[SpHEAR-devel] SpHEAR-devel - (PCB, new Octathingy, calibrationand more)

Thu Jun 28 04:56:02 PDT 2018

Hi all,

I’ve been following this mailing list since Fernando published the SpHEAR Project Paper at AES SFC In Guildford.

I’ve yet to post to list so I hope this message finds you all correctly!

I’m a PhD candidate at the University of Derby, My PhD is focussed on the development of higher order spherical arrays. I’ve see various posts on here and the SurSound forum and felt it time to chime in!

I’m currently developing and evaluating various calibration approaches for higher order arrays - all of these approaches are relevant to FOA mics! I’m presenting a paper on this work at AES AV/AR in Seattle in August - I’ll also be posting updates regularly on ResearchGate.

The calibration approaches considered use a large set of impulse responses measurements conducted in a Hemi-Anechoic chamber. The chamber is obviously useful but In the past I’ve measured in non-anechoic conditions and removed unwanted reflections with some success!

Some approaches considered rely on the use of On-Axis Responses measurements with others utilising Diffuse Field Equalisation as covered in Aaron Heller and Eric Benjamins Paper at AES 133rd Convention - These various approaches are evaluated against a microphone simulation that has been developed.

Once published I’ll be posting the Matlab scripts online for file prep/generating calibration filters and plotting scripts to view results!

I’d be very interested in all your feedback!

Also Marc a few things I thought I’d ask/mention....

- I’ve borrowed John Leonards Twirling 720 Lite - I’ll be testing this in the chamber next week - along with his OctoMic at somepoint...

- Does anyone anyone have any contacts or more details on the AAMA project....I’d like to build one - along with plans to try the higher order SpHEAR mics along with my own prototypes using TSB165A and TSB140-AT capsules!

