John Reekie
Technologicality, at work and play

iPad Measurement Mics

I was curious about how the internal iPad microphone fared for measurements, and what else could be used. There are a few twists and turns here, so if you’re in a hurry, feel free to jump straight to the conclusion.

Note: these measurements are not intended to be authoritative or definitive. I took them to satisfy my own curiosity. If you know of measurements that are consistent with or contradict these measurements, please do post a link to them in a comment below.

Preliminaries: measurements in REW

I first tested three microphones with REW. These were all done in-room, approximately at a suitable listening position, since the iPad measurements will be simple 1/3rd-octave RTA. All microphones were pointed directly at the speaker and the appropriate (on-axis) calibration file used.

Firstly, a comparison of measurement sweeps using an Earthworks M30 vs my CSL-calibrated Dayton EMM-6 microphone:

REW Sweep, Earthworks M30 (red) vs CSL-calibrated EMM-6 (blue), 1/6th octave smoothing

REW Sweep, Earthworks M30 (red) vs CSL-calibrated EMM-6 (blue), 1/6th octave smoothing

The ¬†two mics are very consistent across the range, with the EMM-6 reading slightly lower from 6 to 17 kHz. I’m not 100% convinced that the M30 is quite on the mark, for reasons I won’t go into here, but regardless, pretty close.

Next, a measurement sweep comparing the M30 to a miniDSP UMIK-1:

REW Sweep, Earthworks M30 (red) vs miniDSP UMIK-1 (green), 1/6th octave smoothing

REW Sweep, Earthworks M30 (red) vs miniDSP UMIK-1 (green), 1/6th octave smoothing

The UMIK-1 reads slightly higher than the M30 below 50 Hz and above 8 kHz.

Now, I mentioned above that the iPad measurements will be an RTA (real time analysis), not a measurement sweep. Here’s how the REW RTA function (1/3rd octave) compared to the sweep, both with the UMIK-1:

Measurement sweep in REW (green) vs RTA in REW (red)

Measurement sweep in REW (green) vs RTA in REW (red)

They’re pretty close, with the RTA drooping slightly at the top and bottom (the same effect was observed with the other two microphones). Note that the RTA does tend to bounce around a little, so some variation at individual 1/3 octave bands should be ignored.

iPad measurements

The measurements on the iPad were done using the RTA function of AudioTools. To compare measurements, the RTA was stopped and saved as a file, which was then moved to my laptop and imported into REW. The actual display in AudioTools is a bargraph, but they come out as smooth curves in REW. Good enough to compare though.

I was unable to use the EMM-6 and the M30 with the iPad as my MOTU Microbook II USB audio interface doesn’t work with the iPad. Bummer. I guess it saved me some time. So I used the UMIK-1 (with the Apple Lightning-USB adapter), a Dayton iMM-6, and the internal iPad mic. The UMIK-1 and iMM-6 had their respective calibration files loaded, while the internal mic used the AudioTools default (which apparently didn’t do much, see below).

First off, here’s the RTA measurement using the UMIK-1 taken on REW, compared with the RTA measurement using the UMIK-1 on the iPad:

RTA from REW (red) vs RTA from AudioTools (purple), both using UMIK-1

RTA from REW (red) vs RTA from AudioTools (purple), both using UMIK-1

Pretty close, although there seems to be some drop-off in the AudioTools measurement below 90 Hz and I don’t think I’d trust the two lowest bands (i.e. below 30 Hz).

How does the AudioTools measurement compared in absolute terms? In other words, let’s assume that the sweep is the most accurate measurement. Here’s the sweep taken in REW using the UMIK-1, compared to the RTA measurement in AudioTools:

Measurement sweep in REW (green) vs RTA in AudioTools (purple), both using UMIK-1

Measurement sweep in REW (green) vs RTA in AudioTools (purple), both using UMIK-1

Oooo… the AudioTools RTA reads about 3 dB lower from 70 Hz downward. Above 90 Hz, it’s as close as you could want. I can’t really explain this – if I can get the other mics working with the iPad at some point, I’ll see if I can verify whether this effect is consistent or just an anomalous measurement.

How does the internal mic compare? Here is the RTA from AudioTools using the UMIK-1 compared to the RTA using the internal mic:

Audiotools RTA using UMIK-1 (purple) vs the internal iPad mic (green)

Audiotools RTA using UMIK-1 (purple) vs the internal iPad mic (green)

As you can see, a fair bit of a drop below 200 Hz and above 6 kHz.

The little Dayton iMM-6, though, comes with a calibration file. Here it is compared to the UMIK-1:

Audiotools RTA using UMIK-1 (purple) vs the Dayton iMM-6 (blue)

Audiotools RTA using UMIK-1 (purple) vs the Dayton iMM-6 (blue)

It wobbles around a bit but is generally a decent match, with the exception of the 100-400 Hz range where it varies more than I’d be comfortable with for serious measurements or EQ. (I repeated the measurement and got a similar result.)

