Sound Waves & Frequency
The vibrations our ears decode
Summary
Sound is a pressure wave travelling through air (or another medium). Frequency is how many times the wave repeats each second (Hz), and amplitude sets its loudness. The human ear hears the range from 20 to 20,000 Hz.
In depth
Sound waves are longitudinal waves: cycles of compression and expansion parallel to the direction they travel. A loudspeaker cone moves forward and squeezes the air, then pulls back and creates a region of expansion. These pressure changes reach our eardrum and set it vibrating, which moves the tiny bones of the middle ear; hair cells in the cochlea respond to different frequencies and send signals to the brain. Frequency corresponds to perceived pitch (high/low), and amplitude to loudness (loud/quiet). The speed of sound depends on the medium: 343 m/s in air, 1,480 m/s in water, 5,960 m/s in steel. That is why the sound of a train arrives through the rails long before it arrives through the air. Wavelength is inversely proportional to frequency: λ = v / f.
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