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Sine Wave A 440 Hz Concert Pitch for Ten Hours
Stop your brain from overthinking. You don't need an app. Ten hour Sine wave with no overtones which is called pure as it is only the fundamental frequency.
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"Some rhythms are impossible for human to perform. But as samples, human voice can be as precise and relentless as any machines. To further the possibility of voice, transcending the voice beyond physical limitations is an inevitable trajectory" - Rully Shabara