Spectroscopy of Sound Transmission in Solid Samples - Journal of

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Spectroscopy of Sound Transmission in Solid Samples Dean J. Campbell,* Joshua P. Peterson, and Tamara J. Fitzjarrald Mund-Lagowski Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinois 61625, United States S Supporting Information *

ABSTRACT: These laboratory experiments are designed to familiarize students with concepts of spectroscopy by using sound waves. Topics covered in these experiments include the structure of nitinol alloys and polymer chain stiffness as a function of structure and temperature. Generally, substances that are stiffer or have higher symmetry at the molecular level transmit sound more effectively, and the higher pitched sounds that they produce upon impact can be heard and visually observed using a laptop microphone and SpectrumView freeware. This lab is designed for use in an introductory undergraduate chemistry course, although the techniques can be used for more and less advanced courses. Additional related classroom demonstrations are also presented.

KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Laboratory Instruction, Polymer Chemistry, Hands-On Learning/Manipulatives, Materials Science, Spectroscopy

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students as well as graduate students, but this laboratory was written for undergraduates in a general chemistry laboratory course.

he vast majority of spectroscopy used in chemistry involves electromagnetic waves, including those associated with visible light. The laboratory experiments described in this paper are designed to introduce students to concepts of spectroscopy using sound as a medium. The motivation for this is that learning is enhanced when information is presented in more than one sensory mode, e.g., as visual and auditory information.1,2 Chemistry can be readily connected to all five senses.3 Just as students have been introduced to visual color concepts in art classes before taking general chemistry, they have been given some understanding of sound in music classes. Sound is composed of longitudinal mechanical waves propagating through matter.4 The wave nature of sound can be readily connected conceptually to the wave nature of light, and both kinds of waves lend themselves to spectroscopic analysis. Sound has been explored in chemistry education in a variety of ways. Different phases of nickel−titanium alloy have been shown to produce different sounds when struck.5 The theory of using sound absorption for finding rates of fast chemical reactions has been discussed.6 Sounds associated with fast, explosive chemical reactions have been used to emphasize the instability of compounds.3 Sound, waveforms, and spectroscopic data (e.g., infrared and nuclear magnetic resonance spectra) have all been interconverted in a variety of ways.2,7 The purpose of these laboratory experiments is to explore the transmission of sound through metals and polymers to understand how these sound transmissions are affected by molecular-level structure. In addition to typical sound observations using hearing, the sounds are visually analyzed using a computer program such as SpectrumView. The experiments described could be run by junior high school © XXXX American Chemical Society and Division of Chemical Education, Inc.

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THE SPECTRUMVIEW SOFTWARE The SpectrumView freeware8 was chosen for this set of experiments because it featured sufficient sound spectral analysis capabilities and was readily downloaded from CNET.com onto our laboratory laptop computers (Dell Latitude E5430). The microphones used were those in the laptops (IDT High Definition Audio CODEC). Many of the features mentioned on the display screen, shown in Figure 1A, are described in more detail in the laboratory procedure.

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EXPERIMENTAL PROCEDURES

Introduction to the Software with Beaker “Bells”

The first experiment is designed to describe the various modes of operation of the SpectrumView software. One could demonstrate these features by playing an instrument or simply whistling, but the first sound source used in this chemistry lab is gently tapping empty beakers with a metal spatula to make it ring like a bell. The students are first directed to select the “Time” display option, which displays sound wave intensity as a function of time. The students then tap a large (e.g., 400 mL) beaker to produce a sound waveform that they capture by stopping acquisitions and then sketch in their lab report, Figure 1A. The software also allows saving of the image as a wave file or a graphics file. The students repeat the experiment with a small (e.g., 150 mL) beaker. They should be able to observe

