According to Fourier Analysis, a square wave is actually a combination of an almost infinite colleection of sine waves, a fundamental and many harmonics. The more harmonics you eliminate or reduce, the more the wave sounds like the pure tone of the fundameental sine wave. But eliminating harmonics removes energy from the wave, decreasing its apparent loudness. A 1 V peak-to-peak square wave has an RMS value of 1 V. For a sine wave to have the same RMS value, the peak-to-peak value is 1.4 V.
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