Parabolic microphones.

[Greg Miller]

Bob, thanks for your detailed report of your very interesting experiment to date!

Originally Posted by Bob Hart

With the offset feed I found I was unable to depress the dish low enough to pick up sound from the horizon level.

If you invert the whole affair, so the mic is physically above the dish, then you would not need to depress the dish toward the ground to aim it at the horizon ... you would need to aim the dish so the mic is just slightly above the top of the dish. Remember, the actual aim is not toward the pickup, it is shooting just slightly past the pickup, so the pickup does not cast a shadow on the dish.

However, I suspect that a small dish with an off-center feed is even less efficient for audio than it is for RF ... certainly less efficient than a proper center-fed dish.

Originally Posted by Bob Hart

I might rehang the support arm from above or remake it longer and more central to the dish.

I doubt that moving the mic more central to the dish will be successful, at least in theory. Visualize a parabola rotated through 360º around its axis, forming a 3-D parabolic reflector. Now visualize the focal point within that reflector, located on the central axis. Finally, visualize slicing off a small section of that reflector, making the slice NOT perpendicular to the central axis. That's what you have. Making the sliced section does not change the correct location of the focal point. If you were to move the mic relative to your [off-axis] slice of reflector, you will be moving it away from the correct location.

I wish I lived out in the country, and had an old decommissioned satellite dish, 6' or 8' in diameter. It would be interesting to place a mic at the [central] focal point, and aim the dish at the neighbors' house a mile away, and give it a listen.

I suggest you keep looking for a proper center-fed reflector, preferably bigger than what you have now. I think that will be an improvement over your already promising experiments. But, indeed, if you are going for very low frequencies (I don't really have any idea what you're after, in terms of frequency) a parabolic reflector might be impractically large.

[Bob Hart]

Jay and Greg.


Thank you for your responses. In regard the shape of the satellite dish I had wondered if there was an oblique crop of a true parabolic shape or it the shape itself was modified to cope with a north-south oscillation of a geosynchonous satellite in an orbit slightly off true equatorial.

In the last exercise, moving the mike to the centre did not seem to improve gain but there are so many variables in my rough-hewn setup that would mask this. There seemed to be more tolerance for off-axis orientation of the disk on subject in the vertical direction with the mike centred.

Where the gain "snapped in" most apparently was in the horizontal pans across the already vertically centred subject.

The noise from the disturbed airflow at about 1km distance was about that of a sound system outdoors when an unmuffed mike is lightly wind buffeted, but more subtle. I imagine that the dominant frequencies would be lower, in the ballpark from very low, up to about 2000Hz. That seems to be in the lower gain ramp of both the Sony and Audix mikes on the low-frequency side.

Over the 1km or so distance, I would not expect there to be much of power in the 20Hz to 100Hz zone would make the distance. It was not apparent to me at the time.

What I am basically after is the airflow detachment, the buffeting and a slightly different sound from the airbrakes in the wings when extended on final. There is a soft thud as they hit their limit stops. Their disturbance sounds may not travel well over the 200 metres or so from the aircraft to the camera position.

For those of you who aviate or are groundfowls who would like to.

BOB GRIMSTEAD. FOURNIER CROSSWIND LANDING. - YouTube

[Jim Michael]

Not my ideal flying day. It's hard to hear much with all that wind buffeting. Won't the wind be a factor keeping the dish pointed in the right direction?

[Bob Hart]

Jim.


Regarding the clip. - Windy. Yes it was not ideal. I actually took my socks off later, rolled them up with the toe-ends remaining baggy and made a wind muff for the on-camera mike. I was hand-shielding the camera mike during the shot. I also have a piece of car-seat woolly fabric which I use as an impromptu muff.

The dish mike will have to get the woolly muff treatment. I'm not sure what to do about wafting sounds coming from the rim of the dish. Maybe muff material is needed there too and maybe acoustic insulation on the rear of the dish to reduce through-the-dish pickup of local ambience.

When there is any wind anywhere even if on the ground it is still, the sound becomes very "airey". This is something that the C76 mike seems to do as well. When pointing across open ground towards trees opposite, it is apparent. It makes sense that if nature is generating distant noise in the line of fire, so to speak, then the hyper-directional mikes of whatever persuasion must pick it up.

What is becoming rapidly obvious is that the parabolic mike is likely to be a niche device. The satellite dish is certainly not agile and would share a tripod head with a camera grudgingly.

