From time to time, I do ground-to-air videography of aircraft in flight more often than not doing aerobtics.


At Serpentine ( YSEN for the aviators ) one day I observed that at a certain position from one of the corrugated iron hangars, whilst a low-powered Fournier RF4 pilot was practicing for a display, there were moments when the disturbance of airflow detachment could be heard in some more extreme manouvres like the avalanche.

With something like the F18 Hornet in an extreme tight turn it is easy to hear, even with a jet engine thundering.

With a 33hp Fournier RF4D, everything is more subtle, specially since the Hercules high-efficiency propellors were introduced.

The interesting acoustic return from the hangar wall prompted me to think of the parabolic microphone. I trued making one years ago in the 1970s using a studio photoflood reflector. It worked in a sort of a fashion, great for hearing neighbours having a domestic but the audio seemed very tinney, dry and unattractive.

Tracking audio from an aircraft at safe aerobatic altitude could be both interesting and difficult with a mike system which has a very narrow focus. The delay due to speed of sound would make using an optical sighting system difficult.

Just wondering if anyone has any thoughts on the worth of going to the trouble of making a parabolic setup versus using a highly directional mike like the Sony C76. I'm not overfond of the C76 as it seems to struggle for gain and seems to also have internal noise.

A studio photo reflector might not work well if fabric (too flexible for good uniform sound reflection). In the worst case fabric would be like a speaker grill cloth (acousticly transparent with little reflection). A more rigid surface like fiberglass would probably do better. And larger is probably better for lower frequencies.

Commercial products are available, but costly, probably due in part to low sales volume.

The C76 specs to not look bad with respect to gain and noise level for a shotgun. Recall that a shotgun mic's purpose is to suppress sound from the sides and rear (i.e., isolate pickup to sound from the front direction). However, if the sound of interest reaching the mic is low volume than the mic's self noise may become apparent when the mic output is amplified. The reflector can provide acoustic gain that will helps overcome the mic's noise floor (over certain frequencies at least)..

For a recent DIY approach:
How to Build a Parabolic Mic Dish | Videomaker.com (http://www.videomaker.com/article/17144-how-to-build-a-parabolic-mic-dish)

The reason parabolic mics sound "tinney, dry and unattractive" is because the size (diameter) limits the low-frequency response (acoustic physics, it's not just a good idea, its the LAW!) because of the wavelength of sound at lower frequencies. Parabolic mics are popular with bird-watchers because they are dealing with high-frequencies. But if you are interested in a more full-range frequency response, you need something that is directional at lower frequencies. Note that virtually all conventional mics, even those that are highly directional at voice frequencies, become omni-directional at low frequencies.

One exception was the Electro-Voice 643 which we sometimes referred to as "the bazooka". because of its resemblance to the shoulder-mount portable weapon. All current "shotgun" mics are scale-models of this behemoth.

Electro-Voice Model 643 (http://www.coutant.org/ev643/index.html)

http://javierzumer.com/wp-content/uploads/2012/07/electro_voice_model_6431.jpeg

Don and Richard.


Thank you for your responses.

The Sony C76 looks good on paper and is good if you are close to subject. That is probably what it is really for, rejecting a fairly noisy ambience. I found the companion Sony C74 more user-friendly. Rode blimps have finally caught up with it with their blimp extension kit. A double extension kit to keep the wind off the C76 looks very uwieldy but works. Needs very stout neoprene support rings though.

Much of our Pay TV over here is reticulated via satellite. On roadside chuckouts, sometimes there are parabolic dishes. These unfortunately are cropped parabolas, quite flat with offset feed and do not seem suitable.

The older full parabola dishes never seem to turn up on roadsides. If they are steel they probably end up as oversize woks for family eatathons. They'd be a fair substitute for the old Volkwagen beetle bonnets some folk improvised years ago for bushcooking seafood on sand pit fires. If aluminium they'd get traded pretty quick for scrap.

