frequency
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The number of vibrations per second that determine a sound's pitch, measured in Hertz.
Frequency measures how many times a sound wave completes a full cycle per second. Higher frequencies produce higher pitches, lower frequencies produce lower pitches. The standard tuning note A4 vibrates at 440 Hz — 440 complete cycles per second.
Human hearing spans roughly 20 Hz to 20,000 Hz, though the upper limit decreases with age. The lowest note on a piano (A0) vibrates at about 27.5 Hz, while the highest (C8) reaches 4,186 Hz. The relationship between frequency and pitch is logarithmic: doubling the frequency raises the pitch by exactly one octave.
Dogs can hear frequencies up to 65,000 Hz — more than three times the upper limit of human hearing. Some dog whistles exploit this by producing sounds above 20,000 Hz that humans cannot detect.