Sound begins when an object vibrates. A guitar string, loudspeaker or vocal cord pushes and pulls on nearby matter, producing alternating regions of higher and lower pressure. Those disturbances travel as mechanical waves through air, water or solids. Without a material medium, ordinary sound cannot travel.
Frequency, amplitude and speed
Frequency is the number of wave cycles per second. People usually perceive higher frequencies as higher pitches and greater amplitudes as louder sounds, although human hearing is more complex than either measurement alone. Sound speed depends on the medium and its conditions. In air, temperature matters; in many liquids and solids, vibrations transfer faster than they do through air.
From motion to nerve signals
The outer ear directs sound toward the eardrum. Its movement is passed through three small middle-ear bones to the fluid-filled cochlea. Inside the cochlea, movement along the basilar membrane bends sensory hair cells. That mechanical motion opens tiny channels and produces electrical signals carried by the auditory nerve.
Different parts of the cochlea respond best to different frequencies. The brain combines this frequency information with timing and intensity differences between the ears to help identify a sound and estimate where it came from.
Hearing is interpretation
The ear converts vibration into neural activity; the brain gives that activity meaning. Context and experience help us recognize a voice, follow speech in a noisy room or hear a melody instead of unrelated tones. This is why hearing is not simply a microphone-like recording.
The same physics supports technologies such as medical ultrasound, sonar and acoustic design. It also helps animals communicate and navigate: bats use returning echoes, while many whales exploit the efficient transmission of sound through water.
Sound therefore connects physical motion with perception. A vibration becomes a pressure wave, the ear converts it into electrical signals, and the brain organizes those signals into the sounds we understand.
Sources
- NIH/NIDCD, How Do We Hear?: https://www.nidcd.nih.gov/health/how-do-we-hear
- OpenStax, Sound: https://openstax.org/books/physics/pages/14-1-speed-of-sound-frequency-and-wavelength