The studio photoflood was an approx 15" diameter spun aluminium parabola with a big hole in the centre for the edison screw bulb. I was able to cover that with a teapot lid of about the right shape. I might renovate that if I can still find it in the shed.

Modern microphones are so much better than the one I originally used with it, an old dynamic Toa if I recall correctly. This is why I am tempted to give the idea another shot.

They used to sell a round plastic disc for kids to use in the snow. Oh wait a minute - here you go Amazon.com : Snow Sled Saucer Heavy Duty (Purple) : Sports & Outdoors (http://www.amazon.com/Snow-Sled-Saucer-Heavy-Purple/dp/B00HGJAKEE/ref=sr_1_11?ie=UTF8&qid=1420922870&sr=8-11&keywords=sled)

The most readily available , and cheap , true parabolic dish you could try to experiment with would be a TV satellite dish , which also has the advantage of an arm to which you could easily fix a mic at the focus point in place of the LNB .

I have no idea how well it would work , but since old satellite dishes could be found for literally nothing on derelict buildings etc , it has to be worth a try ?

Jim and Derek. Thank you for your inputs.


A bit of an update.

As chance would have it, a satellite dish was on the roadside and the householder wasn't leaving it out for Foxtel to take back. It is the style which is slightly ovoid with an offset feed. My guess it won't be all that good as an audio reflector.

I gave it a quick try last night with a handheld NT2a mike switched in omni and cardioid. It seems to work best for gain in omni with nose of mike towards the dish but slightly less surrounding environmental noise getting in with the cardioid pattern switched on and the mike pointed correctly side-on towards the dish.

There did not seem to be a difference with the mike in the offset position versus versus directly in centre but it is very early days yet. Except for "apparent" gain, it seems no more directional than the Sony C76.

Any furthur advice will be appreciated.

I'd think you'd want a 1-meter or bigger full circumference satellite dish. Overall area counts toward gain, so they might easily have 6dB or more gain over the small off-center dish you have now. And diameter determines the LF cutoff.

Also, the closer you can get to a true parabola the better. The "snow coaster" dishes are probably a sperical section so they theoretically won't focus as well as a parabola. They will have one near focal point (the mic) and one far focal point (the subject) but if the subject is closer or further from the far point, gain will be greatly reduced. And finally you want to locate the mic as nearly as possible to the exact focal point of the reflector. Of course if the reflector is smooth and shiny you can locate the focal point visually; otherwise it will be more of a challenge.

I did build a small version of the "bazooka" as a science fair project back in high school. As I recall it worked surprisingly well. Of course back then anything beyond two tin cans and a string was considered high tech.

I'll ponder this some more, and will let you know if I have any inspirations. Meanwhile keep your eyes open for a 1-meter full-circumference dish. These were popular for ku band perhaps 10 years ago, so they have probably all been scrapped by now, but you might get lucky.

I see various items offered on e-bay.
Parabolic Project Reflector Dish Microphone Science Outdoor Nature Recording | eBay (http://www.ebay.com/itm/Parabolic-Project-Reflector-Dish-Microphone-Science-Outdoor-Nature-Recording-/331436841820)
is among the lowest cost. But shipping overseas may be problematic.

Start - Telinga Microphones (http://www.telinga.com/)
presents some commercial options with international distribution.

Parabolic microphone - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Parabolic_microphone)
for some additional comments.
Per this a 1m dish is good for sound above 1 kHz.

Yes, but remember what Mr. Crowley and I have pointed out. Gain is related to overall area. A 24" diameter dish would have 1/4 the area of a 4' diameter dish, so -12dB less gain. More importantly, low frequency cutoff is related to diameter. Those little eBay reflectors are fine for bird calls, or for capturing the neighbors' conversation with some intelligibility. But if you want to hear lower frequencies, such as aircraft engines as mentioned by the OP, then I think you want a significantly bigger aperture ... maybe so large as to be unworkable. Read the second paragraph of the Wikipedia article linked by Mr. Palomaki.

