ch_17_physics_of_hearing.pdf - CHAPTER 17 | PHYSICS OF...

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17PHYSICS OF HEARINGFigure 17.1This tree fell some time ago. When it fell, atoms in the air were disturbed. Physicists would call this disturbance sound whether someone was around to hear it ornot. (credit: B.A. Bowen Photography)Learning Objectives17.1. Sound17.2. Speed of Sound, Frequency, and Wavelength17.3. Sound Intensity and Sound Level17.4. Doppler Effect and Sonic Booms17.5. Sound Interference and Resonance: Standing Waves in Air Columns17.6. Hearing17.7. UltrasoundIntroduction to the Physics of HearingIf a tree falls in the forest and no one is there to hear it, does it make a sound? The answer to this old philosophical question depends on how youdefine sound. If sound only exists when someone is around to perceive it, then there was no sound. However, if we define sound in terms of physics;that is, a disturbance of the atoms in matter transmitted from its origin outward (in other words, a wave), then therewasa sound, even if nobody wasaround to hear it.Such a wave is the physical phenomenon we callsound.Its perception is hearing. Both the physical phenomenon and its perception are interestingand will be considered in this text. We shall explore both sound and hearing; they are related, but are not the same thing. We will also explore themany practical uses of sound waves, such as in medical imaging.
17.1SoundFigure 17.2This glass has been shattered by a high-intensity sound wave of the same frequency as the resonant frequency of the glass. While the sound is not visible, theeffects of the sound prove its existence. (credit: ||read||, Flickr)Sound can be used as a familiar illustration of waves. Because hearing is one of our most important senses, it is interesting to see how the physicalproperties of sound correspond to our perceptions of it.Hearingis the perception of sound, just as vision is the perception of visible light. But soundhas important applications beyond hearing. Ultrasound, for example, is not heard but can be employed to form medical images and is also used intreatment.The physical phenomenon ofsoundis defined to be a disturbance of matter that is transmitted from its source outward. Sound is a wave. On theatomic scale, it is a disturbance of atoms that is far more ordered than their thermal motions. In many instances, sound is a periodic wave, and theatoms undergo simple harmonic motion. In this text, we shall explore such periodic sound waves.A vibrating string produces a sound wave as illustrated inFigure 17.3,Figure 17.4, andFigure 17.5. As the string oscillates back and forth, ittransfers energy to the air, mostly as thermal energy created by turbulence. But a small part of the string’s energy goes into compressing andexpanding the surrounding air, creating slightly higher and lower local pressures. These compressions (high pressure regions) and rarefactions (lowpressure regions) move out as longitudinal pressure waves having the same frequency as the string—they are the disturbance that is a sound wave.

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Term
Fall
Professor
JOHN DELL
Tags
Wavelength, flickr

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