Sound Waves and Their Properties
How vibrations become the sounds we hear every day
How vibrations become the sounds we hear every day
Sound is actually invisible to our eyes, even though we hear it constantly. Put your hand on your throat and say "hello" out loud. Feel that vibration? That is your vocal cords vibrating — they are producing sound waves that travel through the air and eventually reach other people's ears. Every sound you have ever heard started with something that was vibrating, or moving back and forth very quickly.
When something vibrates, it pushes the air molecules around it. Those molecules then push the molecules next to them, and so on, creating a chain reaction that travels outward like ripples in a pond when you throw a stone. The molecules themselves do not travel across the room — they just push their neighbors and then return to where they were. It is the pattern of disturbance, not the molecules themselves, that moves. This is why a sound can travel across a room even though the air itself is not moving as a wind.
There are two main types of mechanical waves. The wave created when you shake a rope from side to side is called a transverse wave — the rope moves up and down while the wave travels left to right. Sound waves are different. In a sound wave, the molecules in the air compress together and then spread apart in the same direction the wave is traveling. This type of wave is called a longitudinal wave.
Imagine squeezing a spring toy (like a Slinky) on one end and then releasing it. You would see sections of the spring compress together and then stretch apart as the compression travels down the spring. That is exactly what a sound wave looks like in the air. The compressions are areas where air molecules are crowded together, and the rarefactions are areas where they are spread apart. Sound waves cannot travel through a vacuum because there are no molecules to compress and stretch — they need a medium, which can be a gas, a liquid, or a solid.
Sound travels through different materials at different speeds. In general, sound moves fastest in solids, slower in liquids, and slowest in gases. This is because molecules in solids are packed very tightly together, so they can pass the vibration along quickly. In air at room temperature, sound travels at about 343 meters per second (roughly 767 miles per hour). In water, it travels about 1,482 meters per second. In steel, it travels about 5,960 meters per second.
Think about how you can hear a train approaching by putting your ear to the track, even if it is far away. The sound travels much faster through the steel rail than through the air. That is why soldiers in old movies put their ears to the ground to hear hoofbeats from far away — the sound travels faster and farther through the earth. This principle is also why your voice sounds different when you speak underwater in a swimming pool — the sound travels differently through water than through air.
The loudness of a sound is determined by the amplitude of the wave. Amplitude is basically how big the vibrations are — how hard you push the air molecules. When you pluck a guitar string gently, it vibrates only a small amount and makes a soft sound. Pluck it hard and it vibrates much more, creating a louder sound.
Scientist measure sound loudness in decibels, abbreviated dB. The quietest sound a human can typically hear is 0 dB. A normal conversation is about 60 dB. A lawnmower is around 90 dB. A rock concert can reach 110 to 120 dB. At 85 dB and above, sounds can start to damage your hearing if you are exposed to them for long periods. That is why it is important to wear hearing protection at loud events or when using power tools.
Pitch is how high or low a sound seems to us. A whistle sounds high-pitched. A drum sounds low-pitched. Pitch is determined by the frequency of the sound wave — how many vibrations per second. Frequency is measured in units called Hertz (Hz). One Hertz equals one vibration per second. A sound at 440 Hz vibrates 440 times per second. Humans can generally hear sounds between about 20 Hz and 20,000 Hz.
High-pitched sounds like a squeaky voice have a high frequency — many vibrations per second. Low-pitched sounds like a bass drum have a low frequency — fewer vibrations per second. If you have ever watched a car or a siren move past you, you have experienced the Doppler effect — the pitch seems to drop suddenly as the source passes you. That happens because when the source is moving toward you, the sound waves get bunched up, increasing the frequency. When it moves away, the waves spread out, decreasing the frequency.
Some animals use sound to navigate and find food in complete darkness. This is called echolocation. Bats are the most famous echolocators. They emit high-pitched clicks or chirps that humans usually cannot hear because the frequency is above 20,000 Hz (this is called ultrasound). These sounds bounce off objects and return to the bat as an echo. The bat's brain processes this echo to figure out where objects are — even objects as small as a mosquito.
Dolphins use the same technique underwater. They emit clicks that travel through water and bounce back from objects, allowing them to "see" with sound even in murky ocean water where they cannot see well. Some species of whales use low-frequency sounds that can travel thousands of kilometers through the ocean to communicate with each other. Scientists have even developed sonar technology, inspired by animal echolocation, to map the ocean floor and detect submarines.
Ultrasound, which is simply sound with a frequency too high for humans to hear (above 20,000 Hz), has many practical applications beyond animal echolocation. In medicine, doctors use ultrasound machines to look inside the human body. The most common use is for checking on unborn babies during pregnancy — parents get to see their baby and doctors can check that everything is developing normally, all without any surgery or radiation.
Ultrasound is also used to clean delicate objects like jewelry and eyeglasses. The high-frequency vibrations create tiny bubbles in a cleaning solution that scrub the surface of objects. Engineers use ultrasound to detect cracks in metal structures like bridges and airplanes, a technique called non-destructive testing. Some phone screens can even use ultrasonic sensors to detect your fingerprints through the display glass.
Your ears are remarkable organs, but they can be damaged, and the damage is often permanent. The cells in your inner ear that detect sound are delicate and can be destroyed by exposure to extremely loud sounds. Once those cells die, they do not grow back. That is why many older adults have hearing loss — years of accumulated damage from loud noises.
The good news is that hearing loss from loud sounds is almost entirely preventable. Wear earplugs or earmuffs when you are around loud noises like concerts, fireworks, construction sites, or when riding a motorcycle. Turn down the volume on headphones and earbuds — if someone sitting next to you can hear your music, it is too loud. Give your ears a break after being in a noisy environment. And remember that sounds above 85 dB can cause damage over time, so it is worth protecting your hearing now so you can enjoy music and conversation for decades to come.