If you search eBay for "parabolic antenna" you will find some dishes around 3' diameter, or even a bit bigger. (Needless to say, you need a solid dish; the mesh designs won't work for audio.) Prices start around $100 for under three feet, and go upward quickly. Don't forget shipping cost for something this big! That would at least give you something for an experiment, to see whether a parabolic mic would be at all useful for your specific application.

This Foxtel-style dish is about 750mm diameter but very shallow, a crop of a much larger diameter parabola or sphere. The acoustic focal point is about 150mm more distant from the dish than the pickup of the RF receiver.

I tried a Sony ECM-55B neck mike on it tonight and that was a significant improvement over other types. A rotation of about 7 degrees either way drops the gain but the whole thing is not something to write home about.

I used an old ticking alarm clock in a hanging wire basket to try to avoid reverbs and echoes when zoning it in and trying it tonight --- then the alarm went off after about 20 minutes. ---- I had forgotten to reset the time or switch off the alarm as I neglected the clock over the holiday break.

Mechanical alarm clocks which have been allowed to run down often stop not far short of going off due to the set-off cam beginning to ride and add friction. ---- With the gain wound right up on the headphones it was a bit loud.

I wonder what would explain the difference in focal point.

Since you were using an omni mic, I wonder whether you were locating the mic at a position where its rear pickup was in phase with reflections from the dish ... in other words at some multiple of the predominant frequency in question.

I like your idea of using a ticking clock as the sound source. I wonder if your results would be the same if you used a small hanging speaker driven from a pink noise source.

If it weren't 10ºF here this week, I would be tempted to try some tests of my own. I think any experimentation at my end will need to wait five or six months.

I'm wondering if I should research more what might have been a unique acoustic environment at Serpentine airfield. The apron in front of the hangar has an approx 10 degree slope and extends about 30 metres. I am about 5' 8" tall. I was about 3 metres or about 15ft out from the hangar wall. The aircraft from that viewpoint was about 35 degrees above horizon.

Unfortunately, I was handholding a small videocam jury rigged onto a KS8 gyro, so the audio is all gyro and no subject otherwise I may have scooped the sound of airflow detachment with the handheld camera audio.

The video was for purpose of the pilot studying the display on the ground afterwards. The original mission had been to shoot some air-to-airs but it was called off late in the piece when another party failed to arrive and I had taken only the gyro, no tripod.

For the aviators among you, this may be of interest. The pilot, Bob Grimstead, flew this test after my advising him of the sound. Although the video title refers to "flick rolls", the three figures were actually avalanches. Bob later explained that the "whoosh" noise was not just the turbulence of airflow detachment but the entire starboard wing being driven almost directly downward through the air.

Bob flies airshow displays on 33hp Volkswagen beetle car engine power, albeit an engine suitably modified for aircraft and renamed the Rectimo. To lose weight, the electric starter motor was eliminated and the engine is hand-propped for starting. For in-flight restarts, the biggest lawnmower-style pull-starter in the entire universe is a cord and handle in the cockpit for the pilot to tug on.

To conserve the engine during aerobatics when oil falls away from the oil-pump pick-up, Bob uses a molybdenum-based oil additive. A collateral benefit has been the extension of the lifetime of the valve rocker gear due to better lubrication, requiring fewer valve adjustments.

Three Fournier flick rolls - YouTube

Furthur to my earlier reply, I made a mount for the lapel mike from foam which is a tidy fit in the original receiver holder.

The "senstive" area of the dish seems to be about one inch or 25mm furthur down inside the fitting which now supports the microphone. The "zone" for want of better definition is very tight across the dish and about 40mm or one and a quarter inches as a vertical stripe if you get my drift. If the mike is placed centrally relative to the dish, then that "stripe" seems to extend longer. It is hard to know for sure as all manner of variations can occur in backyard engineering.

