Hi,
Mics placed around a log spiral can be beam formed this I think is the=
traditional application for beam forming (log) also for the linear array.=
In the context below I more refer to the effect of the boundary itself on t=
he mics - I experimented with a parallel boundary mic that utilizes log spi=
rals with the mic placed at the center.
The idea originally investigated by the Neumann paper I link to via the blo=
g entry. .It shows the effect of diffraction effects at the edge of a bound=
ary, rectangle, square, circle, triangle and the log spiral. The geometries=
were investigated and they found the log spiral to give the least influenc=
e caused by diffraction effects in fact it boosted key frequencies by 6 dB.=
The distance from the mic to the edge is always different thus pressure bu=
ild up leading to peaks or nulls in the response are avoided. Neumann went =
on to develop the concept further using computer analysis to come up with t=
he triangle boundary layer mic, the GFM132.
The first rig included an area to place the recorder:
http://www.urlme.net/blog/?p=3D1150
The second was investigating the boundary without a center structure - in t=
his regard the rice crispies box comment was related to using cardboard rat=
her than the plastic or acrylic plates. The second mic used corrugated plas=
tic and a tube between the mics.
http://www.urlme.net/blog/?p=3D1166
Perhaps the best examples there would be the "urban din" and papa walking p=
ast the rig.
Ive not had much time to get out down to the local river where I normally t=
est rigs.
For more processing Im working on a dipole mic: Which is proving to be an =
interesting contraption considering the mics are next to each other
http://www.urlme.net/blog/?p=3D1175
The last mp3 player has a very crude test, imaging is so so and needs furth=
er work to EQ the capsules response. I was glad to see reference to the Ozo=
ne 4 mastering plugin which may help tremendously with that effort.
Kind Regards,
Mike.
--- In "gmo_dunes2" <> wro=
te:
>
> Mike:
>
> Could you please explain what a "log spiral array" is? Is it an array of =
microphones with processing to do beamforming or two mikes, one on each sid=
e of a large conch shell? How do you make it out of a box of rice crispies =
(do you HAVE to eat those dry little bits first in order to make it work?)?
>
> Inquiring minds want to know!
>
> Thanks!
>
> -Greg
>
> --- In "Mike Rooke" <yg@> wrote:
> >
> > Hi,
> > I think the snow recording is in a very diffuse environment - i che=
cked the cross correlation and there wasnt much to conclude besides the dif=
fuseness :) - Previous recordings Ive heard had a decent center. Ive found =
a log spiral array to give the best center and also gives a boost to the cr=
ispyness. You could even make it from a box of rice crispies :)
> >
> > Using matrixing to adjust parallel boundary arrays is shifting the phas=
ing around which I think is as intended but it may require quite a processi=
ng chain, preeq & mic response balance, matrix, m/s eq, matrix / post eq & =
secret sauce. - Ideal setup is to ensure the mics are matched and placed so=
the effects of phase are minimal, which means exactly at the same position=
either side of the rig, thats also shaped exactly the same.
> >
> > I can assist you testing the mic Klas using my mad science if it helps.=
Im enjoying the development and the small changes that are made. Muriemike=
has a lot of potential.
> >
> > BR
> > Mike
> >
> >
>
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