The human ear is capable of hearing soundwaves of frequencies approximately between 20Hz (20 cycles per sec) and 20kHz (20,000 cycles per sec). This range of frequencies is known as the audio spectrum.
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The frequency range of 20Hz to 20kHz is equivalent to about 10 octaves in pitch. An octave on a piano equals 12 notes (semitones). The higher the frequency the higher the pitch. A4 on a piano is 440Hz, therefore A5 is 880Hz and A3 is 220Hz (see diagram below).