Chirp!

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ronv

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First let me say I have a champagne appetite but a beer budget.

I have a friend that records birds professionally for schools and museums.

Right now he uses a shotgun mike, tripod and a good recorder. I'm thinking I could extend his range with a parabolic refector type of system.

I would love to find out someone has experience with this, but failing that maybe some questions and answers.

I found parbolic dishes at Edmunds - I'm thinking 12 or 18 inch. Any idea of the gain I could expect from these? Is it worth the bulk to go to the 18" dish?

Any thoughts on the fidelity of the refelcted wave? Quality sound is important.

I've been looking at electret mike elements because they are easy, but so far the best I have found are some Panasonics that look pretty good from 50Hz to 16Khz. I would like better - any ideas.

Since I have no idea what output to expect I thought I would just build the capability to really gain it up. But doing that will mean I need something to keep from popping ear drums. So I'm in search of a cute curcuit. I'm thinking I would keep the earphones separate from the recorder but if I do that I need something to make sure I'm in the sweet spot for the recorder (1 volt I think).
It's all a brainstorm right now so any and all input would help.
 
Use a scrounged satellite TV dish and make sure the microphone is well shielded from direct backside sounds as they will arrive slightly earlier than the indirect sound from the dish surface. Make a class A preamp and include amplitude limiting in the circuit.
 
 
A satellite dish is big so it will have good bass response. Bass from birds??
Since birds chirp in high frequencies then a small dish is fine.

Nobody makes a "class-A" audio opamp. They are all class-AB.
 
You should be able to run the opamp as "class-A" by putting a resistor from its output to gnd (or to the -ve PSU if it is dual rail PSU).

Someone suggested it recently as a method to remove the AB crossover distortion caused by some old style opamps.
 
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National Semi (now TI) recommends adding the resistor in their datasheet for the lousy old LM358 and LM324 opamps because they are low power and the outputs operate in high distortion class-B instead of low distortion class-AB.

People at Headwize use the resistor to reduce the 0.00008% distortion from OPA2134 opamps in their heaphones amplifiers.
 

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4pyros, Yes a hi fi version is what I'm after.

AG, so maybe a 2134 for the preamp?
 
Gain

Well, I know how to calculate the gain now and understand the effect of size on frequency response, but now I'm not sure where the improvement over just a shotgun mic comes from. If I use a low pass filter to get the gain close to flat again it seems like I have lost most of the benifit of the dish. If I try to get it flat to say 500Hz I guess I could pick up 5 or 10 dB with a good sized dish. Is that all there is? I guess the dish gain would be without noise so that might be another small plus.
Am I missing something?
 
I did not know that a pretty big dish microphone has a very high gain at only very high audio frequencies.
I also did not know that a dish microphone needs equalization to obtain a flat frequency response.

Like a dish, a little shotgun mic picks only high frequency squeaks. A very long shotgun mic is good for lower frequencies.
 
Reading this implies the gain is a plus over a flat response. Interesting.

**broken link removed**
 
The Telinga article is full of contradictions:
1) A shotgun mic has "frequency independent acoustical amplification of sounds in front of it and frequency independent attenuation of what is around it". The article does not say that a parabolic dish mic also attenuates sounds around it.

2) A recording engineer needs a distant sound to be more "closeup" when using a shotgun mic because the surrounding sounds are attenuated. Then he turns up the treble?? Why not simply turn up the gain so that the sound is louder like a closeup sound?
I think when you hear with your ears a distant sound then background sounds around you are loud and if the sound is close then background sounds around you are less loud but the article says the opposite.

3) A parabolic dish mic has a ''knee" frequency depending on its diameter and I believe it. I believe the amplification is flat above the knee frequency, like a shotgun mic. But the graph of a parabolic dish mic shown recently does not show any "knee".

4) The article says a parabolic dish mic provides amplification that increases with increasing frequency. But they did a test with pink noise 30m away and the same mic without the parabola 3m away and say the frequency spectrums were similar!!
Without the parabola the mic response should be flat, not peaked at high frequencies.

5) "A parabolic dish mic is an acoustical filter, compensating for distance". I know that wind blows away some distant high frequencies and humidity attenuates some distant high frequencies but the distances in the article are not far (50m then 30m) and wind and humidity were not mentioned in the article.

Maybe there were translation errors. The Telinga inventor Klas Strandberg is from Sweden.
 
I'm so confused.

Here is another one that supports the curves. I just don't see where the performance improvement comes from. It seems like they should have more gain than just a mike in a tube.

**broken link removed**
 
Some curves and two articles say that a parabolic dish mic boosts high frequencies and needs a second-order lowpass filter to make its frequency response flat.
After the frequency correction I don't know if it provides any gain compared to a bare mic.
 
audioguru said:
...
After the frequency correction I don't know if it provides any gain compared to a bare mic.
Click to expand...

It should boost all frequencies? It focuses pressure variations from a large area into a very small area. Whatever the pressure variation is at the dish surface, it should be many times higher at the focal point where the mic is placed.
 
Arrg! Light goes on.

I don't have to put the filter at zero. I can put it anywhere on the curve. So if I have a dish with 12 dB of gain at 1Khz I can make it flat from 1Khz to 20Khz with the filter. Gives 12dB of noise free (mechanical?) gain. I supose some young guy could hear the difference.
 
I am a "young guy" because I can hear well to 14kHz. I am 67 years old and have never used guns and protected my hearing from acid rock and other loud noises.

The parabola boosts sounds so the mic and preamp can have less gain and therefore produce less noise (hiss).

A response flat from 1kHz to 20kHz will sound "thin" and tinny, won't it?
But I guess birds do not produce any low frequency sounds.
 
audioguru said:
...
A response flat from 1kHz to 20kHz will sound "thin" and tinny, won't it?
But I guess birds do not produce any low frequency sounds.
Click to expand...

Possibly. But for recording birds I think a flat frequency response woul dbe ideal to record the "true" sound of the bird. You could always EQ it later.

Just keep in mind that very few mics have flat freq response, and those cheap little electret inserts are probably very bumpy. You can probably get a mic freq chart from the manufacturer?
 
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