configuring the reference power (dB) in the Spectral Measurement express VI

[akjryarcamo]

What is the reference power (dB) or pressure in obtaining the dB FFT response from the Spectral Measurement express VI? We need to obtain the SPL of a cellphone speaker and SPL is measured when the reference pressure/power is 20uPa or 1pW.... The data result is in negative dB but we need to have a positive result. Im thinking that maybe the reference is at 1mW.. 

[johnsold]

You can convert Express VIs to regular subVIs. Then open the block diagram and look at what they have done.

In this case when you get six levels down into subVIs you find one called ma_Convert to dB.vi. It has inputs for dB reference for ch X and ch Y. The default values are 1.00. These inputs are not wired in the SubVIs of the Express VI.

To change the reference FIRST Make a Copy if the converted Express VI with a new name. Put it into your project, not in vi.lib. Similarly, make copies with new names of any subVIs you need to change. Then make the changes you need on these copies. Document those changes so anyone looking at the code knows why the standard Express VI was not used and what the differences are. Put the changed VI in your main VI and get the SPL values.

Lynn

[tahir09]

sorry but can you elaborate a little bit more.

I have a sound signal waveform to which I need to find out the single sided power spectral density. I attach the "power spectrum and PSD" express VI. The graph I get in dB is all in negative but I need to get the positive dB values to carry out further frequency analysis. I do not know how to configure or modify the program so that it shows me positive dB values of the sound signal waveform.

Thank you.