need some assistant on the op-amp circuit attached in the thread.

[yaantey]

Hi,
Can I directly connect the electret microphone to oscilloscope to estimate the maximum output voltage from it so that I would know how much amplification is needed. I know noise and stuff will effect and I am going to test considering it. I'm going to scream at the microphone and try some loud music too. Attach is the sort of microphone i am going to use to giv u an idea (but not exactly the same model). Do u think I can hook it directly to oscilloscope to test?

Thanks.

[Grumpy_Mike]

It depends on two things:-
1) The maximum sensitivity of the oscilloscope input.
2) If the microphone requires a voltage to make it work.

[yaantey]

Attached is the microphone type i am using. You think it can work if i connect to oscilloscope?

[dc42]

If you connect a 10k or so resistor from the +ve side of the mic to the +ve side of a 9v battery, and connect the -ve side of the 9v battery to the -ve side of the mic, and switch your oscilloscope input to AC, then you should be able to see the mic signal on the scope and thereby estimate the signal strength.

[Techone]

@yaantey

I happen to have a few of those mics, so I did this experiment using my scope I have. Well I set my voltage setting at 2 mV/div and I did get something, ( I was blowing on it ) and I place close to the speaker of my DIY AM radio, I did get something ( a sound waveform ) and try to measure it, it between 2 mV to 5 mV ( peak-to-peak ). If you feel the op-amp circuit do not amplify enough, you can still add another amp.  Usualy, when I design this type of system, I add  a transistor for the fisrt stage and the second stage a op-amp. Then I add a filter ( in your case a band pass filter set around the baby cry frequency ) and amplyfy and buffer to going into a analog input of the arduino.

And I will use a electronic circuit simulator to check the design and change some values ( resistors and capacitors ) if needed - if it improve or not.

 

[yaantey]

I manage to test the voltage output from the electret mic and attach are the output from the mic when I shouted at it, played music and did nothing. All outputs were produced when I shouted and played music in very closed proximity to the electret mic, almost touching it. But unlinke Techone I found the my electret mic produce 23 mVpp. I got 8 mVpp without music and shouting, and by shouting I got 31 mVpp, hence 23 mVpp.

Attached are the output and the simple connection used, does the reading seem good?

[Techone]

My experiment simply measure the output of the mic without voltage and resistor.  After net surfing, search : electret microphone ,I come across this site : http://www.openmusiclabs.com/learning/sensors/electret-microphones
Which I did know about the inside JFET, so DC42 is correct, it need a resistor and a voltage supply. I wonder if you will get better result by changing the resistor value ( lower or higher ) ? Hum , maybe...

@yaantey

The reading seem right.

[ajofscott]

Electret mics make voltage by the effects of altering dielectric thickness by sound waves. There has to be a charge across the capacitance for dielectric changes to manifest as voltage changes. C=Q/V, where C= capacitance in Farads, Q= charge in coulombs, and V = plate differential in Volts. Rearranged we get V=Q/C, thus anything that increases C reduces V and visa versa. If V=0 because the cap was never charged we will never see that C changed at all.

[yaantey]

Hi,

I have downloaded a baby crying voice from (http://www.soundjay.com/baby-crying-sound-effect.html) and used audacity to analyse the frequency. I have got the graph but I am not quite sure how I can find the fundamental frequency. I have attached the .txt file and the graph. Can someone who know the stuff point me to the fundamental frequency. Thanks.

[Grumpy_Mike]

It's a baby crying, it doesn't have a fundamental frequency.

[yaantey]

Atleast there should be a frequency of baby's voice so that I will be able to make a band pass filter for the circuit. Or else how will i be able to do it.

[Grumpy_Mike]

At least there should be a frequency of baby's voice

No it is all the frequencies in that graph.

Or else how will i be able to do it.

You can't on an arduino.

[Techone]

@yaantey

You need a band pass filter using op-amps. By looking at the graph, from 500 Hz to 11 000 Hz is where the baby is the loundest ( higher db value : -50 db ( lound ) is higher than -95 db ( silence ) ) The graph represented the a fourier analysis responce of a baby craying.  So design a op-amp circuit - could be more than one op-amps at those frequency mention. And I notice the lounder point is around 1 KHz whish make sense.
 

[ajofscott]

Play it through an 11 band EQ tweak on the controls and find the combination that gives the dryest result. "Dry" loosely meaning harsh and lacking in lower frequencies. And a single passive notch filter could be made using the second amp in the dual package for the filter.

[yaantey]

I am trying to come up with my own circuit to amplify the signal from mic. I would like to know these terms because when ever I see op-amp amplifying circuits it comes up. I tried going trough somethings on the internet and keep getting confused. So can someone explain to me in a simple way what dc biasing (is it same as bias voltage?), dc offset and dc coupling means.

Thanks