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The students repeat the preceding procedure using the “Frequency” display option, which displays sound intensity as a function of frequency, Figure 1B. The sound from the ringing beaker is then displayed as a series of peaks, with each peak corresponding to a different frequency. The students are asked to sketch the observed sound spectra and to label the lowestfrequency peak. This lowest frequency peak (which is sometimes a challenge to see against the background noise of the spectrum) is the fundamental frequency; the higher frequency peaks are the overtones.4 Again, the students should be able to observe that the sound peaks produced by the large beaker have lower frequencies. The sounds produced by tapping a beaker can fade away quickly, making them difficult to observe. The students repeat the preceding procedure using the “Waterfall” display option, which helps alleviate the sound loss problem. In the “Waterfall” display mode, each data acquisition is displayed with intensity represented by colors and with frequency on the x-axis. Each acquisition stacks on top of the other, so the display continuously moves downward along the y-axis over time (and looks like a waterfall), Figure 1C. Sharp peaks show up as bright spots in the display, and students sketch the spectra and circle the location of the bright spots. Here, too, the students should be able to observe that the sound produced by the large beaker has a lower frequency. Nickel−Titanium Alloy “Ring” and “Thud” Rods

Alloys of nickel and titanium known as nitinol (short for nickel−titanium Naval Ordinance Laboratory) have unusual properties that depend on the arrangements of their atoms. Some of the alloys can change between a low-temperature martensite solid phase and a high-temperature austenite solid phase at about room temperature; the specific transition temperature depends on the specific element composition. The acoustic properties of these alloy phases have been previously described in this Journal.5 Rods made from these alloys make different sounds when dropped on a solid surface, depending on their phase. Sound is more easily transmitted through the high-temperature austenite-phase nitinol rods because the atoms have a more symmetrical arrangement, and the rods “ring” when they are dropped on a hard surface. Sound is less easily transmitted through the low-temperature martensitephase nitinol rods because the atoms have a less symmetrical arrangement, and the rods produce a “thud” when they are dropped on a hard surface. In the lab, students drop each type of nitinol rod five or more times in a row on a lab bench. They can hear the sound difference between the rods, and in the waterfall spectra, they can see that the ringing austenite-phase rods exhibit more peaks and more overall signal at higher frequencies than the thudding martensite-phase rods, Figure 2A.

Figure 1. (A) Display screen for the SpectrumView program, showing the available control options. The spectral display shows the “Time” option for a 400 mL beaker being gently tapped with a metal spatula. (B) The spectral display for the same sound source in the “Frequency” option. (C) The spectral display for the same sound source in the “Waterfall” option. The beaker is tapped repeatedly, and then stopped, four times.

that the sound produced by the large beaker has an audibly lower pitch and that the waveform of the sound has a visibly longer wavelength and lower frequency. The lab points out that in chemistry there are sometimes relationships between the size of structures and their interactions with electromagnetic waves. For example, the wavelengths of light absorbed by π-conjugated molecular structures increase as the number of bonds in conjugation (and therefore the size of the structure in conjugation) increases.9 In nanoscale chemistry, the wavelengths of light absorbed or emitted by nanoparticles tend to increase as the sizes of the particles increase.10 The students then repeatedly tap the large and small beakers at the same time to observe that combining the individual wave patterns can produce a more complicated wave pattern. The lab points out that these complex patterns are often more clearly expressed by plotting the frequency or wavelength of the simple wave patterns along the x-axis rather than time, and that the conversion from time to frequency representations involves mathematical techniques such as Fourier transformations.

Polymer Stiffness and Sound: Different Polymers

Stiff materials often transmit sound effectively because the sound waves do not significantly deform their molecular structure. Less stiff materials do not transmit sound as effectively because some of the energy of the sound waves is lost to deforming their internal molecular structure.11 Polymers are excellent structures for connecting the concepts of molecular structure and sound behavior because they are readily available in a variety of chemical compositions and structures. In this laboratory exercise, students flex plastic containers to see if they can audibly distinguish the sound produced and then B

