Baffled
stereo is a generic term for a number of different stereo techniques
which use an acoustic baffle in between a pair of matched microphones
to enhance the channel separation of the stereo signals. You
can use a baffle in all the main stereo set ups, in spaced or
A-B stereo, ORTF, DIN or NOS stereo, and the shadow effect from
the baffle has a marked effect on the attenuation of off-axis
sound sources, enhancing the channel separation. A number of
baffles are available commercially made from an acoustic absorbent
and non-reflective material but the most well known is the Jecklin
Disc developed by the Swiss sound engineer Jürg Jecklin.
The disk is basically a circular central plate about 10 mm thick,
covered on both sides with thick foam intended to suppress high
frequencies and various materials have been tried as coverings
for the disk most noticably lambs wool. Two mics (ideally small
diaphragm omni condensers) are mounted, one on each side of
the disc, with their capsules 165 mm apart which is calculated
to give the correct delay time difference between the two channels.
How the disc works is that as the frequency increases, the two
microphones are more and more separated. Below around 200 Hz,
the two microphones record the same but frequencies above this
are attenuated by the acoustic muffling of the disc resulting
in a frequency response difference of the two channels depending
on the angle of impact of the sound. In addition, there is also
a sound diffraction around the disc rim which is dependent on
frequency and angle.
I have never used a disc though I have come across a number
of mobile recording outfits using home made versions of the
disc with a pair of very high quality small diaphrgm omnis like
Earthworks. I have also seen versions of the disc sprouting
a hemispherical lump on each side which is obviously begining
to look very much like a dummy head/Binaural setup and it does
generally seem to be a great area for home made experimentation.
So if you have any experience with a disc get in touch. There
is a full technical breakdown of the Jecklin disc on the Josephson
microphones website.
September
2008) The Hebden Sound omnis have become the mic set up of choice
for me when recording a choir and while I still haven't had
the opportunity to compare them to the more established small
diaphragm omnis like the DPA range I am completely sold on these
mics. They have performed faultlessly for the last year and
have given my mobile recordings a sense of space that coincident
pairs or even single fixed piont stereo mics like the mighty
Soundfield can't achieve. I have recently come across
a very nice Jecklin disc and I plan to try out the Hebdens with
the disc on an up coming session. Watch this space!!! JR
