Parabolic microphones.

[Bob Hart]

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.

[Don Palomaki]

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

[Richard Crowley]

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://javierzumer.com/wp-content/up...odel_6431.jpeg

[Bob Hart]

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.

[Jim Michael]

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

[Derek Heeps]

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 ?

[Bob Hart]

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.

[Greg Miller]

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.

[Don Palomaki]

I see various items offered on e-bay.
Parabolic Project Reflector Dish Microphone Science Outdoor Nature Recording | eBay
is among the lowest cost. But shipping overseas may be problematic.

Start - Telinga Microphones
presents some commercial options with international distribution.

Parabolic microphone - Wikipedia, the free encyclopedia
for some additional comments.
Per this a 1m dish is good for sound above 1 kHz.

[Greg Miller]

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.

[Bob Hart]

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.

[Greg Miller]

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.

[Bob Hart]

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

[Bob Hart]

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.

[Jay Massengill]

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.