Lab6Sound

# Lab6Sound - 6.1 Physics 2010 Sound Waves Experiment 6...

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6.1 © University of Colorado at Boulder, 2000 Physics 2010 Sound Waves Experiment 6 INTRODUCTION Sound is a pressure wave in air. When we hear a sound, we are sensing a small variation in the pressure of the air near our ear. The speed of a sound wave in air is about 340 m/s or about 5 seconds to travel one mile, and this speed depends only on the properties of the air (temperature, composition, etc.) and not on the frequency or wavelength or amplitude of the wave. Consider a sinusoidal sound wave in air with frequency f and wavelength λ . The speed v is related to f and λ by (1) v = f λ . To see where this relation comes from, think: The time it takes for one wavelength of the sound to go by is the period T, so v = λ /T. But so v = λ f . Note that as f increases, λ decreases, but the speed v stays the same. The frequency range of human hearing is about 20 Hz to 20,000 Hz. (The upper end drops as we age; for people over 60, it is about 12 kHz, while dogs can hear up to about 35 kHz.) In this experiment, we will be studying standing waves , which should not be confused with traveling waves . A traveling sinusoidal wave can be thought of as a sine (or cosine) curve that is rigidly moving to the right or to the left. The figure below shows a right-going traveling wave at two different times. The solid line is the wave at an earlier time; the dashed curve is the wave at a slightly later time. A standing wave occurs when two travelling waves of the same wavelength λ , the same frequency f, and the same amplitude A, but moving in opposite directions, pass through each other. The two traveling waves interfere, producing a standing wave which oscillates between large amplitude (when the two waves are in phase) and zero amplitude (when the waves are out of phase).

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6.2 © University of Colorado at Boulder, 2000 One way to produce two travelling waves of identical λ , f, and A, but moving in opposite directions, is to generate a travelling wave that reflects from a surface. The reflection, or echo, then combines with the original wave to produce a standing wave. The figure below shows a standing wave produced on a taut string which has its right end attached to a wall. (The amplitude of the string wave is greatly exaggerated for clarity.) The figure shows snapshots of the standing wave at two different times; the solid curve is the wave at an instant when it has maximum amplitude; the dashed curve is the wave at an instant one half-period later. The standing wave oscillates between these two extremes. Points along the standing wave where the amplitude of motion is zero are called nodes ; between the nodes are antinodes where the amplitude of the motion is a maximum. Notice that the nodes are one-half wavelength ( λ /2) apart.
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