Dynamic Loudspeaker Principle

A current-carrying wire in a magnetic field experiences a magnetic force perpendicular to the wire.

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.

Loudspeaker basics
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Loudspeaker Basics

The loudspeakers are almost always the limiting element on the fidelity of a reproduced sound in either home or theater. The other stages in sound reproduction are mostly electronic, and the electronic components are highly developed. The loudspeaker involves electromechanical processes where the amplified audio signal must move a cone or other mechanical device to produce sound like the original sound wave. This process involves many difficulties, and usually is the most imperfect of the steps in sound reproduction. Choose your speakers carefully. Some basic ideas about speaker enclosures might help with perspective.

Once you have chosen a good loudspeaker from a reputable manufacturer and paid a good price for it, you might presume that you would get good sound reproduction from it. But you won't --- not without a good enclosure. The enclosure is an essential part of sound production because of the following problems with a direct radiating loudspeaker:


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The sound from the back of the speaker cone will tend to cancel the sound from the front, especially for low frequencies. The free cone speaker is very inefficient at producing sound wavelengths longer than the diameter of the speaker.
Speakers have a free-cone resonant frequency which distorts the sound by responding too strongly to frequencies near resonance. More power is needed in the bass range, making multiple drivers with a crossover a practical necessity for good sound.
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Loudspeaker Details

An enormous amount of engineering work has gone into the design of today's dynamic loudspeaker. A light voice coil is mounted so that it can move freely inside the magnetic field of a strong permanent magnet. The speaker cone is attached to the voice coil and attached with a flexible mounting to the outer ring of the speaker support. Because there is a definite "home" or equilibrium position for the speaker cone and there is elasticity of the mounting structure, there is inevitably a free cone resonant frequency like that of a mass on a spring. The frequency can be determined by adjusting the mass and stiffness of the cone and voice coil, and it can be damped and broadened by the nature of the construction, but that natural mechanical frequency of vibration is always there and enhances the frequencies in the frequency range near resonance. Part of the role of a good enclosure is to minimize the impact of this resonant frequency.

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Types of Enclosures

The 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.

The term "infinite baffle" is often encountered in discussions of loudspeaker installations. It visualizes a loudspeaker mounted in an infinite plane with unlimited volume behind it, but in practical use may refer to a loudspeaker mounted in the surface of a flat wall with considerable volume of air behind it. Because of the elastic properties of the loudpeaker suspension, it will still exhibit its natural free-cone resonance, but will be free of the diffraction effects observed with a small box speaker, and essentially free of the effects of the compression of the air behind the loudspeaker cone.


Sample bass response curves
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Reference
Cohen
 
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Use of Multiple Drivers in Loudspeakers

Even 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

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Line Array or Column Loudspeakers

One way to achieve a degree of directionality with bass loudspeakers and to achieve a modest increase in potential acoustic gain is to use several identical bass speakers in a "line array" or column geometry.

It may be surprising that the effect of this geometry is to spread the sound in the horizontal plane and narrow its spread in the vertical plane. This is the geometric effect of sound diffraction, and is analogous to the spreading of light through a narrow slit in the direction perpendicular to the narrow dimension.

The directional patterns below show the spreading of the sound in the horizontal plane on a polar scale indicating the diminishing of the sound in decibels compared to the centerline direction of the loudspeakers. For a column of bass speakers of size 15" or 38 cm, λ=4D corresponds to a frequency of about 225 Hz while the shortest wavelength, λ = 0.25D corresponds to a frequency of 3570 Hz.

Example of a line array loudspeaker collection which is ceiling-mounted in an auditorium. It points generally at the audience and spreads the sound perpendicular to the array.

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Reference
S. H. Stark
 
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Directionality of Loudspeakers

While 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.

Index

Sound reproduction concepts

Loudspeaker concepts

Reference
S. H. Stark
 
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