Loudspeaker

A loudspeaker, or simply speaker, is an electromechanical device which converts an electrical signal into sound. The term is used to refer to both the transducer, or driver itself, and a complete system consisting of one or more transducers in an enclosure. The loudspeaker is the most variable element in an audio system. The audible differences between speaker systems are considerable.

Variations on the dynamic loudspeaker

One problem with loudspeakers is that the original soundwave usually radiates outwards in a spherical wavefront that reaches both ears; this is difficult to replicate with the usual, essentially planar loudspeaker designs as it is difficult to create either a point source for the sound or a sphere that varies in size with the amplitude of the desired pressure wave. Several approaches have attempted to remedy this by approximating the sphere.

Related Topics:
Spherical - Wavefront - Planar - Point source

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Amar Bose of MIT spent many years trying to reproduce this spherical wavefront by constructing a one-eighth sphere covered in small drivers that would be situated in the corner of a room, thus mimicking one-eighth of a spherical wavefront emanating from that corner; in practice this idea never became workable, but Bose's experience with combining multiple small drivers in one loudspeaker cabinet gave rise to the popular Bose speakers which use multiple four-inch drivers, either to direct sound rearwards to reflect it from a wall behind the speakers, for home use, or to provide high power capacity when aimed directly at the listeners, for professional use.

Related Topics:
Amar Bose - MIT - Bose

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For high frequencies, a variation on the common dynamic loudspeaker design uses a small dome as the moving part instead of an inverted cone. This design is typically used for tweeters and sometimes for mid-range speakers. Because the wavelength of high-frequency sound is short (approximately 15 mm at 20 kHz), tweeters must have a physically small moving component or they will create a "beam" of sound rather than sending sound omnidirectionally (as is usually desired). Making the moving component in the form of a dome rather than an inverted cone also helps direct sound evenly in all directions. The dome moving forwards and backwards provides a very simple approximation to the ideal shape of a sphere that enlarges and contracts.

Related Topics:
Dome - Tweeter - Mid-range speaker - Wavelength - KHz - Sphere

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The ribbon loudspeaker consists of a thin metal-film ribbon suspended between two magnets. The electrical signal is applied to the ribbon which vibrates creating the sound. The advantage of the ribbon loudspeaker is that the ribbon has very little mass; as such, it can accelerate very quickly, yielding good high-frequency response (although its shape is far from ideal). Ribbon loudspeakers can be very fragile but recent designs have the metal film printed on a strong lightweight material for reinforcement. Ribbon tweeters often emit sound that exits the speaker concentrated into a flat plane at the level of the listeners' ears; above and below the plane there is often less treble sound.

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The Ohm model "F" speakers invented by Lincoln Walsh feature a single driver mounted vertically as though it were firing downwards into the top of the cabinet, but instead of the normal almost flat cone, having a very-much extended cone entirely exposed at the top of the speaker. This turned normal speaker driver design problems on their head; whereas the normal problem with designing a driver is how to keep the cone as stiff as possible (without adding mass), so that it moved as a unit and did not become subject to traveling waves on its surface, the Ohm drivers were designed so that the entire purpose of the electromagnetic driver was to generate traveling waves that traversed the cone from the electromagnet at the top downwards to the bottom. As the waves moved down the truncated cone, the effect was to reproduce the omnidirectional soundwave, as with a cylinder that changed diameter. This created a very effective omnidirectional radiator (although it suffered the same "planarity" effect as ribbon tweeters for higher-frequency sounds) and eliminated all problems of multiple drivers, such as crossover design, phase anomalies between drivers, etc. However, in practice it was found necessary to use a very complex cone made up of various materials at different points along its length, in order to maintain the waveform traveling evenly. See more details here.

Related Topics:
Lincoln Walsh - Traveling wave - Cylinder - Diameter

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Other technologies

Other technologies can be used to convert the electrical signal into an audio signal. These include piezoelectric, electrostatic, and plasma arc loudspeakers.

Related Topics:
Piezoelectric - Electrostatic - Plasma arc loudspeaker

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Piezoelectric speakers

Piezoelectric transducers, frequently used as beepers in watches etc., are often used as tweeters in cheap speaker systems. Computer speakers and portable radios are common examples. Piezos have several advantages over conventional loudspeakers when applied to such purposes:

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• Piezoelectric transducers have no voice-coil, therefore there is no electrical inductance to overcome; it is easy to couple high-frequency electrical energy into the piezoelectric transducer, especially under the low-power, non-critical applications in which they are usually employed.
• Piezoelectric transducers are physically small yet powerful, leading to good dispersion, although the fidelity of such devices remains in question when it comes to critical listening.
• Piezoelectric transducers are resistant to overloads that would normally burn out the voice coil of a conventional loudspeaker.
• Because piezos comprise a capacitive load, they usually do not require an external cross-over network; they can simply be placed in parallel with the inductive woofer/midrange loudspeaker(s).

