalent self noise from 94 db SPL and give SNR. For instance a mike with self noise of 15 db would have an SNR of 94-15 or 79 db. Now maybe that is still good for such a small condenser microphone.

Its output impedance is listed here as 100 ohms which is more normal. The other place mentioning 4.4 kohm probably was referencing the specs were what you get with a mic pre having 4.4 kohm input impedance or load on the microphone.

 

[SIY]

B88, you're not missing anything. That noise spec IS for a 1 Hz bandwidth, and Dan is trying to compare it to the noise specs of sound cards over a 20 kHz bandwidth. That's where I think his disconnect is. The sound cards will not be the noise limitation in his setup.

Dan, what you need to do in order to have apples to apples is to divide the 20k noise voltage specs by sqrt(20,000).

 

[DDF]

I had the opportunity to work with an RTX6001 this week, and thought I would share some test results of the instrument.

Due to time constraints I was limited to using RMAA. The loopback performance from unbalanced out to balanced in was decent at 24bit 96kHz:

Performance was mostly limited by mains and it`s harmonics:

I used the default RMAA settings so the so the graphs have FFT gain, but the SNR is shown in the table.

I found it very interesting that I could improve the unbalanced mains noise by 10 dB by either using a much heavier gauge AC cord, or running a ground wire from my power bar earth ground directly to the output jack ground. The results above were with the heavy duty ac cord.

Good news is that the power supply noise was common mode and balanced loopback was better:

Results at 16 bit 44.1 decreased as expected with the bit depth noise masking the mains noise. Here are results 16/44.1 unbalanced:

Neither outcome was quite as good as hoped (leaving some dB on the table) but I find these prosumer setups are extremely sensitive to PC noise and ground and these were best results off 2 different PCs and best cables in my collection, with careful dressing. All results optimized the ADC for lowest input gain possible without sacrificing noise from dropping the level on D/A too far digitally.

 

[Robertlars]

Dear DDF,

Thank you for posting your RMAA results on the RTX6001’s performance. I note that if I compare your posted RMAA results to that of the ones shown on this URL link https://www.virtins.com/RTX6001.shtml that the two differ.

Since the time the RTX6001 was released there have been two different modifications to the hardware to improve its performance it would be interesting to know if the RXT6001 unit you used had both of the physical modifications made to it or not. Please see the attached two PDF files for details. Last but not least, there was an RTX6001 firmware update, can you please address what RTX6001 firmware was used for your RMAA testing as well?

Furthermore, I do know that some have questioned the RMAA audio analyzer software insofar as being suspect, please see: http://nwavguy.blogspot.com/2011/02/rightmark-audio-analyzer-rmaa.html and http://nwavguy.blogspot.com/2011/02/testing-methods.html (I do note that while I love nwavguy’s postings, he [she?] is an elusive enigma insofar as knowing whom they actually are, see: https://spectrum.ieee.org/tech-history/silicon-revolution/nwavguy-the-audio-genius-who-vanished).

If I may be so bold as to suggest that you post your RMAA RTX6001 results to both of the following websites:

• https://www.diyaudio.com/forums/equ...diy-audio-analyzer-ak5397-ak5394a-ak4490.html
• https://www.diyaudio.com/forums/gro...-autoranging-autoscaling-soundcard-users.html

Doing the above will allow pretty much all the RTX6001 owners like Demian Martin and others as well as the main engineer who designed the RTX6001 (Jens) to chime in on your RMAA results and allow for a great discussion of your results.

Finally, please note I am not saying your RMAA results are wrong, I simply would like to see the vast majority of RTX6001 owners via the above diyaudio URLs be able to chime in on your results and allow a real discussion of them to play out to potentially ferret out why the delta between your RMAA results vs. that of the aforementioned (as well as many RTX6001 end users via the first RTX6001 diyaudio URL link above many of whom have also tested and posted their RTX6001 results using many different audio analyzer software packages).

