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| October 15, 2002 Sound and Music: Understanding How Loudspeakers Work The first feature in this series explained how vibrations travelling as pressure variations through the air create the sounds we hear. To be audible these must occur within the range to which the human ear is sensitive, which is anywhere between 20 and 20,000 vibrations (or cycles) per second (abbreviated as Hz).Musical instruments work by creating these vibrations, and therefore the role of the loudspeaker is to recreate them as faithfully as possible. There are various means of doing this. Most involve creating controlled vibrations within some form of diaphragm, such as the cones and domes found in most speakers. To comprehend this, take a look at the way sound was originally recorded and replayed. The modern recording studio might involve an intimidating complexity of electronics and computer tools, but the very earliest steps in sound recording were actually quite simple. When Thomas Edison shouted, "Mary had a little lamb" into the bell end of a horn, the energy created by his voice caused a diaphragm at the narrow end to vibrate. This was connected to a pin that transcribed the pattern of those sound-generated vibrations onto metal foil wrapped around a rotating cylinder. When this process was reversed, so that the indentations in the rotating foil vibrated the pin and hence the diaphragm, the horn acted as an acoustic amplifier and the original speech was reproduced. True, it was barely recognizable, and there was definitely no attempt at anything approaching fidelity, but one could more or less comprehend the words that were said, and that alone was a major breakthrough in human history. What had happened was that Thomas Edison had found a way to store audio signals in mechanical form, using a simple acoustic-mechanical-acoustic chain. This basic approach, using both cylinders and discs, and wind-up gramophones with large horns, proved very successful through the first decades of the 20th century, but the process of recording acoustically was always a serious limitation and handicap. The next big step was the invention and development of electronic recording techniques, including microphones and amplifiers. Adding an extra stage of complexity to the recording/reproducing chain, which now runs acoustic-electronic-mechanical-acoustic, greatly improved the flexibility and capability of the recording process. Crucially, the microphone converts the initial acoustic vibration into an electrical "model," which follows the same pattern as the acoustic vibrations. During the early years of electronic recording, discs still had to be cut "live," and replay was normally direct-acoustic, via the same sort of horn gramophone that had been used for acoustic recordings. Ultimately, of course, the tape recorder added much more recording flexibility, including editing, while amplifiers and speakers took over from the acoustic horn at the end of the replay chain. Microphones at the beginning of the chain and loudspeakers at the end are both forms of transducers, which effect the change between acoustic and electrical versions of the same signal. The two processes are opposite sides of the same coin, the technologies are to some extent interchangeable, and the difference between microphone and loudspeaker usually has much more to do with size and power than technology. When an engineer makes a recording, the microphone’s diaphragm moves in response to the air-pressure changes caused by the sound, and then converts that diaphragm motion into an electrical model. When replaying a recording, we amplify the electrical sound signal, which then drives the loudspeaker. The loudspeaker turns the electrical signal into the diaphragm motion that recreates a sound wave in the adjacent air molecules. There are a number of possible techniques for doing this, which will be examined in more detail in future articles in this series. However, the most popular approach is the so-called moving-coil drive unit. This is based on the phenomenon that a wire sitting within a permanent magnetic field moves when a varying electric current is passed through it. In practice the wire is formed into a voice coil, which sits within a magnetic field. The voice coil is attached to the cone or dome diaphragm that actually creates the sound by vibrating the air molecules. Much of the difficulty involved in loudspeaker engineering arises from trying to generate sufficient loudness. A pair of microphone capsules strapped to the side of the head could serve as headphones, in theory at least. But try to fill a room with the sound they generate and you’ll just fry the voice coils. Filling an ear is one thing, filling a room quite another, and addressing the requirements of a concert hall or open-air festival takes the power problems a few stages further still. Happily, the problems of full-scale public address are beyond our concerns; we will confine ourselves to the reproduction of music in the home. Personal taste and the size and acoustic properties of the listening room will all have some effect upon any given circumstances, but the range of basic real-world requirements is actually not that enormous. That said, there’s a remarkably rich variety of choices out there in the market to suit a huge range of audio desires and aspirations. Crucially, every loudspeaker represents a complex, yet endlessly fascinating mixture of compromises between often-conflicting criteria. For example, achieving a healthy loudness capability is simplistically about moving plenty of air, especially at the bass end of things, and to move plenty of air you need a relatively large loudspeaker. Trouble is, small loudspeakers have their advantages too. Small enclosures mean that cabinet coloration is both lower and much easier to control than that found in larger speakers. Small speakers also tend to provide superior stereo imaging, with tighter focus and more precise perspectives. And they don’t take up as much of your precious living space either. That said, a good "biggun" will usually beat a good "littlun" when it comes to getting across the emotional and visceral impact of the music. Ultimately, it’s all about making informed choices between conflicting variables, and these articles aim to provide the information necessary to make those choices. ...Paul Messenger
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