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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: enter image description here

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

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  • $\begingroup$ It's just plotting the sample values - so it's somewhat like an oscilloscope display in the real world. $\endgroup$ – Paul R Feb 9 '16 at 17:43
  • $\begingroup$ @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? $\endgroup$ – GaussTheBauss Feb 9 '16 at 17:45
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    $\begingroup$ 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. $\endgroup$ – Paul R Feb 9 '16 at 17:47
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What you are observing is the digital representation of the voltage, which in fact represents the acoustic pressure. Workflow would be something like:

  • Vibrating larynx is producing Acoustic Pressure [Pa]
  • Variations of that pressure are converted by the microphone into Voltage [V]
  • Voltage is being sampled and quantized by the ADC (Analog to Digital Converter) and represented as series of numbers.

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.

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