Waveform and Envelope
The amplitude may be constant (in which case the wave is a continuous wave) or may vary with time and/or position. The form of the variation of amplitude is called the envelope of the wave.
If the waveform is a pure sine wave, the relationships between peak-to-peak, peak, mean, and RMS amplitudes are fixed and known, as they are for any continuous periodic wave. However, this is not true for an arbitrary waveform which may or may not be periodic or continuous.
For a sine wave the relationship between RMS and peak-to-peak amplitude is:
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For other waveforms the relationships are not (necessarily) arithmetically the same as they are for sine waves.
Read more about this topic: Amplitude
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