I took it up to our local footy oval ( sports ground in Amer-English ) to find a flat open area. There were still a few trees around the boundary and the car park where I propped for the test.

With the offset feed I found I was unable to depress the dish low enough to pick up sound from the horizon level. I might rehang the support arm from above or remake it longer and more central to the dish. In trees about 50 feet distant, there were some summer cicadas going off. Over here, they make a clicking sound of about 6Hz repetition. They all syncronise their clicks.

You can throw them off-beat if you have a diesel truck with a distinct louder click on one of the cylinders by findng their repetition rate with the idle knob then slowly winding the idle speed up slightly. Then turn the engine off and the closer insets will be out of sync with their neighbours for about two seconds.

Into the microphone the sound of one about 50 feet away was so intense that on the third mark of the SD302 input level control, the limiter was triggering. The primary trimmer was at about 45. The remaining ambience was well down. The centering was very tight. Just a few degrees horizontally each way would drop the sound significantly. There seemed a little more tolerance in the vertical direction.

The limiter continued to trigger with the primary trimmer wound back to about 28. The lower frequency sounds like cars on the roadway, kids in the skatepark about 400 yards away was an indistinct dull roar with a few higher pitched breakthroughs of voices. The directional sensitivity was nowhere near as tight as with the cicadas. Distant white cockatoo calls came in fairly strongly.

The microphone was very sensitive to air movement. The dish would rumble with slight breeze going across it after the mike was muffed. Any objects like stone retaining walls and tree trunks created quite defined echoes of the cicadas when the insects were off-miked behind the dish.

All in all, an interesting experiment but not outstanding. The acute performance at high frequencies was pretty much consistent with the comments already offered here with lower frequencies not so good. I am not sure the most powerful sound of the airflow over the aircraft wing would dominate local ambience as it is of a lower audio frequency.

Search for the ECM-55b frequency graph and you'll see the typical bass roll-off for lavalier mics to keep the chest resonance from becoming overbearing in typical placements. The knee point starts at 100 hertz and slopes down sharply from there.

Do you have access to a small instrument mic with a flatter frequency response in the low end that would still be easy to place due to small size?

Something like the Audix ADX20I Cardioid Condenser Instrument Microphone? I've never used this mic, but it came up quickly in a search of the style of mic I'm thinking of.

Also a hanging choir mic could work, like an Audio-Technica U853R UniPoint Series Cardioid Condenser Hanging Microphone.

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

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.

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.

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

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?

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.

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.

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 (http://greenpowerscience.com/PARABOLICSHOP/32AUDIODOME.html)

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.

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 (http://www.canford.co.uk/Products/Archive/53-399_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.

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.

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.

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.

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.

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.

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?

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

How to Build a Parabolic Mic Dish | Videomaker.com (http://www.videomaker.com/article/17144-how-to-build-a-parabolic-mic-dish)

Interesting hobby article, except for the misleading use of the word "Parabolic" in the title. "Curved" would have been more accurate. e.g. I doubt that the skillet or trash can lids were really parabolic.

Based on the photo of the trash can lid, which is relatively flat, I would have guessed that the actual focal point (if there is one at all) would be much farther out. But the photo could be misleading.

Actually I disagree somewhat with the comment that the collander would not be useful because of its punched holes. (But a strainer, made from screen, would be entirely different from a collander with some small punched holes.) As long as the hole dimension is a small fraction of the shortest wavelength desired, and the overall perf area is a small fraction of the overall area, perforations would have little appreciable effect on the reflection. That's why perforated satellite dishes, and WiFi dishes constructed of wire segments, still work effectively despite the apparent open spaces. Of course holes would allow a small amount of leakage -- mostly HF -- from behind the dish.

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


Maybe visit a fabric store and check out the fake fur. I've not tried it, but it might work. specially if used to line the edges

QUOTE:


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

That can be arranged. I have a roll of deep fur-fabric I originally got hold of for custom deadcat muffs for the wider enclosures I made for the Sony mikes.

