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Old March 31st, 2015, 02:51 PM   #46
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Re: Parabolic microphones.

Quote:
Originally Posted by Bob Hart View Post
The audio seems more mellow than with the smaller dish. There does not seem to be any more actual audio gain as such but discrimination between on-axis and off-axis sound seems improved.
What you are describing is that larger diameter dishes perform as parabolic reflectors at lower frequencies (longer wavelengths). That is what you are characterizing as "more mellow" (i.e. improved LF response) and "improved discrimination" (i.e. improved LF directionality). Exactly what we would expect from a larger diameter reflector.
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Old April 3rd, 2015, 06:11 AM   #47
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Re: Parabolic microphones.

Dang, you found a 4-foot dish along the road side! Some place you live. Around here we mostly see tire fragments, a bit of road kill, and the occasional used & stained mattress along the road side - but never something useful.

You must lead the good life! Enjoy your continued researches.
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Old April 3rd, 2015, 01:43 PM   #48
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Re: Parabolic microphones.

There are interesting things turn up on the roadside collection, lots of rubbish too. This dish is approx 2mm pressed or spun aluminium. It looks like it might have been either an OB or old private Reuters data link from the late 1980s or thereabouts.

The previous owner of my place had one. The concrete footing is still in the yard. A few people here had them for private share trading etc. Otherwise it could have been an OB dish or private business link. There's bits of it missing and the original iron framed base was all cut up.

It is grey with a blue lightning logo. It is turning into a bit of an ordeal to rework it. The fastener screws are all monel but have been secured with permament loctite. An impact screwdriver can shift them - just.

I wonder what I could have done with the big 25ft troposcatter dish at Mt. Tom Price in our Pilbara region when it was decommissioned.

Last edited by Bob Hart; April 4th, 2015 at 12:05 PM.
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Old April 4th, 2015, 08:07 AM   #49
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Re: Parabolic microphones.

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Originally Posted by Bob Hart
An impact screwdriver can shift them - just.
Sounds like it's time for a Dremel tool or an angle grinder, depending on the size of the hardware. ;-)
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Old April 4th, 2015, 11:50 AM   #50
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Re: Parabolic microphones.

I ended up doing a rough bushie's job on the retainer for the feedhorn base. - just turned the three of the original threaded pieces I could get loose so they act as clamps and refastened - more or less.

Tried the 4ft roadside collection dish with the mounting arrangement and small Sony ECM-55B ( fairly good for dish mikes ) mounted on the feedhorn support. I've made three of these things over the years and this comes up as by far the best.

The ticking clock outdoors at 22 metres is just audible above the environmental noise floor of night-time small birds and bats. Automotive tyre noise attenuates off-axis but lower frequencies endure to about 30 degrees. This means airflow noises of aerobatic aircraft should reproduce without need to be spot-on although spot-on would be better.

Front-to-back noise suppression seems better. The aluminium dish is not "ringy" when bumped like the smaller steel Foxtel dish, so probably blocks a bit more noise from the rear. It may not need foam across the back surface.

I have it mounted to an old black Miller fluid head to an angle extrusion mounted across the lower centre rear of the dish. Hopefully it will also reduce its risk as a windcatcher to pull the tripod over. It is a bit of a big ask for the tripod head but it does lock off. With counter-mass it should pan and tilt normally and remain within weight limits for the head.

Now to invent some sort of sighting system and fix the dead red Commodore wagon. The Echo is about two inches too narrow in the rear hatch door space.

I placed a small portable AM radio out on the rose bush with the clock and listened to it directly with my left ear, whilst I moved my right ear into the sweet spot of focus. The "stereo-effect" is quite intriguing.

Despite the bulk of my body masking some of the dish, the reflected sound came up subjectively to about two-thirds more power to my right ear. It may be doing better than that. I am about 8db down on my right ear and less sensitive to lower frequencies in it as well. - Suffered a mild "custard ear" as a kid fortunately only for about two days before fixed so no real damage was done.
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Old April 5th, 2015, 07:17 AM   #51
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Re: Parabolic microphones.

In making furthur experiments I am obviously re-inventing the wheel but no harm done. I tried the 4ft dish at 60 metres today.

Environmental ambience was high due to light intermittent breeze in the trees. The clock can very barely be heard ticking and only when there is a bit of favouring wind. It is right down there in the noise floor and only discerneable because I know the sound.

The small transistor radio at its lowest loudness setting was audible with all the mixer gains right up. With the environmental noise floor gained up to -12db, the radio speaker was returning about -5db at best. Direct to ear, I was not able to hear the radio beyond 8 metres.

A human male voice at normal close conversational modulation from 60 metres fares less well. Only occasional words were discernable and only because I have done a bit of HF radio listening in the past and am hearing-trained for difficult conditions. This might have been in part because I was able to decline the dish very close but not quite low enough.

The wings of a magpie flying though the sound path were deafening at those settings.
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