PODCAST: explore significant technological advancements from the mid-20th century. One video from AT&T Archives highlights the Shive Wave Generator, a teaching tool developed at Bell Labs in the 1950s to illustrate universal wave behaviors across various physics disciplines, from mechanical to electrical systems, and explains concepts like reflection, superposition, resonance, and impedance matching. Another video from AWA Communication Technologies Museum details the meticulous manufacturing process of quartz crystals for radio frequency control during World War II, emphasizing the importance of piezoelectricity and the precise cutting, testing, and finishing techniques required for these critical components. Finally, an AT&T Tech Channel documentary from 1953 introduces the transistor, invented at Bell Telephone Laboratories, showcasing its revolutionary advantages over vacuum tubes in terms of size, power consumption, heat generation, and reliability, and anticipating its profound impact on miniaturization, communication, computing, and military applications.
The fundamental behaviors of various wave types, including mechanical, electrical, acoustical, and optical waves, exhibit striking similarities because they all behave fundamentally alike. Dr. John Shive’s Wave Generator, invented at Bell Labs in the 1950s, was designed as a teaching tool to illustrate these universal wave motion concepts across disciplines.
Here are the parallels in fundamental wave behaviors and their practical applications:
- Total Reflection
- Behavioral Parallel: Waves can be totally reflected in two primary ways: either right-side up if the reflecting end of the medium is completely free, or upside down if the reflecting end is completely restrained. This consistency holds true across different wave systems.
- Applications/Examples:
- Mechanical Waves: A wave on the Shive machine reflects right-side up if the end is free, and upside down if the last crossarm is clamped.
- Electrical Waves: Electrical waves on a transmission line reflect differently depending on whether the far end is terminated by an open circuit (analogous to a free end, reflection is right-side up) or a short circuit (analogous to a restrained end, reflection is upside down).
- Acoustic Waves: Waves in an acoustic tube reflect based on whether the end is closed by a rigid termination (upside down reflection) or the acoustic analog of a free end (right-side up reflection).
- Principle of Superposition
- Behavioral Parallel: When two waves traveling in opposite directions on the same medium pass through each other, the instantaneous amplitude of the resultant wave is the algebraic sum of the amplitudes of the two constituent waves. This principle applies even when one wave is positive and the other negative, which can lead to momentary cancellation.
- Applications/Examples:
- Standing Waves: Continuous trains of periodic waves, when reflected back upon themselves, superpose with oncoming waves to produce patterns of built-up amplitude and complete cancellation, appearing to stand still. These “dead spots” or nodes are exactly half a wavelength apart.
- Vibrating Strings: A common example is a vibrating string, which naturally vibrates in segments separated by such nodes.
- Resonance
- Behavioral Parallel: Resonance occurs when new waves continually sent out superpose upon previously emitted and multiply reflected waves in just the right phase, causing the amplitude to build up to an abnormally large value. This condition can be achieved by tuning the length of the medium or the frequency of the generator.
- Applications/Examples:
- Mechanical Systems: Examples include the escape wheel of a watch and a child’s swing.
- Acoustical Systems: A sounding organ pipe.
- Electrical Systems: The resonance circuit of an electrical radio transmitter. Resonant systems exhibit large oscillation amplitudes and contain significant energy built up over many cycles.
- Impedance and Impedance Matching
- Behavioral Parallel: The impedance of a wave medium is defined as the ratio of the originating cause of the waves (e.g., oscillating torque for mechanical, AC voltage for electrical) to the resulting effect (e.g., angular velocity for mechanical, AC current for electrical).
- Applications/Examples:
- Matched Load: When the impedance of a terminating load equals the impedance of the line itself, all wave energy is absorbed by the load, and no reflection takes place. This is called a “matched” condition.
- Partial Reflection: If the load impedance does not match the line impedance, partial reflection of wave energy occurs, leading to a partial standing wave.
- Standing Wave Ratio (SWR): In cases of partial reflection, the ratio of the maximum to minimum amplitude of the standing wave envelope is called the standing wave ratio (SWR). This value is significant because it allows for the calculation of the percent reflection taking place at a termination or discontinuity. This formula, originally developed for AC electricity, is universally applicable.
- Impedance Discontinuities: Waves are partly reflected not only at mismatched terminations but also at points where the impedance of the transmission medium changes abruptly. This is similar to light emerging from glass into air.
- Impedance Matching Devices: To avoid wasteful partial reflection at impedance discontinuities, various devices are used to “match” impedances:
- Quarterwave Matching Transformer: A short section of the wave medium, exactly a quarter of a wavelength long, with an intermediate impedance, can effectively promote wave energy transmission across a discontinuity without reflection loss. An optical analog is the non-reflecting coating on camera lenses. This type is effective for continuous waves over a narrow frequency range.
- Tapered Section Transformer: A gradual taper in the impedance (like a megaphone for sound or a tapered waveguide section) can match impedances over a relatively wide band of frequencies and is effective for single waves and pulses.
- Natural and Engineered Applications of Impedance Transformation:
- Electrical Engineering: Transformers range in size from tiny components to large units found in hydroelectric stations.
- Acoustics/Biology: The three tiny bones in the mammalian ear (hammer, anvil, stirrup) provide an impedance linkage between the low impedance of the air in the outer ear and the high impedance of the liquid in the inner ear.
- Mechanics: Mechanical advantage machines like gear trains, levers, and pulleys are also impedance transforming devices.
Understanding these fundamental similarities means that knowledge gained from studying waves in one discipline can be readily applied and felt “at home” in any other branch of physics or engineering where waves play a role.