My inclination would be to deadcat the rim and the ECM-55B, its holder and maybe its support spar which is made of square furniture tube. To reduce the front-to-back mechanical acoustic couple through the dish itself, I might try some old carpet cut into pizza slice segments and glued onto the rear surface

Well here's a quick unscientific test.The MixPre and Zoom H4n recorder were each gained up a fair way, about 60% of the gain cointrol on the MixPre and 52/100 on the Zoom. Normally the Zoom's gain would be kept down to about 0.5 - 1 when taking feed from the mixer.

The dish was aimed blind through the bushland to see what was there. It happened to point towards a main road junction about 3km distant. Rejection from off the axis was good. There was a gas cannon operating on an orchard about 2km distant to the right and it was not coming through in the audio.

EDIT. Youtube spat the dummy on the first upload which would not process. Excep for cuts and crossfades, no other process was done on the sound.


PARABOLIC TEST - EARLY MORNING BIRDS. - YouTube

Hey, Bob, that sounds pretty good! Glad to see you're having success there.

Just for the sake of reference, what are the rough dimensions of the dish? (Forgive me if you mentioned this above, it's midnight and I don't have the fortitude to read through the entire thread again.)

Greg.


The dish is ovoid in shape. The dimensions are in metric as I could not find my imperial tape measure. The dimensions are 660mm narrowest and 720mm widest of the active reflecting surface. The 720mm width is oriented in the upright direction. Add about another 32mm diameter for the rolled edge. Actual depth of dish as measured from a line running directly rim to rim is only about 65mm.

The profile is an angled flat faced crop of a larger parabola. Focal point to dish is about 490mm for best acoustic performance.

The satellite transducer was about 25mm closer but the active element is probably furthur inside the plastic shell, which would make it the same or near to.

The thickness of the material can only be guessed as it has a near double-rolled edge. It looks to be about 1.5mm pressed steel rough-finish powdercoated.

My guess is a proper full parabola dish would work better but this one is adequate as is.

I gave it a spray coat of gloss black to make it go faster, ( just kidding ), to smoothen the surface for better reflectance.

The stand is from a 2 x 500watt halogen worklight pair. ( Your Home Depot brand equivalent ).

The dish has been inverted. The nuts which lock off the vertical arc of adjustment have been replaced with wingnuts for faster adjustment.


The birds were not in sight. They were very distant to hear by ear, possibly in the ballpark of about 50 metres in the trees or garden on the opposite side of the road. The crow, I did not hear by ear at all. However this is not a fair judgement. Leaving the muff off one ear is probably too tricky for the brain to cope with. The really loud bird in the last segment had flown closer to about 20 metres for water I had sprayed up into my own trees to draw the birds in.

Bob,

Thanks for the dimensions. Don't worry, I can convert to imperial, even though the US dropped the ball on official changing to metric back in the 1970s. I studied a fair amount of science and engineering, so "25.4" and "39.37" are firmly ingrained in my aging gray matter. (And somewhere in my shop there's a wooden folding rule that I picked up in Germany back in 1979.)

I guess any original "wrinkled" or "bubbled" finish might de-focus the dish very slightly, but probably not enough to worry about. Let's assume the original RF frequency was 10 GHz; that's a wavelength of 0.03 meters = 1.18 inches = 0.0983 ft. Assuming speed of sound to be 1100 ft./sec., that corresponds to one wavelength at ~~ 11,180 Hz. My point is that if the dish was flat enough for 10 GHz RF, it is flat enough for > 11 kHz audio.

Another way to look at it: a dimensional error of 0.04 wavelength will cause negligible gain decrease; of course a smaller error will cause even less decrease. So unless the original "roughness" exceeded about 0.047 inches (sorry, ~ 1.2 mm) it shouldn't cause much gain reduction until you get up to 11 kHz.

