Chapter 17 Sound Sound and Hearing. Properties of Sound Waves 1/20/2017. Pearson Prentice Hall Physical Science: Concepts in Action

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1 Pearson Prentice Hall Physical Science: Concepts in Action Chapter 17 Sound Standing Waves in Music When the string of a violin is played with a bow, it vibrates and creates standing waves. Some instruments, like flutes, create standing waves in a column of air. The Behavior of Waves Resonance The Behavior of Waves resonance - object is made to vibrate by absorbing energy at its natural frequency If enough energy is absorbed, the object can vibrate so strongly that it breaks apart Sound and Hearing Objectives: 1. Describe the properties of sound waves and explain how sound is produced and reproduced 2. Describe how sound waves behave in applications such as ultrasound and music 3. Explain how relative motion determines the frequency of sound as the observer hears What causes sound? Every sound is produced by an object that vibrates. For example, your friends voices are produced by the vibrations of their vocal cords, and music from a carousel and voices from a loudspeaker are produced by vibrating speakers. Properties of Sound Waves Sound waves are compressional / longitudinal waves They have compressions are rarefactions Many behaviors can be explained by the properties of speed, intensity, loudness, frequency and pitch At 20 C in dry air, the speed of sound is 342 m/s Sound waves travel fastest in solids, slower in liquids and slowest in gases 1

2 Sound Waves Sound waves are compressional waves. A compressional wave is made up of two types of regions called compressions and rarefactions. Sound Waves When a radio speaker vibrates outward, the nearby molecules in the air are pushed together to form compressions. Sound Waves When the speaker moves inward, the nearby molecules in the air have room to spread out, and a rarefaction forms. Sound Waves As long as the speaker continues to vibrate back and forth, compressions and rarefactionsare formed. Traveling as a Wave Compressions and rarefactions collide with air molecules as energy is transferred. Compression- Air molecules close together Rarefaction- Air molecules spread apart A series of compressions and rarefactions travel to your ear that we interpret as sound Moving Through Materials Most sounds you hear travel through air to reach your ears. If you ve ever been swimming underwater and heard garbled voices, you know that sound also travels through water. 2

3 Moving Through Materials Sound waves can travel through any type of matter solid, liquid, or gas. The matter that a wave travels through is called a medium. Sound waves cannot travel through empty space. The Speed of Sound in Different Materials The speed of a sound wave through a medium depends on the substance the medium is made of and whether it is solid, liquid, or gas. The Speed of Sound in Different Materials Sound travels the slowest through gases, faster through liquids, fastest through solids. The Speed of Sound in Different Materials The denser the material the the closer the molecules are together which makes it easier and faster to transfer sound The Speed of Sound in Different Materials Speed of sound doesn t depend on the loudness. All sounds travel through a medium at the same speed. A Model for Transmitting A line Sound of people passing a bucket is a model for molecule transferring the energy of a sound wave s 3

4 A Model for Transmitting Sound When the people are far away from each other, like the molecules in gas, it takes longer to transfer the bucket of water from person to person. A Model for Transmitting Sound The bucket travels quickly down the line when the people stand close together The closer the particles, the faster they can transfer energy from particle to particle. Temperature and the Speed of Sound As the temperature of a substance increases, its molecules move faster. Molecules more likely to collide with each other increasing sound speed. Click image to view movie Human Hearing Vocal cords and mouths move in many different ways to produce various kinds of compressional waves. Your ears and brain work together to turn the compressional waves caused by speech, music, and other sources into something that has meaning. Human Hearing 1 st Ear gathers the compressional waves. 2 nd Ear amplifies the waves. Gathering Sound Waves The Outer Ear 3 rd Waves are converted to nerve impulses that travel to the brain. 4 th Brain decodes and interprets the nerve impulses. 4

5 For us to hear, the outer ear gathers & focuses sound into the middle ear where the vibrations are received and amplified The inner ear uses nerve endings to sense vibrations and send signals to the brain Sound is recorded by converting sound waves into electronic signals that can be processed and stored Sound is reproduced by converting electronic signals back to sound waves Gathering Sound Waves The Outer Ear Outer ear, Middle ear, and Inner ear. The outer ear is where sound waves are gathered. Gathering Sound Waves The Outer Ear The eardrum is a tough membrane about 0.1 mm thick. Then incoming sound waves vibrate the eardrum Converting Sound Waves The Inner Ear cochlea filled with liquid and contains tiny hair cells. Converts sound to nerve impulses Question 1 What type of wave is a sound wave? A sound wave is a compressional or a longitudinal wave. Question 2 In which of the following environments would sound waves not travel? A. at altitudes of 10,000 15,000 m B. in solid aluminum C. on the Moon D. under water 5

