Waves Structure and Function

Origin of a Wave  Waves always start when something moves  Motions that create waves are repeated again and again  Most of the time we call these motions “Vibrations”  These vibrations cause other motions which are transmitted as waves

Wave Types Transverse Waves Transverse waves are created through motion which is at right angles to the direction the wave moves Example: Attach a rope to a solid object, hold the other end making the rope taut Shake the rope up and down or side to side Observe the wave created

Transverse Wave Applet  This website demonstrates transverse wave motion if your computer has the proper software.  Try it. If it doesn’t work, it won’t hurt your computer.  On the Website - Click on the Applet Menu and select Waves/Transverse Waves 

Wave Types Longitudinal Waves  Longitudinal Waves are created through motion which is in line with the direction the wave moves  Example:  Attach a slinky to a solid object. Stretch it by pulling it to a distance that doesn’t exceed it’s stretch limits  Push and pull quickly to produce a longitudinal wave.

Wave Size  Large vibrations produce large waves (The Tsunami in Indonesia was produced by a large earthquake).  Wave size is called “Amplitude”  Amplitude is measured by how far the crest or the trough of the wave is away from the midpoint between the crest and the trough.  Powerful waves are large amplitude waves

Distance between Waves  Each transverse wave has a crest and a trough  The distance from the start of a crest to the end of a trough (a complete cycle of both crest and trough) is called one wavelength.  Waves that repeat within a short distance are short wavelength waves

Numbers of Waves  Slow motion vibrations produce only a few waves in a given time  Rapid motion vibrations create many waves in a short time  The number of waves created in a second is called the “Frequency” of the wave  The measure of frequency is cycles/second (sometimes called hertz)

Frequency is Inversely Related to Period  If a vibrating object sends out 6 waves in a second, the frequency of the waves is 6 cycles/second or 6 hertz  So each wave is 1/6 of a second  The time for one wave is called the period of the wave  To find the time for a wave when the frequency is known, we find the reciprocal of the frequency  Written as f = 1/p  1/6 = 0.16 seconds/wave

The Speed of a Wave  Different waves travel at different speeds  Water waves rarely exceed a few meters per second  Sound waves travel at about 340 m/s  Frequency and wavelength are related to wave speed by the relationship  Wave Speed = Frequency x Wavelength

Speed of Sound  Sound’s speed is known to be 330 m/s at 0 degrees Celsius.  At room temperature it is about 340 m/s  Anyone who has ever watched a track meet knows that you see the smoke from the starter’s gun before you hear the sound of the gun  The distance from the starter can be calculated by the time difference between the smoke and the sound  Time in seconds x 340 m/s = Distance to starter

Sound can be reflected  Visitors at the Grand Canyon are famous for shouting into the canyon and listening for the “echo” of their voice off the wall of the canyon.  These echoes are reflections that work just like light reflecting off a mirror  The angle that the sound strikes a surface (the angle of incidence) is equal to the angle that sound bounces off that surface (the angle of reflection)  These angles make it possible to design rooms that are quiet or noisy (quiet rooms absorb sound, noisy rooms reverberate) the balance of these is called the acoustics of the room

Sound can be Refracted  Bending of sound waves is called refraction  Sound travels faster in warmer air (as we have discovered)  Where air temperature changes, sound is bent toward the cooler region as it “drags” along the cooler air boundary  This “bending” of sound makes it possible to hear distant sounds clearly at times when closer sounds are not heard

Waves can Interfere With Each Other  Waves pass through other waves in the same media (think of several people talking at once).  This is not the same as saying they don’t effect each other  Waves can enhance each other (reinforce) or diminish each other (cancel)  Reinforcement occurs when one wave’s crest and trough overly another waves crest and trough  Cancellation occurs when one wave’s crest overlies the trough of another and vice versa

Music Relies on Wave Interference  One instrument’s frequency interferes with another instrument’s frequency  These interferences create cancellations (destructive interference) and reinforcements (constructive interference) repeatedly throughout a musical piece  These successive interferences are called beats  Your toe tapping is probably synchronized with these beats

Doppler Effect  A train passing a crossing sounds its horn as it approaches and departs  The apparent frequency of the train horn depends upon the motion of the train  When the train is approaching an observer the successive sound waves are compressed by the forward motion of the train  When the train is leaving the successive sound waves are rarefied (stretched) by the motion of the train