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Chapter 10 – Part I - Sound. Lesson 1: The Nature of a Sound Wave Sound is a Mechanical Wave Sound is a Longitudinal Wave Sound is a Pressure Wave Lesson 2: Sound Properties and Their Perception Pitch and Frequency The Speed of Sound Lesson 3: Behavior of Sound Waves
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Chapter 10 – Part I - Sound Lesson 1: The Nature of a Sound Wave Sound is a Mechanical Wave Sound is a Longitudinal Wave Sound is a Pressure Wave Lesson 2: Sound Properties and Their Perception Pitch and Frequency The Speed of Sound Lesson 3: Behavior of Sound Waves Interference and Beats The Doppler Effect and Shock Waves Boundary Behavior Reflection, Refraction, and Diffraction Lesson 4: Resonance and Standing Waves Natural Frequency Forced Vibration Standing Wave Patterns Fundamental Frequency and Harmonics
The Nature of a Sound Wave- objectives • Sound is a Mechanical Wave • Sound is a Longitudinal Wave • Sound is a Pressure Wave
Sound is a MechanicalWave • A wavecan be described as a disturbance that travels through a medium, transporting energy from one location to another location. A wave is created by vibratingobjects. • Mechanical wave is a wave that propagates through a mediumfrom one location to another. • The medium is simply the material through which the disturbance is moving; it can be thought of as a series of interacting particles. • The coil in a slinky • The water particles in the ocean • The people in the stadium
The tines of a tuning fork vibrates at a very high frequency. If the tuning fork that is being used corresponds to middle C on the piano keyboard, then the tines are vibrating at a frequency of 256 Hertz; that is, 256 vibrations per second.
Some tuning forks are mounted on a sound box. In such instances, the vibrating tuning fork, being connected to the sound box, sets the sound box into vibrating motion. In turn, the sound box, being connected to the air inside of it, sets the air inside of the sound box into vibrating motion. • As the tines of the tuning fork, the structure of the sound box, and the air inside of the sound box begin vibrating at the same frequency, a louder sound is produced. In fact, the more particles which can be made to vibrate, the louder or more amplified the sound.
Mechanical waves vs. electromagnetic waves. • Electromagnetic waves are waves that have an electric and magnetic nature and are capable of traveling through a vacuum. Electromagnetic waves do not require a medium in order to transport their energy. • Mechanical waves are waves that require a medium in order to transport their energy from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through a vacuum. The sound produced by the bell cannot be heard since sound cannot travel through a vacuum. ..\..\sound_en.jar
Check Your Understanding 1. A sound wave is different than a light wave in that a sound wave is a. produced by an oscillating object and a light wave is not. b. not capable of traveling through a vacuum. c. not capable of diffracting and a light wave is. d. capable of existing with a variety of frequencies and a light wave has a single frequency.
Sound as a Longitudinal Wave • Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport.
Sound is a Pressure Wave • vibrating tuning fork is capable of creating such a longitudinal wave. As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally creates high-pressure area and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.
High pressure regions are known as compressions and low pressure regions rarefactions. • The wavelength is commonly measured as the distance from one compression to the next adjacent compression. • Since a sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave.
example • A sound wave is a pressure wave; regions of high (compressions) and low pressure (rarefactions) are established as the result of the vibrations of the sound source. These compressions and rarefactions result because sound • is more dense than air and thus has more inertia, causing the bunching up of sound. • waves have a speed which is dependent only upon the properties of the medium. • is like all waves; it is able to bend into the regions of space behind obstacles. • is able to reflect off fixed ends and interfere with incident waves • vibrates longitudinally; the longitudinal movement of air produces pressure fluctuations.
example • As a sound wave travels through air, there is a net transfer of • energy, only • mass, only • both mass and energy • neither mass nor energy
example • The diagram shows a tuning fork vibrating in air. The dots represent air molecules as the sound wave moves towards the right. Which diagram below best represents the direction of motion of the air molecules? A B C D
example • An electric bell connected to a battery is sealed inside a large jar. What happens as the air is removed from the jar? • The electric circuit stops working because electromagnetic radiation cannottravel through a vacuum. • The bell's pitch decreases because the frequency of the sound waves is lower in a vacuum than in air. • The bell's loudness increases because of decreased air resistance. • The bell's loudness decreases because sound waves cannottravel through a vacuum.
