Intensity and Loudness of Sound Jean Brainard, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required)
To access a customizable version of this book, as well as other interactive content, visit www.ck12.org AUTHOR Jean Brainard, Ph.D. CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform. Copyright 2014 CK-12 Foundation, www.ck12.org The names CK-12 and CK12 and associated logos and the terms FlexBook and FlexBook Platform (collectively CK-12 Marks ) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the CC License ), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: April 15, 2014
www.ck12.org Chapter 1. Intensity and Loudness of Sound CHAPTER 1 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 1.1 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. 1
www.ck12.org FIGURE 1.1 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: http://www.youtube.com/watch?v=i 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 1.2. 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 2 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.
www.ck12.org Chapter 1. Intensity and Loudness of Sound FIGURE 1.2 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. http://www.physicsclassroom.com/class/so 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. References 1... CC-BY-NC-SA 3.0 2... CC-BY-NC-SA 3