Plasma arc loudspeakers

The most exotic speaker design is undoubtedly the plasma arc loudspeaker, using electrical plasma as a driver http://www.plasmatweeter.de/home.htm, once commercially sold as the Ionovachttp://www.belgaudio.com/ionophone.htm. Since plasma has minimal mass, but is charged and therefore can be manipulated by an electric field, the result is a very linear output at frequencies far higher than the audible range. As might be guessed, problems of maintenance and reliability for this design tend to make it very unsuitable for the mass market; the plasma is generated from a tank of helium which must be periodically refilled, for instance. A lower-priced variation on this theme is the use of a flame for the driver http://www.madsci.org/posts/archives/feb98/888372043.Ot.r.html, flames being commonly electrically charged. Unfortunately, the recent marketing of plasma displays as high-end television sets and computer monitors has caused the me-too labeling of many speakers as "plasma" which have nothing whatsoever to do with plasma http://www.proaudiosuperstore.com/electrovoice-plasma-speakers.html, much as the advent of digital audio caused the marketing of a large number of "digital" headphones and speakers.

Related Topics:
Plasma arc loudspeaker - Plasma - Ionovac - Electric field - Plasma display - Digital audio

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Digital speakers

Actual digital speaker driver technology not only exists, but is quite mature, having been experimented with extensively by Bell Labs as far back as the 1920s. The design of these is disarmingly simple; the least significant bit drives a tiny speaker driver, of whatever physical design seems appropriate; a value of "1" causes this driver to be driven full amplitude, a value of "0" causes it to be completely shut off. (This allows for high efficiency in the amplifier, which at any time is either passing zero current, or required to drop the voltage by zero volts, therefore theoretically dissipating zero watts at all times). The next least significant bit drives a speaker of twice the area (most efficiently, but not necessarily, a ring around the previous driver), again to either full amplitude, or off. The next least significant bit drives a speaker of twice this area, and so on.

Related Topics:
Bell Labs - 1920s - Least significant bit

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There are two problems with this design which led to its being abandoned as hopelessly impractical, however; firstly, a quick calculation shows that for a reasonable number of bits required for reasonable sound reproduction quality, the size of the system becomes very large. For example, a 16 bit system to be compatible with the 16 bit audio CD standard, starting with a reasonable 2 square inch driver for the least significant bit, would require a total area for the drivers of over 900 square feet. Secondly, since this system is converting digital signal to analog, the effect of aliasing is unavoidable, so that the audio output is "reflected" at equal amplitude in the frequency domain, on the other side of the sampling frequency. Even accounting for the vastly lower efficiency of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonics accompanying the desired output. In electronic digital to analog conversion, this is addressed by the use of Low-pass filters to eliminate the spurious upper frequencies produced; however, this approach cannot be used to solve the problem with this digital loudspeaker, since it is the last link in the audio chain.

Related Topics:
Audio CD - Aliasing - Sampling frequency - Ultrasonic - Digital to analog - Low-pass filter

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Flat Panel speakers

There have also been many attempts to reduce the size of loudspeakers, or alternatively to make them less obvious. One such attempt is the development of flat panels to act as sound sources. These can then be either made in a neutral colour and hung on walls where they will be less noticeable, or can be deliberately painted with patterns in which case they can function decoratively. There are two, related problems with flat panel technology; firstly, that the flat panel is more flexible than the cone shape and therefore fails to move as a solid unit, and secondly that resonances in the panels are difficult to control, leading to considerable distortion in the reproduced sound. Some progress has been made using such rigid yet damped material as styrofoam, and there have been several flat panel systems demonstrated in recent years. An advantage of flat panel speakers is that the sound is perceived as being of uniform intensity over a wide range of distances from the speaker. Flat panel loudspeaker designs also work well as electrostatic loudspeakers.

Related Topics:
Styrofoam - Electrostatic loudspeakers

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Electrostatic loudspeakers (ESL)

Some speakers are electrostatically driven rather than via the usual electromechanical voice coil, thereby giving a more linear response; the disadvantage, however, is that the signal must be converted to a very high voltage and low current, which can be problematic for reliability and maintenance as they attract dust, and develop a tendency to arc, particularly where the dust provides a partial path; the point where the arc occurs often becomes more prone to arcing, as carbon builds up from the burned dust.

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Electrostatic Loudspeakers (ESL)

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Converting ultrasound to audible sound

A transducer can be made to project a narrow beam of ultrasound that is powerful enough, (100 to 110 DBSPL) to change the speed of sound in the air that it passes through. The ultrasound is modulated-- it consists of an audible signal mixed with an ultrasonic frequency. The air within the beam behaves in a nonlinear way and demodulates the ultrasound, resulting in sound that is audible only along the path of the beam, or that appears to radiate from any surface that the beam strikes. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small, well-defined area. A listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam.

Related Topics:
Ultrasound - DBSPL - Modulated

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There are some criticisms of this approach. Anyone or anything that disrupts the path of the beam will disturb the dispersion of the signal, and there are limitations, both to the frequency response and to the dispersion pattern of such devices.

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This technology was originally developed by the US (and Russian) Navy for underwater sonar in the mid-1960s, and was briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 (105th AES Conv, Preprint 4853, 1998) fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.

Related Topics:
Sonar - 1960s - 1980s - Massachusetts Institute of Technology - 1998

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The technology, termed the Audio Spotlight, was first made commercially available in 2000 by Holosonics, a company founded by Dr. Pompei.

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There are currently two devices available on the market that use ultrasound to create an audible "beam" of sound: the Audio Spotlight and Hypersonic Sound. See AudioSpotlights.com for more information.

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See also sound reproduction, electronics

Related Topics:
Sound reproduction - Electronics

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