Waves in air, fluids and solids

A wave transfers energy from one place to another without transferring matter — the wave itself moves, but the substance it travels through does not travel with it. A water wave, for example, moves across the surface, but a floating object just bobs up and down rather than being carried along.

• •Increasing the frequency of a wave (for a fixed wave speed) decreases its wavelength, and vice versa — since v = f × λ, if v stays constant, f and λ vary inversely.
• •Period and frequency are inversely related: a smaller period means a higher frequency.

Measuring wave speed: for sound in air, make a sound roughly 50 m from a solid wall and time the echo, then use speed = distance ÷ time (using the total distance travelled there and back); alternatively use two microphones connected to a data logger a known distance apart, and time the delay between the sound reaching each one. For ripples on a water surface, a stroboscope flashing at the same frequency as the waves can make them appear frozen, so wavelength can be measured directly and combined with the known frequency using v = f × λ.

When a wave reaches the boundary between two different materials, it can be reflected, absorbed, or transmitted (pass through).

A smooth surface produces a single, clear reflected wave (specular reflection). A rough surface scatters the reflected wave in many different directions (diffuse reflection) — this is why rough surfaces appear matt rather than shiny, even though reflection is still happening at a microscopic level.

A wave is transmitted through a material if the material allows it to pass through and re-emerge on the other side (it may still change direction, by refraction, while doing so). A wave is absorbed if the material's particles take in the wave's energy without re-emitting it as the same kind of wave — often the energy is converted to heat instead.

Sound waves are longitudinal pressure waves that can travel through solids, liquids, and gases, causing the particles of the medium to vibrate.

1. 1The outer ear collects sound and channels it down the ear canal as a pressure wave.
2. 2The sound wave reaches the eardrum, a tightly stretched membrane. Compressions push the eardrum inward and rarefactions pull it outward, so the eardrum vibrates at the same frequency as the sound wave.
3. 3Tiny bones (ossicles) connected to the eardrum vibrate at the same frequency and amplify the vibration, passing it on to the fluid in the inner ear (the cochlea).
4. 4As the fluid vibrates, tiny hair cells lining the cochlea move. Each hair cell is tuned to a particular frequency, so different sounds make different hair cells move the most.
5. 5When a hair cell moves enough, it sends an electrical impulse along a nerve to the brain, which interprets it as sound.

When ultrasound reaches a boundary between two different materials, part of it is reflected back and the rest is transmitted onward. Because the speed of ultrasound in a given material is known, timing how long it takes reflected pulses to return lets you calculate distances — this is used for medical imaging (e.g. scanning a foetus non-invasively) and for detecting flaws like cracks inside solid materials (a crack reflects some ultrasound earlier than the surrounding solid material does).

Sonar works the same way: a ship sends a pulse of ultrasound downward, and the time taken for it to reflect off the seabed (or a shoal of fish) and return is used to calculate the depth.

On the opposite side of the Earth from a large earthquake, only P-waves are ever detected — S-waves are never detected there. Since S-waves can't travel through a liquid, this is evidence that part of the Earth's core is liquid.