example • A student plucks a guitar string and the vibrations produce a sound wave with a frequency of 650 hertz. The sound wave produced can best be described as a • transverse wave of constant amplitude • longitudinal wave of constant frequency • mechanical wave of varying frequency • electromagnetic wave of varying wavelengths
Lesson 2: Sound Properties and Their Perception • Relate Pitch and Frequency • Relate Intensityand the amplitude • Explain the factors affecting The Speed of Sound
Pitch and Frequency • A sound wave, like any other wave, is introduced into a medium by a vibrating object. • Regardless of what vibrating object is creating the sound wave, the particles of the medium through which the sound moves is vibrating in a back and forth motion at a frequency same as its source. • The frequency of a wave refers to how often the particles of the medium vibrate when a wave passes through the medium. If a particle of air undergoes 1000 longitudinal vibrations in 2 seconds, then the frequency of the wave would be ____________ vibrations per second. A commonly used unit for frequency is the Hertz (abbreviated Hz), where 1 Hertz = 1 vibration/second • The period of the sound wave is the time between successive high pressure points. The frequency is the reciprocal of the period.
The sensation of a frequency of sound is commonly referred to as the pitch. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave. • The ears of a human (and other animals) are sensitive detectors capable of detecting the fluctuations in air pressure which impinge upon the eardrum. The human ear is capable of detecting sound waves with a wide range of frequencies, ranging between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound. Ear drum
The energy of a sound wave is its intensity (loudness) • The amount of energy which is transferred to the medium is dependent upon the amplitudeof the wave. • For example, if more energy is put into the plucking of the string (that is, more work is done to displace the string a greater amount from its rest position), then the string vibrates with a greater amplitude. • The amount of energy which is transported past a given area of the medium per unit of time is known as the intensity (loudness) of the sound wave. The greater the amplitude of vibrations of the particles of the medium, the more intense that sound wave is.
As a sound wave carries its energy through a medium, the intensity of the sound wave ____________ with increasing distance from the source. The mathematical relationship between intensity and distance is sometimes referred to as an inverse square relationship. The scale for measuring intensity is the decibel scale. decreases
example • A system consists of an oscillator and a speaker that emits a 1,000.-hertz sound wave. A microphone detects the sound wave 1.00 meter from the speaker. The microphone is moved to a new fixed location 0.50 meter in front of the speaker. Compared to the sound waves detected at the 1.00-meter position, the sound waves detected at the 0.50-meter position have a different • wave speed • frequency • wavelength • amplitude
example • Light is to brightness as sound is to • color • loudness • period • speed
The Speed of Sound • Like any wave, the speed of a sound wave refers to how fast the disturbance is passed from particle to particle. While frequency refers to the number of vibrations which an individual particle makes per unit of time. Speed and frequency are different quantities. • speed = distance/time • The faster a sound wave travels, the more distance it will cover in the same period of time.
Factors Affecting Wave Speed • The speed of any wave depends upon the properties of the mediumthrough which the wave is traveling. • Typically, sound wave travels faster in denser materials. • vsolids > vliquids > vgases Sound waves travel faster in solids than they do in liquids than they do in gases.
example • What occurs when sound passes from air into water? • Its speed decreases, its wavelength becomes smaller, and its frequency remains the same. • Its speed decreases, its wavelength becomes smaller, and its frequency increases. • Its speed increases, its wavelength becomes larger, and its frequency remains the same. • Its speed increases, its wavelength becomes larger, and its frequency decreases.
