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sensitive parabolic michrophone


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Hi Konrad,
The resistor at the bottom right is 10 ohms. It is a load on the LM386 at frequencies above 160kHz where the speaker's inductance makes it a very high impedance. It keeps the LM386 from oscillating at a very high frequency. There isn't much ultrasonic power at frequencies so high, and the microphone won't pick them up anyway so an 1/8W or 1/4W resistor will be fine.

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You don't need an amplifier when you connect a microphone to the mic-input on a computer.
If you want to record the faint sound of bubbles popping and the computer's gain isn't high enough, then you need a microphone preamp circuit. An LM386 can be used as a mic preamp if a low-value coupling cap replaces the high-value cap at its output, feeding a switched or a pot attenuator to reduce its level and hiss. The attenuator would feed a low-value coupling cap that connects to the computer's mic-input.
The value of the 1st cap matches the value of the attenuator or pot's resistance and the 2nd cap matches the input impedance of your computer's mic-input at the lowest frequency you want.

Your computer's mic input has a bias voltage on it for electret mics so whatever you connect must be designed not to be affected by it, the function of the 2nd cap above.

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konrad, you indicate in your original post that you want a parabolic mic for very close range.  The characteristic of a parabola is that it will focus all rays (optical or acoustic) originating at INFINITY to one point.  It is a special case of an ellipse, that has two finite foci.

You don't indicate the distance or frequency range that you intend to operate at, but I presume you mean high frequencies at a few inches to a few feet.  If that is the case, you would be better off using an ellipsoidal reflector with the noise source at one focus and the mic at the other focus.

Now, all this becomes somewhat academic, depending upon what you are trying to accomplish with your reflector, because the finite size and imperfect placement of the micropone diaphragm broadens the central lobe of the polar pattern of the reflector so the exact mathematical theory also becomes blurred.

Additionally, while buying or fabricating (by spinning a bowl of epoxy at a constant speed as it hardens) a parabolic reflector is not too difficult, fabricating an ellipsoidal reflector might be a little more difficult, perhaps requiring cutting out an ellipsoidal template and using it to form a mold that you cast a reflector in.

The gain/directivity of a reflector mic is directly related to the size of the reflector relative to the wavelength of the sound.  I built a 1 meter diameter reflector once that worked excellently according to theory, but had almost no directivity below 1KHz.

Have fun.

awright

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Dear AudioGuru

Go to this project in Project section. "Amplifier 2x30W with STK465 "

at the bottom of the schematic I have attached, in 8th line, it is written:

"Finally, at the same time with the exit exists networking RC (0,1mF - 4,7 Ohm) that it attends to the minimisation of phenomenon  crossover. "

ThanX again
Shahriar

post-1483-14279142631845_thumb.jpg

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Hi Shahriar,
That project has a terrible translation into English.
Its 0.1uF cap and 4.7 ohm resistor begin applying a load to the output of the amp at 340kHz. Crossover distortion occurs at all audio frequencies and is caused by not having enough idle current through the output transistors.

I found the same explanation for having the RC network at the output of amps as I have in my old databook. The LM380 is similar to the LM386 and nearly all of National's audio power amp ICs have the RC network at the output:

post-1706-14279142631943_thumb.png

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