Waves are created by disturbances which cause vibrations.

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2 Wave Motion Waves are created by disturbances which cause vibrations. Vibrations produce a back-and-forth type motion called an oscillation. wave simulation The number of vibrations (or waves) that occur in one second is defined as the frequency of the wave. The time it takes to complete one vibration (back-and-forth) is called a period.

3 Parts of a typical wave: Crest wavelength Trough Amplitude Crest highest point of the wave. Trough lowest point of the wave. Wavelength the distance between identical parts of a wave. Amplitude height of the wave measured from the horizontal reference point.

4 Wave Motion The frequency of the wave is the inverse of the wave period. Example: If 5 waves go by in 1 sec, the frequency is 5 Hz The period will be 1/5 th of a second. Frequency = waves per second frequency = 1 period Period = seconds per wave Period = 1 frequency

5 Practice: The rocking chair pictured here is oscillating at a rate of 1 wave per second. What is it s frequency? What is the period? Back and forth Back and forth Back and forth

6 Practice: The Sears Building in Chicago sways back and forth at a frequency of about 0.1 Hz. What is its period of vibration?

7 Practice: Stringed instruments produce sounds by vibrations of the strings. If a harp string makes one vibration in 2.3 x 10-3 seconds, what the frequency of the sound?

8 The speed of the wave is dependent on the material through which it travels. For example: A Sound wave in air travels around 340 m/s, but in water, the wave travels much faster about 1500 m/s, and in Iron 5130 m/s!!! However, all waves follow this relationship: Speed of the wave = frequency x wavelength =

9 For Practice: A bumblebee can flap its wings at a rate of 130 flaps per second. If the speed of sound in air is 340 m/s, calculate the length of the wave created.

10 For Practice: While on the beach, you observe that the wave crests reach the shore about every 2 seconds. If the distance between the waves is 5 meters, at what speed are the waves traveling? The waves are really slow today..

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12 Types of Waves wave simulation Transverse waves the motion of the medium is at right angles to the direction of the wave. Direction of the wave motion The boat pictured here moves up and down while the wave moves forward. A8A5IMkW-Kg

13 Other examples of transverse waves include: People doing the wave in a large stadium.

14 TKF6nFzpHBU Strings on a musical instrument

15 Light and radio waves are transverse waves. And so are other electromagnetic waves.

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17 Longitudinal waves The particles in the medium move back and forth in the same direction as the wave

18 Wavelength the distance between similar points on the wave Transverse wave Longitudinal wave

19 Sound waves are longitudinal waves.

20 Interference Interference is caused by two or more waves overlapping. Interference can increase the amplitude of a waves, decrease the amplitude of the waves, or cancel each other out. When the waves increase in amplitude, it is called constructive interference. When the waves decrease in amplitude, it is called destructive interference.

21 Constructive interference Destructive interference

22 Can you find the points where these waves cancel each other out?

23 Standing waves When two waves intersect each other, sometimes a standing wave is produced. Standing waves look like this: The higher the frequencies of the waves, the more nodes that will form.

24 Standing waves are produced in musical instruments, whistles, and when you blow in a pop bottle. Toot! Toot!

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26 Sound Waves Sound waves are produced by a vibrating object, which forms a longitudinal wave in the surrounding medium. Ex. A guitar string, a bell, a tuning fork, your vocal chords The frequency of the vibration determines the pitch. The higher the frequency, the higher the pitch. The lower the frequency, the lower the pitch. Do Re Mi

27 The average human ear can sense frequencies that range from 20 Hz to Hz. Speech frequency range 100 Hz to 7500 Hz Music frequency range 50 Hz to Hz

28 Loudness (Intensity) Loudness is determined by the amplitude of the wave. The intensity of the sound can be measured by an oscilloscope. An oscilloscope can show the wave patterns for sound vibrations and provide an intensity level reading in units called decibels. Source of Sound Level (db) Jet engine 140 Pain threshold 120 Loud rock music 115 Average factory 90 Busy street traffic 70 Normal Speech 60 Close whisper 20 Normal breathing 10 Hearing threshold 0

29 Materials that transmit sound: A sound wave is a longitudinal wave. In order for the wave to continue, particles must be present. Sound will not travel in a vacuum. (There are no particles present.) Sound will travel through air by moving the air molecules back and forth. Sound travels through liquids and solids louder and faster than it travels in air.

