What is sound?

Sound consists of pressure and motion changes carried by mechanical waves through a medium. A vibrating source disturbs nearby matter, which transfers the disturbance onward. Human hearing detects only certain ranges of frequency and intensity. Producing, transmitting and perceiving sound are distinct stages.

How do sound waves form and travel?

1. Source and medium A speaker membrane moves back and forth, creating compressions and rarefactions in air. Air particles do not collectively travel all the way from speaker to ear; they move around equilibrium positions and pass on the disturbance. Sound also travels through liquids and solids, but not an ideal vacuum. 2. Frequency and pitch Frequency counts oscillations per second and is measured in hertz. Higher frequencies generally sound higher in pitch. Pitch perception is more complex than one number but closely related to frequency. An instrument’s timbre also depends on the distribution of overtones alongside its fundamental frequency. 3. Amplitude and intensity Amplitude relates to the size of pressure variations. Intensity is power passing through a unit area. Decibels express logarithmic levels rather than a linear scale. Physical intensity and perceived loudness are not identical.

Echoes, Doppler shifts and applications

4. Propagation speed Sound speed depends on properties including elasticity and density; air temperature also matters. In ordinary conditions, a higher frequency does not necessarily travel faster in the same medium: its wavelength changes. 5. Reflection and motion Reflected sound can produce an echo. Relative motion between source and listener can change the detected frequency, producing the Doppler effect. An approaching and receding siren can sound different without its source producing different tones. 6. Acoustics and measurement Auditorium design considers reflection, absorption and reverberation time. Ultrasound uses frequencies above human hearing. Noise assessment depends on distance, duration and measurement method; one instantaneous decibel reading does not describe an entire exposure. Blocking transmission and reducing room echoes are also different goals.