Anyway, kudos on your experiments. I will have to start looking for "road kill" dishes around here, and maybe try to replicate what you've found, once the weather warms up.

Greg.


"Roadkill" That's a new one for me. I shall have to remember it.

What surprised me that the lower-frequency off-axis rejection was subjectively better than I expected. I distinctly by ear heard but not in the recording, the gas cannon going off ( tweetie blaster"). These are used as an alternative to lethal force against protected wildlife which will bankrupt stone-fruit growers otherwise.

Whether this rejection is attributable to directivity of the dish or less efficient capture of the lower frequencies or both aspects being interactive, I do not know.


FOOTNOTE:

In the early morning cool drift down the gully on a still day, the sweet scent of ripening peaches can be caught here. When the Fenthion ban is brought in, the peach trees will likely be pulled as the crop is the most sensitive to Meditteranean fruitfly.

Bob,

I wonder whether it was low frequency off-axis rejection, or just low-frequency attenuation. The dish has a lot of gain at high frequencies, but is essentially non-functional at low frequencies. So if you use the high frequency output level as an audible reference, the low frequencies certainly will be attenuated.

In other words, at low frequencies (certainly below the frequency where the dish size < one wavelength) the dish gain is negligible, so you have an omni pickup pattern (just the mic element) with resulting unity gain of 0 dB. At higher frequencies you have a very directional pattern with a gain of some unknown amount ... probably +6 dB or perhaps a lot more than that. The gain tends to rise with rising frequency. So all the low frequencies, both off-axis and on-axis, are very much lower in level relative to the high frequencies.

That dish has a minor axis of 660 mm ~~ 2.16 feet. That's one wavelength at ~~ 500 Hz, so your system should be pretty much unity gain below that, and rising gain above that.

ACOUSTIC FRESNEL REFLECTOR ??.


Those of us who are older may be familiar with the effect of a large vertical area of corrugated wall or fence upon a sharp explosive sound. There is an artifacted echo return which is time-stretched but also reproduced as a sharp high pitch. Corrugated iron is a sinewave shape.

My assumption is that the high pitch is created by the incremental arrival of each short intense echo in rapid succession over time from each of the angular corrugated surfaces which collectively direct their echo towards a common point. That point is not apparently critical along the wall's length so long as there remains sufficient wall area to provide the return.

More critical it seems is the distance from the wall of the listener in attenuating that return. Too close and each peak in the corrugation will mask the echo from its neighbour, thus eventually attenuating by reducing the effective reflective area to the listener.

Too far and the return is more akin to a normal echo, far less time-stretched as all reflecting elements of each corrugation are more equidistant from the listener. There seems to be a sweet spot distance from such walls.

So for the scientists among you, my question is this, could much lower audio frequencies be amplified to a listening point by superimposition of each individual increasingly delayed echo from the sine-shaped upright corrugations. I guess there would be all manner of frequency bands of amplifying and nulling.

If unwanted higher "artifact" peaks were graphed out of the audio recording, could an amplified lower frequency recording be constructed from that echo return additive to the direct sound from a distant object like an aircraft?

I definitely heard an enhanced return from a corrugated airport hangar wall, of sound of turbulent airflow over an aircraft's stalled wing. My subjective estimate is that there might have been as much as 12db worth.

The aircraft was in a favorable position for sound to be reflected off a gently sloping but almost planar paved apron angled towards the aircraft, reflected up at a shallow angle to the corrugated iron wall then concentrated back to my head-height where I was camera-operating.

I have been possibly chasing my tail playing with a parabolic reflector mike which amplifies higher audio frequencies.

A mike, or even a row of them along a "sweet spot" line parallel to the corrugated hangar wall and flight of the performing aircraft in a "sweet zone" might do the trick with the lower frequencies of air-buffeting and airflow detachment from a wing of an aircraft flying aerobatic figures.

Any advice will be appreciated.

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.

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."

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.

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.

.

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.

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.

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.

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.

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.

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. ;-)

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.