Thank you again for posting your RTX6001 RMAA results and hoping you’ll post your results to the above diyaudio forums to allow for the broadest type of feedback from the entire RTX6001 owner community on them.

Regards,

Robertlars

 

[gvl]

Can the RME ADI-2 Pro be used to its full potential for unbalanced measurements? A passive unbalanced to balanced adapter is half the voltage which is less than ideal, right?

Edit: according to specs TRS inputs are fully TS/RCA compatible, so using unbalanced is not an issue it seems.

 

[Don Hills]

Can the RME ADI-2 Pro be used to its full potential for unbalanced measurements? A passive unbalanced to balanced adapter is half the voltage which is less than ideal, right?

Edit: according to specs TRS inputs are fully TS/RCA compatible, so using unbalanced is not an issue it seems.

Archimago has just reported a series of measurements using the RME ADI-2 Pro to ADC the outputs of several DACs, I assume unbalanced.
http://archimago.blogspot.com/2019/03/measurements-look-at-audio-ultra-high.html

 

[March Audio]

Archimago has just reported a series of measurements using the RME ADI-2 Pro to ADC the outputs of several DACs, I assume unbalanced.
http://archimago.blogspot.com/2019/03/measurements-look-at-audio-ultra-high.html

Thanks for the link, interesting stuff

VI. In Conclusion...
With the <US$100 SMSL iDEA, the >US$1000 Oppo UDP-205, both using ESS DACs, and adding the TEAC UD-501 from 2013 with its TI/Burr Brown DAC in the mix, I think we can say based on the data from the RME ADC with 384kHz bandwidth that there's really not much in the "high end" at all during PCM playback when using appropriate digital filters. Sure, likely there's ultrasonic stuff below the limits of the RME's AKM AK5574 ADC modulator noise floor, but we'd be looking for signals below about -85dBFS at frequencies significantly above 100kHz!

As discussed in late 2018, I'm far from impressed by audiophiles who seem to believe that there's all kinds of noise, jitter and whatnot affecting the sound output from otherwise competent DACs connected to computers through USB. Anxiety over things that appear to have no basis in reality amounts to perpetuation of unsubstantiated fear, uncertainty, and doubt. I hope magazines and on-line sources seek to find balance and evidence when these kinds of ideas are presented.

 

[L0rdGwyn]

Hi - question for ya'll relevant to this thread.

I do a large portion of my listening out of my OTL tube amplifier. I'd love to have an affordable measurement setup such that I can quantify the harmonic distortion profiles of various input/output tubes and see if there is a correlation to my subjective listening preferences.

Would the QuantAsylum QA401 be a good choice for this use case? The price is reasonable and it seems to be getting very positive feedback.

Also, I have never taken audio measurements before, so I will have to do a fair amount of learning. I am not an electrical engineer, although my first degree is in a STEM field (now work in healthcare), I'm confident I can figure it out.

Thanks!

 

[SIY]

A sound card plus an interface like Pete Millett's will be able to get you capabilities to get those spectra at a very low cost.

If it helps, I wrote a series of articles on this in AudioXpress. Here's part one of the series, with links to the other parts at the end.

 

[gvl]

A sound card plus an interface like Pete Millett's will be able to get you capabilities to get those spectra at a very low cost.

If it helps, I wrote a series of articles on this in AudioXpress. Here's part one of the series, with links to the other parts at the end.

Any suggestions for a reasonably priced competent USB-based ADC solution? I've been thinking about getting the RME ADI-2 Pro just because it is such a Swiss army knife, but can't justify paying the price for my modest needs.

 

[SIY]

Any suggestions for a reasonably priced competent USB-based ADC solution? I've been thinking about getting the RME ADI-2 Pro just because it is such a Swiss army knife, but can't justify paying the price for my modest needs.

Depends on how deep down you want the measurement to go. For most normal stuff, a Scarlett 2i2 is more than good enough.