Sound is made by vibration. Any object that vibrates, whether it’s a guitar string, a vocal cord, or a slamming door, pushes against the air molecules around it. Those molecules bump into their neighbors, which bump into their neighbors, creating a chain reaction of pressure changes that ripples outward as a wave. When that wave reaches your ear, your brain interprets it as sound.
How Vibrations Become Sound Waves
Picture a tuning fork after you strike it. The two prongs move back and forth rapidly, and each time they push outward, they squeeze the nearby air molecules closer together. This creates a tiny zone of high pressure called a compression. When the prongs swing back the other direction, they leave a gap of low pressure behind them, called a rarefaction. These alternating zones of high and low pressure spread outward from the tuning fork in all directions, like ripples in a pond.
This is why sound is called a mechanical wave. It needs physical matter to travel through. Each air molecule doesn’t actually travel from the source to your ear. Instead, it nudges the molecule next to it, which nudges the next one, passing energy along the chain. In a vacuum, with no molecules to carry the vibration, there is no sound.
What Determines Pitch and Volume
Two properties of a sound wave shape what you actually hear: frequency and amplitude.
Frequency is how many complete waves pass a point each second, measured in hertz (Hz). One hertz equals one wave per second. Higher frequency means the vibrating object is oscillating faster, producing more waves in the same amount of time. Your brain perceives higher frequency as higher pitch, like a whistle compared to a bass drum. Human hearing spans roughly 20 Hz to 20,000 Hz, though that upper limit drops with age.
Amplitude is the intensity of the wave, essentially how far the air molecules are being pushed from their resting position. Greater amplitude means more energy in the wave, and your brain perceives that as louder sound. Volume is measured in decibels. Normal conversation sits around 60 decibels. Repeated exposure at or above 85 decibels can cause permanent hearing loss. Even lower levels, around 55 decibels sustained over a full day, have been linked to stress responses, increased blood pressure, and cardiovascular effects.
Sound Below and Above Human Hearing
Sound waves exist well outside the range your ears can detect. Infrasound refers to frequencies below 20 Hz. Earthquakes, volcanic eruptions, and large weather systems produce infrasound. These waves can travel enormous distances because their long wavelengths lose energy slowly. Ultrasound covers everything above 20,000 Hz. Medical imaging is the most familiar application: ultrasound devices bounce high-frequency waves off internal structures to create images of organs or a developing fetus.
How Sound Travels Through Different Materials
Sound doesn’t just move through air. It travels through any material that has molecules to carry the vibration, and it moves at very different speeds depending on what it’s passing through. In air at room temperature (20°C), sound travels at about 343 meters per second (roughly 767 miles per hour). In water, it jumps to about 1,482 meters per second. In steel, it reaches approximately 5,900 meters per second, more than 17 times faster than in air.
The reason comes down to how tightly bonded the molecules are and how quickly they snap back into position after being disturbed. In a solid like steel, atoms are packed closely and held together by strong forces. When one atom gets pushed, it transfers energy to the next atom almost instantly. In a gas, molecules are spread far apart and loosely connected, so passing energy between them takes longer.
Density also plays a role, but in the opposite direction you might expect. If two materials have similar stiffness, the denser one will actually carry sound more slowly because heavier molecules take more energy to set in motion. Aluminum and gold have nearly the same elastic stiffness, but sound moves about twice as fast through aluminum because aluminum is roughly seven times less dense.
How Temperature and Humidity Affect Sound
Warm air carries sound faster than cold air. When air heats up, its molecules move more energetically and collide more readily, speeding up the transfer of vibration. This is why sound can seem to carry differently on a hot summer day compared to a cold winter night.
Humidity also has a subtle effect. Water vapor molecules are lighter than the nitrogen and oxygen molecules they replace in the air. Adding moisture to air effectively lowers its average molecular weight, which increases the speed of sound slightly. The effect is modest in everyday conditions, but it matters in precision work like acoustic thermometry, where scientists use the speed of sound to measure temperature.
How Your Ear Converts Sound to Hearing
The journey from pressure wave to conscious perception involves a remarkable chain of conversions. Sound waves funnel into your ear canal and hit the eardrum, a thin membrane that vibrates in response. Those vibrations pass to three tiny bones in the middle ear (the smallest bones in your body), which amplify the motion and transmit it to a fluid-filled, snail-shaped structure called the cochlea in the inner ear.
Inside the cochlea, the vibrations ripple through fluid, causing thousands of microscopic hair cells to bend. These hair cells are the critical link between mechanical motion and electrical signaling. When they bend, ion channels at their tips open, triggering an electrical impulse. Different hair cells respond to different frequencies: cells near the base of the cochlea pick up high-pitched sounds, while cells near the tip respond to low-pitched ones. The electrical signals travel along the auditory nerve to the brain, where they’re decoded into the sounds you recognize, all within milliseconds of the original vibration.
These hair cells do not regenerate. Once they’re damaged by loud noise, aging, or certain medications, the hearing loss they cause is permanent. This is why the 85-decibel threshold matters: it marks the point where sustained exposure begins to physically destroy the structures your ears depend on.