Parabolic'ing a highschool football game. Am I doing this right?

[Paul R Johnson]

How will most people reading that article work out if their bin lid or saucepan is remotely parabolic? At best, some of those products are merely shields from sounds coming from other directions. Gain from proper dishes with a real focal point are good, but moving the feed point on a real dish just an inch or two wrecks the performance, so using a plastic lid is totally random!

[Gary Nattrass]

Best way to get audio from the heart of the action is to do what we do on Rugby and put a radio mic on the ref.

Of course you get all the decisions too and even some choice language but it actually does add to the game coverage if mixed in correctly.

As for parabolic mic's they all sound dreadful with narrow bandwidth and tend to be only used to record birdsong in the UK, we tend to use 416 and 816 mic's on most sports in the UK.

As a sidenote I tend to find the NFL audio for TV very tiring to listen to and they seem to add huge amounts of gain momentarily to get the kick off and the crowd is distant and very mushy and ill defined.

It all ends up as just noise most of the time with little definition and after sitting on the end of it all for up to 7 hours of coverage at work it is very exhausting.

[Don Palomaki]

The Videomaker article provides a point of departure, and some ideas for a low budget experimenter. It was not suggested as an authoritative source. It is up to the end user to decide whether or not it meets his/her needs. It certainly will not meet the needs of networks, except perhaps as a way to do it in a stealth-mode.

[Bob Hart]

I have played with home-brewed parabolic arrangements. To render a normal audio frequency range comfortable for the human hearing requires reflectors larger than are practicable to use.

For a "must get it right" mission critical event coverage, as I have myself been previously counselled here, you owe a respect to the people who have commissioned you to make product for them. The advice of others here suggesting options besides a parabolic arrangement are worth considering seriously.

There's nothing wrong with recording from a parabolic source to its own dedicated audio channel so long as you have other audio going to its own channel. ANY air movement is going to be a deal killer. Please DO NOT contaminate your audio with a parabolic source going into a live mix.

All that lamentation just said, there is an arrangement which is easy to construct. It uses a fairly common pay TV satellite dish made of powder-coated steel. The dish is not an entire parabola, but has been muchly cropped at the sides, has an offset feed relative to the total dish size but is a faitfhul parabolic segment.

To make it work as a mic reflector, the dish has to be inverted on its four-bolt mounting frame. In use across a flat field, the dish front rim is inclined upwards so that the feed post is relatively level to the ground.

The best mics I had in my kit I found to be lavalier type, Sony ECM55B. It was a simple task to cut some soft foam to fit in the space where the transponder fits to a bent post. A small hole was cut in the foam to support the mic and insulate it acoustically from the metal frame.

Mounting the dish to a tripod is not hard but preventing handling noise clanking its way around the dish into the mic is another matter.

Fixed adjustment of the original hinge bolts and clamping bolts can be made easier by replacing the existing fasteners with bolts, adding flat washers between all rubbing surfaces and using wingnuts.

A sandwich of flat washers and spring washers under the wingnut is desirable to enable tensioning of the fasteners without firming down hard. This enables trim adjustments of the angle to be forced against a light friction without the dish flopping freely.

To mount the dish on a conventional camera tripod required a short metal pole with a flat base to match the tripod baseplate with a tripod screw thread tappen into the bottom. There needs as mentioned above in previous posts, a rear countermass to balance the weight of the dish across the tripod tilt and pan axial centres.

You also need some sort of sighting device which can be as simple as a piece of PVC tube to look through or an old telescope gunsight.

My version shown in this video clip was temporarily mounted on a garage light stand.

As well as the concentration effect of the dish, the signal from the mic was fed and heavily amplified via a Sound Devices MixPre to a Zoom H4n recorder.

PARABOLIC TEST - EARLY MORNING BIRDS. - YouTube

[Paul R Johnson]

Confused - I get something like that with any microphone in my garden? The point is the increase in wanted sound compared to unwanted sound, and there is a hell of a lot of ambient background noise. How about pointing it at yourself 30m away and speaking in a normal voice? Honestly - nothing in that clip allows a listener to determine anything whatsoever without some kind of comparison, or images, or structured test.

Here's an alternative
http://www.limelight.org.uk/Birdchirp.mp3
Is it a dish, a cardioid or an omni? Impossible to tell - it's actually my iPhone.

