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.