Re: Wide Sound Stage Parabolic Reflector
Re: Wide Sound Stage Parabolic Reflector
"Rob Danielson" danielson_audio
Fri Jan 18, 2008 9:51 pm ((PST))
At 10:37 PM +0000 1/18/08, tk7859 wrote:
>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)
>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
>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.
>Testing the arrangement with the "standard 9V power supply" proved
>....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
>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.
>The next photo shows a head on view of the sextruplets mounted in the
>final parabolic/barrier assembly
>and a photo showing the detail of mounting the left hand sextuplet
>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.
>The second recording had the barrier in circuit with 15V being used to
>power the sextuplets
>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.
>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
You might have set a record for the number of mics involved in a
stereo rig. :-) As for the tests,I'm having trouble determining
which radio sound is established to be the right and which on the
left. I do a difference but the changing sounds make it harder to pin
down. I also agree you that the reflections from the walls and
corridors inside are probably muddying/unbalancing the image.
Are all six mics making up each side/channel all wired in parallel?
As the focal point mics are optimized for center pick-up, would it
make sense to direct the mics on the protruding barrier in front
towards the sides, perhaps at an angle of 115 degrees or even
opposing each other. Rob D.
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