Sound in air • The speed of a sound wave in air depends upon the properties of the air, namely the temperature and the pressure. • The speed of sound wave at STP is 331 m/s • At normal atmospheric pressure, the temperature dependence of the speed of a sound wave through air is approximated by the following equation: v = 331 m/s + (0.6 m/s/oC)•T • where T is the temperature of the air in degrees Celsius. • Using this equation, we can determine the speed of a sound wave in air at a temperature of 20 degrees Celsius:
Using Wave Speed to Determine Distances • At normal atmospheric pressure and a temperature of 20oC, a sound wave will travel at approximately 343 m/s; • The speed of a sound wave is slow in comparison to the speed of a light wave. Light travels through air at a speed of approximately 3 x 108 m/s; • The time delay between the arrival of the light wave (lightning) and the arrival of the sound wave (thunder) allows a person to approximate his/her distance from the storm location. • For instance if the thunder is heard 5 seconds after the lightning is seen, then sound has traveled a distance of d = v • t = 345 m/s • 5 s = 1715 m, which means the storm is about one mile away. Every 5 seconds is about a mile.
Another phenomenon related to the perception of time delays between two events is an echo. • For instance if an echo is heard 1.40 seconds after making the holler, then the distance to the canyon wall can be found as follows:
The Wave Equation Revisited • Like any wave, a sound wave has a speed which is mathematically related to the frequency and the wavelength of the wave. • Speed = Wavelength • Frequency v = f • λ • Even though wave speed is calculated using the frequency and the wavelength, the wave speed is not dependent upon these quantities. • An alteration in wavelength does not affect (i.e., change) wave speed. Rather, an alteration in wavelength affects the frequency in an inverse manner. • A doubling of the wavelength results in a halving of the frequency; yet the wave speed is not changed. • The speed of a sound wave depends on the properties of the medium through which it moves and the only way to change the speed is to change the properties of the medium.
example • A sound wave is produced by a musical instrument for 0.50 second. If the frequency of the wave is 360 hertz, how many complete waves are produced in that time period?
Check Your Understanding • An automatic focus camera is able to focus on objects by use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If a sound wave (speed = 340 m/s) returns to the camera 0.150 seconds after leaving the camera, how far away is the object?
2. On a hot summer day, a pesky little mosquito produced its warning sound near your ear. The sound is produced by the beating of its wings at a rate of about 600 wing beats per second. • What is the frequency in Hertz of the sound wave? b. Assuming the sound wave moves with a velocity of 350 m/s, what is the wavelength of the wave?
3. Doubling the frequency of a wave source doubles the speed of the waves. a. True b. False 4. Playing middle C on the piano keyboard produces a sound with a frequency of 256 Hz. Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to the note of middle C.
5. Most people can detect frequencies as high as 20 000 Hz. Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to this upper range of audible hearing. 6. An elephant produces a 10 Hz sound wave. Assuming the speed of sound in air is 345 m/s, determine the wavelength of this infrasonic sound wave.
Behavior of Sound Waves objectives • Understand Interference and Beats • Explain The Doppler Effect and Shock Waves • Recognize Boundary Behavior - Reflection, Refraction, and Diffraction
Interference and Beats • Wave interference is the phenomenon which occurs when two waves meet while traveling along the same medium. The interference of waves causes the medium to take on a shape which results from the net effect of the two individual waves upon the particles of the medium. • Constructive interference occurs if two waves are moving in the same direction at the same time meet with each other. Constructive interference result a ________ displacement. • destructive interference occurs if two waves are moving in the opposite directions at the same time when they meet. Destructive interference result in a __________ displacement. larger smaller
rarefactions • Sound is a pressure wave which consists of compressions and _______________, or a high pressure region and a low pressure region. • The interference of sound waves causes the particles of the medium to behave in a manner that reflects the net effect of the two individual waves upon the particles. • If a compression (high pressure) of one wave meets up with a compression (high pressure) of a second wave at the same location in the medium, then the net effect is that that particular location will experience an even greater pressure. This is a form of ______________ interference. • If two rarefactions (two low pressure disturbances) from two different sound waves meet up at the same location, then the net effect is that that particular location will experience an even lower pressure. This is also an example of ______________ interference. constructive constructive
Now if a particular location along the medium repeatedly experiences the interference of two compressions followed up by the interference of two rarefactions, then the two sound waves will continually reinforce each other and produce a very loud sound. The loudness of the sound is the result of the particles at that location of the medium undergoing oscillations from very high to very low pressures. locations along the medium where constructive interference continually occurs are known as __________. anti-nodes
destructive • Now if two sound waves interfere at a given location in such a way that the compression of one wave meets up with the rarefaction of a second wave, ___________ interference results. As a result, the particles would remain at their rest position as though there wasn't even a disturbance passing through them. The two sound waves will continually cancel each other and no sound is heard. Locations along the medium where destructive interference continually occurs are known as nodes.
