Propagation of mechanical waves in media. Uses of ultrasound and infrasound in daily life and technology
The spreading of an oscillation through a medium is called a wave; mechanical waves do not travel through a vacuum because they need a medium whose particles interact. In a wave the particles oscillate but the matter itself is not carried along, while energy is transmitted. If the particles oscillate along the direction of the wave, it is longitudinal (it travels in gases, liquids and solids); if they oscillate perpendicular to it, it is transverse (only in solids, because it needs elasticity of shear). The distance between the two nearest points oscillating in the same phase is the wavelength λ: λ = υT = υ/ν. Humans hear waves of about 20 Hz to 20 000 Hz as sound; below 20 Hz is infrasound, above 20 kHz is ultrasound. The speed of sound is about 340 m/s in air (a little less at 0 °C), about 1500 m/s in water and ~5000 m/s in steel. Ultrasound is used in sonar (echolocation), flaw detection in metal, medical scans (ultrasonography) and cleaning; infrasound in monitoring earthquakes and natural events.
Only in class: oscillate one end of a long spring (Slinky) or rope to watch a transverse wave; compress and release the spring to see a longitudinal wave. Do not use loud devices; protect your hearing.