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noise, whereas less stiff bits of polymer such as PET produce a softer, lower-pitched sound. The waterfall sound spectrum for PS also exhibits a greater sound intensity at higher frequencies, Figure 2B. Students also perform the shaking experiment on two different kinds of “packing peanuts” (polymer foam pieces used as padding). Students compare the sound produced by shaking PS foam “peanuts” in a 400 mL beaker to the sound produced by starch-based foam “peanuts.” Students should again notice that the stiffer PS peanuts produce a louder, higher-pitched shaking noise, and that the waterfall sound spectrum also exhibits a greater sound intensity at higher frequencies. Polymer Stiffness and Sound: Different Temperatures

Sound also propagates differently through polymers at different temperatures, because polymer chains vary in flexibility depending on temperature (cold polymer chains tend to be less flexible than warm polymer chains). To study this, students shake polymer samples (5-cm lengths of rubber tubing suspended from threads) at different temperatures and examine the resulting waterfall sound spectra. At room temperature, shaking the rubber tubing produces little noise, as the polymer lacks sufficient stiffness to propagate sound waves well. Students then suspend the rubber tubing segments by the threads in liquid nitrogen for a few minutes. The tubing cools from a rubbery state to a glassy state, and the polymer becomes much stiffer. Shaking these very cold tubes produces a glassy jingling sound that can be easily detected on the waterfall sound spectrum. As the glassy state polymer warms back to room temperatures, the peaks in the waterfall sound spectrum shift to lower frequencies, and the sound intensity becomes weaker and fades away, Figure 2C. It should be pointed out that cooling rubber from room temperature all the way down to the low temperature of liquid nitrogen produces the most dramatic results. Small temperatures changes through the glass transition (e.g., by heating polystyrene in boiling water) can produce changes in the sound spectra, but these changes are more subtle (and could potentially involve dealing with hot, wet polymer samples).

Figure 2. (A) Waterfall sound spectrum for dropping nitinol rods on a benchtop. The lower portion exhibiting peaks corresponds to the austenite phase and the relatively signal-free upper portion corresponds to the martensite phase. (B) Waterfall sound spectrum for shaking PS and PET pieces in a 400 mL beaker. The lower portion with greater signal at higher frequencies corresponds to PS and the upper portion with less signal at higher frequencies corresponds to PET. (C) Waterfall sound spectrum for shaking rubber tubing suspended from threads as they warm up after being cooled to liquid nitrogen temperatures. The signals corresponding to the coldest temperatures are at the bottom of the display. The upper portion corresponds to the tubing after it has returned to room temperature.

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HAZARDS Glass containers should be tapped gently to avoid cracking the glass. Liquid nitrogen, and objects cooled to liquid nitrogen temperatures, present a frostbite hazard. PS heated above its glassy state can cause burns.

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are directed to characterize the sound produced by shaking bits of polymers in beakers. Transparent, colorless plastic food containers, such as those used to package baked goods like cookies, can be easily flexed and also cut into 1−2 cm sized pieces for shaking. For many years, polystyrene (PS) was used to package baked goods. More recently, polyethylene terephthalate (PET) has often been used for these containers. PS contains bulky pendant phenyl groups that decrease the flexibility of the polymer.12 One quantitative description of the stiffness of the polymers is the modulus of elasticity, which is the ratio of the force on an object divided by its cross-sectional area to the change in length of the object divided by its original length.13 PS, with a modulus of elasticity of 3.0−3.5 GPa, is more difficult to deform than PET, with a modulus of elasticity of 2.0−2.7 GPa.13 The PS containers tend to flex with a sharper crackling noise; the flexing noise of PET containers is a more dull sound. Students shake two 400 mL beakers, each containing either PS or PET polymer bits. Stiffer bits of polymer such as PS produce a harder, higher-pitched shaking

DISCUSSION As shown in Experimental Procedures, sound waves propagate much more effectively through austenite-phase nitinol alloy, which has a more symmetrical atomic arrangement. Sound waves were also shown to propagate more effectively through polymers with stiffer structures, either produced by polymer chains having greater structural rigidity, or by cooling the polymer chains to where they lose a significant amount of their flexibility. These experiments (with the exception of the more recently developed exercise involving rubber tubes in liquid nitrogen) were added to basic lab exercises (e.g., using analytical balances and pipets) on the first day of the General Chemistry II laboratory course in the Fall of 2013 at Bradley University. Pairs of students completed these sound spectra experiments in about 1.5 h. This laboratory course is separate from, but loosely follows, the lecture course. The concepts of spectroscopy, C

