[SpHEAR-devel] SpHEAR Progress

Tue Feb 27 16:47:01 PST 2018

Hi all, Len asked a few questions and I thought I would answer to the 
list as well...

On 02/25/2018 09:28 AM, Len Moskowitz wrote:
...
> I see that you've built and tested a complete tetrahedral microphone using
> the EM182.

Are you talking about the one pictured in the paper? That was a very 
preliminary prototype (just a "proof of concept", really). The current 
design is very different and includes a proper phantom power interface 
and is physically very different (includes shock mount, windscreen, etc).

>Have you built one with the EM200? If so, how did it turn out?

My current prototypes all use the EM200 capsules. Big difference in the 
low end performance, much better and balanced bass response. You can see 
that in the raw curves that come in the spec sheet of both capsules, the 
EM182 starts rolling off bass at 600-700Hz, or so, and you do not see 
that in the EM200 - to be expected, of course.

Noise performance is also reasonable given the cost (well, better than 
reasonable).

I have used the microphones (both four and eight capsule designs) to 
record several concert performances and in field and studio recording 
sessions, and the results have been very good. Better ears than mine 
have listened to them and liked them (this includes the raw performance 
of the capsules but most importantly the calibration and subsequent A2B 
encoding).

My latest pieces (ambisonics, two layers, 5th and second order) all use 
sounds captured with my microphones and processed in SuperCollider. No 
complains there either :-)

> In your 2016 paper
> (https://ccrma.stanford.edu/~nando/publications/sphear.pdf) you discuss
> the desire to build an Octathingy. Have you built and tested one? If so,
> have you done this using the EM182 or the EM200 capsules? Are you seeing
> acceptable performance?

The project has seen a lot of progress since that paper (a LOT). I have 
been meaning to update the list on this but never find the time. Commits 
of most of the stuff are going to git, of course.

Yes, I built one Octathingy and another (better) one is almost done, the 
first prototype was finished and first tested in June 2017. Time flies. 
It took me a while to be able to calibrate it, specially extracting the 
second order components. All of that needs a lot more work.

I used EM200 capsules for the build and the performance is surprisingly 
good. As outlined in Eric Benjamin's paper the first order performance 
at high frequencies in the horizontal plane is much better that a 
tetrahedral design. At least one more octave before "bad stuff" begins 
to happen, and even then it is less bad :-)

The second order components are a bit noisy, as is to be expected from 
difference microphones. Capsule self-noise obviously limits how low the 
second order components can go (and then there's also spatial aliasing 
at high frequencies). Depending on the nature of the recordings the 
noise can be a problem. My workaround is to just transform the 
microphone into first order at very low levels :-) This is a hack, and 
can introduce artifacts, but is very effective.

I was hoping to have another paper ready with current results by now, 
but I will probably wait until I finish and measure the second prototype 
(which has the proper elevation angle for the upper and lower 
capsules... long story). Time time time, where does it go??

I also need to built a better measurement rig. Second order needs 
measurements above and below the horizontal plane - for the tetrahedral 
microphones I was just using horizontal plane measurements (as the 
microphone is 3D symmetrical) and those are easy. My current rig can do 
multiple horizontal layers but is a pain to use. I have a new design 
that should be able to automate the whole process (not only to calibrate 
but to also better characterize the whole array as built and 
calibrated), but I need time to do the mechanical design and building.

> Have you tested a front/back response for the individual capsules as
> mounted in your assembled microphone holders? How did that turn out?

Hmm, not sure I follow. Just the capsule holder? Or the holder in the 
array with all other capsules installed? I think I could derive the 
later from the calibration measurements. I did measure at some point one 
capsule in my test rig but I did not notice anything weird (except that 
it is not as cardioid at high frequencies - again, to be expected).

> Could you post a few photos too, please?

I'm attaching a few:
sphear_1.jpg: previous generation tetrahedral prototype, no windscreen 
and older shock mount design
octa-3.jpg: the first Octathingy prototype, without the windscreen
octa-2.jpg: with the windscreen
octa-5.jpg: showing the windscreen assembly (no foam insert inside, no 
dead cat outside)

I'll try to post some curves later which might be of more interest :-)

What is your experience so far with your (Core Sound) eight capsule 
microphone? Anything you could share with us?

Best regards,
-- Fernando
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