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Excited frequency in sine sweep not matching FFT



So I'm generating a sine wave sweep and taking the FFT of the data I am capturing through microphones. I noticed that the frequency being excited is not the frequency on the FFT. I attached an image of my front panel to show what I just described and I also attached my VI. Does anyone know why this might be happening?




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Message 1 of 7



Your application seems pretty simple. In the attached screen shot the current frequency is 2000Hz, but the FFT has a peak at somewhere around 1920Hz. Is this the problem you are referring to? Generally when a sweep test is done, multiple cycles are generated for each frequency (no.of cycles will depend on the requirements for the project) and every new frequency starts from zero line, i.e there should be enough samples for each frequency component to complete a full cycle, otherwise when transitioning from one frequency to the next there could be sudden glitches in the signal generated which could affect the FFT measurement also. The no.of cycles can be just 1 also , but make sure that enough samples are generated to complete at least one full cycle before jumping to next frequency. It is always better to use DAQmx VI's instead of  express vi's for daq assistant. But if you are at university, that's fine for now.


To understand it better can you provide a more detailed info about what you are trying to do.

  1. What hardware are you using to generate the sound?(PC sound card and speakers?).
  2. Which microphone are you using? (model, make?).
  3. How are you acquiring output voltage from microphone? Which DAQ device? ( Model ?)
  4. how close have you placed the mic?
  5. can you show us a simple diagram for your setup?


Deepu Jacob

Waiting For the inner calling 🙂

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Message 2 of 7

You make measurements when changing the tone frequency. Several frequencies are included in the spectrum measurement array. Since calculating one, you get the average frequency. In addition, it is highly desirable that the polling frequency be a multiple of the generation frequency. And ideally, both would work, from a single clock generator.

Labview 4.0, 5.0, 6.1, 8.6, 2009, 2011, 2012, 2014
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Can you please share the logged data file also?

Borjomy said a valid point.

Waiting For the inner calling 🙂

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Message 4 of 7

Hey Guys, 

Thanks for all the help! So a brief explanation of what I would like labview to do.  Ultimately, I would like to perform a smooth sweep by increasing in intervals of 1 Hz. The range starts at 200 Hz and should end at 2000 Hz.  Using Labview, we generate the tone through the audio amplifiers which has a speaker to excite the PVC pipe.  The microphone captures voltage readings and labview should write/save this information to a txt file as it runs through the sine sweep.  I have also attached a simple diagram that demonstrates my setup and includes the model of the hardware I am using.  Please let me know if there is any other information you need. 





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Message 5 of 7

Hi Michelle,


Thanks for the info and the diagram.

Swept sine frequency analysis is often used to determine the characteristics of device under test such as 3dB gain, phase etc. What do you plan to do with the microphone voltage readings logged in the file. what analysis you intend to perform? Also is it necessary that you need to perform FFT online. Is it okay to do the FFT for each frequency separately from the data file once the sweeping is over. Why i'm asking is because while reading the microphone data it is possible that the data array can contain data corresponding to more than one frequency unless the generation and reading process are synchronized properly. also would it be possible to upload a sample data file.





Waiting For the inner calling 🙂

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Message 6 of 7

The main purpose of my project is to investigate the resonating frequencies of my setup. With the data captured, I take the power spectrum density and see which frequencies resonate i.e have the highest amplitudes. I think my data set is too large so it wont let me attach it. Is there something in specific you would like to know in regards to the data set?

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