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Sound Sensor Experiment Guide Sound Sensor Experiment Guide Sound Sensor Introduction: Part of the Eisco series of hand held sensors, the sound sensor allows students to record and graph data in experiments on the go. This sensor has two modes of operation. In slow mode it can be used to measure sound-pressure level in decibels. In fast mode it can display waveforms of different sound sources such as tuning forks and wind-chimes so that period and frequency can be determined. With two sound sensors, the velocity of propagation of sound in various media could be determined by timing a pulse travelling between them. The sound sensor is located in a plastic box accessible to the atmosphere via a hole in its side. Sensor Specs: Range: 0 - 110 dB | 0.1 dB resolution | 100 max sample rate Activity – Viewing Sound Waves with a Tuning Fork General Background: Sound waves when transmitted through gases, such as air, travel as longitudinal waves. Longitudinal waves are sometimes also called compression waves as they are waves of alternating compression and rarefaction of the intermediary travel medium. The sound is carried by regions of alternating pressure deviations from the equilibrium pressure. The range of frequencies that are audible by human ears is about 20 – 20,000 Hz. Sounds that have a single pitch have a repeating waveform of a single frequency. The tuning forks used in this activity have frequencies around 400 Hz, thus the period of the compression wave (the duration of one cycle of the wave) that carries the sound is about 2.5 seconds in duration, and has a wavelength of 66 cm. In that distance the air goes through one complete period of compression and rarefaction. By playing two single toned instruments that have dissimilar pitches playing simultaneously, one notices the sounds interfering with one anther. The periodic variation that can be heard is called a beat, and is created by the sound waves interfering with one another. The waves constructively and destructively interfere at the peaks and troughs of the waves combining and cancelling depending on how the two waves overlap. The beat is observed as a sort of ringing, and the spacing between the sounds of the ringing is called the beat frequency. Mathematically, the audible beat frequency is given by the difference between the two frequencies being played, �!"#$ = �! − �!. In this activity, you will use graphical representations of the waveform to discover the exact frequency of the tuning fork, how modifying the tuning fork will change the frequency of the sound, and how two overlapping sounds create interference patterns in their waveforms. Required Materials: Eisco Sound Sensor & Handheld Unit Eisco Pair of Tuning Forks [PH0742A] Computer and graphing software (such as Microsoft Excel) Procedure 1. To set up the tuning fork apparatus, insert a tuning fork into each of the resonance boxes. Attach the moveable mass to one of the tuning forks, screwing it in tightly to the top of one of the tines of the fork. 2. Switch the Eisco Sound Sensor into waveform mode by touching the dB (decibel) symbol in the upper right corner of the touchscreen and setting the unit to Arb (arbitrary). 3. Set the Eisco Sound Sensor to record 50 milliseconds, with 10,000 samples/sec. Situate the sound sensor so it points into the open end of the resonance box of the tuning fork without the attached mass. 4. Using the rubber mallet, strike the tuning fork without the attached mass. Immediately hit record on the touchscreen. 5. Check the graph that is created. If a clean waveform with clear sinusoidal behavior is not evident, repeat step 4 again until clear data is recorded. 6. Repeat steps 4-6 for the other tuning fork. You should be able to hear the difference in frequency of the tuning fork with the attached mass. Extension – Beat Frequency: 7. Set up the resonance boxes and sound sensor as shown in the figure below. Set the Eisco Sound Sensor to record 0.25 seconds. 8. Quickly strike each of the tuning forks and then hit record on the touchscreen. It may be necessary to practice striking the tuning forks in quick succession before successfully recording an interference pattern with the sound sensor. Analysis 1. Connect the Eisco Force Sensor to your computer via a micro-USB cable. Start the Eisco Sensor Software. Import each recorded dataset (bare tuning fork, tuning fork with attached mass) and then export as a .csv (comma-separated- value) file. 2. Import each .csv file into a graphing program of your choice. Make a plot of each file, graphing the arbitrary unit axis (amplitude of the waveform) versus the time in seconds. 3. For each waveform, determine the number of complete periods the wave makes in the amount of time recorded. A period is one cycle of the sinusoidal shaped wave, above the equilibrium point then below the equilibrium point and back to the original value. To determine the frequency of each tuning fork, divide the time span recorded (0.05 seconds) by the number of periods. It is possible that there is a partial period remainder, approximate this as a fraction of a period (i.e. ½ or ¼ of a period). Extension – Beat Frequency Analysis 4. With the Eisco Force Sensor connected to your computer via a micro-USB cable, start the Eisco Sensor Software. Import the recorded beat frequency dataset, and then export as a .csv (comma-separated-value) file. 5. Import the .csv file into a graphing program of your choice. Make a plot, graphing the arbitrary unit axis (amplitude of the waveform) versus the time in seconds. 6. You should see that the interfering waveform is produced of envelopes of sound. The entirety of each envelope is one beat, and the ringing one hears is the ascending and descending of the envelope of sound. For the interfering waveform, determine the start and stop time between several complete envelopes. To determine the beat frequency, divide this time span recorded by the number of envelopes. 7. Compare this result to the frequency calculated by the equation �!"#$ = �! − �!. Sample Results These are examples of possible results. Similar results should be seen. Unmodified Tuning Fork Soundform There are approximately 20 periods in the span of 0.05 seconds. So the frequency of the unmodified tuning fork is given by 20 � = = 400 ��. ! 0.05 � Modified Tuning Fork Soundform There are approximately 18.25 periods in the span of 0.05 seconds. So the frequency of the modified tuning fork is given by 18.25 � = = 365 ��. ! 0.05 � Interfering Tuning Forks Soundform From 0.018 seconds to 0.102 seconds on the above plot, there are 3 envelopes of interfering waveforms. The beat frequency the number of envelopes divided by the duration of time for those envelopes, so 3 � = = 35.7 ��. !"#$ 0.102 � − 0.018 � This is nearly identical to the difference between the frequencies of the two tuning forks, �! − �! = 400 �� − 365 �� = 35 ��. .
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