This lesson uses the Physics of Sound LabVIEW Instrument from the Ergopedia - Essential Physics Curriculum.

Transcription

1 Physics of Sound This lesson uses the Physics of Sound LabVIEW Instrument from the Ergopedia - Essential Physics Curriculum. Learning Objectives As students complete this lesson, they ll have the opportunity to meet the following learning objectives: Physics Students explore and understand concepts of sound, waves, and frequency. Mathematics Students explore sine functions, various plots, and Fourier Transforms in a concrete way. Materials Physics of Sound LabVIEW Instrument (exe) Windows Computer with a microphone Optional Materials External microphone that can be plugged into your computer Various Musical Instruments Tuning forks Various Coins Smart Phone / ipad Male to Male Stereo Cable Slide Whistle A Balloon Notes The Physics of Sound instrument uses your computer s microphone to listen to sound. Computer microphones can have a wide variety of quality, range, and sensitivity. Your computer may not be able to hear very high or very low sounds very well. In some cases your computer will edit the incoming sound to try and make it better. Turn off all microphone enhancements and effects to get the most objective sound. If you want to input a song or tone from a phone or MP3 player, connect the headphone jack of your device to the microphone input of your computer using a male to male audio cable. This activity asks students to whistle to create sounds. If no one in your class can whistle download a frequency generation app for your smart phone so you can generate tones. How Sound Work? When you hear a sound your ear drum is being pushed back and forth by waves in the air. Your brain turns the motion of your ear drum into sound. Every sound you hear can be recreated by adding sine waves of various frequencies together. Your brain can detect more than 15,000 different frequencies. In this activity you will look at sound using three different plots that each reveals information about how sound works.

2 Activity 1 - Waveform Graph The first plot is the Waveform Graph, this shows sound in a Amplitude vs Time graph. You can think of this graph as the position of your ear drum over time. As the amplitude changes it moves your ear drum back and forth. You can use the Amplitude Range and Time Range controls to change how much is shown on the graph. Computer microphone are all a little different so if yours is too quit or too loud you can adjust the Gain knob so loud sounds reach an amplitude of -1 to 1. You can use the Pause button to freeze a sound wave on the screen. 1) Set the Time Range to.02s and set the Amplitude Range so that the wave is on the screen. 2) Whistle into your computers microphone and you should see an image similar to the one shown here. A whistle is close to a pure sine wave so we can see a regular pattern that looks like the sine wave you are familiar with from math class. However since the air passing through your lips also make other frequencies we can see variations in the amplitude over time. Use the Pause button to capture a clean sine wave like the one shown on this graph then press the Analyze button. Figure 1 - Waveform Graph When you click the Analyze button the instrument will capture the current waveform and sent it to the Analyze Waveform tab. You goal on is to edit the value of the green sine wave to match the waveform as closely as possible. 3) Move the T1 and T2 markers so they are both on the crest (top) of the sine wave. You can count the number of pulses between the two then divide by the Delta-t (dt shown on right) to find the frequency of your whistle. Once you set the frequency you will need to play around with the Amplitude, frequency and Phase in order to get your Match % (shown in yellow). Try and get a match % above 90. Figure 2 - Analyze Waveform Bonus activity) The analyze tab is set up to match waves that combine two different sine waves. Capture 2 students whistling two very different tones at the same time and try to get your Match % above 85%. Turn on the red sine wave and turn on Show Sum to display the combination of the green and red sine waves. You will find this is more challenging that the single tone. Physics of Sound Pg 2

3 Activity 2 Spectrum Figure 3 - Analyze Waveform with matched waveforms The Spectrum tab shows a plot of amplitude vs frequency. Using a Fourier Transform the program reads the waveform and sorts it into the individual frequencies represented in the sound. Think of this as a bar chart showing how loud each frequency is. Humans can hear from ~20hz to ~20,000hz. As you age the highest frequency you can hear will continue to decrees; a typical 50 year old can only hear up to ~12,000Hz. Check out this hearing test video to see how high you can hear. Note: Your speakers may not be able to play the high pitch sounds in this video. Use the spectrogram to see if your computer can hear it. 1) Get 5 students and have them each whistle an individual tone, you should see 5 unique spikes for each tone. 2) Ask a student to sing or play a note on an instrument and write down the fundamental frequency (biggest peak). Then ask them to play the same note an octave higher. What is the relationship between these two frequencies? Is it the same for other notes that are an octave apart? 3) Use a tuning fork and verify its frequency using the spectrum. Figure 4 - Spectrum Bonus Activity) Ask your students to bring in different band instruments and ask them to play concert A (440 Hz) one at a time and capture it on the Spectrum using the pause button. Write down the Peak frequencies and their amplitude for each instrument. You can also right-click on the graph and select export>simplified image to save a picture of each graph. Examine the mathematical relationship between each set of peaks. The strongest frequency is known as the fundamental the other frequencies are known as harmonics. The same note played on different instruments should have the same fundamental frequencies but will have different harmonics patterns. Activity 3 Spectrogram The Spectrogram chart is a 3 dimensional plot meaning it has the typical X and Y axis but it also uses color to represent a Z axis. The X Axis is time, the Y Axis is frequency and the color represents the intensity of each frequency. Imagine we took the spectrum chart and lat it on its side so we can streak it over time. Use the Set Z button to reset the Z scale, so you can see the maximum amount of information. 1) The easiest way to demonstrate how a spectrogram works is to whistle a changing frequency. (Try the bomb drop whistle sound) 2) Ask your student to play concert A (440 Hz) on various instruments, Because we can now see the spectrum over time it is easier to see the how the instruments are different. Physics of Sound Pg 3

4 3) Have a student read the following text Yesterday all my troubles seemed so far away. Look what happens on every s sound, compared to the rest. Are there other letters that make a unique shape on the spectrogram? 4) Have each student step up to the microphone and say Hello How do different voices make different patterns? Figure 5 - Spectrogram 5) Clap your hands, what frequencies are represented in a clap? 6) Play a song into your microphone or connect your smart phone directly to the computer microphone using a male to male stereo audio cable. Try different types of music with different instruments. 7) Fill up a balloon, pinch the opening and let the air out slowly so it makes a high pitched sound. Be sure the frequency range is set to the maximum 20Hz so you see high pitched sounds. How many harmonic frequencies do you see. 8) Set your computer on a hard surface and drop a quarter next to it and observe the peak frequencies. Try other coins and see how their unique frequencies are different. Does it matter how you drop it? Try dropping multiple coins at the same time, can you see each coin s frequencies? Try using foreign currency, washing machine tokens or any metal object that rings when you drop it. Figure 6 - Spectrogram - A quarter dropping on the table. Check for Understanding Now that you know more about sound, answer the following questions to apply that knowledge. 1) What is the mathematical definition of an octave? 2) While G-sharp has a frequency of 417 hertz, the musical note A has a frequency of 440 Hertz. If the two notes were graphed on the same waveform graph, how would the two curves differ? A. The A curve would be taller. B. The A curve would be shorter. C. The crests of the A curve would be closer together. D. The crests of the A curve would be farther apart. 3) One of these three graphs shows a sound that contains two different frequencies. Physics of Sound Pg 4

5 A. Which graph is it and how do you know? B. What is the lower frequency in this sound? C. What is the higher frequency in the sound? 4) For which of the following would a spectrogram be able to represent different parts of sound? A. speech B. music C. bird songs D. all of the above 5) At which frequency listed below is the sound represented on this spectrogram the loudest? A. 500 Hz B Hz C Hz D Hz Physics of Sound Pg 5