30 Speed of sound in common materials: Material Speed (m/s) Air 0 C 331 Air 25 C 346 Helium 0 C 972 Hydrogen 0 C 1290 Water 1490 Alcohol 1140 Rubber 54 Copper 3560 Iron 5130

31 For practice: How far away is the storm cloud if the thunder is heard 3 seconds after the lightening is seen?

32 For practice: Why does your voice sound higher when you breath in the helium from a balloon? While Irish eyes are smiling.

33 Natural frequency & forced vibrations All materials have a natural frequency at which the molecules vibrate. This explains why forks and knives clatter, wind chimes chime, and rubber mallets thud. And drums drum.

34 Resonance When an object is forced into vibration, the intensity of the natural frequency wave becomes greater. These are examples of natural frequencies being forced into vibration: -Musical instruments -Opera singer s breaking glass trick -The TacomaNarrows Bridge disaster musical glasses UGftsRH7A2w Galloping Gerdie

35 Interference in Sound Waves Constructive interference waves add together and sound intensity increases. Ex. Surround sound Destructive Interference waves cancel each other out and sound intensity decreases. Ex. Dead spots in an auditorium

36 Beats A periodic variation in loudness that occurs when there is both constructive and destructive interference. This occurs when two frequencies are very close. Pattern: loud, soft, loud, soft, loud, soft The combs shown here illustrate this concept. Destructive (Dark areas) Constructive (Light areas)

37 The number of beats heard can be calculated by finding the difference in the two interfering frequencies. Ex. A 100 Hz sound wave and a 102 Hz sound wave will produce a 2 Hz beat frequency. This concept is helpful when tuning musical instruments. If beats are heard, then the instruments are not in tune and need to be adjusted. Most people can hear up to a frequency difference of 10 beats.

38 Practice: While tuning a violin a frequency of 3 beats is heard? After slightly tightening one of the strings, 2 beats are heard. What should the person tuning the violin do?

39 Practice: Two notes are sounding, one which is 440 Hz. If a beat frequency of 5 Hz is heard, what it the frequency of the other note?

40 The Doppler Effect The Doppler effect occurs when the source of a wave is in motion. Sound waves travel in all directions from the source. The waves are equally spaced according to the frequency of the sound.

41 If the source is moving, the pattern of the waves might look similar to this: Direction of motion Waves behind become spread out. A lower frequency is heard. Waves in front become compressed. A higher frequency is heard.

42 Calculating Doppler Effect Frequencies The Basic Equation: f = f s (v + v o )/(v v s ) where v o = velocity of observer v s = velocity of source v = speed of sound f s = frequency of the sound Helpful Hints: v o = + if moving toward the source v o = - if moving away from the source v s = + if moving toward the observer v s = - if moving away from the observer

43 Let s Review: Two types of waves Draw a tranverse wave Draw an arrow showing the direction of the movement of particles in the medium in that wave. Which direction do the particles move in a compressional wave? What typ of wave is a sound wave? Draw another wave above that wave with a higher frequency. Draw another wave below with a lower frequency. Draw a diagram showing Doppler Effect.

44 Reflection A wave strikes an object and bounces off it. All types of waves reflect Examples: Echo Light from a mirror Water waves hitting an object

45 SONAR Reflection of a Sound Wave Sound travels through Water at a speed of 1500 m/s.

46 A sonar pulse returns in 3 seconds from a sunken ship that is directly below. How deep is the sunken ship?

47 SONAR Sound Navigation and Ranging

48 Ultrasound

49 Read p Do # 4-7 on page 326