Re: [LAU] ambisonics: fons ambdec and muse.demon.co.uk ambidec

From: Jörn Nettingsmeier <nettings@email-addr-hidden-hochschule.de>
Date: Sun Jan 09 2011 - 23:14:18 EET

On 01/08/2011 03:30 AM, fons@email-addr-hidden 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...

_______________________________________________
Linux-audio-user mailing list
Linux-audio-user@email-addr-hidden
http://lists.linuxaudio.org/listinfo/linux-audio-user
Received on Mon Jan 10 00:15:03 2011

This archive was generated by hypermail 2.1.8 : Mon Jan 10 2011 - 00:15:03 EET