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8/23/2005 6: EXPERIMENTS WITH SOUND PULSES Sound waves propagate (travel) through air at a velocity of approximately 340 m/s ( 1115 ft/s ). As a sound wave travels away from a small source of sound such as a vibrating speaker or tuning fork, its amplitude decreases as its energy is spread over larger and larger area. These waves also reflect from surfaces and in reflection lose amplitude through absorption. The purpose of these experiments is to explore these ideas by using short pulses of sound. We will generate these short pulses of sound by sending short voltage pulses to a speaker. Physics Concepts Involved: The velocity of sound in air depends on the temperature of the air. The formula: v = 331.5 m/s + (0.6 m/s) T (1) gives the velocity in m/s (meters per second) where temperature T is in degrees Celsius. Sound reflects from a flat surface much as light reflects from a mirror. In analogy to light one can draw rays of sound where the incoming and reflected angles are equal. From this view you can calculate the time it takes for a sound wave from the speaker to bounce from a flat surface and back to the mike. Assume the room temperature to be 20 degrees Celsius. A. Velocity of Sound: A convenient arrangement for these experiments is one where the directional speaker is mounted on a ring stand about 0.5 m above the lab table. The computer microphone must be mounted on a ring stand in fixed position for careful measurements. See the drawing. Distance = 2 L board mike speaker
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1. Simple velocity measurement: Look at the “ VAR iable OUT put” of the 4001 Pulse Generator on channel 1 of your oscilloscope. Convenient settings of the pulse generator are: power and run buttons pushed in, pulse spacing = 10ms , pulse width = 100 s , spacing vernier = 1, and width vernier = 1. Adjust the amplitude of the pulse to approximately 0.5 volt . When you are happy with your pulse, attach your speaker in parallel with the oscilloscope. You should hear a (gentle) buzzing. Loud buzzing is unnecessary. To measure the velocity of sound, you need only measure the distance the sound travels and the time it takes to get there. 2. To measure the time, record a series of clicks on your computer (?-0). Zoom in on the recorded signal so you can see a close pair of pulses (the first pulse represents the sound signal from the speaker passing the microphone, the second is the reflected sound). Point the cursor at the first pulse and click the mouse, then point it at the corresponding part of the reflected pulse and click again. The time (in seconds) and signal level (in volts) of each point you clicked are recorded in the bottom window. To find the time between the pulses, subtract the two times. The distance (in meters) between the speaker and microphone is also needed. Calculate
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