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heat source and strikes hard surfaces, it produces a rather dull sound. When the PS cools back to room temperature, it strikes hard surfaces with a glassy clinking sound. Still another demonstration that can be heard as well as visually observed using SpectrumView is a simple model of single bond versus double bond oscillation constructed from LEGO bricks, Figure 3C. When the LEGO “atom” attached to the holder by a single strut is plucked, it oscillates at a lower frequency than the LEGO atom attached to the holder by two struts. This demonstration can be used in discussions of vibrational spectroscopy, in which multiple bonds vibrate at higher frequencies than single bonds.9 These vibrations can also be detected using SpectrumView, Figure 3D. Many demonstrations using sound have been used to illustrate the speed of sound in various gases, which is proportional to the square root of the molar mass of the gas molecules.4 A simple demonstration using SpectrumView was used to illustrate this phenomenon to a classroom. A dry, 2-L PET soda bottle is filled with hydrogen gas. The bottle is opened and then quickly held upright loosely by its neck with one hand at the same time as the thumb and middle finger of the other hand rapidly drums alternately on opposite sides of the bottle. Over the course of a few minutes, as the less-dense hydrogen leaves the bottle and the more-dense air enters the bottle, the sound frequency of the drumming noise decreases. This change in sound frequency can often be heard, and is also visible in the waterfall spectra on SpectrumView on a nearby laptop computer. In one case, the peak sound frequency of the drumming dropped from about 200 to about 100 Hz. This demonstration might spare demonstrators from inhaling balloons full of helium and sulfur hexafluoride in order to raise and lower the pitch of their voices. There is a free version of SpectrumView, produced by Oxford Wave Research, that is available for use with iPhones, iPod Touches, and iPads. A few students successfully used this program with their own devices to collect the waterfall spectra for this laboratory experiment. An increasing number of students bring their own hand-held devices to class, and we find it intriguing that they could use these devices to readily collect data both inside and outside of the chemistry laboratory. Finally, it should be noted that using SpectrumView or similar software for the spectral analysis of sound might enrich observation opportunities by hearing-impaired students,2 although this concept has not yet been tried using these laboratory experiments.

including Beer’s law, are covered throughout the laboratory course, so these experiments have been designed to start students thinking about spectroscopy concepts in the very first lab of the course. Multiple stations, each containing laptops running SpectrumView, were placed in a quiet, unoccupied lab near the main General Chemistry lab to minimize exogenous noise. The students seemed to grasp the software and theexperiments with little instructor help. The lab instructions have since been modified to include more guidance for better responses to questions and to clarify laboratory directions. In addition to the laboratory activities, the SpectrumView software can be used with sound-producing classroom demonstrations. Building on the idea of the tinkling sound of rubber tubes cooled to a glassy state, an X-shaped cut was made in a racquetball, a glass marble was added to its interior, and a wire was attached to suspend the ball, Figure 3A. When the

Figure 3. (A) A racquetball jingle bell. (B) Waterfall sound spectrum for a shaking racquetball jingle bell as it warms up after being cooled to liquid nitrogen temperatures. The signals corresponding to the coldest temperatures are at the bottom of the display. The upper third corresponds to the bell after it has returned to room temperature. (C) A LEGO model of an oscillating single and double bond. (D) Waterfall sound spectrum produced by plucking the single bond and double bonds of the LEGO model. The lower portion with less signal at higher frequencies corresponds to the single bond, and the upper portion with greater signal at higher frequencies corresponds to the double bond.

