An electrodynamic or moving-coil loudspeaker is an electromagnetic transducer for converting electrical signals into sounds. When the original version of this book was published in 1954 under the title Acoustics, the only practical amplifying device available was the vacuum tube, so output power was expensive. Hence, the efficiency of the electrodynamic loudspeaker was one of the most important factors. As the cost of amplifier watts has decreased, there has been a steady trend toward smaller loudspeakers coupled with ever more powerful amplifiers, which are needed to compensate for the reduced radiating efficiency of the smaller diaphragms. Two developments have spurred this trend: the replacement of the vacuum tube with silicon transistor and the introduction of the so-called digital amplifiers using various coding schemes, the most popular being pulse-width modulation or Class D. In mobile devices, which rely on battery power, the efficiency issue has not gone away. The most important development for mobile devices has been the introduction of rare-earth neodymium magnets, which have enabled significant miniaturization. One trend which has enabled loudspeakers of all sizes to be used with smaller enclosures has been the development of high-compliance suspensions which are stable enough not to cause rocking modes. The term “acoustic suspension” has been dubbed [1] to describe a loudspeaker which has an enclosure so small that the air inside the enclosure is stiffer than the suspension.