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Patrick Cooper

Ideas for a DIY parabolic dish

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I'm thinking of getting hold of a parabolic dish for recording the sounds of nature outdoors like bird calls. Though instead of purchasing a commercially made one, I'd like to go the low budget DIY route and obtain a cheap house old item that has the same basic shape and convert that. I thought locating such an item would be quite easy but I'm having a hard time finding something suitable. Ive spent ages looking on eBay and although Ive found a number of items on there that have the shape of a parabolic dish, they are all very small. The largest one I found was a lid for a kitchen pot but it was only 40cm in diameter. Ive also considered trash can lids but they tend to be cone shaped rather than curved. Any suggestions for other items to look out for that would do the job? I am in Australia by the way so there might be some things here available here that could be suitable. 

Ive also looked at second hand satellite dishes on eBay but a lot of them are not very deep and in some cases, almost flat. Plus many are sold interstate and are 'pick-up only' with regards to payment transactions. 

If all else fails, I could try making a parabolic dish out of thick cardboard though obviously, that wouldn't be as good as one made of stiffer / harder material. 

I would probably use an omnidirectional microphone with it. 

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I don't think cardboard would work - it would absorb low frequencies.

 

What about making one from Fibre Glass? You can get kits for boat repairs etc... Then you could make any size you want and it would probably be easy to source a former to make the shape - a Yoga exercise ball or similar could work.

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Bear in mind that a parabolic reflector shouldn't ideally be a spherical section. Much like a lens, it should really be - well - a spun parabola, for best efficiency.

You can get pretty close by fastening some sort of slightly elastic membrane (a mylar blanket is common) over a circular hole and drawing a partial vacuum on the other side, then backing it up with fibreglass. That will naturally form something very close to a parabolic section.

P

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Thanks for the replies and suggestions. There is a youtube videos which shows someone testing their cardboard parabolic dish. The guy did a comparison test with and without the dish (recording audio of white noise.) And there was a very noticeable boost in amplification with the cardboard dish vs without. I'm not sure about the presence of low frequencies with that kind of sound source. Though yea harder material would be even better, I would imagine.

The fibreglass suggestion sounds like a good one. Ive actually never worked with this stuff before. Is it straight forward to apply? Yea I would need a mold of some kind. There was another guy on youtube who used a mound of rocks and then layed cement over that and shaped it.Probably overkill. I wonder if I could use clay instead though that would probably require a huge amount of clay. The yoga exercise ball is also a good idea. 

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In the thrift stores and garage sales of the United States you can usually find discarded dish antennas from the satellite TV companies. They are definitely parabolic and usually still have the bracket.

The big users of parabolics are the solar cooking enthusiasts... if I recall correctly they sell fairly inexpensive dishes.

The other shape you might try is the giant plastic salad bowl; they come in a variety of shapes and if you're really into it you could put one in the oven at a temp that's just hot enough to make it pliable again and shape it over a true parabola.

dishNET-1-879x485.jpg

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Posted (edited)

You do see satellite dishes on the occasional roof here in Australian suburbs. Though I don't think Ive ever seen one in thrift stores or garage sales. The only scond hand ones Ive seen have been on eBay.

Yea those solar cookers are really tempting. And I see some of them are reasonably priced. Though the only possible downside with them is that I notice the focal point is often outside of the dish. And obviously with sound recording, it would be preferable to have the focal point inside the dish to protect against wind noise and other extraneous sounds. 

Though I guess that's always a risk with adapting any item for this kind of application. There's always the chance that the focal point could be outside of the dish. I'm not sure of the maths involved for working out the focal point for any random curved object. I guess it's a combination of the diameter and the depth. 

Edited by Patrick Cooper

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Today, in my family's kitchen, I discovered a large glass bowl tucked away in a cupboard which has a nice curvey shape. Very deep too. It looks roughly about 50cm in diameter or close to that. I think this thing has potential. 

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I researched these briefly some years ago and even purchased a small-ish eBay one for testing.

They are not as easy to make as one would think if you want to get good quality audio out of them. I don't have the links anymore which I used for research but one of the things is that you can really tune these things to work well at only one pretty narrow frequency range at a time. The other frequencies are not amplified as much and they don't have the same directionality characteristics so you will amplify very narrow frequency range directional spot from the front and then all the other sounds and noise will surround it from all over the place. For most applications the end results sounds pretty weird. For example it is not good to try to record dialogue with these things because they miss the low frequencies of the human speech and over-amplify the mids.

Better low frequency response means that you need larger parabolic disc. at some point it becomes  impractical to use and challenging to make. And too heavy or otherwise awkward to use efficiently. Picks up wind, wobbles and is challenging to transport.  Thus the existing solutions tend to be heavy compromises between practicality and usability, usable frequency response and directionality and cost.

And you'll still need the mic which you would probably not want to manufacture by yourself (it is possible but not practical) 

It needs to be real parabolic (spherical does not work, it does not have defined enough focal point and it causes phase differences to the sound waves which reflect from different parts of the disc) and it is pretty precisely tuned construction even if it looks simple and easy to make. The mic placement and mic pickup pattern needs to be precisely set too according to the disc cracacteristics. I think the eBay disc I purchased was pretty OK working for the price but the mic placement had to be set by about 5mm accuracy to get the best sound out of it. And it only worked well at very narrow frequency range... I metered it back then but can't remember right now. I think it was something like about 600 to 1300 Hz frequency range where the system worked OK and the rest of the frequencies were not amplified and sounded weird.

I know couple of nature cinematographers who use Telinga parabolic mics. I have understood that they are pretty good for that kind of stuff https://www.telinga.com      

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for microphone, a cardioid mic is better than omnidirectional in this application. You will not want the mic to pick up too much sound from the back (the direction of the sound source) because that sound arrives to the mic earlier than the sound the parabolic reflects you really want to record. All the sound waves which are picked up should be in same phase. Thus you need to know the focal point where the mic is placed and then you should select the microphone's pickup pattern so that it covers as much disc area as possible but nothing extra from the sides and nothing at all from the back. So that the mic "sees" only the disc surface and nothing more. This gives you the best amplification and the best S/N ratio

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