FFT Amplitude into Sound Pressure Level (SPL)

[jamison.suade]

Hi everyone,

I have a problem with my program. Basically, i have a chirp signal to be outputted into my DAQ and being fed to my speaker.

The microphone will sense anything coming out from the speaker, and acquire it using my DAQ.

I want to display a frequency response vs. SPL.

My method is that, I'm using FFT on the received Chirp Signal from the microphone and display its frequency response.

My questions are?

1. Is by doing mere FFT will give me an output of SPL vs Frequency?

2. If not, how do I convert this value of the amplitude of the FFT to SPL db?

3. Why is the output amplitude of my FFT is negative?

[MarcDub]

The FFT is a mathematical tool that outputs complex values that contains frequency and  phase information. If you want to obtain the amplitude of your signal as a function of frequency, it is easier to use the "Power and Phase Spectrum" vi in the -> Signal processing -> Spectral palette.