DIY Stereo Field Dish Microphone (WIP) (1 Viewer)

[Pee-wee Pete]

Hi. Please be patient with me on this Work In Progress (WIP) project. I am not as young in mind and vitality as I used to be.

Originally wanted to buy a parabolic dish, but it seems to costly for me. So I decided to look for alternatives. I saw on the internet that a Forrest Cook, some 20 years ago, made such a device from a Wok cover/lid; and I had the same thing at home. I never use the lid, so I had no excuse to not give it a go.

I was not sure if the lid was a parabolic curve or not, so I did a laser test (see images). It proved that it was not. Anyway, I decided to explore further, and find out exactly how this lid will perform in terms of relecting sound to some sort of focal area. It turned out that about half of the reflections do come to a narrow section, but not where one would expect from a parabolic focal point as calculated by... radius squared divided by 4 x dish depth. That is, in my case: 160mm^2 /(4x95mm) = 67.4mm from dish apex.

The geometric lines of reflections from a circulular curvature is different, as shown in the following diagrams.

Also shown is my current findings and plan to use four microphones, two microphones (in parallel connections) per stereo pick-up. I decided to use a partition as well, as many stereo dishes have.

Since it is a work in progress, more info will be posted later on in this thread.

 

[pianoman]

Looks interesting and I look forward to seeing how it progresses.

I can't resist:
This is a Wok in Progress

 

[Pee-wee Pete]

Looks interesting and I look forward to seeing how it progresses.

I can't resist:
This is a Wok in Progress

Pan fried microphones.

 

[Pee-wee Pete]

I am reconsidering an option (something I do a lot of these days), on how to wire the microphones. At first I mentioned using printed circuit board and copper tracks along the dish's microphone support. My new option is to stick with coaxial wire. This will shield the wiring a bit more from any stray electromagnetic fields (noise) entering the microphone wiring.

I redrew the dish profile and reflections to full scale. In doing so, it helped me to figure out exactly how and where to mount the microphones.
I will be using 2mm polyesterene (high impact) sheet for both as a mic support and stereo dish partition... Knowing me, I may change my mind about this too. That's what happens while learning on the job.

 

[Pee-wee Pete]

Well, I stuffed up! The wok dish is not a circular dish at all. It is altogether something else.
Oh well. Back to the drawing board.

 

[Pee-wee Pete]

So far, all I seem to have done with this WIP is establish that the dish is neither a parabolic or sphere/circle. Slow beginnings for sure!
Anyway, this is where I am at at the moment...

 

[Pee-wee Pete]

I finally got the best ideas as to where to put the microphones. I am sure many would be thinking it would be easier to save up and just buy a parabolic dish... and I would agree. But this dish has now intrigued me. How good or bad can a wok lid be, for gathering distant bird songs.

Due to yesterday's strange geometric reflections, I had some doubt about my rendering. I wanted a more accurate truth about the dish profile. So I cut out the stereo partition from polyesterene sheet. Filed it as close as I can without reversing the progress by filing too much. I used this partition as a template, and redrew the reflections.

1. Interestingly, the outer portions of the dish show very little input to picking up distant sound.
2. The main concentration of soundwaves is not at the centre, but about a fifth of the dish's radius from the centre, and about halfway down into the dish. Because it is not central, in order to catch the waves (lines 5-13), the microphone needs to be a torroid. So, in a way, that becomes impractical.
3. A microphone close to the centre could pick up more concentration of less reflections (lines 1-6).
4. I will try using both focal points. Six electret microphones in all. Three per stereo side; one for lines 1-6, and two for lines 5-13.

Yesterday, most of electronic parts arrived for the mic stereo preamp. Some time today I will assemble them on a protoboard to test and fine-tune the preamp circuit before assembling it.

 

[Jon.Bryant]

Just to throw another spanner in the works. I note that if you buy a stereo parabola, the mics are matched. All mics have a sensitivity of + or - so many dB. The HighSound I recently bought had a certificate to say that the mics were tested and had matching sensitivity.

I think this means you will really need to by 2 matched pairs. The alternative is that all recording may sound skewed to the left or right - I understand that stereo parabolas are near co-incidental, so stereo effect (mainly relating to the ambient ‘background’) is created by loudness only.

Anyway, good luck with the project.

 

[Pee-wee Pete]

Just to throw another spanner in the works. I note that if you buy a stereo parabola, the mics are matched. All mics have a sensitivity of + or - so many dB. The HighSound I recently bought had a certificate to say that the mics were tested and had matching sensitivity.

I think this means you will really need to by 2 matched pairs. The alternative is that all recording may sound skewed to the left or right - I understand that stereo parabolas are near co-incidental, so stereo effect (mainly relating to the ambient ‘background’) is created by loudness only.

Anyway, good luck with the project.

Thank you Jon.Bryant for your input. I understand the matching, though I have not considered it at any great depth. And I also understand the co-incidentalness of picking up noticeable stereo effects. Well, I will find out when I try it in the field.

Now, the preamp that I am going to build (if it works) will have adjustable mic gain for both sides. I have noticed some circuits have a Hi and Lo impedence switch for different types of mics. But I decided to have this as well as a potentiometer. So I hope to fine tune a balance of sorts at the time of recording.

Secondly, I found out that stereo mic set ups come in various designs, some dishes without and some with various kinds sort of partitions. I am hoping that the partition I am making may make a noticeable difference to sounds coming from the sides.

I will be having stereo VU meters to monitor the signal strengths. It will be interesting to see how co-incidental the readings may vary.

Thanks again for sharing your thoughts on this project. You bring new perspectives for me to think about.

 

[Pee-wee Pete]

Today I had some time working out how I am going to mount the button elecret microphones.