6 The answer is C. Sound waves require a medium through which to travel. So, sound waves cannot travel through empty space. Question 3 Which region of the ear amplifies sound waves? A. ear drum B. inner ear C. middle ear D. outer ear The answer is C. The bones of the middle ear amplify sound waves. Intensity and Loudness What happens to the sound waves from your radio when you adjust the volume? The notes sound the same as when the volume was higher, but something about the sound changes Def: intensity is the rate at which a wave s energy flow through a given area The decibel (db) compares the intensity of different sounds Def: loudness is a physical response to the intensity of sound modified by physical factors As intensity increases, loudness increases Loudness also depends on the health of your ears and how your brain interpret sounds Def: pitch is the frequency of a sound as you perceive it Intensity and Loudness The difference is that quieter sound waves do not carry as much energy as louder sound waves do. 6

7 Intensity and Loudness The amount of energy a wave carries corresponds to its amplitude. For a compressional wave, amplitude is related to the density of the particles in the compressions and rarefactions. Intensity and Loudness strongly vibrating objects makes sound waves with tight, dense compressions. Weak vibrations make sound waves with less dense compressions. Intensity and Loudness The density of particles in the rarefactions behaves in the opposite way. It is important to remember that matter is not transported during the compression and rarefaction of a compression wave only energy is transported. Matter compresses and expands as the wave of energy passes through the matter. Intensity Intensity- amount of energy that flows through a certain area. Intensity Volume Increases = Intensity Increases Volume decreases = Intensity decreases Intensity Intensity = how far away sound can be heard. If you and a friend whisper a conversation, the sound waves you create have low intensity and do not travel far. 7

8 Intensity Decreases with Distance Intensity influences how far a wave will travel because some of a wave s energy is converted to other forms of energy when it is passed from particle to particle. low intensity = less distance sound travels. High intensity = farther distance sound travels Loudness Loudness is the human perception of sound intensity. When sound waves of high intensity reach your ear, they cause your eardrum to move back and forth a greater distance than sound waves of low intensity do. Loudness The bones of the middle ear convert the increased movement of the eardrum into increased movement of the hair cells in the inner ear. A Scale for Loudness Sound intensity is measured in decibels (db) As a result, you hear a loud sound A Scale for Loudness Pitch If you were to sing a scale, your voice would start low and become higher with each note. Pitch- how high or low a sound seems to be. pitch is related to the frequency of the sound waves. 8

9 Frequency and Pitch High Pitch = High Frequency Low Pitch = Low frequency Frequency and Pitch This figure shows different notes and their frequencies. A healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz. How Sound Waves Behave & Relative Motion Ultrasound is used in a variety of applications, including sonar and ultrasound imaging Def: Sonar is a technique for determining the distance to an object under water The pitch of a sound is determined by the frequency Higher pitch means faster frequency As the source of the waves moves it changes the frequency (this is the Doppler Effect) As it moves toward you the pitch rises and away from you the pitch lowers = Doppler Effect Ultrasonic and Infrasonic Waves ultrasonic - waves above 20,000 Hz, High pitched waves humans cannot hear Ultrasonic and Infrasonic Waves Ultrasonic waves are used in medical diagnosis and treatment. They also are used to estimate the size, shape, and depth of underwater objects. Ultrasonic and Infrasonic Waves Infrasonic waves frequencies below 20 Hz too low for most people to hear. 9

10 Ultrasonic and Infrasonic Waves These waves are produced by sources that vibrate slowly, such as wind, heavy machinery, and earthquakes. Echolocation At night, bats swoop around in darkness without bumping into anything. Their senses of sight and smell help them navigate. Echolocation is the process of locating objects by emitting sounds and interpreting the sound waves that are reflected back. Sonar Sonar system that uses the reflection of underwater sound waves to detect objects. Ultrasound in Medicine One of the important uses of ultrasonic waves is in medicine. Using special instruments, medical professionals can send ultrasonic waves into a specific part of a patient s body. Ultrasound in Medicine Reflected ultrasonic waves are used to detect and monitor conditions such as pregnancy, certain types of heart disease, and cancer. Ultrasound Imaging Like X rays, ultrasound can be used to produce images of internal structures. The sound waves reflect off the targeted organs or tissues, and the reflected waves are used to produce electrical signals. A computer program converts these electrical signals into video images, called sonograms. 10

