Hi
It has been a while since I contributed to the Naturerecordists Group
so this is a lengthy post to make up for lost time.
Recently I have been dabbling in surround sound but have been
remarkably unsuccessful. However a spin off of this is the making of
a couple more sets of Rapid capsule triplets.
Reading John Hartog's post "Split Parabola Boundary Array", and
subsequent comment, led me to believe I should have a go at something
like that using available bits and pieces.
My plan was to use an existing parabolic reflector (powered by a
stereo pair of Rapid capsule triplets)
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Stereo_Tripl=
et_Parabolic.JPG
and add a parallel boundary arrangement at the front of the reflector.
It seemed sensible to have the same number of capsules on the barrier
as in the parabola thereby having an equal playing field, soundwise,
for both the boundary and the reflector. I had available two triplet
arrays
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/new_triple_a=
ssembies.JPG
so the logical thing to do was to attach these via longish leads to
an existing powered stereo triplet arrangement. It was first
necessary to see if such an arrangement would actually work so a
sextuplet barrier was quickly put together.
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/sextuples_%2=
0on_%20barrier.JPG
Testing the arrangement with the "standard 9V power supply" proved
very dissappointing....
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Sextulpes_9V=
_power.JPG
....as the sensitivity proved a lot less than a triple on its own.
This is exactly the result I got when I first tried using triplets
with the standard Sony PIP supply (this necessitated the 9V supply
arrangement). I reasoned the sextuplets required a larger voltage.
I then tried 12V but it was still not satisfactory. I finally tried
two 7.2V(nominal) model racing car packs in series. The resulting
15+ volts did the trick with the sextuplets appearing to be 3 to 4 dB
louder that the 9V powered triplets
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Sextuples_15=
V_power.JPG
The next job was to set up a spaced barrier at the front of the
parabolic reflector. A previously made "corrux" spaced barrier was
very light and was easily and relatively securely suspended, using
elastic bands, in front of the divider holding the stereo triplets at
the focal point.
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Parabolic%20=
+%20Spaced%20Barrier.JPG
The next photo shows a head on view of the sextruplets mounted in the
final parabolic/barrier assembly
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/parabolic_fr=
ont_view.JPG
and a photo showing the detail of mounting the left hand sextuplet
capsules
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/mounted_left=
_side_sexuplet_capsules.JPG
Having completed the assembly the next step was to see if the
theoretical concept actually gave a wide sound stage whilst
concentrating on a distant object.
At the moment the weather is bad - rain and wind for the next couple
of days or so. An indoor test was therefore necessary. The main
target of the test was a radio receiver giving "white noise". The
receiver was placed in one bedroom whilst the reflector was positioned
44 feet away in a second bedroom with a corridor in between the two
rooms. In the same room as the reflector a further two radio
receivers were placed. One was to the left in the same plane as the
rim of the reflector and 7 feet away. The other was 7 feet away to
the right of the reflector, again in the same plane as the rim. These
two radios were tuned to different stations. The volume of the
"target", white noise, radio was less than that of the "side" placed
radios. Because of the line of sight between the "target" radio and
the reflector was via two doorways it was necessary to place the left
side radio against a wall whereas the right side radio was in the
middle of the room - the sound reflections of the radios in this room
situation were complex.
A first recording was made (HiMD minidisc) with the barrier
disconnected, using a 9V battery to power the triplets.
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Parabolic%20=
Triplets%20A%20mp3.mp3
The second recording had the barrier in circuit with 15V being used to
power the sextuplets
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Parabolic%20=
Sextuplets%20B%20mp3.mp3
To my ears there is more of a stereo feel to the sextuplets than the
triplets yet the source radio is just as evident.
Here is 10seconds (or so) of the parabolic immediately followed by
25seconds (or so) of the parabolic plus barrier.
http://ad2004.hku.nl/naturesound/TomR/Wide%20Stage%20Parabolic/Triplets_Sex=
tuplets%20mp3.mp3
Given the situation in which the recordings were made I feel there is
some mileage in carrying out further test recordings out in the field
when the weather is conducive.
Comparing the traces of the recordings in Audacity it seems the
sextruplest are actually a little quieter than the triplets. It will
be easy to cobble up an 18V power supply to see if this improves the
sextrupets.
Regards
Tom Robinson
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