sensitive parabolic michrophone

[konrad]

Does anyone have a curcuit diagram for a sensitive parabolic michrophone for very close range?
I need this for a science project to listen to quiet bubble popping.

[prateeksikka]

hey!
i dont understand 1 thing!
do u want to build up a parabolic microphone?
why are u doing it when they are commercially available?
or u want the rest of the circuit?

[konrad]

yah i need to build one for a science fair project and i would do better if i made one

[audioguru]

An LM386 little power amplifier IC has a gain of up to 200. Its circuit is simple, it is cheap and amplifies an electret microphone very well.

[konrad]

Do you know where to get the curcuit for this?

[audioguru]

This circuit:

[steven]

hello audio guru you should of posted  this circuit agers ago to my topic seeking sensitive parabolic mic circuit , it may of been handy but anyhow i got it now so ill biuld it to  if its any thing similiar to the last one

[audioguru]

hello audio guru you should of posted

[konrad]

hey audioguru on the curcuit diagram the resistor on the bottom left is not shown the power of, do u kno wut it is? 1k or 10k or?

[audioguru]

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.

[konrad]

thanks for all your help auidoguru

[konrad]

do u kno how to hook up this lm368 to the computer?

[audioguru]

1st of all, it's an LM386.
2nd of all, you don't say if you want it to amplify the computer's output so you can hear a speaker play what's stored on the computer, or if you want to amplify something then connect the output of the LM386 to a line input on the computer so it can be recorded.

[konrad]

sorry i wanted to hook it up to the computers mic outlet thing so i could record the sound on a sound recorder on the computer, and analyze data

[audioguru]

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.

[awright]

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

[Shahriar]

Dear Audioguru

On a circuit explanation, I remeber that was written that the 10 Ohm resistor and the cap are to decrease Cross over Phenomena!!! I don't know how?
Does the resistor and cap play any roll in Cross Over either?

ThanX
Shahriar

[audioguru]

Hi Shahriar,
No, the RC at the output doesn't affect crossover distortion that is low in an LM386 anyway. My old databook has many more pages than the datasheets on the web and explains that it has oscillation without the RC.

[Shahriar]

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

[audioguru]

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:

[Staigen]

Hi

Audioguru is right, the RC-snubber is for stopping oscillating, not to reduce crossover! :)

//Staigen