What kind of signal amplitude can I expect from an electret microphone?
A google search on "CZ034D" yielded a datacheet:
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I found a basic circuit for powering the mike. If I use an R1 that is higher than 2.2K, what will happen? Am I right in assuming that the only difference will be a higher 3dB frequency of the mike?
The electret mic needs a certain amount of current to power its internal circuitry. If R1 is too high, it will not be able to pass enough current. The datasheet for a particular element will tell you how much current it needs.
I haven't done much playing with them yet, but it looks like mV are what is to be expected. From looking around lately it looks like a wide variation exists.
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The impedance will go up with the value of the resistor.
I have one that I've only just toyed with that uses a 10k resistor and I'm using a 5V supply.
Anywhere from 3 to 10 millivolts per Pascal of sound-pressure-level, depending on the particular electret element. This is assuming you bias the microphone with the normal operating current according to the manufacturer (usually 2 to 5 volts at around 500 microamps), using a circuit similar to the one shown in your post.
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The datasheet specifies max 500 microamps, and I have measured the DC resistance to 1.8K ohm. My power supply is 12V, so I need an R1 of at 22.2K ohm, to get a 500 microAmp bias current. Am I right? I'm in doubt because it's so far from the circuit I found.
Another question. Sensitivity (0dB=1V/pa, 1KHz)
-40dB for my specific mike. Which I translate into 10mV/pa. I have no idea what the sound pressure level is. Does anyone know how many pascal you would get in a motorcycle helmet? Or just in an ordinary noisy environment.
0=2E0002 dyne/cm^2 is defined the threshold of human hearing, or 0 dB_spl
That microphone is good for 100 mV / dyne / cm^2 (at 1 kHz), so at 0 dB_spl it will produce 20 microvolts.
At 80 dB_spl (a relatively noisy environment), it should produce 200 mV.
At 100 dB_spl (a very loud environment), it should produce 2 V, but now you have to make sure the circuit has enough "headroom" in the power supply, the bias and the load.
I would change the 2k2 resistor to at least 22k. In most electret circuits I have seen, the bias resistor is at least twice as large (47k). The load resistance is probably OK, just make sure that the load resistance of the amplifier is taken into account, also.
Great! In general, keep the current through the electret to 1 mA or less. You can always depart from that general figure when you have a datasheet for the element. The 10 uF is normally polarized - you want its (+) side connected to the capsule in your diagram.
Next: It turns out that it doesn't much matter what amplitude the electret provides. All electrets produce signals that must be amplified in order to do anything worthwhile with them. The output from the electret is fed into an amplifying stage (often called a pre-amp). You get the level you need for whatever circuit you are driving from the pre-amp. (Sometimes the pre-amp is part of the circuit you are driving.) There is no need to control the signal level coming out of the element itself - controlling the amplitude in the pre-amp is all that is required. Here's a link:
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For a kit that has everything you need, including an electret element, get # 3378 RB from MPJA
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(You may need to put 3378 in the "stock no. direct" box, sometimes links to MPJA take you to their home page) The kit includes a pre-amp and an amp that delivers 1.5 watts output, all on one small PC board, and costs $10.95.
1 Pascal are 93.97dB SPL. this is quite high, but should be in the ballpark if your mike is close. With 12V you should split the required resistor of
22k into 2 parts and use first an 18k into a grounded cap(10u) and then the recommended or a bit bigger load resistor like 4k7 or 6k8. You do not need R2, it will only dampen the signal.
No, here it is. the bias current is always 0.5mA, as the output behaves more like a current source. the recommended voltage across the Mic is usually less than 5V, so your circuit might damage certain mics.
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