Tech Directions - October 2007 - (Page 14)

Fig.1—Mass and spring A mass and spring illustrates simple harmonic motion as well as an inverse relationship between stiffness and mass. Unlike a real pendulum, period is independent of amplitude. Moving-coil loudspeakers can be shown as a moving mass with a given amount of compliance (the inverse of stiffness). Fig. 2—Measuring for impedance curve Calibrate for 1 V across 10 Ω to equate voltage to resistance, i.e. 1 V = 10 Ω. Measure and record I x R drop as frequency changes throughout the mechanical resonance range. Finish with an X-Y graph to show the impedance curve. y ky -mg Fig. 4—Characteristics of parallel circuits Frequency resonance Fr = 1 / 2π √LC Current is minimum at resonance, Z is maximum Fig. 3—Characteristics of series circuit Impedance Z = √ R2 + (XL – Xc )2 Current is maximum when Z is minimum tics are exhibited when driven by and can be varied widely to bring oscillating electrical currents in large voltage signals. students in contact with seemingly resonant LC circuits, relating meIt is also well established that mysterious electrical phenomena. chanical waves to electrical waves this type of transducer is surprisDepth of instruction and learner (again, thanks to F. H. Olson’s model). ingly inefficient at changing forms of activities can be controlled to meet Teacher-led discussions can highenergy, often as low as 1 percent, teacher goals and objectives by breaklight principles in wave theory with because almost all input power gets ing the subwoofer up into its teachconstruction of a ripple tank—a lost as heat in the energy transforable parts: electrical, mechanical, great class project that allows stumation process. Call it beautiful, or and acoustical. dents to see waves in action. Seeing call it beautifully flawed, but a thunSpecific units of study can be surface waves and longitudinal derous 12"-dia. subwoofer with flat designed and planned to address waves can help students better suspension, rubber surround, multiple learner intelligences up and understand invisible electrical wave molded polypropylene cone, and down Bloom’s Taxonomy of Learning phenomena. Many basic electrical stacked 68-oz. magnets makes for and should include practical assessconcepts can be revealed with apmany teachable minutes in a techment strategies. Recognizing the propriate lab activities that use nology-based classroom. impedance electrical equivalent subwoofers, such as measuring a model of a loudspeaker as analloudspeaker’s unique impedance ogous to parallel RLC components, curve (Fig. 2) to illustrate the changVersatile Teaching Tool teachers can introduce the mechaniing nature of impedance in accorThe scope of instructional activical aspects of diaphragm mass as dance with changing frequencies. ties involving subwoofers is scalable capacitance, diaYou can easily show phragm and spider the meaning of Q in Fig. 5—Measuring for compliance as electrical circuits and free air resonance (Fs) Measure for maximum inductance, and components through I x R drop, characterized diaphragm susclose examination of by a distinct voltage pension stiffness the impedance curve. It peak, and/or look for and air loading as is well established that diagonal line on oscilloresistance. This low-frequency loudscope in x-y mode when gives students a speakers exhibit linear at resonance (Fs), with sine wave input. chance to rethink characteristics relative characteristics of to input power vs. RLC circuits, like output power when minimum current driven by small voltage flow and maximum signals. Conversely, impedance at resnonlinear characteris- 14 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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