11 Treating with Ultrasound Sometimes small, hard deposits of calcium compounds or other minerals form in the kidneys, making kidney stones. Ultrasonic treatments are commonly used to break them up. Treating with Ultrasound Bursts of ultrasound create vibrations that cause the stones to break into small pieces. These fragments then pass out of the body with the urine. Question 1 The process of locating objects by emitting sounds and interpreting the sound waves that are reflected back is called. A. acoustics B. echolocation C. infrasonic tracking D. reverberation The answer is B. Echolocation is used by some animals such as bats and dolphins. Question 2 Which of the following is not a use of ultrasonic technology in medicine? A. examination of the gallbladder B. examination of bones C. fetal monitoring in utero D. kidney stone treatment The answer is B. Ultrasound is not as useful as X rays for examining bones, because hard tissues absorb ultrasonic waves instead of reflecting them. 11

12 Question 3 What is sonar? Sonar is a system that uses the reflection of underwater sound waves to detect objects. The Doppler Effect Doppler effect- change in pitch or wave frequency due to a moving wave source Moving Sound As a race car moves, it sends out sound waves in the form of compressions and rarefactions. The race car creates a compression, labeled A. Compression A moves through the air toward the flagger standing at the finish line. Moving Sound By the time compression B leaves the race car, the car has moved forward. Because the car has moved since the time it created compression A, compressions A and B are closer together than they would be if the car had stayed still. Moving Sound As a result, the flagger hears a higher pitch. A Moving Observer The Doppler effect happens any time the source of a sound is changing position compared with the observer. It occurs no matter whether it is the sound source or the observer that is moving. The faster the change in position, the greater the change in frequency and pitch. 12

13 Using the Doppler Effect Radar guns use the Doppler effect to measure the speed of cars. Weather radar also uses the Doppler shift to show the movement of winds in storms, such as a tornado. Question 1 Each unit on the scale for sound intensity is called a. The answer is decibel, abbreviated db. Question 2 Sound frequencies above 20,000 Hz are called waves. A. infrasonic B. infrared C. subsonic D. ultrasonic The answer is D. Subsonic and infrasonic are waves with frequencies below 20 Hz. Question 3 Describe the Doppler effect. 13

14 The Doppler effect is the change in pitch due to a moving wave source. effect Sound, Perception, and Music Key Question: How is musical sound different than other types of sound? \ Most musical instruments vary pitch by changing the frequency of standing waves Def: resonance is the response of a standing wave to another wave of the same frequency Musical instruments often use resonance to amplify sound 12.3 Sound, Perception, and Music A single frequency by itself does not have much meaning. The meaning comes from patterns in many frequencies together. A sonogram is a special kind of graph that shows how loud sound is at different frequencies. Every person s sonogram is different, even when saying the same word Music The pitch of a sound is how high or low we hear its frequency. Though pitch and frequency usually mean the same thing, the way we hear a pitch can be affected by the sounds we heard before and after. Rhythm is a regular time pattern in a sound. Music is a combination of sound and rhythm that we find pleasant. Most of the music you listen to is created from a pattern of frequencies called a musical scale. 14

15 12.3 Consonance, dissonance, and beats Harmony is the study of how sounds work together to create effects desired by the composer. When we hear more than one frequency of sound and the combination sounds good, we call it consonance. When the combination sounds bad or unsettling, we call it dissonance Consonance, dissonance, and beats Consonance and dissonance are related to beats. When frequencies are far enough apart that there are no beats, we get consonance. When frequencies are too close together, we hear beats that are the cause of dissonance. Beats occur when two frequencies are close, but not exactly the same Harmonics and instruments The same note sounds different when played on different instruments because the sound from an instrument is not a single pure frequency. The variation comes from the harmonics, multiples of the fundamental note. Application: Sound from a Guitar Accoustics When an orchestra stops playing, does it seem as if the sound of its music lingers for a couple of seconds? reverberation The echoing effect produced by many reflections of sound During an orchestra performance, reverberation can ruin the sound of the music. 15

16 Accoustics Acoustics- study of sound Some scientists and engineers specialize in acoustics to make concert halls and other theaters pleasant EXAMPLES!! Accoustics They know that soft, porous materials can reduce excess reverberation, so they might recommend that the walls of concert halls be lined with carpets and draperies. 16