Cal file conundrum

After I took the RTA with the iMM-6, it occurred to me to check the calibration file. Loaded into REW, it looks like this:

Dayton iMM-6 calibration file

Dayton iMM-6 calibration file

See all those wobbles above 4 kHz? They really shouldn’t be there. Dayton (or rather, their supplier) are doing something funny with the calibration. Still, for 1/3 octave RTAs, it makes no practical difference, but I wouldn’t use these cal files for measurement sweeps without smoothing. Here’s the Dayton cal file for my EMM-6 compared to the cal file from Cross-Spectrum Labs:

EMM-6 calibration files: from Dayton (green) and from Cross-Spectrum Labs(blue)

EMM-6 calibration files: from Dayton (green) and from Cross-Spectrum Labs(blue)

(In case it’s not obvious, the Dayton cal file simply can’t be correct.)

Conclusions/Recommendations

Based on the above measurements, here is what I think…

  1. The internal microphone in the iPad isn’t really suitable for measurement work. It is probably possible to generate a calibration file that would make it work much better for this kind of RTA, at least above 40 Hz, but the default setting in AudioTools wasn’t it.
  2. The little iMM-6 performed OK. If you’re just looking at the general trend and not trying to EQ something to the last dB, this is great value at $17. Or, it would be a great tool for learning the ropes without spending very much. If it weren’t for the funny response in the 100-400 Hz region, it would be a complete no-brainer at that price.
  3. The external USB mic, the UMIK-1, performed consistently with the measurements from REW albeit with some droop below around 90 Hz. I can’t explain that but I’d assume that it’s the software not the mic itself. In either case, something to keep in mind if doing EQ. The Dayton UMM-6 would I assume be a good alternative, except for…
  4. The Dayton cal files. If spending a bit more for a USB measurement mic (and the needed USB adapter for the iPad), then I would recommend getting the mic from Cross-Spectrum Labs. (Actually, I’d recommend that for the UMIK-1 as well as the UMM-6 – for a slight increase in cost over the stock mic you also get a 90-degree calibration file, which is more than a little handy if you’re trying to EQ an HT system i.e. with speakers all around you).
  5. I don’t have a USB audio interface (i.e. with mic preamps) compatible with the iPad. For some reason I had assumed that the Microbook II would work, until I tried it and remembered that it does require a driver even on the Mac. Any recommendations on an iPad-compatible interface? Please post a link below if so.
  6. iPad software is more limited than measurement programs running on a regular computer. It wins on convenience and portability but not on features or analysis capabilities. Nothing wrong with that, just be realistic.

 


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Readers' comments

    John-
    Thank you for taking the time to do this test and the write up! I’ve been looking for a portable measuring solution and it seems I may have found it. I typically use REW/TrueRTA on my laptop with a usb phantom power interface (MicMate) and my Behringer ECM8000 mic. I was thinking it would be nice to be able to have one solid, calibrated mic to use with that setup, but also to be used for a portable device- namely my ipad mini. The UMIK-1 looks like an excellent solution, especially since it can be used without any ancillary interfaces (outside of the lightning to usb adapter) which makes for a very lightweight, portable and fairly accurate measuring system when combined with AudioTools. When I contacted Herb of CSL, he had mentioned that the Dayton UMM6 and UMIK-1 didn’t work with iOS devices, but it seems that they do, in fact work with them. (MiniDSP also shows the compatibility and setup in a page here: http://www.minidsp.com/applications/acoustic-measurements/using-the-umik-1-with-audiotools-on-ipad )
    Thanks again.

  • I have compared the Umk-1 with the mini DSP cal file, the Dayton Audio EMM-6 with the CSL cal file using an Art dual mic preamp, both on the Ipad pro, with an Ivie IE-10 A and a lowly Tenma 72-942 SPL meter.
    All calibrated with a B&K 4230 calibrator at 93.8 dB.

    My Octave band SPL results using Audio Tools were within 2-3 dB of all except for the Umk-1, which read considerably higher at ambient room tone measurements. I had hoped that the Umk-1 and my ipad would have been my inexpensive portable iOS rig. I regrettably am not comfortable with the Umk-1 doing any serious measurements.
    I have a feeling that the Audio Tools iTest mic may be the best solution for class 2 portable acoustic testing on iOS devices at this time.
    I’ll post again when I buy and test it.

  • My EW M30 and iMM6 calibrated are a very good match up to above 10k, and this is feeding quite different RTAs, the dbx RTA1 and an iPhone5S running S6D AT. Nothing like any major delta 100-400Hz. In prior comparison M30 was v close match to B&K quarter- and half-inchers.
    Not seeing how cal file is clearly off either, so I am missing something wrt ripple >4k.
    Finally, 90deg cal might be nice (for grazing work?) but do not see why needed, really, for HT EQ regardless.

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