# What does MATLAB display when plotting a sound signal?

When you load the speech file 'mtlb' in MATLAB (which is by the way the voice of a woman saying the word 'MATLAB'), and plot the data, you get the following plot:

My question is this: what does the y-axis represent? What is MATLAB actually plotting here? Thanks!

• It's just plotting the sample values - so it's somewhat like an oscilloscope display in the real world. – Paul R Feb 9 '16 at 17:43
• @PaulR OK. but what do those sample values measure? intensity? amplitude?... This is where I am confused. For example an oscilloscope in an electric circuit may measure voltage. But what about sound? – GaussTheBauss Feb 9 '16 at 17:45
• Sound pressure waves are converted to a voltage signal by a microphone. This analogue voltage signal is then sampled by an A-D converter, giving you discrete sample values which can be plotted as above. – Paul R Feb 9 '16 at 17:47

• Vibrating larynx is producing Acoustic Pressure [Pa]
• Variations of that pressure are converted by the microphone into Voltage [V]
Now knowing your microphone's sensitivity [mV/Pa] you can relate the voltage and acoustic pressure. Additionally if ADC's parameters are known (or measured) then you can directly relate sample values to Pascals.