Holt Science Spectrum Holt, Rinehart and Winston presents the Guided Reading Audio CD Program, recorded to accompany Holt Science Spectrum. Please open your book to the chapter titled Sound and Light. Be sure to follow along in your book and to look carefully at the illustrations while you listen. Sound and Light In this chapter, you will explore the properties of both sound and light. You will examine how musical instruments produce sound, and how sound is detected by the human ear. You will also learn about the use of sound waves in ultrasound and sonar technologies. In addition, you will discover that light has the properties of both waves and particles. This chapter includes information about why objects appear to have different colors, and how mirrors, lenses, and prisms work. ******************************** Let s begin by turning to the section titled Sound. In this section, you will examine various properties of sound, including the speed of sound, loudness, pitch, harmonics, and resonance. You will also learn how the human ear functions. And finally, you will learn how sound is used in sonar and ultrasound technologies. When you listen to your favorite musical group, you probably hear a variety of sounds. You may hear the steady beat of a drum, the twang of guitar strings, the wail of a saxophone, chords from a keyboard, or human voices. Although these sounds come from different sources, all of them have one thing in common they consist of longitudinal waves produced by vibrating objects. How does an instrument or a stereo speaker make sound waves in the air? What happens when those sound waves reach your ears? Why does a guitar sound different from a violin? Let s take a look at some of the properties of sound. Figure 1A shows what happens when a drummer hits a drum. The head of the drum vibrates up and down. Each time the drumhead moves upward, the drumhead compresses the air above itself. As the drum head moves back down again, the drumhead leaves a small region of air that has a lower pressure. As this happens over and over, the drumhead creates a series of compressions and rarefactions in the air. Figure 1B shows these compressions and rarefactions as light and dark bands. The sound waves produced by a drum are longitudinal waves. These sound waves are like the waves along a stretched spring. And like the waves on a spring, sound waves are caused by vibrations, and they carry energy through a medium. However, unlike waves along a spring, sound waves in air spread out in all directions away from the source. When sound waves from a drum reach your ear, they cause your eardrums to vibrate. Please look at the next page. Holt Science Spectrum English Audio CD Program Script Sound and Light p.1
Imagine that you are standing a few feet away from a drummer. It may seem that you hear the sound from the drum at the same time that the drummer s hand strikes the drum head. Sound waves travel very fast, but not infinitely fast. The speed of sound in air at room temperature is about 346 meters per second. Table 1 shows the speed of sound in various materials and at various temperatures. Examine Table 1. At what temperature in the table does sound travel fastest in air. If you said 100 degrees Celsius, you are correct. In what gas in the table does sound travel fastest? Of the gases listed in Table 1, hydrogen is the gas through which sound travels fastest. Which solid is not very effective for conducting sound waves? Table 1 shows that the speed of sound is slowest in rubber. The speed of sound in a particular medium depends on how well the particles of the medium can transmit the sound waves. For example, in a gas such as air, the speed of sound depends on how often the molecules of the gas collide with one another. At higher temperatures, gas molecules move faster and collide more frequently than at lower temperatures. An increase of 10 degrees Celsius increases the speed of sound in a gas by about 6 meters per second. Sound waves generally travel faster through liquids and solids than through gases. The particles in a liquid or solid are much closer together than the particles in a gas. As a result, the vibrations in liquids and solids are transferred more rapidly from one particle to the next. However, some solids, such as rubber, dampen vibrations. When this happens, sound waves travel very slowly through the material. This is why materials like rubber can be used for soundproofing. The intensity of a sound determines its loudness. How do sound waves change when you increase the volume on your stereo or television? The loudness of a sound depends partly on the energy that is contained in the sound waves. The energy of a mechanical wave is determined by its amplitude. So the greater the amplitude of a sound wave, the greater its energy, and therefore, the louder the sound. Loudness also depends on one's distance from the source of the sound waves. The intensity of a sound describes its loudness at a particular distance from the source of the sound. Turn to the next page. A sound with twice the intensity of another sound does not seem twice as loud. Humans perceive loudness on a logarithmic scale. This means that for one sound to seem twice as loud as a second sound, the first sound must be 10 times the intensity of the second sound. The relative intensity of sounds is found by comparing the intensity of one sound with the intensity of the quietest sound that a person can hear, which is referred to as the threshold of hearing. Relative intensity is measured in units called decibels. The abbreviation for decibel is d-b. Notice that the d is lower case, while the B is upper case. A difference in intensity of 10 decibels means a sound seems twice as loud. Holt Science Spectrum English Audio CD Program Script Sound and Light p.2