A smaller one would probably be a handy tool to locate small noisy creatures for wildlife documentary production. I was able to zone in on the individual cicada the other day.

[Bob Hart]

Furthur to above, I managed to invert the whole dish on its existing mount and it is now fine for subjects on the same level. For the sake of curiosity, whilst I had the mike out of its holder, I parked my eyeball at the mike position and moved the dish to pick up a broad light source, a ceiling flouro tube pair in a diffuser.

From the offset mike position, the entire area of the dish flashed bright as it lined up on the source. From a centre position, the brightness "walked" across the dish to an overall even brightess, then walked off the opposite edge. Hardly a scientific test though.

I connected the parabolic dish and the Sony C76 to a MixPre and tried each in turn on the alarm clock which I parked in the corner of a carpeted room.

The C76 required more gain from the MixPre for the same perceived level. Interestingly in a rather crude front-to-back noise rejection test, the dish was able to cope with more noise from an operating television directly behind than the C76. This is not a fair test as there would have been all manner of reflections happening in the room but interesting nevertheless.

It would be interesting to see how a small parabolic dish recorded to one channel, selectively mixed in post with a conventional studio mike channel could be used to improve indoors audio in post. It is sometimes the crisp high audio frequencies which suffer in the wide shots indoors.

[Greg Miller]

Bob, thanks for keeping us up to date with your explorations!

I'm glad to hear that the inverted dish solved one of your problems. And your "eyeball" experiment confirmed what I expected about focal point. The small dishes use an off-center feed, to prevent having the relatively large (relative to dish size) LNA / LNB creating a significant shadow on the dish.

Looking around the internet, I found a lot of info about parabolic reflectors hiding under the "solar power" and "solar cooking" searches. One website sells a variety of dish sizes, almost all of them mirrored for solar cooking. You wouldn't want to use those for audio; if you accidentally pointed it at the sun, you'd vaporize your mic!

However, they do have one 32" transparent plastic dish intended for audio collection. The link includes a very unscientific demo, which nevertheless illustrates the fact that this size dish is pretty bad at anything other than HF pickup.
HUGE 32" PARABOLIC AUDIO DOME

[Bob Hart]

Greg.


Thanks for that. All good stuff, especially the solar cookers. One would think that the offset satellite dish style might be a better contender for a solar cooker.

The polycarbonate dish certainly looks robust enough. For a "maybe it will work, maybe it won't" experiment for my application, purchase plus international shipping is a bit of an expense.

One would also fret about it not being flattened or cracked on its journey. If the metal dish works well enough but that little more is needed, the polycarbonate dish looks like a good contender.

I had also considered much smaller spun aluminium-backed glass carbon-arc projector rear reflectors for my original experiment so many years back. They have a glass front. They also have a hole in the centre. The photoflood reflector was less fragile, slightly wider and lighter so I went with that instead.

For a horizon level subject, the satellite dish is now tilted upwards about 30 degrees. This gives plently of allowance for pointing downhill. About 45 degrees of tilt range in the original mount remains available. this should be more than adequate for an aircraft being filmed at a workable angle with a tripod camera.

[Paul R Johnson]

Canford audio have been selling the Big Ears and Little Ears parabolic reflectors for years now, and they're very popular of OB use, but the specs are quite revealing. Sadly, they're not made any longer.
The old info is here
BIG EARS PARABOLIC REFLECTOR Clear

Big Ears
Overall size: 675 ×650 ×250mm
Parabola size: 585mm
Weight: 3.3kg
Working range: 100–15000Hz (note 1)
Pick-up pattern: 800mm diameter target at 30 metres (tunable)
Useful range: 1.5 metres to 90+ metres

Frequency response and effective range isn't huge - ideal for sports, but for aircraft, I think you'd really be talking 2m at least, and then the problems of aiming such a large and unwieldy device.

[Bob Hart]

Paul.


Thanks for that. As for "Big Ears". My guess is that there may be insufficient market to consistently support more than one manufacturer of the product. A 2 metre diameter dish definitely would be a deal-killer. It is highly likely that I would never be allowed to take it airside in case the wind caught it and blew it away out of control across the airfield.

The old galvanised round dustbin lids used to get up and roll for many metres ( yards ) sometimes when the strong easterlies set them going over here, as do wide-brimmed felt hats. Now we have swing-top bins, baseball caps, araphats and soft cloth giggle hats, that little aspect of our culture is now extinct.

