Re: Parabolic microphones.
 

An interesting notion which came up was that of the WW2 and prior long-range acoustic detection devices which preceded early VHF ground-to-air radars.

These acoustic systems consisted either of huge squarely constructed conical feedhorns or large shallow parabolic reflectors. Neither of these structures were orientable or portable.

By the nature of its ability to focus off-axis sources to focal points in a limited area around the natural point of focus, the parabolic listening station was able to plot to a limited ability, off-axis direction and altitude of sources. The parabolic listening station reflector was uncannily similar to the cropped parabola of the modern Foxtel style dish, except on a much larger scale.

It would be technically feasable but highly impractical to construct a parabolic dish of concrete in a dug pit under the aerobatic display box at the airfield. Likewise, an aerobatic display might be scheduled to occur within the focal zone of a radio-telescope dish or a decommissioned older generation large communications dish in order to make a one-off specimen of a unique sound. Most of those dishes are now long gone from here.

Using a communications dish which is still in service would be hazardous to human health and recording equipment due to high-energy RF at the focal point where the microphone would be placed.

This is all of course swerving well off the course of practical application into speculative theory but an interesting excursion nevertheless.

Re: Parabolic microphones.
 

Bob,

I have read about, and have seen photos of, some of the dual-feedhorn acoustic locators. The ones I remember were all steerable, at least in the horizontal plane, and probably vertically as well. Big massive contraptions with ear tubes leading down to a centrally located listener.

I have also seen photos of some cast concrete dish reflectors (maybe parabolic, but maybe spherical sections) that obviously were not moveable.

One problem with a giant parabola is that the beam width is a function of the ratio of (wavelength / diameter). In other words, for a given frequency, as the dish gets bigger, the beam width becomes narrower, and aim becomes more critical. (Remember, gain rises at ~ 6dB/octave. And you get more gain by narrowing the beam width.)

OTOH as the diameter becomes smaller (allowing for easier aim), the LF cutoff frequency goes higher and higher.

I suspect the combination of these two factors is one reason that most reflectors for microphone use tend to max out in the range of 24" to 32". This seems to be a good tradeoff. If you make them bigger for better LF performance, they become difficult to focus. If you make them smaller for wider beam and easier focus, the LF cutoff rises up into the voice range (or even higher).

A parabolic mic seems to be good for specific uses (where you can adjust dimensions to suit your needs), but is not a good general purpose long-range "miracle mic."

Re: Parabolic microphones.
 

Here are some pictures I took of the "sound mirrors" for this purpose in Kent (UK) - just to show you how large they are.

They are open to the public one Sunday a year in July.

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Re: Parabolic microphones.
 

John.


Thanks for that. If must have been a fair old nightmare, drawing, making and setting up the formwork for the concrete dishes.

It seems a pity there is no apparent conservation work being done to the deteriorating concrete.

Re: Parabolic microphones.
 

A footnote to my endeavours above, as luck would have it, on the roadside today I found an old 48" or 4ft diameter aluminium satellite dish. There are no brand names on it, only a stylised blue lightning logo.

It was around two inches too broad to go in the back of the small hatchback so it had to ride upon cushions on the roof, tied down with strips of nylon spiral wire wrap. It was a fraught but successful journey.

This is a centre-fed dish. Initial tests with the Sony ECM-55B mike hand-held in front of the temporarily aimed dish, suggest it is far more critical in its acoustic "focus" than the smaller steel dish which is fed off-axis.

With the audio gains turned up, outdoors, it will pick up the ticking clock at 18 metres. The clock comes in faintly but distinctly over the environmental noise floor.

There is less forgiveness for off-axis aim. The distance from the dish centre towards the feed horn, seems to be about twice as critical as with the offset dish. The "best" zone is about a 25mm area along that centre radius line.

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.

The aluminium dish is quite light but much more awkward for agile use on a tripod head. It does not have adjustments built in for aiming so I will have to invent these unless I use a heavy camera tripod.

Re: Parabolic microphones.
 

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.

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.

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.

Re: Parabolic microphones.
 

Sounds like it's time for a Dremel tool or an angle grinder, depending on the size of the hardware. ;-)

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.

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.