Infrasound Presentation

1. Astronomical and Audio Engineering Societies Chris Orban David Petruncio Ben Niemoeller Jarrod Waldeck Jeff Zahos Infrasound Presentation Monday, March 10, 2003 ***Please click on hyperlinks to go to fun demonstrations***

2. What Is Sound? AES If a tree falls in the forest, and there is no one to hear it, will there be a sound?... "Sound is an organized movement of molecules caused by a vibrating body in some medium - water, air, rock, etc.” "Sound is the auditory sensation produced through the ear by the alteration ... in pressure, particle displacement, or particle velocity which is propagated in an elastic medium."

3. More on Sound The Nature of a Sound WaveSound originates when a body moves back and forth rapidly enough to send a coursing wave through the medium in which it is vibrating. Demonstration:A simple form of sound wave is produced by an explosion of a small balloon of compressed air. By bursting the balloon, potential energy (energy of position) is converted to kinetic energy (energy of motion). Sound GeneratorsAny motion that is repeated is called periodic motion. Examples of periodic motion are the moon's orbit of the earth, a beating heart, the whirring tail of a frightened rattlesnake, the wings of a hummingbird, and the movement of the valve on a revolving bicycle tire. A vibrating body in contact with the atmosphere will produce sound waves. One simple example is a vibrating piston.

4. Infrasound • Normal Human audible range is between 20Hz and 20kHz Hertz. • Infrasound is frequencies below 20Hz • Imagine a world in which you could not only hear nearby conversations and the noise of traffic a few blocks away, but also the sound of blasting coming from a quarry in Missouri, or the erupting blast of a volcano in Hawaii, or even the roar of a typhoon half-way around the world. • “Infra”comes from the Latin word for “below” • Interest in atmospheric infrasound originated in Cold War to detect atmospheric disruptions due to nuclear weapon testing.

5. Natural Sources of Infrasound Waterfalls (98) Geothermal activity (99) Seismic movements of tectonic plates (99) Winter seasonal sources oceanic waves (100) marine storms (100) wind storms (101) Summer and other seasonal sources thunder/lightning storms (102) auroral disturbances (103) geomagnetic activity (104) atmospheric turbulence (105) Acute sources of natural infrasound are volcanic eruptions (106) seismic shock waves of earthquakes (107)

6. Animal Sources and Listeners Infrasound receptors have been identified in special infrasound sensitive neurones in the cochlear ganglion Birds Elephants (14Hz) Giraffes (11Hz) Rhinoceros Okapi (11Hz)

7. Research - Resonator Need to boost Low frequencies with a Helmholtz Resonator Let's put in somenumbers: for a 1 liter bottle, with S = 3 square centimeters and L = 5 centimeters, the frequency is 130 Hz, which is about the C below middle C

8. Research - Microphone All modern microphones are trying to accomplish the same thing as the original, but do it electronically rather than mechanically. A microphone wants to take varying pressure waves in the air and convert them into varying electrical signals. There are five different technologies commonly used to accomplish this conversion: Carbon microphones Dynamic microphones Ribbon microphones Condenser microphones Crystal microphones Condenser (or capacitor) microphones use a lightweight membrane and a fixed plate that act as opposite sides of a capacitor. Sound pressure against this thin polymer film causes it to move. This movement changes the capacitance of the circuit, creating a changing electrical output. In general, most microphones are geared for human hearing applications, 20Hz – 20kHz.

9. Research - Analog to Digital When you sample the wave with an analog-to-digital converter, you have control over two variables: The sampling rate- Controls how many samples are taken per second The sampling precision- Controls how many different gradations (quantization levels) are possible when taking the sample In the following figure, let's assume that the sampling rate is 1,000 per second and the precision is 10: The green rectangles represent samples. Every one-thousandth of a second, the ADC looks at the wave and picks the closest number between 0 and 9. The number chosen is shown along the bottom of the figure. These numbers are a digital representation of the original wave. When the DAC recreates the wave from these numbers, you get the blue line shown in the following figure:

10. Research – Analog to Digital In the following figure, the rate and the precision have been doubled again (40 gradations at 4,000 samples per second): You can see that as the rate and precision increase, the fidelity (the similarity between the original wave and the DAC's output) improves. In the case of CD sound, fidelity is an important goal, so the sampling rate is 44,100 samples per second and the number of gradations is 65,536. At this level, the output of the DAC so closely matches the original waveform that the sound is essentially "perfect" to most human ears.

11. Research - Analysis The Fourier transform, in essence, decomposes or separates a waveform or function into sinusoids of different frequency which sum to the original waveform. It identifies or distinguishes the different frequency sinusoids and their respective amplitudes. The Fourier transform of f(x) is defined as

12. How We did it - Resonator We found a 725 gallon tank! Since the volume and the cross-sectional area were fixed, we calculated the length of the port.

13. How We Did It - Microphone Shure, Inc lent us a SM81 Capsule. The electronics filter out frequencies below 20Hz We had to make our own circuit due to high impedance at 3 Hz Remember X = 1/(j*w*C) and the capsule has a C of 55pF

14. How We Did It – ADC We borrowed it! PSN-ADC-Serial Samples 16 bits at 200 samples per second Runs with WinSDR Used by professional seismologists

15. How We Did It - Analysis Exported data from WinSDR to file. Imported with Matlab!!! Plot(abs(fft(signal)))

16. Final Product

17. All Work and No Play

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20. All Work and No Play

21. Special Thanks • EOH • George Swenson • Shure, Inc • Fertilizer Dealer Supply of Illinois • OICS • ECE Department • Larry Cochrane and Public Seismic Network

22. References 31 Schernmuly L, Klinke R. Infrasound sensitive neurones in the pigeon cochlear ganglion. J Comp Physiol A 1990; 166: 355-363.   98 Donn WL, Rind D, Natural infrasound as an atmospheric probe.Geophys J R Astron Soc 1971; 26:111-133. 99 Infrasound and low frequency vibration; Tempest W (ed). Academic Press, London. 1976 100 Arendt S , Fritts DC. Acoustic radiation by ocean surface waves. J Fluid Mechanics 2000; 415: 1-21 101 Bowman HS, Bedard AJ. Observations of infrasound and subsonic disturbances related to severe weather. Geophys J Roy Astron Soc 1971; 26:215-242. 102 Uman MA. Lightning. Dover Publications , New York. 1969. 103 Wilson CW; Auroral infrasonic waves J Geophys Res. 1969; 74: 1812-1836 104 Chrzanowski PG, Greene G, Lemmon KT, Young JM. Travelling pressure waves associated with geom agnetic activity.J Geophys Res 19 61; 66 3727-3733 105 Bedard AJ; Infrasound originating near mountainous regions in Colorado. J Applied Meteorology 1978;17:1014-1022. 106 Goerke VH, Young JM, Cook RK. Infrasonic observations of the May 16 1963 volcanic explosion on the island of Bali J Geophys Res 1965;70:6017-6022 107 Donn WL, Posmentier ES. Ground coupled airwaves from the great Alaskan earthquake. J Geophys Res 1964;69:5357-5361