September 15, 2002
Sound and Music: An Introduction to Loudspeaker Technology
The loudspeaker is a mimic. Its prime task is to imitate the sound of every kind of musical instrument, and the human voice, too, as accurately and faithfully as possible. In one sense, therefore, it behaves like a musical instrument, but from another perspective it can be regarded as the exact opposite.
To understand how a speaker works, we first need to understand something about sound. Its a form of energy, created when things vibrate. The vibrating source causes the air molecules next to it to vibrate, and these in turn pass the vibrations on from molecule to molecule until they reach your eardrums. (In space, no one can hear you scream, simply because there are no air molecules to transmit the sound!)
To qualify as sound, these vibrations must fall within the range of frequencies to which human ears are sensitive, but thats a pretty broad range. It generally runs from 20 vibrations per second (20Hz) in the very deepest bass up to 20,000 per second (20kHz) at the topmost limit of audibility.
Musical instruments dont go quite that low in practice. However, a large auditorium will have resonance down into the subsonic regions, and the explosions found on movie soundtracks will also have very low-frequency components. Musical notes as such dont go up to the highest audible frequencies either, but the higher harmonics of the higher-pitched instruments certainly do -- cymbals may offer the most obvious example.
Though its not always evident at first glance, all musical instruments work by generating vibrations of some sort. This usually (but not always) involves combining some sort of actuator that the musician actually plays, such as the string of a guitar, with some sort of resonator, which amplifies the vibration of the string and adds the tonal richness, such as the instrument s wooden sounding board.
Orchestral stringed instruments like violins and cellos, usually played with a bow rather than a plectrum, use a similar arrangement. The piano is similar too, though its use of hammers (and dampers) adds a percussive element to the sound. The resonators of the various wind instruments -- brass, woodwind, the organ -- consist of tubes of air, adjustable in length according to the note being played, while the initial vibration is created either by the lips alone or with the help of a reed (and in the case of the organ, a pump). Similarly, the human voices vocal cords act rather like a reed, and the mouth and nasal passages as the resonator.
Although there are myriad variations, most acoustic musical instruments follow this broad modus operandi, relying heavily on some form of natural resonance to create adequate volume to make themselves heard, and to provide an attractive tonality. Indeed, the development of instruments down the ages has much to do with finding ways of getting more volume out of them -- which is precisely why the pianoforte took over from the harpsichord.
However, electric and electronic instruments are the dominant forces in the many forms of popular music today, and these instruments do operate in a rather different way. The electric guitar, for example, is still played by plucking strings to create the initial vibration, but its solid body is not designed to resonate acoustically like a Spanish guitar. Rather, pickups built into the solid guitar body sense the motion of the (metal) strings electromagnetically, and loudspeakers process, amplify, and finally reproduce this output -- using technology closely related to that used for hi-fi reproduction.
"Electrically assisted" music changes the rules completely, of course, removing any need for mechanical/acoustical amplification. It also offers unlimited possibilities for manipulating the sounds by electronic processing. Indeed, the sheer immensity of the possibilities here poses its own difficulties. The question arises of retaining creative musicianship in an era where the computer sometimes seems to be threatening to take over.
The ultimate long-term success of the electric guitar has much to do with its ability to combine the musicianship of the plucked/struck string with the flexibility of electronic processing and amplification. Unlike hi-fi equipment, however, where neutrality is a prime consideration, the amps and speakers used by the electric guitarist are often regarded as part of the tonality of the instrument. Valve amps may be deliberately over-driven to add harmonic distortions, and the colorations of necessarily high-sensitivity loudspeakers all contribute to the overall character of the instrument.
Although electronic instruments of all kinds are able to bypass the acoustic stage completely, the microphone remains the key tool of the recording engineer, and the microphone and loudspeaker are very close relatives, indeed. They are so close, in fact, that you can (in a pinch) use a loudspeaker as a mike, or vice versa. Technically speaking theyre interchangeable, though each has evolved separately to suit its particular task.
The microphones job is to "read" the sound waves traveling through the air, converting that acoustic energy into an electrical signal that can then be stored (i.e., recorded) and/or amplified. To do this, the microphone has a tiny diaphragm that moves in response to the sound waves and converts that vibrational movement into an electrical form using some kind of transducer.
The loudspeaker does the opposite. But because it has to generate sound vibrations rather than merely respond to them, the diaphragms have to be much larger, and the engineering as a whole must be on a much bigger scale. A microphone diaphragm that's just half an inch across will pick up all the sounds you want, but try to use it to reproduce music and youll probably blow it up. In truth, a headphone capsule is actually much closer to a microphone than a loudspeaker.
To do full justice to whatever music its trying to replay, the loudspeaker needs sufficient loudness capability to cover a realistically wide dynamic range. It must do so across the complete audio spectrum, from deepest bass to highest treble, without adding significant quantities of distortion and coloration. This is actually a very demanding task for a whole clutch of different and quite complex reasons, which we will address next.
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