Cheers
Charles Middlicott

Sent from my iPad

> On 27 Jun 2018, at 20:00, "sphear-devel-request at ccrma.Stanford.EDU" <sphear-devel-request at ccrma.Stanford.EDU> wrote:
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> Today's Topics:
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>   1. Re: latest musings (PCB, new Octathingy, calibrationand more)
>      (Marc Lavall?e)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 26 Jun 2018 22:23:38 -0400
> From: Marc Lavall?e <marc at hacklava.net>
> To: "sphear-devel at ccrma.Stanford.EDU"
>    <sphear-devel at ccrma.Stanford.EDU>
> Subject: Re: [SpHEAR-devel] latest musings (PCB, new Octathingy,
>    calibrationand more)
> Message-ID: <b36c9c6f-080a-1968-1343-808dc4fd4862 at hacklava.net>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
>
> Hi Umashankar (and all).
>
> I did not forget the Matlab scripts I converted to Octave... I just
> postponed my ambisonics related projects, partly because of the
> difficulties to properly measure and calibrate a microphone. I must
> admit that I never try to calibrate a microphone, and I yet have to
> understand the Matlab scripts (converting a script does not imply
> mastering it)... But it's never too late! I just found the precious
> scripts in a folder named "brahmatlab"... ;-) Publishing the scripts
> (properly edited and tested) as free software would be great (if Angelo
> would allow it). Also, there's probably other methods to calibrate a
> microphone, but I'm not holding my breath; as Fons recently wrote on
> sursound: "So unless you want to spend a few months full time to develop
> and test all of this, I'd strongly advise to not even try it."
>
> Because building an ambisonics microphone is a lot of work (even the
> ones from the SpHEAR project), I got a cheap Twirling720 Lite; it looks
> more a toy than a proper microphone, but I like its form factor, its
> built-in USB interface, and maybe its a good enough device to hack in
> order to learn the subtle art of measurement and calibration. I just
> hope there's no processing between the capsules and the USB interface
> (something to discover).
>
> Fernando, I was asking about the article I found because I like the idea
> of using a reference microphone to calibrate another one (with a
> transfer function). So i wondered if a set of individually calibrated
> capsules (using the same reference microphone) could be used without
> further measurements to make a calibration profile based on a
> theoretical ambisonics microphone, knowing the distance between the
> capsules and the center of the array; I guess it would fail because the
> capsules holder (and other capsules) are not acoustically transparent.
> The devil is in the details (n'est-ce pas?). Please excuse my na?ve
> questions (this is the best way for me to learn).
>
> Marc
>
>
> Le 2018-06-26 ? 08:32 AM, Umashankar Mantravadi a ?crit?:
>>
>> Dear Fernando
>>
>> I am enclosing photograph of my 16 position rotator, based on an
>> Arduino card and stepper motor. It works reliably and most importantly
>> has almost no reflections when recording test sweeps. (it was quite a
>> problem in earlier designs.) ?I measure an earthworks Measurement
>> microphone before and after the test sweeps. I use angelo?s log sine
>> sweep method, and for an eight position 1^st order measurement, the
>> matlab scripts needed to create the filtermatrix. One of the first
>> steps the matlab scripts do is use *invert kirkeby* to match the
>> measurements to the earthworks microphone. Angelo thinks that with 16
>> horizontal positional measurements it should be possible to create a 8
>> x 8 filtermatrix for a second order microphone in the octathingy style.
>>
>> Nevertheless I am working on a two axis rotator, using three steppers,
>> With rigid carbon fiber rods and minimal structures at about 30 cm
>> from the array under test, I should be able to control reflections.
>> Angelo?s method is to cover all the surfaces with acoustic foam, but I
>> think it would be better to minimize the reflections. Marc has seen
>> the matlab scripts (he had in fact converted them to Octave, but he
>> seems to have forgotten.
>>
>> Will send you the rotor design when I finish it.
>>
>> umashankar
>>
>> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
>> Windows 10
>>
>> *From: *Fernando Lopez-Lezcano <mailto:nando at ccrma.Stanford.EDU>
>> *Sent: *Monday, June 25, 2018 4:36 AM
>> *To: *Marc Lavall?e <mailto:marc at hacklava.net>;
>> sphear-devel at ccrma.Stanford.EDU <mailto:sphear-devel at ccrma.Stanford.EDU>
>> *Subject: *Re: [SpHEAR-devel] latest musings (PCB, new Octathingy,
>> calibrationand more)
>>
>>> On 06/23/2018 02:20 PM, Marc Lavall?e wrote:
>>>
>>> Hi, and thanks for the news!
>>
>> Hi Marc,
>>
>>> I started a related project: building a binaural microphone, for
>>
>>> measurements and recording, using very small Knowles capsules. One thing
>>
>>> I have to learn is how to calibrate microphone capsules, and I found an
>>
>>> interesting article about a calibration technique that doesn't require
>>
>>> an anechoic environment, and I wondered if a similar technique could be
>>
>>> used for the measurement and calibration of ambisonics microphones:
>>
>>>
>>
>>>
>> https://www.scribd.com/document/321928725/Microphone-Calibration-by-Transfer-Function-Comparison-Method
>>
>> They use SMART which is a (expensive) professional software to run a
>>
>> realtime transfer function measurement between the two microphones.
>>
>>> Of course it'd be impossible to place a reference microphone at the
>>
>>> center of an ambisonics microphone, but I guess it'd be fine to first
>>
>>> measure the reference microphone (a few times for averaging) then the
>>
>>> ambisonics microphone by making sure its center is at the same spot than
>>
>>> the previously measured reference microphone. Does it make sense (or am
>>
>>> I too optimistic)?
>>
>> It does make sense and that is what I am doing (and presumably everybody
>>
>> else), but not in realtime as described in the article. I have a emm-6
>>
>> calibrated microphone that I use to have a reference measurement by
>>
>> placing it at the exact same position as the microphone being
>>
>> calibrated[*] - that measurement is used to create an inverse filter
>>
>> that "equalizes" the speaker, and that filter is then used to calibrate
>>
>> all measurements of the Ambisonics microphone...
>>
>> After that the fun begins - making sense of the data :-)
>>
>> -- Fernando
>>
>> [*] well, not exactly the same position but as close as I can get it
>>
>> _______________________________________________
>>
>> SpHEAR-devel mailing list
>>
>> SpHEAR-devel at ccrma.stanford.edu
>>
>> https://cm-mail.stanford.edu/mailman/listinfo/sphear-devel
>>
>
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