On 01/08/2011 03:30 AM, fons@xxxxxxxxxxxxxxx wrote:
On Sat, Jan 08, 2011 at 01:39:17AM +0100, Jörn Nettingsmeier wrote:
Not exactly. For a well-designed decoder the pressure at LF will
be proportional to W, even without NFC, and should not have any
bass boost.
need to double check, but iirc i did hear a very pronounced (read:
unbearable) boominess on my organ recordings when listening to them
on my<4m array in second order without NFC...
That is very well possible. It means that you don't have the real
pressure, not even at the sweet spot. Which is to be expected if
your room isn't free of LF reflections, as most are.
What you get in that case is a velocity field that
does not match the pressure in level,
this i understand.
and more important - in
phase, and this sounds very unnatural.
this is news to me. where does this phase delay come from?
It's just a property of the higher order components in the near
field. If you look at the amplitude and phase plots of the higher
order NF effects, it's very evident that there is quite a large
phase shift even before the amplitude starts to rise by any
significant amount. Even for first order the +3dB point has
45 degrees phase shift - like a first order filter.
hmm. i need to think about this some more.
Also if you look at Moreau's 'useful bandwidth' filters for
synthesising near sources, you'll see that none of them has
more than 3 or 4 dB gain before going down steeply for lower
frequencies.
you mean this one?: daniel/moreau, "Further Study of Sound Field Coding
with Higher Order Ambisonics"
i'm wading through it... :)
This means that it is not the very high LF gains
of the higher order components that create a near source, but
the phase shifts.
enlightening. i always found it utterly counter-intuitive that LF boost
was all there is to it, but i could never quite grasp why...
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