Distortion in electret microphone

[deepsetan]

Hi,

I pumped in 1KHz to one electret mic and measure the distortion using audio analyzer. I found that the distortion is almost ~4%. I put one variable resistor in series with the mic to see its behaviour. During my measurement, I found that the distortion still keeps ~4%. Why the distortion didn't reduce even though the voltage input to the mic getting higher? I'm thinking to change the JFET inside the microphone.

 

[jiripolivka]

How much audio power do you "pump" into your poor electret? Does it line up with its specifications?

 

[deepsetan]

How much audio power do you "pump" into your poor electret? Does it line up with its specifications?

I set up my amplitude -32.2dB at 1KHz. I never calculate the audio power. Normally this electret mic have distortion almost ~3%.

 

[BradtheRad]

The mic's output voltage should occupy a middle region between the supply rails. Try experimenting with the series resistor. In typical schematics the resistor is located between the mic and positive rail.

I made a microphone with an electret. The battery was 1.5V. It worked fine, until one day I let the battery get low. The recording had terrible distortion (known as shatter). I think the signal was clipped due to the low supply voltage.

 

[schmitt trigger]

How much audio power do you "pump" into your poor electret?

My same concern.
It could be a translation issue, but the way that the question is phrased, it appears that the OP is attempting to drive the microphone like a speaker.

If this is not correct, and the audio is actually sound "pumped" via an amplifier and a speaker, how do we know that those devices are not introducing the distortion?

 

[deepsetan]

How much audio power do you "pump" into your poor electret? Does it line up with its specifications?

My same concern.
It could be a translation issue, but the way that the question is phrased, it appears that the OP is attempting to drive the microphone like a speaker.

If this is not correct, and the audio is actually sound "pumped" via an amplifier and a speaker, how do we know that those devices are not introducing the distortion?

How can i calculate that audio power? My dc supply to this electret mic is 3V with a 2.7k resistor.

 

[jiripolivka]

How can i calculate that audio power? My dc supply to this electret mic is 3V with a 2.7k resistor.

Typical electret microphones have the DC voltage specified, and the load resistor, too.
If we assume the DC circuit is correct, please reduce the audio "pump" power by adjusting source speaker distance to 1...10 meters while observing the distortion. If it is caused by audio overdrive, it will decrease with lower audio power.
To check the audio source distortion, best is to use a calibrated microphone as a reference.
I would suggest to use an audio fork tuned to "a", this is said to have no distortion.

 

[Audioguru]

All the 2-wires electret microphone datasheets I have seen and all the 2-wires electret mics I have used have a current of 0.5mA and produce no distortion unless the sound is VERY loud. Then with a 2.7V power supply the resistor should be (2.7V/2)/0.5mA= 2.7k which you have. The voltage is low so the sound level should also be fairly low.

If you are blasting the mic with an extremely loud sound like inside a drum or inside a piano (or a rock and roll screamer) then a 3-wires electret mic should be used with a 9V supply. The Jfet in a 3-wires electret mic is a source-follower instead of the common-source Jfet in a 2-wires electret mic. Online you might find the article and modification of a 2-wires to a 3-wires project from Linkwitz. I have never used a 3-wires electret mic.

 

[FvM]

I set up my amplitude -32.2dB at 1KHz. I never calculate the audio power. Normally this electret mic have distortion almost ~3%.

dB is no amplitude unit, just a relative value. Are you talking about dBm(600 ohm), dBV or what?
Where do you get the distortion specification?

How do you measure the distortion? Can you be sure that it's generated in the microphone?

 

[deepsetan]

dB is no amplitude unit, just a relative value. Are you talking about dBm(600 ohm), dBV or what?
Where do you get the distortion specification?

How do you measure the distortion? Can you be sure that it's generated in the microphone?

Sorry for my mistake. It is in dB V. I measure the distortion using audio analyzer. I pumped in 1-KHz into this electret microphone and measure the distortion before and after the variable resistor. Normally during maximum input SPL, the total harmonic distortion of the electret mic is ~<3%.

- - - Updated - - -

Typical electret microphones have the DC voltage specified, and the load resistor, too.
If we assume the DC circuit is correct, please reduce the audio "pump" power by adjusting source speaker distance to 1...10 meters while observing the distortion. If it is caused by audio overdrive, it will decrease with lower audio power.
To check the audio source distortion, best is to use a calibrated microphone as a reference.
I would suggest to use an audio fork tuned to "a", this is said to have no distortion.

Thanks for your answer! I put my speaker almost 5mm to this electret mic and reduced my amplitude from -32.2 dB V to -29.2 dBV and the distortion reduced to ~2%. :lol:

- - - Updated - - -

All the 2-wires electret microphone datasheets I have seen and all the 2-wires electret mics I have used have a current of 0.5mA and produce no distortion unless the sound is VERY loud. Then with a 2.7V power supply the resistor should be (2.7V/2)/0.5mA= 2.7k which you have. The voltage is low so the sound level should also be fairly low.

If you are blasting the mic with an extremely loud sound like inside a drum or inside a piano (or a rock and roll screamer) then a 3-wires electret mic should be used with a 9V supply. The Jfet in a 3-wires electret mic is a source-follower instead of the common-source Jfet in a 2-wires electret mic. Online you might find the article and modification of a 2-wires to a 3-wires project from Linkwitz. I have never used a 3-wires electret mic.

Yup, thanks for reminding me about the current. I measured the current in the mic. It is almost 250uA when I pumped in this 1-KHz. I can assumed that my input is very low. :thumbsup:

 

[FvM]

reduced my amplitude from -32.2 dB V to -29.2 dBV

??? According to my math, -32.2 dBV (24.5 mV) is less than -29.2 dBV (34.7 mV).

What's the respective SPL according to microphone specification?

Regarding your original question, consider that the JFET amplifier stage is working as current source with the current set by the threshold voltage and a source resistor. As long as Vds is sufficient, the distortion will mainly depend on the electret output voltage (respectively the SPL), not the amplifier output voltage.

 

[deepsetan]

??? According to my math, -32.2 dBV (24.5 mV) is less than -29.2 dBV (34.7 mV).

What's the respective SPL according to microphone specification?

Regarding your original question, consider that the JFET amplifier stage is working as current source with the current set by the threshold voltage and a source resistor. As long as Vds is sufficient, the distortion will mainly depend on the electret output voltage (respectively the SPL), not the amplifier output voltage.

Yes, you re correct. From my previous measurement, the SPL condition level is 1Pa, 0dB=1V/Pa.