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exp8 - Acoustic Plane Waves Experiment 8 Julian Weathersby...

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Acoustic Plane Waves Experiment 8 Julian Weathersby Course: AME341 Lab Date: Tuesday, November 27, 2007 Lab Partners: Joe Lubinski
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Julian Weathersby Acoustic Plane Waves Abstract Acoustic plane waves are theoretical which can make them complicated to observe. The waves propagate and vary in only one direction, which means observing them calls for innovating ways to confining the acoustic disturbances in order to get a smooth decipherable signal. The most popular device for doing this is a loudspeaker. The vibrations from the coils propagate away from the speaker at the speed of sound and in part, produce sound. Introduction Acoustic waves have a few properties that define them. They are low amplitude pressure waves that circulate through fluid and they are governed by the wave equation, c 2 t 2 = c 2 2 x 2 (1) where is the fluctuating pressure and c is the wave speed. The wave speed is also considered the speed of sound and is given as, c = w k (2) c = grT (3) where γ is the ratio of specific heats, r is the universal gas constant, T is the temperature in Kelvin, ϖ is the frequency, and k is the wavenumber which is given by, k = 2 p l (4) where λ is the wavelength. The phase can be represented as φ = kx (5) where x is the distances, and becomes c φ x = k (6) which states that k is the rate at which φ increases with x. Materials and Methods A speaker cone, a microphone, a waveform generator, and computer-based instrumentation of a VScope and DSA were used to perform the experiment. The waveform generator was used to power the speaker and a power supply was used to power the microphone. When connecting the circuit, a LM747 dual op-amp was used and the pinouts were connected according to figure 1. The speaker was then tested along with the microphone to prevent problems during the experiment. Once everything was checked out, the microphone was placed inside the tube to complete the setup of the experiment. Using VScope, the input and output signals were observed at a frequency between 100 and 400 Hz, in this case 200 Hz was used. The microphone was also moved around the tube to see how the signal varies. A sample time trace was saved to disk at a microphone position that was about 30 cm away from the speaker cone. Using the same frequency and position, DSA was used to observe the power spectra of both the input and II
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