11 - Sound 11 Sound is one of our most important forms of...

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B ASICS 269 J. Newman, Physics of the Life Sciences , DOI: 10.1007/978-0-387-77259-2_11, © Springer Science+Business Media, LLC 2008 Sound is one of our most important forms of communication. The science of sound is known as acoustics. In this chapter we learn about the physical properties of sound and how to describe sound in the language of waves. We study how sound can be pro- duced in speech as well as musical instruments, and how our ear works to detect sound and transform its energy into electrical signals to be interpreted by our brain. Depending on the relative motion of the sound source and detector, the frequency of sound is changed according to the Doppler effect, studied next in this chapter. Ultrasound is simply sound at frequencies beyond the detection capabilities of our ears. It has a number of medical and scientific applications that we study, including ultrasonic imaging, routinely used for fetal monitoring and for imaging internal organs of the body. 1. BASICS What happens when someone is speaking to you that enables you to hear them? The sound you hear is first generated by the person forcing a set of vocal chords in their larynx to vibrate while expelling air. The intonation and pitch are controlled by var- ious muscles, the tongue, lips, and mouth. Sound emitted by the person then travels through the air to your ears where in a series of remarkable steps it is converted into an electrical signal that travels to the auditory center of your brain. We interpret sound to have several properties, including loudness, pitch, and tonal qualities or tim- bre, but what is sound, how does it travel through the air, and what physical qualities does it have that correspond to the properties just mentioned? When vocal chords vibrate, they force molecules of air in the larynx to vibrate through collisions that periodically transfer momentum to the surrounding air (Figure 11.1). Consider a zone or band of air molecules in the vicinity of a vocal chord and let’s follow those particular molecules through one oscillation in Figure 11.2. The vocal chord’s motion to the right increases the local momentum of our neighboring band of molecules thus increasing the local pressure (in the figure we code the increased local momentum or pressure with a darker band). There is also a corresponding increase in the local density above the mean density as our mole- cules collide with those just to the right and a subsequent corresponding decrease in the local pressure and density below the mean of the band of molecules just to the left of the vocal chord. As momentum of our band on the right is transferred through col- lisions with neighboring molecules farther to the right and the vocal chord oscillates to the left, our band of molecules slows down, reducing its pressure and density, and a net restoring force to the left is applied from the pressure (and density) imbalance.
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11 - Sound 11 Sound is one of our most important forms of...

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