Destructive interference of sound waves becomes an important issue in the design of concert halls and auditoriums. The rooms must be designed in such as way as to reduce the amount of destructive interference. One means of reducing the severity of destructive interference is by the design of walls, ceilings, and baffles that serve to absorb sound rather than reflect it. • The destructive interference of sound waves can also be used advantageously in noise reduction systems. Ear phones have been produced which can be used by factory and construction workers to reduce the noise levels on their jobs. Such ear phones capture sound from the environment and use computer technology to produce a second sound wave which one-half cycle out of phase. The combination of these two sound waves within the headset will result in destructive interference and thus reduce a worker's exposure to loud noise.
Musical Beats • Interference of sound waves has widespread applications in the world of music. In fact, the major distinction between music and noise is that noise consists of a mixture of frequencies whose mathematical relationship to one another is not readily noticeable. On the other hand, music consists of a mixture of frequencies which have a clear mathematical relationship between them. • Note: the diagrams on this page represent a sound wave by a sine wave. Because the variations in pressure with time take on the pattern of a sine wave. Sound is not a transverse wave, but rather a longitudinal wave.
The beat frequency refers to the rate at which the volume is heard to be oscillating from high to low volume. For example, if two complete cycles of high and low volumes are heard every second, the beat frequency is 2 Hz. The beat frequency is always equal to the difference in frequency of the two notes which interfere to produce the beats. So if two sound waves with frequencies of 256 Hz and 254 Hz are played simultaneously, a beat frequency of 2 Hz will be detected.
Doppler effect • We are most familiar with the Doppler effect because of our experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was _______; and then suddenly after the car passed by, the pitch of the siren sound was ______. That was the Doppler effect - a shift in the apparent frequency for a sound wave produced by a moving source. high low
Explaining the Doppler Effect • The Doppler effect is observed because the distance between the source of sound and the observer is changing. • If the source and the observer are approaching each other, then the distance is decreasing and the waves is compressed into the smaller distance. The observer perceives sound waves reaching him or her at a more frequent rate (_______ pitch). • If the source and the observer are moving apart, then the distance is increasing. the waves can be spread apart; the observer perceives sound waves reaching him or her at a less frequent rate (____pitch). • It is important to note that the effect does not result because of an actual change in the frequency of the source. The source puts out the same frequency; the observer only perceives a different frequency because of the relative motion between them. high low • The Doppler effect is a shift in the observed frequency and not a shift in the actual frequency at which the source vibrates.
Shock Waves and Sonic Booms • If a moving source of sound moves at the same speed as sound or faster than sound, then shock waves will be produced.
Check Your Understanding 1. Suppose you are standing on the passenger-loading platform of the commuter railway line. As the commuter train approaches the station, what pitch or changes in pitch will you perceive as the train approaches you on the loading platform?
Boundary Behavior • As a sound wave travels through a medium, it will often reach the end of the medium and encounter another medium through which it could travel. The behavior of a wave upon reaching the end of a medium is referred to as boundary behavior. • There are essentially three possible behaviors which a wave could exhibit at a boundary: • ____________ (the bouncing off of the boundary), • _____________ (the bending around the obstacle without crossing over the boundary), • ________________ (the crossing of the boundary into the new material or obstacle), • ________________ (occurs along with transmission and is characterized by the subsequent change in speed and direction). reflection diffraction transmission refraction
Reflection - echo or a reverberation. • A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. The reflected sound wave has a very short delay, it seems to the person that the sound is prolonged. You might observe reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower. • Echoes occur when a reflected sound is perceived as a second sound rather than the prolonging of the first sound.