First of all, to make it a bit easier on myself I decided to give away the need to find focal points. Instead of moving the mic to hunt for best reception, I decided to trust my previous measurements and follow that exactly. That is, make the brackets and glue them straight to the partition.

The following shows the patterns of 2mm condensed polystyrene sheet I need to cut out.

The mics will be tight-fitted, and glued, into the rectangular cutouts. And the patterns glued to the dish partition.

 

[Pee-wee Pete]

Started to cut out the lower four mic holders.

 

[Elkhornsun]

The mic should be placed where the reflected sound will be the greatest and so being able to adjust the distance from the dish is important so you can be trial and error determine the sweetspot.

I have three commercial dishes and each has the mic in a different spot as they have varying amounts of curvature.

Don't forget the handle to be able to hold the dish or mount it to a stand.

 

[Pee-wee Pete]

The mic should be placed where the reflected sound will be the greatest and so being able to adjust the distance from the dish is important so you can be trial and error determine the sweetspot.

I have three commercial dishes and each has the mic in a different spot as they have varying amounts of curvature.

Don't forget the handle to be able to hold the dish or mount it to a stand.

Thank you Elkhornsun for your advise on finding the sweetspot. I will be testing for it when the time comes.

The handle bit is still unresolved for me. Sure, I will have a handle, but not yet determined what else, and how, it will be attached to it.

 

[3dpan]

Have you read this DIY parabolic mic project from Tom Benedict ?

DIY Stereo Parabolic Microphone

I’ve had a parabolic dish I bought off Ebay sitting around for the better part of a year. The whole idea was to build a parabolic mic out of it based on the dishes from Telinga, but for one r…

tombenedict.wordpress.com

You may pick up some ideas for a handle, (and other things too).

 

[Pee-wee Pete]

Have you read this DIY parabolic mic project from Tom Benedict ?

DIY Stereo Parabolic Microphone

I’ve had a parabolic dish I bought off Ebay sitting around for the better part of a year. The whole idea was to build a parabolic mic out of it based on the dishes from Telinga, but for one r…

tombenedict.wordpress.com

You may pick up some ideas for a handle, (and other things too).

Awesome. Thank you 3dpan.
The article resolved a few thinks and some things I did not even know about. Your refferal has saved me a lot of time and energy. Thanks again.
Oh, I do have a drill handle, I never used, somewhere in my garage. I will seek it out tomorrow morning and see how I could use it.

 

[3dpan]

Just thinking, laterally.
You have been drawing your "ray diagram" with parallel lines striking the face of the parabola. This would be reasonably correct if your target (bird) was a long way from your parabola, eg hundeds of metres.
But what if the bird was much, much closer, just a few metres, then wouldn't the sound source be a point source ?
So, what would your ray diagram look like then ?
You might even get a better focus point for your microphone ? And as a result you might have a "macro-focusing" parabola, good for recording the sound of crickets, cicadas, etc, up close.

And maybe not either,

 

[Pee-wee Pete]

Just thinking, laterally.
You have been drawing your "ray diagram" with parallel lines striking the face of the parabola. This would be reasonably correct if your target (bird) was a long way from your parabola, eg hundeds of metres.
But what if the bird was much, much closer, just a few metres, then wouldn't the sound source be a point source ?
So, what would your ray diagram look like then ?
You might even get a better focus point for your microphone ? And as a result you might have a "macro-focusing" parabola, good for recording the sound of crickets, cicadas, etc, up close.

And maybe not either,

Strange, I never considered using the dish for close encounters of a few metres. For close up I guess I would use the inbuilt microphone of the digital recorder. Interesting though, I will find out about this stuff in due course.
Thanks for sharing your thoughts. It has given me some food for thought and experimentation.

 

[3dpan]

Some insects, like wasps, are not very sociable, and some, like cicadas, won't stay if you get too close. The parabolic allows you to get acoustically close at a safe working distance, while at the same time reducing surrounding noises.

and, another thought,
If the Wok gets too much, then this may be an economic option,

Parabolic Microphone

The home of affordable Parabolic Microphones, useful for Wildlife Recording and the most intricate of sound bites. Essential kit for any Sound Engineer's tool-belt; whether an amateur hobby or industry professional. Free Shipping in the UK!

www.parabolicmicrophone.co.uk

Cheers,
Alec

 

[Lerxst]

You have been drawing your "ray diagram" with parallel lines striking the face of the parabola. This would be reasonably correct if your target (bird) was a long way from your parabola, eg hundeds of metres.

Moreover, there is the frequency dependence to consider. The waves diffract upon entering the dish aperture, so none of the "rays" actually strike the surface along the dish normal/perpendicular, and they are not parallel. In the high frequency limit the effect is negligible, but not for low frequencies. Hence the low-frequency performance loss. One could deviate from a parabolic surface in order to compensate for this, to get better low-frequency gain, but then you'd hose up the higher end of the spectrum. There are no solutions, there are only trade-offs.

 

[Pee-wee Pete]

Some insects, like wasps, are not very sociable, and some, like cicadas, won't stay if you get too close. The parabolic allows you to get acoustically close at a safe working distance, while at the same time reducing surrounding noises.

and, another thought,
If the Wok gets too much, then this may be an economic option,

Parabolic Microphone

The home of affordable Parabolic Microphones, useful for Wildlife Recording and the most intricate of sound bites. Essential kit for any Sound Engineer's tool-belt; whether an amateur hobby or industry professional. Free Shipping in the UK!

www.parabolicmicrophone.co.uk

Cheers,
Alec

Thank you for the tips. I will log that down in my field note book, on 'things to remember'.

Thanks for the link. I checked it out and there is no price onj the dish because they sold out. It is now bookmarked in my bird watching links folder.