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Unformatted text preview: Physics 1240 Hall Chapter 5 Notes 1 Focus questions and learning goals Chapter 5 focus questions: 1. What physically changes when a sound becomes louder? 2. How can this be measured? Chapter 5 learning goals. After studying this chapter, you should be able to: 1. Decide whether energy, power, or intensity is the relevant quantity in a basic physical situa- tion, and/or relate them. 2. Explain why it makes sense that intensity of a given sound is independent of the area of the detector, but energy absorbed from that sound does depend on the area. 3. Describe why and how sound intensity level (decibels) is related to, but different from, inten- sity. 4. Describe what a given change in decibels sounds/feels like as you change the intensity. 5. Explain qualitatively why we use decibels. What does this tell us about our perception of intensity? 6. Explain how the decibels change if you change the intensity, and vice versa. 7. Describe the impact on your ears (how much more energy flow?) when raising a sound by some number of decibels. 2 Intro to sound loudness Weve skirted around this topic all term. Weve talked about loudness and intensity and sound volume as though they were synonyms...but theyre not. Its time to start making this story a little more precise! This chapter may feel a little more mathy, but most of the ideas (and math) are not that hard, and pretty similar to things weve already done. (With one exception, which Ill explain carefully!) Sound intensity is a big dealin terms of your experience with music, and the health of your ears. Its a subtle topic too. First of all, theres the physics (what can you measure about a sound wave?) and then theres the perception (what does it sound/feel like to you?). Weve dealt with this before, e.g., the difference between frequency (a measurable aspect of a periodic wave) and pitch (the perception that is closely associated with frequency). In this chapter, were going to focus on the physics, and start to connect it to perception; in the next chapter well talk more about human perception. (So well save the ear mechanics for Chapter 6 too!) This chapter will be mostly about measurable quantities. 1 Physics 1240 Chapter 5 notes There are two obvious ways to talk about sound strength. One is the amplitude of the sound wave. This is what weve discussed before, it simply tells you how much overpressure (or underpressure) there is for each wiggle of the sound wave. (See the quick review in the next section.) Amplitude is measured in units of pressure, N/m 2 (Newtons [of force] per square meters [of area]). But it turns out that a perhaps more useful measure, and relatively easy to measure, is the intensity of the sound wave. Thats a measure of how much energy the wave carries with it....
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- Spring '08