Experiment 10: Introduction to Waves

Transcription

1 N ame Partner(s): Experiment 10: Introduction to Waves Objectives Equipment Pre-Lab M easure the period and frequency of sound w aves Computer with LoggerPro and Labview, speakers, microphone A list of Activities is to be completed before this Lab. Completing these Pre-Lab assignments w ill expedite your progress during the lab period. A s part of your preparation you are also expected to read through the lab manual before the lab. You will be pressed for time during the lab. Since successful completion of all lab activities counts towards your final lab grade it will be important to be well prepared by doing Pre-Lab assignments and reading the entire lab before attending the lab. Points earned today Lab Challenge Total Instructor Initials Physics 1200 X-1

2 Pre-Lab for LAB#10 Complete the following before you attend class: Below is a graph of displacement vs. time for an object that experienced SH M. 1. The period (T) is the time it takes the object to complete one full oscillation. Calculate the period for the motion shown below. For better precision and accuracy, try measuring the time it takes for more than one period of the oscillation, and then dividing by the number of oscillations to calculate the period. Period = s Physics 1200 X-2

3 List of Today s Activities Introduction Problem solving Lab Activity Brief review of key concepts and Pre-Lab questions Discuss concept questions Properties of sound w aves The speed of sound measuring echoes in a tube The frequency of sound measuring the musical scale Test your ears pitch, peak sensitivity and frequency range Physics 1200 X-3

4 Activity 1 Concept Questions Laboratory These schematics illustrate the positions of molecules during the motion of two different sound waves through air at room temperature. Which wave: - Has a higher frequency? A B Equal - Has a longer wavelength? A B Equal - Has a higher velocity? A B Equal Using the scale given, estimate the wavelength in A. The motion of one molecule at a moment in time is indicated by the arrow in A. The wave in A is moving: Right Left Can t tell Sketch the horizontal position of this molecule as a function of time, averaged over many wave cycles. You may want to consult this link: Physics 1200 X-4

5 Activity 2 Lab Activity M easure the Speed of Sound A microphone is placed at the mouth of a tube whose other end is closed. A sharp noise sounds nearby. Which of the follow ing best indicates the microphone s signal versus time? A ssume sound travels through air at a speed of ~340 m/ s. 1.7m snap microphone signal A B C time (ms) Your instructor will now set up such a tube in the class to directly measure the speed of sound in air. By snapping your fingers, you will trigger LoggerPro to start recording the microphone s response as a function of time. The length of the tube will be varied for each group by adjusting the position of a stopper at one end. Record the time betw een echoes, and the tube s length. Use this information to calculate the speed of sound in the tube. Tube Length (m) = Time (ms) = Speed of sound (m/ s) = Class average (m/ s) = Question: If the tube is instead filled w ith He, which is much lighter than N 2 or O 2, w ould the speed you measured increase or decrease? A ssume the density of gas is comparable. (Hint: compare sound traveling in a gas of bowling balls to sound traveling in a gas of ping pong balls; for a given force, which molecule would have a higher acceleration?) Physics 1200 X-5

6 Activity 3 Lab Activity M easuring Sound Frequency The common Tempered Scale divides our audible range into a series of octaves, each of w hich is divided into 12 intervals. On a piano, these are the 7 w hite keys and 5 black keys. Your instructor will play a series of notes on an electronic keyboard. Open the LoggerPro template microphone.cmbl, which will allow you to monitor your microphone s response as each note is played. Start the program by clicking on the green Collect button at the top of the window. Your microphone is operating correctly if you see background noise on the screen. You and your partner(s) w ill each measure the period of a note, and then average your results. By measuring the period of the microphone signal, you can calculate the frequency, f, of each note (i.e. the note s pitch ). Period = T frequency = 1/ T Each group will measure a different note, so that the class as a whole will be able to fill in the following table: Note Period (s) Frequency (H z) A ccepted freq (H z) C 4 (4 th octave) C # D 4 D # E 4 F 4 F # G 4 G # A 4 A # B 4 C 5 (5 th octave) Physics 1200 X-6

7 Activity 4 Lab Activity Test Your H earing Range The nominal audible hearing range is from 20 Hz - 20 khz, although this varies considerably from person to person, and with age. Sound with frequencies outside this range cannot generally be heard by humans. In addition, the apparent loudness of a sound also depends on its frequency. Open the Labview program output sound.vi, which will allow you to generate a single tone from the laptop s speakers. To run the program, hit Ctrl-R, or the right arrow button at the top of the window. To stop, hit Ctrl-, or one of the stop buttons at the top of the window. You can adjust the sound frequency by clicking on the slider control, or by using the digital indicator. Play a tone w ith increasing frequency, starting below 20 H z. When you begin to hear the sound, adjust the volume of the speaker so that it is not too loud. A s you increase the frequency, keep the volume setting the same. Write dow n the approximate frequency which sounds the loudest, and the maximum frequency you can hear: M y peak frequency = My maximum audible frequency = End of Lab 10 When you are finished, close both LoggerPro and Labview. Do not save any changes. Physics 1200 X-7

8 Handy info links to useful web pages, activities, further information etc. Wave motion: / w w w.kettering.edu/ ~drussell/ Demos/ w aves/ w avemotion.html Test your pitch: / jakemandell.com/ tonedeaf/ The musical scale on a piano keyboard: / w w w.vibrationdata.com/ piano.htm / music/ lessons/ unit01.htm Physics 1200 X-8