1.34 Intensity and Loudness of Sound

Transcription

1 1.34. Intensity and Loudness of Sound Intensity and Loudness of Sound Define intensity of sound and relate it to loudness. Compare decibel levels of different sounds. Identify factors that affect sound intensity. A friend whispers to you in a voice so soft that she has to lean very close so you can hear what she s saying. Later that day, your friend shouts to you from across the gymnasium. Now her voice is loud enough for you to hear her clearly even though she s several meters away. Obviously, sounds can vary in loudness. It s All About Energy Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of the sound waves. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (db). Decibel Levels The Figure 1.70 shows decibel levels of several different sounds. As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Therefore, a 30-decibel quiet room is 10 times louder than a 20-decibel whisper, and a 40-decibel light rainfall is 100 times louder than the whisper. High-decibel sounds are dangerous. They can damage the ears and cause loss of hearing. 118

2 Chapter 1. Energy FIGURE 1.70 Q: How much louder than a 20-decibel whisper is the 60-decibel sound of a vacuum cleaner? A: The vacuum cleaner is 10,000 times louder than the whisper! Amplitude and Distance The intensity of sound waves determines the loudness of sounds, but what determines intensity? Intensity results from two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. Amplitude is a measure of the size of sound waves. It depends on the amount of energy that started the waves. Greater amplitude waves have more energy and greater intensity, so they sound louder. For a video demonstration of the amplitude and loudness of sounds, go to this URL: rqfgyd2ukw As sound waves travel farther from their source, the more spread out their energy becomes. You can see how this works in the Figure As distance from the sound source increases, the area covered by the sound waves increases. The same amount of energy is spread over a greater area, so the intensity and loudness of the sound is less. This explains why even loud sounds fade away as you move farther from the source. Q: Why can low-amplitude sounds like whispers be heard only over short distances? A: The sound waves already have so little energy that spreading them out over a wider area quickly reduces their intensity below the level of hearing. Summary Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity, or amount of energy, in sound waves. The unit of intensity is the decibel (db). As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Intensity of sound results from two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. 119

3 1.34. Intensity and Loudness of Sound FIGURE 1.71 This diagram represents just a small section of the total area of sound waves spreading out from a source. Sound waves actually travel away from the source in all directions. Vocabulary decibel (db): Unit of intensity of sound. intensity: Measure of the amount of energy in sound waves, which is determined by the amplitude of the waves and how far they have traveled and spread out from the source of the sound. loudness: How a listener perceives the intensity of sound. Practice Review sound intensity, loudness, and decibels at the following URL. Then answer the Check Your Understanding questions at the bottom of the Web page. Be sure to check your answers. und/u11l2b.cfm Review 1. Define loudness and intensity of sound. How are the two concepts related? 2. What is the unit of intensity of sound? 3. At what decibel level do sounds start to become harmful to the ears and hearing? 4. Relate amplitude and distance to the intensity and loudness of sound. 120

4 Chapter 1. Energy 1.35 Frequency and Pitch of Sound Define the pitch of sound. Relate the pitch of sound to the frequency of sound waves. Identify infrasound and ultrasound. A marching band passes you as it parades down the street. You heard it coming from several blocks away. Now that the different instruments have finally reached you, their distinctive sounds can be heard. The tiny piccolos trill their bird-like high notes, and the big tubas rumble out their booming bass notes. Clearly, some sounds are higher or lower than others. High or Low How high or low a sound seems to a listener is its pitch. Pitch, in turn, depends on the frequency of sound waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time. High-pitched sounds, like the sounds of the piccolo in the Figure 1.72, have high-frequency waves. Low-pitched sounds, like the sounds of the tuba Figure 1.72, have low-frequency waves. For a video demonstration of frequency and pitch, go to this URL: Can You Hear It? The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound. Infrasound is too low-pitched for humans to hear. Sounds with frequencies above 20,000 hertz are called ultrasound. Ultrasound is too high-pitched for humans to hear. Some other animals can hear sounds in the ultrasound range. For example, dogs can hear sounds with frequencies as high as 50,000 Hz. You may have seen special whistles that dogs but not people can hear. The whistles produce 121

5 1.35. Frequency and Pitch of Sound FIGURE 1.72 sounds with frequencies too high for the human ear to detect. Other animals can hear even higher-frequency sounds. Bats, like the one pictured in the Figure 1.73, can hear sounds with frequencies higher than 100,000 Hz! FIGURE 1.73 Q: Bats use ultrasound to navigate in the dark. Can you explain how? A: Bats send out ultrasound waves, which reflect back from objects ahead of them. They sense the reflected sound waves and use the information to detect objects they can t see in the dark. This is how they avoid flying into walls and trees and also how they find flying insects to eat. Summary 122 How high or low a sound seems to a listener is its pitch. Pitch, in turn, depends on the frequency of sound waves. High-frequency sound waves produce high-pitched sounds, and low-frequency sound waves produce lowpitched sounds. Infrasound has wave frequencies too low for humans to hear. Ultrasound has wave frequencies too high for humans to hear.

6 Chapter 1. Energy Vocabulary infrasound: Sound with a frequency below the range of human hearing (less than 20 hertz). pitch: How high or low a sound seems to a listener. ultrasound: Sound with a frequency above the range of human hearing (greater than 20,000 hertz). wave frequency: Number of waves that pass a fixed point in a given amount of time. Practice At the following URL, complete the interactive module to review and test your knowledge of the frequency and pitch of sound. Review 1. What is the pitch of sound? 2. How is the pitch of sound related to the frequency of sound waves? 3. Define infrasound and ultrasound. 123