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chapter4

# 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 affect 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 effects 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 stuff 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 briefly without worrying about the formulas. 2 Reflections To understand the first 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-defined speed. It travels outwards from sources (in an expanding sphere of influence) Sound will reflect or echo from hard, flat surfaces. If the surface is bumpy, sound waves will reflect every which way, in many different directions; this is called diffuse reflection . If the surface is smooth and hard, the sound will reflect much like you would expect billiard balls to reflect off a wall (see Fig 4.1a in your text). 1

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Physics 1240 Chapter 4 notes We need to discuss this figure some more, though. What the heck are those arrows in Fig 4.1? To explain, first go way back to Fig 1.6a. If you focus 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 of constant high pressure in the PhET simulation you used for a homework. The arrows (pointing to the right-ish in that picture) are “rays of sound”, they represent the direction of motion of 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 “flowing with the arrows”, with high pressure fronts following low pressure fronts. (Does that picture make sense?) So now go back to Fig 4.1a. Those incoming arrows represent a sound wave which is heading in
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chapter4 - Physics 1240 Hall Chapter 4 Notes 1 Focus...

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