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racquetball is shaken at room temperature, the marble rattles dully within the ball. When the racquetball is shaken at liquid nitrogen temperatures, the marble clinks within the glassy rubber ball, making it sound like a jingle bell. A video of this demonstration has been posted on YouTube.com.14 The jingling sound can be monitored with a waterfall sound spectrum in SpectrumView. In a similar manner to the rubber tubes, the sound intensity becomes weaker and fades away as the glassy state polymer warms back to room temperature, Figure 3B. Another sound demonstration involving the glass transition uses PS. Below roughly 100 °C, the polymer is in its glassy state (with little motion in its polymer chains) and transmits sound vibrations well.12 Above this temperature, the PS exhibits greater chain mobility. Producing so-called “Shrinky Dinks” from flat PS involves heating the PS above its glass transition temperature.15 When the shrunken PS is first removed from its

ASSOCIATED CONTENT

S Supporting Information *

Student laboratory experiment “An Introduction to Spectroscopy using SpectrumView”. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Kristine Campbell and Bethany Trang for preliminary testing of the SpectrumView experiments. We D

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would like to thank Rachel Snopko and Leslie Stafford for early work with studying sound characteristics of PS and PET containers. We would also like to thank the Bradley University General Chemistry II lab students for doing this lab.

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REFERENCES

(1) Katai, Z.; Toth, L. Technologically and Artistically Enhanced Multi-Sensory Computer-Programming Education. Teach. Teach. Educ. 2010, 26 (2), 244−251. (2) Hovick, J. W.; Murphy, M.; Poler, J. C. “Audibilization” in the Chemistry Laboratory: An Introduction to Correlation Techniques for Data Extraction. J. Chem. Educ. 2007, 84 (8), 1331−1333. (3) Jackson, D. A.; Dicks, A. P. The Five Senses of Christmas Chemistry. J. Chem. Educ. 2012, 89 (10), 1267−1273. (4) Young, H. D.; Freedman, R. A. Sears and Zemansky’s University Physics, 10th ed.; John Addison Wesley Longman, Inc.: San Francisco, CA, 2000. (5) Gisser, K. R. C.; Geselbracht, M. J.; Cappellari, A.; Hunsberger, L.; Ellis, A. B.; Perepezko, J.; Lisensky, G. C. Nickel-Titanium Memory Metal: A “Smart” Material Exhibiting a Solid-State Phase Change and Superelasticity. J. Chem. Educ. 1994, 71 (4), 334. (6) Perlmutter-Hayman, B. Sound Absorption and Fast Reactions in Solution. J. Chem. Educ. 1970, 47 (3), 201−202. (7) Pereira, F.; Ponte-e-Sousa, J. C.; Fartaria, R. P. S.; Bonifácio, V. D. B.; Mata, P.; Aires-de-Sousa, J.; Lobo, A. M. Sonified Infrared Spectra and Their Interpretation by Blind and Visually Impaired Students. J. Chem. Educ. 2013, 90 (8), 1028−1031. (8) Connell, G. SpectrumView, version 1.26; downloaded from http://download.cnet.com (accessed Jun 2014). (9) Solomons, T. W. G. Organic Chemistry, 4th ed.; John Wiley & Sons, Inc.: New York, 1988. (10) Campbell, D. J.; Xia, Y. Plasmons: Why Should We Care? J. Chem. Educ. 2007, 84 (1), 91−96. (11) Jarzynsky, J.; Balizer, E.; Fedderly, J. J.; Lee, G. In Properties and Behavior of Polymers, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2011; Vol. 1. (12) Seymour, R. B.; Carraher, C. E. Polymer Chemistry: An Introduction, 2nd ed.; Marcel Dekker, Inc.; New York, 1988. (13) The Engineering Toolbox. http://www.engineeringtoolbox. com/young-modulus-d_417.html (accessed Jun 2014). (14) Campbell, D. J. Racquetball Jingle Bell. https://www.youtube. com/watch?v=xfIm29nNo8g (accessed Jun 2014). (15) Stewart, L. Shrinky Dink Science. Education.com. http://www. education.com/activity/article/Shrinky_Dink_Science_middle/ (accessed Jun 2014).

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