[Greg Miller]

Bob,

I think most of the off-center dishes will be small. That's why they are made off center: the LNA/LNB is physically large relative to the dish so the shadow would cause noticeable decrease in gain. Once the dish gets big enough to overcome that problem, you don't need to feed it off center.

I have some reflectors from old carbon arc and xenon lamphouses. Some are silvered glass, but some are solid metal. The problem is that I think they're spherical, rather than parabolic. The optical goal is not to produce a beam of light the diameter of the reflector which will stay collimmated over an infinite distance. The goal is to produce a converging beam of light which will be roughly the same diameter as the first (i.e. rear) element of the projection lens. (Any wider beam would just be lost light.) The projection lens then re-converges it, creating a focused image of the focal plane in the process. (But if you focus the arc on the film plane, the result is melted film.) AFAIK a reflector with two focal points is most likely a spherical section, rather than a parabola. OTOH, now that you've stoked my interest, I will probably dig out the reflectors and try them once the weather is warmer (i.e. 6 months from now).

It also occurred to me that this would be a nifty project for 3D printing. But making something 3 or more feet in diameter would be damned expensive. With really good skills and tools, you could even make one out of 1/4" plywood, but that wouldn't be very practical. I do appreciate your reservations about having a plastic one imported, though. As far as blowing across the runway, just tie a rope to the two hand grips and loop it around your waist. If the wind is too strong for that rig, you've got some serious problems.

Meanwhile, if I come across any construction plans that seem to be especially promising, I'll post something here.

[Jim Michael]

Just brainstorming here - a metal segment (think slice of pie) could be replicated and joined at the center, then fan it out to make the reflector. I suppose the 3-D printing option might work for the segments and the whole thing could be fairly portable.

[Bob Hart]

Greg.

The 3D printing if it was large enough would be good even if it had to be re-inforced by webs and ribs on the rear, which could also be 3D printed and glued or fused into place.

My guess for a lot of things in the future, the 3D printer is going to change the way a lot of manufacturing is done, maybe even the selling via on-line streaming of precision scale modelling kits which might make something of a comeback among younger folk.

CNC machining is not going to go away. The next step will see tighter integration of the two systems especially if they get pure metal printing sorted.

My guess on pure metal is that they may have a spinning platter and an initial pure metal base core on which pure molten metal is sprayed quickly to build in only rudimentary precision, then finished off in a CNC machine with far less wear and tear on the machine and far less materials waste.

There is some sort of sintered metal process already which requires an intermediate step for the final metal form to be stabilised. I'm not too sure how that fares with thin flexible segments. Sintered metal has been around for a long time in the form of oilite bearings.

I did notice that having my hand anywhere in the beam path would knock the level down a bit.

As for the thing taking off with me attached, I guess I could go the extra mile, motorise it and have the thing certified airworthy and fly after the aircraft to get my sound. I think somehow I need to get some sleep as the mind is meandering.


Jim.

The metal segment system has been used from way back. In the times of Noah, well almost, there was a small bulb photoflash made by Hanimex I think. It used metal segments which stowed flat but once pulled around and hooked together warped to form the parabola. I think some portable early military satellite systems also used the segmented arrangement.

I vaguely recall at some point there was a scheme to make an inflatable dish with the RF reflective surface being on the inside of the blow-up and fed through the structure which was to be RF transparent. I don't know if it ever went anywhere.

[Don Palomaki]

Perhaps find a metal RF dish (often screen-like and mostly transparent to sound waves in the larger sizes) and use a fiberglass auto body kit to make it solid to sound waves.

Adding a "dead cat" to the mic and dish edges should help wind noise. Low frequency vibration of the disk might end up modulating the audio that is picked up.

As an alternative, make a parabolic section of wood of the desires size, use it aas a guide to sculpt a 3-D wedge, say 60 degrees, out of wood, foam, clay, what ever and then use it as a mold to sections out of DIY fiberglass that can be bolted together.

[Greg Miller]

Don,
I would love to see a 3-foot or 4-foot diameter dead cat! Sort of like a giant furry garter belt.

Now I'm wondering whether wind blowing across the face of the dish (especially a deep dish) would make the air in the dish oscillate, sort of like a very wide, short, stopped organ pipe?

[Phil Goetz]

Ahhh... to go with the Klover brand mic... or the trashcan lid...

How to Build a Parabolic Mic Dish | Videomaker.com