Three Kinds of Waves Transverse waves  Medium moves perpendicular to the wave direction  Can only happen in media where the molecules are connected to one another  Amplitude is how far molecules are displaced from their normal positions  Longitudinal Waves  Medium moves back-and-forth, parallel to the direction of the wave  Can happen in any kind of medium  Amplitude is still how far molecules are displaced from their normal positions - just in a different direction  Earthquake Waves  S (shake?) waves are transverse  P (pressure?) waves are longitudinal  P waves travel faster than S waves.  Torsional Waves  Medium twists around the direction of travel  Can happen only in solids Two Ways to Picture a Wave Snapshot  All points at one time  As a camera would see it  Or, your eyes, if you blink once  Frog-on-a-post  Imagine a nearsighted, obsessive-compulsive talking frog sitting on a post in a pond as a wave rolls by. It reports the height of the water on its post every second Interference in Sound - Beats Do you remember ever seeing…  A school bus windshield wipers drifting in and out of phase?  Turn signal drifting in and out phase in a line of cars waiting to turn left?  These are slow versions of BEATS for sound waves  The 'wah-wah-wah' sound when two very similar frequencies sound together  The result of alternating constructive and destructive interference  The beat frequency is simply the difference between the two frequencies  The slower the 'wahs', the more nearly in tune  It's rare to hear beats in choral singing, as it's hard for singers to sustain sounds at precise frequencies, but instrumental musicians often hear beats and can tune to each other byeliminating them. A Lot About the Doppler Effect Named for Christian Doppler  Austrian mathemetician and physicist  He published his work on the effect in 1842  The Doppler Effect  The apparent shift in frequency due to the relative motion of a source of waves or an observer  Could come from the motion of the source  Fast police car or ambulance going by you  Or from the motion of the observer  Driving by a loud party or parked car with loud stereo  Applies to sound, water, light, and light's 'relatives'  Measuring Speeds of Stars  Spectral lines of fast moving objects shift toward blue if approaching, red if receding  Measuring Rotations of Stars, Planets  One edge of the sun comes toward us while the other edge receds.  Even though we only see a star as one point of light, the edges of the star move in opposite directions, so we see red and blue shifts at the same time. The spectral lines are broadened.  Doppler saw all of this coming  "It is almost to be accepted with certainty that this will in the not too distant future offer astronomers a welcome means to determine the movements and distance of such stars which…until this moment, hardly presented the hope of such measurements and determinations."  Radio waves show the effect, too.  Police radar units detect a change in frequency in the reflected waves which is converted to your speed  Used in weather forecasting  Ordinary radar measures cloud density, rain  Doppler radar measures wind speeds, too  Bow Waves and Sonic Booms  A bow wave in water is exactly analogous to a sonic boom in air.  In both cases, the speed of the source exceeds the speed of the waves in that medium.  The wave energy 'piles up' along the big V edges  Sonic booms are often misunderstood  Not breaking through a barrier just once, but dragging the shock wave along behind What's So Amazing About Our Sense of Hearing? We can hear a wide range of frequencies Most books say 20 - 20,000 Hz  Hard to specify  Individual differences, of course  Age-related roll-off of high frequencies  Relationship between frequency and intensity  Hearing test  You noticed that many seemed much louder  Partly due to frequency response of your computer speakers or headphones  Mostly due to the basics of human hearing  We're not very sensitive to extremes in frequency  We're much more sensitive to high-middle frequencies  WHY?  We can hear an incredible range of intensities  From zero to 120 dB  A decibel is 1/10 of a bel, named for the phone inventor Alexander Graham Bell  So how does it work?  A change of one bel is 10 times more intense  Like the pH scale for acids and bases  Or the Richter scale for earthquakes  A 6.0 earthquake is 10 times more powerful than a 5.0 quake  The most intense earthquake in Northeast Ohio occurred on January 31, 1986  It measured 5.0 on the Richter scale  The Japanese quake on March 11, 2011 measured 9.0  That was 10,000 times as intense as NE Ohio's worst earthquake  A bel is such a large unit that we use decibels, instead  10 decibels means 10 times the sound intensity  How much more intense is 40 dB than 20dB  From 20 dB to 30dB is ten times…  From 30 dB to 40 dB is ten more times…  For a total of 100 times more intense  (Not 2 times, or 20 times)  How much louder is 40 dB than 20 dB  Experiments show that we perceive change in loudness as the square foot of change in intensity So, 100 times more intense would sound about 10 times louder  The details on decibels  The relative intensity level, beta, in dB, between two sounds is:  B = 10 dB [log (I/Io)], where I is the intensity of a sound in W/m2 and Io is the intensity of the 'other' or 'original' sound  Our hearing is unbelievably sensitive  And speaking of the 'threshold of hearing,' we can barely hear a trillionth of a watt, spread out over a square meter!  That's, perhaps, a ten-thousandth of a trillionth of a watt actually going in!  With a sound of that miniscule intensity, your eardrum only moves about the diameter of one air molecule!  We can fuse sounds for 1/10 seconds  Normal reflections off walls arrive at different times, but you hear them as one sound  To hear an echo, sound must travel for more than 1/10 of a second  Since the speed of sound in air is about 340 m/s, sound travels 34 meters in 1/10 sec  To hear your own echo, sound has to travel from you to a reflecting surface and back  The total distance must be at least 34 m, so  You must be at least 17 m or 55 feet away  We can discriminate