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How do I present an audio signal in a time domain plot using MATLAB, such that the y-axis is in decibel (dB) instead of of a linear scale?

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Kwaku Owusu
Kwaku Owusu on 14 Jan 2021
Commented: Kwaku Owusu on 16 Jan 2021
Please, I have a recorded audio signal from experimental work. I have been able to plot the time domain graph (see attached) using Matlab software. By default, the y-axis values are present in a linear scale (Amplitude). I would like to plot the time domain graph such that the y-axis scale is is in decibel. I understand that using 20*log10(signal/ref. signal) should provide the decibel scale on the y-axis, however, this does not seem to work after several attempts.
** My code**
Let say the signa name is TD, so I use the code plot(20*log10(TD/20*10^-6)), the 20*10^-6 is my reference air pressure. The problem is that the entire signal is distorted.
Please, I would be glad if someone can assist me (specifically with a code in matlab to visualise the signal). I might not be doing something right. Thank you.
Thank you.

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Mathieu NOE
Mathieu NOE on 15 Jan 2021
hello
do not forget to take the abs of your time data before doing the 20*log10 action
spl = 20*log10(abs(TD/20*10^-6))
maybe some smoothing may be needed (=> movmean)
make sure that TD signal unit is Pascals

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Accepted Answer

dpb
dpb on 15 Jan 2021
Edited: dpb on 15 Jan 2021
In particular, note that "Signal intensity, power, and energy are always proportional to the square of the signal amplitude.power". When you just use a zero-mean time trace, the log10 is undefined for values <=0 which is what is happening with what you've tried so far.

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