chapter4 - Physics 1240 Hall Chapter 4 Notes 1 Focus...

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Physics 1240 Hall Chapter 4 Notes 1 Focus questions and learning goals Chapter 4 focus questions: 1. How does sound travel? 2. How does the physics of sound propagation aFect our perception of music? Chapter 4 learning goals. After studying this chapter, you should be able to: 1. Predict how a given sound will bounce from a wall (and decide what properties of the wave you need). 2. Describe beats, explain how it would be used musically, predict the beat frequency (given details of the two pitches involved). 3. Predict which wavelength will bend more when coming out of a hole or around a corner. 4. Explain in simple terms the advantage (or disadvantage) to using a small versus a large opening for an instrument, 5. Explain in simple terms the eFects of using a small (or large) roughness on the wall. 6. Explain what physical features of a band shell help improve sound quality for the audience. 7. Predict which of two band shell shapes (or position of musicians) would make the sound louder for audience members in various particular spots. 8. Predict the (change in) pitch of sound arising from a moving source, or heard by a moving listener. This chapter contains some stuF that we won’t focus on (like refraction), and the most essential points are a little hidden. So I recommend reading it in a funny order, starting with 4.1 and 4.5, and then going back to 4.2 (which we’ll cover only very loosely), and then 4.3 which is kind of a cool application! Section 4.4 is fun too, but really not all that important of a point for sound and music, we’ll just talk about it brie±y without worrying about the formulas. 2 Refections To understand the ²rst section of chapter 4, remember that sound is a wave, a pressure wave in a medium (usually air). Sound arises from the steady propagation of a disturbance. The disturbance travels with a well-de²ned speed. It travels outwards from sources (in an expanding sphere of in±uence) Sound will refect or echo from hard, ±at surfaces. If the surface is bumpy, sound waves will re±ect every which way, in many diFerent directions; this is called diFuse refection . If the surface is smooth and hard, the sound will re±ect much like you would expect billiard balls to re±ect oF a wall (see ³ig 4.1a in your text). 1
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Physics 1240 Chapter 4 notes We need to discuss this fgure some more, though. What the heck are those arrows in Fig 4.1? To explain, frst go way back to Fig 1.6a. I± you ±ocus on the curved (but mostly “up and down”) lines in 1.6a, those are the ones we’re used to thinking about to represent sound waves—they look like the lines o± constant high pressure in the PhET simulation you used ±or a homework. The arrows (pointing to the right-ish in that picture) are “rays o± sound”, they represent the direction o± motion o± sound. This is a rather abstract idea: it’s not something physical! It just shows you how the sound is moving . Start at 1.6a, and try to mentally visually sound “²owing with the
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This note was uploaded on 08/24/2010 for the course PHYS 1240 taught by Professor Holland,murray during the Spring '08 term at Colorado.

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chapter4 - Physics 1240 Hall Chapter 4 Notes 1 Focus...

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