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I have plotted many wav files in MALTAB and tried to understand their time domain plot , Magnitude spectrum and Phase spectrum.

But I don't get why time domain plots of speech wav files are symmetrical around time axis. Why is it needed to be so ?

Here is the plot for speech :: The sky that morning was clear and bright blue. sp10.wav

I observed its Magnitude spectrum and got an idea about magnitudes of sinusoids components at various frequencies.

But What Information can be derived from its Phase spectrum relating it to time domain ? I have only understood that these are the phase values of sinusoids telling which is ahead or behind. But What else useful information can be obtained from the Phase spectrum ?

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The time domain signal being roughly symmetric simply means that the signal has no DC component, i.e. the spectrum at 0 Hz is essentially zero. That's almost always the case for audio wave forms, since air can't actually transmit DC.

In fact, significant DC offset in an audio wave form typically indicates a problem or error of sorts.

Phase has no relation to the signal being "symmetric".

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