Dynamic Loudspeaker Principle
An audio signal source such as a microphone or recording produces an electrical "image" of the sound. That is, it produces an electrical signal that has the same frequency and harmonic content, and a size that reflects the relative intensity of the sound as it changes. The job of the amplifier is to take that electrical image and make it larger -- large enough in power to drive the coils of a loudspeaker. Having a "high fidelity" amplifier means that you make it larger without changing any of its properties. Any changes would be perceived as distortions of the sound since the human ear is amazingly sensitive to such changes. Once the amplifier has made the electrical image large enough, it applies it to the voice coils of the loudspeaker, making them vibrate with a pattern that follows the variations of the original signal. The voice coil is attached to and drives the cone of the loudspeaker, which in turn drives the air. This action on the air produces sound that more-or-less reproduces the sound pressure variations of the original signal.
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Loudspeaker Basics
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Types of EnclosuresThe production of a good high-fidelity loudspeaker requires that the speakers be enclosed because of a number of basic properties of loudspeakers. Just putting a single dynamic loudspeaker in a closed box will improve its sound quality dramatically. Modern loudspeaker enclosures typically involve multiple loudspeakers with a crossover network to provide a more nearly uniform frequency response across the audio frequency range. Other techniques such as those used in bass reflex enclosures may be used to extend the useful bass range of the loudspeakers. The nature of the enclosure can affect the efficiency and the directionality of a loudspeaker. The use of horn loudspeakers can provide higher efficiency and more directionality, but in extremes can reduce the fidelity of the sound. Line array enclosures can provide some directionality.
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Use of Multiple Drivers in LoudspeakersEven with a good enclosure, a single loudspeaker cannot be expected to deliver optimally balanced sound over the full audible sound spectrum. For the production of high frequencies, the driving element should be small and light to be able to respond rapidly to the applied signal. Such high frequency speakers are called "tweeters". On the other hand, a bass speaker should be large to efficiently impedance match to the air. Such speakers, called "woofers", must also be supplied with more power since the signal must drive a larger mass. Another factor is that the ear's response curves discriminate against bass, so that more acoustic power must be supplied in the bass range. It is usually desirable to have a third, mid-range, speaker to achieve a smooth frequency response. The appropriate frequency signals are routed to the speakers by a crossover network. |
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Horn Loudspeakers
Reference: Wiki: Compression Driver Wiki: Horn loudspeaker |
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Line Array or Column Loudspeakers
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Directionality of LoudspeakersWhile loudspeakers cannot achieve the extremes of directionality of microphones, efforts to control the directionality of sound from loudspeakers can be productive. Horn type loudspeakers are generally more directional than open cone-type loudspeakers. Speakers in a line array can be arranged to spread the sound in the horizontal plane more than the vertical plane to direct the sound energy more at the listeners. Making the loudspeakers more directional can increase the potential acoustic gain that can be achieved with a sound amplification system. The example of a small loudspeaker shows that high frequencies from loudspeakers are typically more directional than low frequencies. For sound from a single loudspeaker, this means that the bass-treble balance will become more prominent in bass as you move further off-axis. Most sound systems use some kind of cross-over network so that the bass and treble can be controlled separately to achieve a good balance. Often, horns for the bass are used because they are more directional than other bass speakers and can be combined with treble sources of comparable directionality. |
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