On 12/1/21 12:35 PM, Jens Ahrens wrote:
Hi Fons,
I’m attaching Fig. 1 from the JASA article. Please refer to the article itself
if the image does not get through. It compares the SMA radial filters with the
EMA radial filters. Whenever the solid lines deviate from dashed ones of the
same color, then the given order is not available at the frequency range where
the deviation occurs. If I’m not misreading, then the 7th order is available
somewhere between 2 kHz and 3 kHz and higher. Aliasing kicks in at around 4
kHz-ish.
So, indeed, there is not much more than 7th order that can be squeezed out of
an array of this radius.
Cool. The correctly recovered harmonics for 7th order span about 1
octave of useful range, if I understand correctly.
Is it perceptually significant to have 7th order components? Or, in
other words, as you add spherical harmonics to your encoding process,
how does the spatial perception change?
Or from the other end, if you start with a 7th order recording and you
start truncating the order to lower and lower values in the decoding
process, how does the perception of the recording change? Is there a
decrease in order for which you can say, "well, that one did not add
much, did it?"
Just curious...
-- Fernando
Regarding the calibration: Sorry for the imprecise wording! I meant to say that
we didn’t check if there was actually any mismatch between the microphones.
(This means in turn that I can't tell in how far the method is sensitive to
that. My guess is that it is not more or less sensitive than SMAs.)
The reason why I’m not worried is because the array passed the stress test:
When recording sound sources at a distance of, say, a few meters or farther,
the whole setup is rather forgiving. That means, for example, that one doesn’t
need to be too precise in the modelling of the scattering off the microphone
baffle etc. The critical case is very close sources. Recall that in the video,
I’m as close as a few centimetres to the surface of the array. This triggers a
lot of the high orders at low frequencies, and if there is something that is
not ideal, then the low frequencies tend to go through the ceiling. We tested
this with the array, and it behaved.
How would I be noticing if the microphone mismatch is above the tolerance level?
Best regards,
Jens
On 1 Dec 2021, at 17:47, Fons Adriaensen
<f...@linuxaudio.org<mailto:f...@linuxaudio.org>> wrote:
Hi Jens,
Thanks for your reply.
The lower end is limited by the radial filter gain that the user chooses.
The radial filters are not the same like with SMAs, but they are very similar
so that the limitations are the same.
0th and 1st order are available for all frequencies.
2nd order approx. above 200 Hz
3rd order approx. above 500 Hz
etc.
It's the 'etc' I'm interested in...
The gain required at LF increases by 6 * order dB for each octave going
down, which in turn means that for a given maximum gain the LF limit
goes up with order. Which makes me wonder if for order > 4 anything
useful actually remains.
I cannot comment on calibration requirements because we did calibrate
the array… (Nor did we measure how well it was calibrated out-of-the-box.).
Does the 'nor' mean you did _not_ calibrate it ?
I don’t actually think that there are any special requirements.
There are.
The required gain (at LF) means that small differences in capsule
sensitivity are amplified as well, and this distorts the resulting
polar pattern.
The simplest possible example would be a first order component X
being produced from the difference of two omni capsule signals
A and B, so
X = (A - B) * (1 + j (F0 / F))
where F0 will depend on the distance between the capsules.
Now if A has actually 1 dB more gain the difference signal becomes
1.12 * A - B = (A - B) + (0.12 * A)
The second term, amplified by the filter for low F, will add
an omni component to X.
So the maximum gain that can be used depends not only on how
much noise can be tolerated, but also on the calibration
accuracy (and long term stability) of the capsule gains.
Ciao,
--
FA
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