# dB octave band spectrum

When I was at the University I did some courses about Acoustic. I always seen the dB octave band spectra, as the Fourier Transform of the dB in time. Now looking at some reference in Matlab, I noticed that it is a PSD instead. If it is a PSD should not be displayed as unit (dB/Hz)? Why everyone displays (dB) in the unit in the graph? Which should be the procedure to obtain the dB octave band spectra starting from the time history of the pressure?

• What does "Fourier transform of the dB in time" mean? Commented Jul 25, 2021 at 20:23
• It means that I calculate the sound pressure Level in time Commented Jul 25, 2021 at 20:46
• are you sure that the soundlevel is in logarithmic units and not just simply linear pressure over time? Commented Jul 25, 2021 at 21:00
• (by the way, in your question and comments you happily use "dB" as measure for sound pressure – that's not how decibel work; they're always a relative quantity, so maybe reading up on what decibel is might already clear things up?) Commented Jul 25, 2021 at 21:50

dB is a relative logarithmic measure which is a reasonably first order approximation for human perception. It always needs a reference. For many acoustic measurement, it's actually dBSPL (Sound Pressure Level) where the reference pressure is $$20\mu Pa$$.
$$dB/Hz$$ or $$dB/\sqrt{Hz}$$ doesn't make sense for octave bands since each band has a different bandwidth and normalizing to the bandwidth would defeat the purpose of octaves in the first place.