Tech Directions - October 2007 - (Page 16)

resistance. Once students know Ohm’s law, the instructor can design discussions of power generation, delivery, and consumption to lead back to the subwoofer’s other role as a generator. Connecting an oscilloscope to the terminals of a subwoofer lets students see the sinusoidal waveform of voltage (and current) that comes from pushing the speaker’s cone up and down as the transducer works as a generator—mechanical energy in, electrical energy out. It's proof positive that the law of conservation of energy isn’t something just talked about in textbooks. This offers another great opportunity to present all relative motor theory while recognizing counterelectromotive force (CEMF) issues and detailing Faraday’s law of induction. CEMF, as the backward voltage generated as the subwoofer is being played, highlights Lenz’s law to describe the opposite nature of CEMF to the applied voltage. Further, here you can cover the various ac voltages commonly measured in electrical environments, such as peak voltage, RMS voltage, and average voltage. Practicing power calculations will emphasize RMS voltage as the only acceptable voltage rating to use in Watt’s law calculations for effective power. Instructors can delve deeper by considering the subwoofer’s magnetic influence over its currentcarrying conductor, the voice coil. You can use this phenomenon to illustrate a variety of electromagnetic concepts and principles. The essentials of electromagnetic coupling, the discoveries of Carl Friedrich Gauss (17771855), Coulomb’s law of charges, Hall effect sensing, and the temperature coefficient of resistance and power compression, are only a few of the topics instructors can introduce. Because copper is almost exclusively the material used for the voice coil, classroom topics might range from the history of copper and methods of mining and processing the metal, to principles of conductance and the mho as a unit of measure. Examination of voice coil construction techniques can serve as a lead-in to manufacturing environments and issues in productivity. For example, making delicate multilayered voice coils with small-gauge wire and even more delicate flat-wire edge-wound voice coils for increased magnetic strength will involve several types of workers within a production facility. This opens a door to career exploration. The longer subwoofers lie around in the classroom, the less novel they may seem to students—but don’t assume that their educational value will decline. Instructors can easily use them to demonstrate harmonic distortion, intermodulation distortion, and Doppler distortion. Demonstrate the maximum power transfer theorem using an audio amplifier’s relative output impedance to a given subwoofer’s input impedance in voltage divider fashion—equal impedances are needed between source and load for maximum transfer of power (Fig. 7). Teach logarithms to pave the way to real-world expressions showing changes in voltage, current, and power using the decibel unit of mea- surement (Fig. 8). Industry-standard loudspeaker sensitivity ratings relate sound-power output as sound pressure level (SPL), expressed in dBs, to 1 W of input power for the nominal impedance of the subwoofer, which is typically 4 Ω. And, introducing the inverse square law will inform students that for every doubling of distance away from a sound source, listening intensity will decrease by a factor of four. Lab Considerations Laboratory settings vary from school to school, but because our electro-dynamic machine is ultimately a rather simple machine, general testing and measurement Output impedance “Z” Input impedance “Z” R1 For maximum transfer of power to occur—Zout = Zin Approach this in voltage divider fashion so that voltage loss from output to input can be calculated in percent. Zout / Zout + Zin x 100 R2 Fig. 7—Maximum power transfer Voltage in Power in Amp Voltage out Power out Using the decibel unit for measurement Voltage loss or gain dB = 20 log V1/V2 Power loss or gain dB = 10 log P1/P2 Fig. 8—The decibel unit equipment setups can be affordable and straightforward. You can easily handle most student-assigned lab work with a digital multimeter, function generator, dual-channel oscilloscope, frequency counter, and LCR meter. There are many good print sources that explore loudspeaker testing and measurement in great detail. The Internet offers additional resources. Using subwoofers in the lab can energize your instruction—and your students! 16 techdirections ◆ OCTOBER 2007

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Tech Directions - October 2007 Technically Speaking Contents Direct from Washington The News Report Technology Today Technology’s Past Mastering Computers Rock Your Classroom!—Use Subwoofers to Teach Electricity and Science Sure, They Can Build It, But. . . . Manufacturing Students Need Process Planning Skills Teach Graphic Design Basics with PowerPoint Free Teacher Resources Product Central More than Fun

Tech Directions - October 2007

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