Find frequency through fft

[garnicmac]

My partner and I are currently working on a project that involves finding a frequency
though the Fourier transform in LabView. Once the frequency is found, we would like
to send back a signal tone at that frequency through headphones. We are
having trouble making LabView accomplish this through our codes. Attached
is a zip file if you would need to see the block diagram, with our project
being White_Noise_Final.vi.

The test we are running is white noise that will be sent through an enclosed tube, when the tube opens up there is a plateau like effect in the spectrum (Example picture attached). The goal is to find the frequency of the plateau and resend that frequency. We have no way to exactly test this because we have no tubes available, but we believe the code we have will accomplish the goal. Is there a way to set a threshold to find this plateau? We attempted to use the Peaks.VI, but LabView did not agree. Is there something that we are missing?

Also, the requirement for the white noise is to be randomized between 1kHz and 10kHz, so the second signal should be in this range.

[johnsold]

1. What is the frequency of white noise?

2. What is the physical basis for expecting the plateau in the spectrum? I would expect a resonance in the tube to create a peak (possibly broad) rather than a "rectangular" plateau as you sketched.

3. There is something wrong with the way you have the filter VI set up. I see no output from the filter. When I place a Butterworth Filter.vi from the Signal Processing >> Filters palette, I get s filtered output. I do not like the Express VIs because of the way they tend to mask what is going on, so I did not investigate the issue.

4. (Do not use this to answer #1). The spectrum of white noise is flat. The use of the peak detector on the spectrum of noise does not make sense. It may make some sense to use the peak detector if you get a spectral peak from the "plateau." However a plateau implies a range of frequencies. What frequency from that range would you choose for your sound output?

I think you need to do some thinking about your project, particularly from the perspective of the physics of your project.  Clearly define what you expect the output to be when you excite the tube (closed and open) with white noise. Also attempt to estimate the two signals you will measure: the excitation signal and the tube output. It is highly unlikely that you will not get some of the excitation signal at the output.

Lynn