Figure 2 shows some common sounds and their decibel levels. Examine Figure 2. What is the intensity of sound from a vacuum cleaner? A vacuum cleaner produces a sound with an intensity of 70 decibels. What is the intensity of sound from a lawn mower? A lawn mower produces a sound with an intensity of 90 decibels. How much louder does the sound from a lawn mower seem compared to the sound from a vacuum cleaner? Because the sound from a lawnmower has an intensity 20 decibels higher than the sound from a vacuum cleaner, a lawn mower would seem four times louder than a vacuum cleaner. The quietest sound a human can hear is zero decibels. A sound of 120 decibels is the threshold of pain. Sounds louder than this can hurt your ears and give you headaches. Extensive exposure to sounds above 120 decibels can cause permanent hearing loss. The frequency of a sound determines its pitch. Musicians use the word pitch to describe how high or how low a note sounds. The pitch of a sound is related to the frequency of sound waves. Small instruments generally produce higher-pitched sounds than do large instruments. A high-pitched note is made by something vibrating very rapidly, such as a violin string or the air in a flute. A low-pitched sound is made by something vibrating more slowly, such as a cello string or the air in a tuba. In other words, high-pitched sounds correspond to high frequencies, while low-pitched sounds correspond to low frequencies. Many people, especially trained musicians, are capable of detecting subtle differences in frequency, even differences as slight as a change of 2 hertz. Let s move on to the next page. The human ear can hear sounds from sources that vibrate as slowly as 20 vibrations per second, or 20 hertz. The human ear can also hear sounds from sources that vibrate as rapidly as 20,000 vibrations per second, or 20,000 hertz. Any sound with a frequency below the range of human hearing is known as infrasound. Any sound with a frequency above human hearing range is known as ultrasound. Figure 3 shows animals that can hear frequencies of sounds that are outside the range of human hearing. Examine Figure 3. Which animal in the figure can hear sounds with the highest frequencies? If you said a dolphin, you are correct. Which animal in the figure can hear infrasound? An elephant can hear sounds with frequencies as low as 16 hertz, below the range of human hearing. Now let s further our study of the properties of sound by studying how musical instruments produce sound. Musical instruments come in a wide variety of shapes and sizes and produce a wide variety of sounds. Consider the differences in sound and in appearance between a bassoon and a banjo. Although every type of musical instrument is unique, most instruments produce sound through the vibrations of either strings, air columns, or membranes. All musical instruments rely on standing waves. When you pluck a guitar string, particles in the string start to vibrate. Waves travel out to the ends of the string, then reflect back toward the middle. The waves traveling up and down the string interact to form a standing wave on the string. If you look closely at the strings on the guitar in Figure 4, you can see that there is a simple standing wave on one of the strings. The two ends of the string are nodes. The middle of the string is an antinode. Holt Science Spectrum English Audio CD Program Script Sound and Light p.3
Please turn to the next page. Placing your finger on a string somewhere along the neck of a guitar changes the pitch of the sound that the string produces. This happens because a shorter length of string vibrates more rapidly than does a longer length. In other words, the shorter length of string vibrates at a higher frequency. You learned in the chapter titled Waves that standing waves can exist only at certain frequencies on a string. The primary standing wave on a vibrating string has a wavelength that is twice the length of the string. The frequency of this standing wave is called the fundamental frequency of the string. All musical instruments use standing waves to produce sound. In a flute, for example, standing waves are formed in the column of air inside the flute. Opening and closing holes in the flute body changes the length of the air column. This changes the wavelength and frequency of the standing waves that are produced. Figure 5 shows two-dimensional standing waves on the head of a drum. The dark-colored sand collects at the nodes of the standing wave, forming a geometric pattern. Harmonics give every instrument a unique sound. Suppose you play notes of the same pitch on a tuning fork and on a clarinet. The two notes will sound different from each other. If you listen carefully, you may be able to hear that the clarinet is actually producing sounds at several different pitches. The tuning fork produces a pure tone of only one pitch. A tuning fork vibrates only at its fundamental frequency. The air column in a clarinet vibrates at its fundamental frequency and at certain whole-number multiples of that frequency. These are referred to as harmonics. Figure 6 shows the harmonics that are present in a tuning fork and in a clarinet when each sounds the note A-natural. Notice in Figure 6 that only a single harmonic is produced by the tuning fork. The clarinet sounds different because of the combination of the various harmonics that are produced. Notice also in Figure 6 that the wave produced by a clarinet is more complex than the wave produced by a tuning fork. However, the wave produced by the clarinet has the same primary frequency as the wave produced by the tuning fork. Look at the next page. In the clarinet, several harmonics combine to make a complex wave. You observed in Figure 6, however, that this wave still has a primary frequency that is the same as the frequency of the wave produced by the tuning fork. This is the fundamental frequency, which makes the note sound a certain pitch. The unique sound of a clarinet results from the relative intensity of different harmonics in each note that it plays. Every musical instrument has a characteristic sound quality that results from the mixture of harmonics. The sound of musical instruments is amplified through resonance. When you pluck a guitar string, you can feel that the bridge and the body of the guitar also vibrate. These vibrations, which are a response to the vibrating string, are called forced vibrations. The body of a guitar is more likely to vibrate at certain frequencies called natural frequencies. Holt Science Spectrum English Audio CD Program Script Sound and Light p.4
