Home-made condenser microphone

[dcromley]

I thought that I could get some audio output from doing this: I cut a 6-inch hole in the back of a tablet (thin cardboard) and taped 7x7 pieces of tinfoil on each side so the 2 foils were maybe 1mm apart. Then I connected a 9V battery to the 2 sides through a 5Meg resistor. I connected my scope to the 2 foils and clapped my hands and expected some audio output, but couldn't observe any. Am I off base on this?

 

[MikeMl]

Don't the two plates of the capacitor move together? One would have to be stationary, while the other is attached to a diaphragm which moves with sound pressure...

 

[audioguru]

A condenser mic has only a few microns of gap between the moving foil and the fixed perforated backplate. Then a voltage of 48V is applied through a very high value resistor. The output is extremely high impedance and is buffered by a Jfet or vacuum tube. A modern electret mic is the same but has the 48V permanently stored in the electret material and has a Jfet impedance converting buffer inside.

 

[dcromley]

> a few microns of gap
Wow. That sounds difficult for a home project. Thanks.

 

[prprog]

A condenser mic has only a few microns of gap between the moving foil and the fixed perforated backplate. Then a voltage of 48V is applied through a very high value resistor. The output is extremely high impedance and is buffered by a Jfet or vacuum tube. A modern electret mic is the same but has the 48V permanently stored in the electret material and has a Jfet impedance converting buffer inside.

How can 48v, is permanently stored in the electret material ? is this like a capacitor ? does it discharge at some point?

Thanks,
PRPROG

 

[throbscottle]

Surely, with bigger plates, a bigger gap is possible? Other thought is, rather than thin card, a piece of plastic film might be better, like the window off a cake carton or something. One plate could be stuck down to something to make it more rigid.
There are designs for FET scope probes out there - maybe one of these could be used to capture the signal. Otherwise the scope has a low impedance input compared to the "mic".

 

[audioguru]

How can 48v, is permanently stored in the electret material ? is this like a capacitor ? does it discharge at some point?

You will not be able to make or buy electret material but you can read about it in Google.
The charge is permanent and can last for hundreds of years. It has no load current in an electret mic, it is simply an electrostatic charge. Then when the membrane is moved by sounds its capacitance to the fixed plate changes and the Jfet detects the change of capacitance and produces a audio signal.

 

[dcromley]

MikeML: Yes, I changed the setup so that the bottom is a plate, not tinfoil.
Prprog: Yes, it is totally like a capacitor. I thought it had to be kept charged with a "phantom" voltage supply.
Throbscottle: I've attached an image. A scope has a 1Meg impedence. I thought that was high.
Audioguru: I need to check on "The charge is permanent and can last for hundreds of years". What about a "phantom voltage supply"?

A funny thing is that at one point I thought I saw just what I wanted -- a response to my clapping of hands. But now there is only the 60Hz AC noise.

The tinfoil is taped to be taut.

 

[audioguru]

Now you are making a "condenser" microphone. It needs a 48V external power source which is called a "phantom" supply.
Since you have a "stiff" thick foil membrane (an electret mic has an extremely thin metal film deposited on a thin plastic membrane) and you have a huge 1mm space then its output level will be so low that it will probably not pickup very loud explosions.

I guess you did not look at Condenser Microphone in Google to see that the 48V is fed to the diaphragm through a resistor with a value of 100Mega-ohms to tens of Giga-ohms and the preamp needs an input resistance the same or higher.

All microphones feed a preamp with shielded audio cable. The shield blocks mains hum and other interference.