Standing wave detection using a microphone

[SFB]

Hi


I am trying to develop a standing wave inside a rectangular cavity and detect it using a microphone.In one wall of the cavity ,I placed a loudspeaker and driving it at a frequency of 8.3 Khz. The chanel has a thickness for half wavelength standing wave.

I am trying to detect the nodes and antinodes of the standing wave using a microphopne and have already built a circuit to amplify the signal. Though I have cleared out most of the noises using low pass capacitors (picofarads) at the input and output of my capacitor , still the output from the microphone is not completely stables. The waveform in the oscillocpe appears to be a sine wave , but the values (voltage pp , frequency are chaning all the time ), and the peak of the sine wave is not exactly a smooth curve.


Any suggestion on how can I make it more stable.

 

[Ivan Seeking]

Turn down the volume?

Is this a dynamic or condenser mic? Could it be that the mic itself does not have the desired precision? Assuming it is a dynamic mic, it sounds like you could be fighting mechanical limitations within the mic itself.

I can see voltage issues due to insufficient filtering, but frequency drift is another matter. How pure is the tone from your speaker?

Have you checked for harmonics? Maybe you are seeing the additional frequencies. [I don't know how tightly you are filtering]

 

[SFB]

Thanks for your reply .I am using a condenser microphone. I have used the audio amplifier cicruit priovided in the following link (Fig E7.5) with some additional capacitance to eliminate noise from the room where I am running the test.

http://www.ece.sunysb.edu/~ese123/parekh/Experiment7.pdf



I think the microphone is working fine.I tested it by putting in the opposite end to my speakers and the signals in the oscilloscope was quite clear.

You have mentioned about the voltage issues. Can you please explain a bit more on that?

 

[Ivan Seeking]

Thanks for your reply .I am using a condenser microphone. I have used the audio amplifier cicruit priovided in the following link (Fig E7.5) with some additional capacitance to eliminate noise from the room where I am running the test.

http://www.ece.sunysb.edu/~ese123/parekh/Experiment7.pdf



I think the microphone is working fine.I tested it by putting in the opposite end to my speakers and the signals in the oscilloscope was quite clear.

You have mentioned about the voltage issues. Can you please explain a bit more on that?

My thought on voltage was that harmonics or noise might cause some variation in the peak to peak value.

If you can test the mic directly and see no problem, then I am a bit confused. This implies that everything works until you put the mic in your resonant cavity. If that is true, then why do you think the problem is with the mic? [I guess I should have said, "with the electronics?"] Could your data be valid?

 

[SFB]

Thanks for replying again .

Before my actual test ,I wanted to test whether the speakers I am using can generate a sine wave properly. So I put the mic just opposite to my speaker (the gap between them was really small) and got some sine-waves in my oscilloscope. So I had this feeling that both of these electronics are working fine.


Now I am using the speaker in a cavity . On top of the cavity,I moved the microphone along the cavity length to check If a standing wave is formed. This time I am using the microphone with the amplification circuit and the waveforms in the oscillocope were distorted which I mentioned in my previous post.


However , now I am getting a perfect sinewave just by putting a 100 microfarade capacitance after the the power supply . Though I got read of the distortions , the wave is still fluctuating on the screen .I am using a frequency of 8.2 kHz, but frequency reading in the oscilloscope is varying from 6-9 KHz and at times it drops to 2 KHz. I am not sure if its due to overtones. Is there any way to check it?How can I get rid of it ?

 

[Ivan Seeking]

Is it fluctuating as a function of position, or time, or both?

Maybe you could use some low-, high-, and band-pass filters to see what you have?

What is the length of your cavity? How is it constructed? Have you tried varying the frequency of your speaker to see if you can get a clean and steady sine wave?

 

[Ivan Seeking]

I wasn't really thinking about your frequency. Based on what's you've described so far, it sounds like you should be using a much lower frequency for this.

 

[SFB]

Hi

I have my speaker on the side wall , the opposite wall works as a reflector. Its an open cavity (top and bottom surfaces are open). I am applying a frequency of 8.3 KHz , the distance between the side walls are close to 40 mm . The speaker is of 40 mm dia as well , and the heigth of the cavity is approximately 60 mm.

 

[Ivan Seeking]

I think you have a design problem.

What is this project for?