Sound and hearing

Sound as vibration

Sound begins with vibration. A guitar string vibrates, a speaker cone moves back and forth, vocal cords vibrate in the throat, and a drumhead flexes after being struck. These vibrations disturb nearby air molecules, creating regions of compression and rarefaction. The disturbance travels outward as a sound wave.

Sound is a longitudinal mechanical wave. It is longitudinal because air molecules vibrate back and forth in the same direction the wave travels. It is mechanical because it requires matter. Sound can travel through air, water, wood, metal, and many other materials, but it cannot travel through empty space.

Pitch and frequency

Pitch is how high or low a sound seems. Pitch is mainly related to frequency. A high-frequency sound has a high pitch, like a whistle. A low-frequency sound has a low pitch, like a bass drum.

Frequency is measured in hertz. Human hearing typically covers roughly $20 Hz$ to $20,000 Hz$, though the upper limit often decreases with age. Frequencies above human hearing are called ultrasound. Frequencies below human hearing are called infrasound.

Loudness and amplitude

Loudness is related to amplitude. A larger-amplitude sound wave creates larger pressure changes in the air and is heard as louder. However, human hearing is not perfectly simple; perceived loudness also depends on frequency and the ear's sensitivity.

Sound intensity is often described using decibels. The decibel scale is not linear, meaning a small increase in decibels can represent a large increase in physical intensity. Very loud sounds can damage hearing because they transfer too much energy to delicate structures in the ear.

Speed of sound

Sound speed depends on the medium. In air at room temperature, sound travels at about $343 m/s$. It travels faster in water and often faster still in solids because particles are closer together and interactions transmit vibrations quickly.

The wave relationship applies to sound:

$v = flambda$

If the speed of sound in air is fixed for a situation, then higher-frequency sounds have shorter wavelengths, and lower-frequency sounds have longer wavelengths.

Hearing

The ear converts sound waves into nerve signals. Sound enters the outer ear and travels through the ear canal to the eardrum. The eardrum vibrates, passing motion to tiny bones in the middle ear. These bones transfer vibrations into the fluid-filled cochlea in the inner ear.

Inside the cochlea, tiny hair cells respond to different frequencies. Their motion creates electrical signals that travel through the auditory nerve to the brain. The brain interprets these signals as sound.

Echoes and resonance

An echo occurs when sound reflects from a surface and returns after a noticeable delay. Bats and dolphins use reflected sound for echolocation, helping them locate objects. Humans use similar principles in sonar and medical ultrasound imaging.

Resonance happens when a system vibrates strongly at certain natural frequencies. A guitar body resonates with string vibrations, making the sound louder. A playground swing also shows resonance: pushing at the right rhythm builds a larger motion.

The big idea

Sound is vibration traveling through matter. Frequency affects pitch, amplitude affects loudness, and the medium affects speed. Hearing is the biological process of converting pressure waves into nerve signals. Understanding sound helps explain music, speech, echoes, acoustics, hearing protection, sonar, and medical imaging.