VERNIER SOFTWARE CBL MICROPHONE WITH THE TI CBL AND TI-85 OVERVIEW This document briefly describes the MICPHN85.85p program for use with the Vernier Software CBL Microphone (part number MCA-CBL), the TI-85 calculator and the TI Calculator-Based Laboratory (CBL) System. This program will also work with Vernier’s MPLI Microphone (part number MCA- M) when equipped with the CBL-8MC adapter. The program will not work with Vernier’s ULI Microphone (part number MCA-U). Programs for the ULI microphone can be found in the “CBL System Experiment Workbook”. The primary uses of this program are 1) to produce waveform graphs of sound pressure as a function of time and 2) to monitor the frequency of a sound signal. If waveform data is collected, it is possible to model the data with a trigonometric function. The program defines equation Y1 with an appropriate form. We have found that the best waveforms occur when the peak voltage from the microphone is in the range of 0.5 to 1.0 volts. If the signal is much smaller than that, the quality of the waveform decreases. If this is the case, you may need to produce a louder sound or move the source closer to the microphone. When the waveforms are displayed by this program, you can use the arrow buttons to trace the waveform and check the voltage of the signal noting the maximum or minimum y values. GENERAL DESCRIPTION OF THE PROGRAM To begin, connect the CBL Microphone to channel 1 on the CBL. Run the MICPHN85.85p program. (Note: the name of the program will appear as "MICPHN85" on the calculator). After introductory screens, the following main menu will appear: Main Menu WAVEF=waveform TRIGR=waveform(trigr) FREQU=frequency QUIT=quit If you choose the WAVEF option, a screen will appear prompting you to hold a sound source close to the microphone. After you press the [ENTER] key on the TI-85, the CBL will quickly sample the sound source and you will be promoted to press [ENTER] to get the data from the CBL. A display of sound pressure vs. time will be displayed on the calculator. After you press [ENTER] from the graph screen, you will be asked whether you want to repeat the data collection. If not, you will return to the Main Menu. During sampling the time is stored in list L1 and voltage which is proportional to sound level is stored in list L2. The second option in the Main Menu, TRIGR, also produces a waveform but this time the CBL is placed in a trigger mode. It will not start to collect data until the voltage reaches 0.1 V. Once the trigger occurs, sampling begins and a graph of sound pressure vs. time is displayed. After you select this option, a screen will prompt you to press [ENTER] to make the CBL ready. The CBL will then wait for a trigger. The CBL will display “READY” in its display. Any sound producing a large enough signal will now cause the CBL to collect data. After the data is collected, the CBL will display DONE. Press [ENTER] to get the data from the CBL and display the graph. Again the time is stored in L1 and sound level is stored in list L2. The FREQU option in the Main Menu will monitor a sound source held near the microphone and display its frequency in hertz. After you select the option, you will be instructed to press the [ENTER] key to prepare the CBL to collect data. The next screen will prompt you to move a sound source close to the microphone. When the sound intensity reaches a certain level, the CBL will be triggered and the signal will be monitored. If the sound is weak, you may have to hold the source very close to the microphone. After a short time, the frequency will be displayed on the calculator. The frequency will be stored in list L1. After you press [ENTER], you will be given the opportunity to repeat the data collection. EDITING THE TIME BETWEEN SAMPLES When waveforms are collected, this program is set up to collect data as fast as possible. The minimum sample time in this situation is 0.000164 s. You may want to change this value. This is done by editing the Sample and Trigger Command (command 3) in the MICPHN85 program. If you are collecting waveforms using the WAVEF option, locate this command, {3,.000165,99,0,0,0,0,0,1,0}->L6 within the Lbl 1 section. If you are collecting waveforms using the TRIGR option, locate this command {3,.000165,99,2,1,.1,0,0,1,0}->L6, within Lbl 2 section. To increase the time between samples, edit the second number in either of these commands. For help in editing TI-85 programs, refer to the TI-85 Guidebook. POSSIBLE ACTIVITIES 1) Use a tuning fork or tuning forks to collect waveforms. Use the Trace option to estimate the period of the signal and calculate the frequency. Use the frequency option in the program to compare the two methods of determining frequency. 2) An excellent activity involves applying a trigonometric model to sound data collected from tuning forks. To do this, collect a set of data using the WAVEF option in the Main Menu. Quit the program, press the [GRAPH] key, and select the y(x)= menu item. Notice that y1 has been defined with the following equation: y1=A*sin (2*pi*F*x + D). Select the SELCT menu item to turn on this function. The variables, A, F, and D can then be modified in attempting to match y1 to the data. Estimates of A and F can be obtained by pressing the [STAT] button and then selecting the DRAW menu item followed by the SCAT menu item to redisplaying the last graph. Press the [GRAPH] button and select the TRACE menu item. Use the arrow keys to read off x and y values and estimate the amplitude, A, and record the time between consecutive portions of the graph. The reciprocal of the period of the wave will approximate the frequency, F. Store those values in the A and F variables and again press the [STAT] button. Select the DRAW menu item and then select the SCAT menu item. Both the data and the modeling function will be graphed on the same axes. A, D and F can be fine tuned to improve the fit. More complex waveforms can be modeled, such as the superposition of two sine waves. 3) Tune a home-made musical instrument using either the waveform or frequency option in the program. For example, blow across the top of a soft drink bottle and adjust the water level in it to tune it to a tuning fork. Try to tune two bottles such that they are an octave apart. 4) Use the waveform option to compare the patterns produced by different musical instruments. 5) Use the waveform option to collect a waveform when two sound sources are played simultaneously. If they are slightly out of tune, the beat frequency can be determined from the graph. 6) Measure the speed of sound by capturing the waveform of a sound and its echo. Set up a 1 to 2 meter hollow tube like PVC pipe or a carpet role with a microphone placed at one of the openings. Block the other end or leave it open. Use the trigger/wavfrm option of this program so that it is ready to trigger on a sound. When you snap your fingers next to the microphone, the microphone will record a waveform showing the initial sound and the echo reflected from the other end. If the other end is unblocked, the echo will have the same shape as the initial sound; otherwise the echo will be inverted. While viewing the graph and noting the shapes of the sound and its echo, use the arrow keys to determine the time between the initial sound and the reflection. Use this time and the length of the tube to calculate the speed of sound. If the tube is longer than 2 m, you may need to increase the time between samples in order to see the echo. See the section on editing the sample time. 7) Use the frequency mode to investigate the musical scales. Determine the frequency of the notes do, re, mi, fa, so, la, ti and do. For example, you might use an electronic keyboard and measure the frequencies of middle C and the next seven white keys up the scale. Determine the ratio of the note’s frequency to the starting note. The theoretical ratios are 1/1, 9/8, 5/4, 4/3, 3/2, 5/3, 15/8, and 2/1. Compare the ratios of consecutive notes to theoretical values of 9/8, 10/9, 16/15, 9/8, 10/9, 9/8, 16/15. August 28, 1995 Rick Sorensen Vernier Software 8565 S.W. Beaverton-Hillsdale Hwy. Portland, OR 97225-2429 (503) 297-5317