The sound from the guitar will be loudest when the forced vibrations cause the body of the guitar to vibrate at a natural frequency. This effect is called resonance. Resonance is defined as an effect in which the vibration of one object causes the vibration of another object at a natural frequency. When resonance occurs, the sound is amplified. This happens because both the string and the guitar itself are vibrating at the same frequency. Turn to the next page. The natural frequency of an object depends on its shape, size, mass, and the material used to make the object. Complex objects such as a guitar have many natural frequencies, so they resonate well at many different frequencies. However, some musical instruments, such as an electric guitar, do not resonate well. As a result, these instruments must be amplified electronically. Now let s examine how the ear functions. The head of a drum and the strings on a guitar vibrate to create sound waves in the air. But how do you hear these waves and interpret them as different sounds? The human ear is a very sensitive organ. The ear senses vibrations in the air, amplifies them, and then transmits signals to the brain. In some ways, the process of hearing is the reverse of the process by which a drum head makes a sound. In the ear, sound waves cause membranes to vibrate. Vibrations pass through three regions in the ear. Your ear is divided into three regions: the outer ear, the middle ear, and the inner ear. Locate these three regions of the ear in Figure 7. Follow along in Figure 7 as we trace the path of vibrations through the ear. Sound waves are first funneled through the fleshy part of your outer ear and down the ear canal. The ear canal ends at the eardrum, which is a thin, flat piece of tissue. When a sound wave strikes the eardrum, it causes forced vibrations in the eardrum. The eardrum transfers these vibrations to the three small bones of the middle ear. These three bones are called the hammer, the anvil, and the stirrup. The vibrations eventually reach the stirrup, which taps on another, much smaller membrane. When this membrane vibrates, waves are created in the fluid inside the spiral-shaped cochlea. Resonance plays a role in the processes of the inner ear. The cochlea is divided along its length by a membrane called the basilar membrane. This membrane has many hair cells that vibrate at different natural frequencies. As waves pass through the cochlea, they resonate with specific parts of the membrane. Hairs on the membrane begin to vibrate, and the vibrations stimulate nerve fibers that send impulses to the brain. The brain interprets these nerve impulses as sounds with different frequencies. Let's move on to the next page, where we will discuss ultrasound and sonar. If you shout over the edge of a rock canyon, you may hear the sound reflected back to you in an echo. Like all waves, sound waves can be reflected. The reflection of sound waves can be used to determine distances and to create maps and images. Sonar uses sound waves to locate objects under water. Holt Science Spectrum English Audio CD Program Script Sound and Light p.5
How can a person on a ship measure the distance to the ocean floor? This distance may be thousands of meters from the ocean's surface. One way to measure large distances like this is to use sonar. Sonar is a system that uses reflected sound waves to determine both the location of objects and the distance to those objects.. A sonar system determines distance by measuring the time it takes for sound waves to reflect off a surface. For example, a sonar device on a ship sends a pulse of sound downward. The device measures the time, represented by lower case t, that it takes for the sound to be reflected back from the ocean floor. Using the average speed of the sound waves in water, represented by lower case v, the distance can be calculated using a form of the speed equation. Distance equals speed times time, or d equals v times t. Recall that ultrasound waves are those with frequencies above 20,000 hertz. Ultrasound waves work particularly well in sonar systems because they can be focused into narrow beams. As a result, ultrasound waves are easier to direct than other types of sound waves. Figure 8 shows a bat that uses reflecting ultrasound waves to navigate in flight and to locate insects for food. Ultrasound imaging is used in medicine. The echoes of very high frequency ultrasound waves are used to produce computerized images called sonograms. The frequencies of these waves range from one million hertz to 15 million hertz. Using sonograms, doctors can safely view organs inside the body without having to perform surgery. Sonograms can be used to diagnose problems and to guide surgical procedures. Figure 9 shows how a sonogram can be used to view a fetus in the womb of the mother. Turn to the next page. At high frequencies, ultrasound waves can travel through most materials. But some sound waves are reflected when they pass from one type of material into another type of material. How much sound is reflected depends on the density of the materials at each boundary. The reflected sound waves from different boundary surfaces are compiled into a sonogram image by a computer. X rays can also be used to produce images of the inside of the human body, but ultrasound is safer because sound waves are not energetic enough to damage cells as may X rays. To see details inside the body using sound waves, the wavelengths of the sound must be slightly smaller than the smallest parts of the object being viewed. That is why such high frequency ultrasound waves are used. According to the wave speed equation, the higher the frequency of waves in a given medium, the shorter the wavelength is. Sound waves with a frequency of 15 million hertz have a wavelength of less than 1 millimeter when they are in soft tissue. Please turn your attention to the key concepts that are listed in the Summary. The speed of sound waves depends on temperature, density, and other properties of the medium. Pitch is determined by the frequency of sound waves. Infrasound and ultrasound lie beyond the range of human hearing. The loudness of a sound depends on intensity. Relative intensity is measured in decibels. Holt Science Spectrum English Audio CD Program Script Sound and Light p.6
Musical instruments use standing waves and resonance to produce sound. The ear converts vibrations in the air into nerve impulses to the brain. Reflection of sound or ultrasound waves can be used to determine distances or to create sonograms. ********************************* Holt Science Spectrum English Audio CD Program Script Sound and Light p.7