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Part 3. Transducers
PAR T 3
Transducers A transducer is a device that converts one form of energy to another. A lightbulb that converts electricity to light is a transducer, as are electric and gasoline motors that convert electricity and combustion, respectively, to mechanical motion. Even a foam or fiberglass absorber used for acoustic treatment could be considered a type of transducer, because it converts acoustic energy to heat. In that case, however, the energy is discarded rather than reused in its new form. These days most electronic devices have sufficiently high quality to pass audio with little or no noticeable degradation. But electromechanical transducers—microphones, contact pickups, phonograph cartridges, loudspeakers, and earphones—are mechanical devices, and thus are susceptible to frequency response errors and unwanted coloration from resonance, distortion, and other mechanical causes. For example, the cone of a loudspeaker’s woofer needs to be large in order to move enough air to fill a room with bass you can feel in your chest, but it’s too massive to move quickly enough to reproduce high frequencies efficiently and with broad dispersion. Therefore, most loudspeakers contain separate drivers for the different frequency ranges, which is yet another cause of response errors when sounds from multiple drivers combine in the air. At frequencies around the crossover point where two drivers produce the same sound, comb filtering peaks and nulls result from phase differences between the drivers. Where most audio gear is flat within a fraction of a dB over the entire audible range, with relatively low distortion, the frequency response of loudspeakers and microphones can vary by 5 to 10 dB or more, even when measured in an anechoic test chamber to eliminate room effects. Transducers also add much more distortion than most electronic circuits. A microphone doesn’t require separate woofers and tweeters, but its diaphragm can move only so far before bottoming out in the presence of loud sounds. Further, as a microphone’s diaphragm approaches that physical limit, its distortion gradually increases. The diaphragm in a dynamic microphone is attached to a coil of wire whose mass restricts how quickly it can vibrate, which in turn limits its high-frequency response. All microphones have a resonant frequency, resulting in a peak at that
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372 Part 3 frequency as well as ringing. As explained in Chapter 1, a mass attached to a spring resonates at a frequency determined by a combination of the two properties. With a dynamic microphone, the resonant frequency is determined by the mass of the coil and the springiness of the diaphragm suspension. In a sealed capsule design, the air trapped inside can also act as a spring. The same resonance occurs with loudspeakers: They can continue to vibrate after the source stops, unless they’re mechanically or electrically damped. Indeed, the design of transducers is always an engineering compromise between frequency response both on and off axis, power handling capability (for speakers), SPL handling (for microphones), overall ruggedness, and, of course, the cost to manufacture.
Transducers A transducer is a device that converts one form of energy to another. A lightbulb that converts electricity to light is a transducer, as are electric and gasoline motors that convert electricity and combustion, respectively, to mechanical motion. Even a foam or fiberglass absorber used for acoustic treatment could be considered a type of transducer, because it converts acoustic energy to heat. In that case, however, the energy is discarded rather than reused in its new form. These days most electronic devices have sufficiently high quality to pass audio with little or no noticeable degradation. But electromechanical transducers—microphones, contact pickups, phonograph cartridges, loudspeakers, and earphones—are mechanical devices, and thus are susceptible to frequency response errors and unwanted coloration from resonance, distortion, and other mechanical causes. For example, the cone of a loudspeaker’s woofer needs to be large in order to move enough air to fill a room with bass you can feel in your chest, but it’s too massive to move quickly enough to reproduce high frequencies efficiently and with broad dispersion. Therefore, most loudspeakers contain separate drivers for the different frequency ranges, which is yet another cause of response errors when sounds from multiple drivers combine in the air. At frequencies around the crossover point where two drivers produce the same sound, comb filtering peaks and nulls result from phase differences between the drivers. Where most audio gear is flat within a fraction of a dB over the entire audible range, with relatively low distortion, the frequency response of loudspeakers and microphones can vary by 5 to 10 dB or more, even when measured in an anechoic test chamber to eliminate room effects. Transducers also add much more distortion than most electronic circuits. A microphone doesn’t require separate woofers and tweeters, but its diaphragm can move only so far before bottoming out in the presence of loud sounds. Further, as a microphone’s diaphragm approaches that physical limit, its distortion gradually increases. The diaphragm in a dynamic microphone is attached to a coil of wire whose mass restricts how quickly it can vibrate, which in turn limits its high-frequency response. All microphones have a resonant frequency, resulting in a peak at that
371
372 Part 3 frequency as well as ringing. As explained in Chapter 1, a mass attached to a spring resonates at a frequency determined by a combination of the two properties. With a dynamic microphone, the resonant frequency is determined by the mass of the coil and the springiness of the diaphragm suspension. In a sealed capsule design, the air trapped inside can also act as a spring. The same resonance occurs with loudspeakers: They can continue to vibrate after the source stops, unless they’re mechanically or electrically damped. Indeed, the design of transducers is always an engineering compromise between frequency response both on and off axis, power handling capability (for speakers), SPL handling (for microphones), overall ruggedness, and, of course, the cost to manufacture.