Sound Waves Part II

Compressional Waves Compressional waves do not have crests and troughs as the waves don’t go up and down The crests are similar to the compressions of sound waves The troughs are similar to the rarefactions of sound waves

A wavelength in a compressional wave includes one compression and one rarefaction

Compression In the compression, the medium particles (air for us) get pushed together This makes them closer together and this push moves forward through the air and to our ears

Rarefaction In the rarefaction, the particles of the medium are pulled further apart, creating a short “hole” in the medium This “hole” follows along behind the compression

Hearing The sequence of compression and rarefaction carries the sound vibrations through a medium Repeating series of waves make the sounds we hear Demo

Interference Waves can interfere with each other, creating a new wave pattern where they overlap If you watch the wake from your boat on a wavy day on the lake, you can see interference

This interference can be: – constructive – the two waves add together to make a larger wave – destructive – the two waves cancel each other, in part or whole, and create a shorter wave Interference Demo

Sound Interference It is easy to see transverse waves interfering In the case of sound waves, both constructive and destructive interference also take place – Constructive interference will make a sound louder – Destructive interference will make a sound quieter

Seismic Waves When portions of the earth’s crust slip along or push against each other, there will be an earthquake The location that the earthquake happens, usually deep in the crust, is the focus The location on the surface above the focus is known as the epicenter Waves travel out from the focus carrying the vibrations we feel in an earthquake video

Body Waves The two waves that travel through the “body” of the earth are P and S waves The P waves are compressional waves with a speed of 5-7 km/s The S waves are transverse waves with a speed of 3- 4 km/s Measuring the time between the P and S waves helps determine how far away the earthquake was (like watching lightning and listening for thunder)

P waves can travel through solids, liquids, and gases, so they can travel throughout the earth S waves can not travel through liquids, so oceans will stop their travel, as will the liquid outer core of the earth

Surface Waves Surface waves do almost all of the damage caused by an earthquake One form of surface wave shakes the surface side-to-side, while another shakes the surface up and down (like shaking a rug) Surface waves travel at 2-4 km/s

Additional Threats If an earthquake happens underwater, and it causes part of the crust to either lift or drop suddenly, the earthquake can cause a tsunami These “tidal waves” can travel at speeds of 500 mph in the open ocean and reach heights of 100 m when reaching shallow water of a shoreline