I'd genuinely love to hear your dish work - so I am not being picky, but the test isn't a test without some kind of base line to run it against.

[Bob Hart]

Paul.

I don't take your comment as being picky and my unthorough experiment as such might be explained as follows. My recording is not the paragon of perfection and was never intended to be. My comment I have "played with" home-brewed parabolic arrangements discloses the lack of depth in my experiments.

I recommended against the use of a home-brewed parabolic arrangement for a high value professional task and posted this example purely for curiosity interest and for folk taking responsibility for their own experiments to know a few build methods.

I know you can gain the hell out of modern microphones and pre-ampliers to achieve a similar sound. What you were hearing was the mike zoned in upon the small bird in the tree canopy some 50 metres distant.

The narrow band of pickup from the reflector also picked up some crows furthur distant across the road and the highway traffic 2km away down the valley. It was aimed through a forest and there would be some return of off-axis sounds from tree-trunks and leaves.

The dish crispens high frequency sounds and is most pinpoint with them. The lower frequencies from other directions can and do penetrate. I was receiving more ambient background noise when using a Sony C74 directional on the same subject with the same gain levels.

With this dish, the sound of a ticking alarm clock hanging in a rose bush can be zoned in from about 20 metres against ambient background noise.

With a much larger complete satellite transmission dish, the clock can be zoned in from about 60 metres and a fuller sound can be drawn in. However the larger dish is very impractical to use.

My intention with that dish was to aim it to a particular fixed point an aerobatic aircraft was going to fly into and record the unique sound of torn air when the pilot executed an "avalanche" manouvre.

[Gary Nattrass]

That is not a parabolic microphone it is just a metal sat dish designed for video HF transmission signal used with an ECM mic in front of it.

The whole point of a parabolic mic is that it focusses the sound waves to a single point and as sound waves are a lot shorter than video RF waves the dish needs to be a lot more curved.

As Paul has said you are not really getting any benefit from this set-up over a traditional short shotgun and you might as well just take any lump of metal or wood and use it as a "reflector" . Note your body would probably do just as a good a job and you are not actually creating a parabolic anything and just rejecting certain sounds that are 180 degrees from the mic.

The other aspect to a parabolic set-up is that the focal point of the hot spot is phase coherent, just plonking a mic in front of a lump of metal that is full of holes anyway is asking for trouble.

Sorry to sound (no pun intended) negative but this is akin to just plonking a jam jar on the front of your camera and hoping that you will get lovely pictures.

OK fun to play around with but this is just re-inventing a wheel that is actually rather square.

;0)

[Bob Hart]

Gary.


I do not disagree with you. The profile of the sat dish is not an entire parabola that is true. It is a very small area out of a true larger diameter parabola.

It does not have the apparent acoustic gain of an entire parabola but it does have a fine point of acoustic focus which is at the same position where the original waveguide and UHF pre-amp was positioned.

It does not "seem" to have the rear cone of pickup some shotgun mikes have so airiness of overhead tree canopies and back echo from tree trunks "seems" to be less when chasing birds and frogs. To be sure, a precise comparative test would have to be done.

"---- and as sound waves are a lot shorter than video RF waves the dish needs to be a lot more curved." = true.

Acoustic at 3000Hz wavelength = 0.1146m
Acoustic at 800Hz wavelength = 0.43m

Foxtel 700MHz wavelength = 0.429m

The force of the barrier of denial is strong in me. The sat dish was an experiment. If one does not have a decent low-noise shotgun mike, then the Foxtel dish is of some use.

However, a parabolic dish is not something I would use on a sports ground in preference to having the umpire wired with a radio-mike or a good shotgun mike aimed at the action.

A full spun aluminium parabolic OB dish I have is about 1.5M in diameter. It is much more effective as an acoustic reflector but also too much of a handful to be useful.

The little shotgun mike I have did not pick up the ticking clock in the rose bush but my whistling nose whiskers could be heard when I forgot to hold my breath.

[Gary Nattrass]

Don't get me wrong it is always fun to play around with sound and as a recent purchaser of a very nice analogue allen and heath CMC 32 console for less than $200 it puts my $500k Calrec Apollo into some perspective as it still sounds better but in my 37+ years in TV and film I have never ever actually used a parabolic mic and when I hear the incoming sound from NFL Fox and CBS coverage for SKY UK I know why as it just sounds dreadful.