Loudspeaker

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A loudspeaker, or simply speaker, is an electromechanical transducer which converts an electrical signal into sound. The term loudspeaker is used to refer to both the device itself, and a complete system consisting of one or more loudspeaker drivers (as the individual units are often called) in an enclosure. The loudspeaker is the most variable element in an audio system, and is responsible for marked audible differences between systems.

Contents 1 History 2 Dynamic loudspeakers 2.1 Electrical characteristics of a dynamic loudspeaker 3 Loudspeaker types 3.1 Woofers 3.2 Mid-ranges 3.3 Tweeters 3.4 Full-ranges 3.5 Subwoofers 4 Enclosures 5 Phase or polarity 6 Construction and testing 7 Care of speakers 8 Efficiency 9 Specifications 10 Interaction with listening environments 11 Variations on the dynamic loudspeaker 12 Other technologies 12.1 Piezoelectric speakers 12.2 Plasma arc loudspeakers 12.3 Digital speakers 12.4 Flat panel speakers 12.5 Electrostatic loudspeakers (ESL) 12.6 Converting ultrasound to audible sound 13 Home cinema speakers 14 Wireless 15 Multi driver systems 16 See also 17 External links 17.1 Manufacturers

History

Nikola Tesla is believed to have put electrically charged carbon dust in a cup-shaped device to create the first telephone loudspeaker. However, the first documented ^[1] device that might fit this description was created in 1881.

Alexander Graham Bell patented the first loudspeaker as part of his telephone in 1876. This was soon followed by an improved version from Ernst Siemens in Germany and England (1878). The modern design of moving-coil loudspeaker was established by Oliver Lodge in England (1898). [2]

The moving coil principle was patented in 1924 by two Americans, Chester W. Rice and Edward W. Kellogg. There is some controversy in that an application was made earlier by the Briton Paul Voigt but not granted until later. Voigt produced the first effective full range unit in 1928, and he also developed what may have been the first system designed for the home, although using electromagnets rather than permanent magnets.

These first loudspeakers used electromagnets because large, powerful permanent magnets were not freely available at reasonable cost. The coil of the electromagnet is called a field coil and is energized by direct current through a second pair of terminals. This winding usually served a dual role, acting also as a choke coil filtering the power supply of the amplifier which the loudspeaker was connected to.

The quality of loudspeaker systems until the 1950s was, to modern ears, very poor. Developments in cabinet technology (e.g. acoustic suspension) and changes in materials used in the actual loudspeaker, led to audible improvements. For example, paper cones (or doped paper cones, where the paper is treated with a substance to improve its performance) are still in use today, and can provide good performance. Polypropylene and aluminium are also used as diaphragm materials.

The first commercial acoustic suspension loudspeaker was developed by Henry Kloss and Edgar Villchur at Acoustic Research, and further developed by KLH_(company).

Additional improvements to loudspeaker technology occurred in the 1970s, with the introduction of higher temperature adhesives, improved permanent magnet materials, and improved thermal management.

Dynamic loudspeakers

Image:Speaker cross section.PNG Image:SpkFrontCutawayView.png The traditional design is a semi-rigid paper fibre cone and a coil of fine wire (usually copper), called the voice coil attached to the apex of the cone. A "gap" is a small circular hole, slot or groove which allows the voice coil and cone to move back and forth. The coil is oriented coaxially inside the gap made with a permanent magnet. The gap is also where the magnetic field is concentrated. One magnetic pole is outside the coil, whilst the other is inside the voice coil. In addition to the magnet, voice coil, and cone, dynamic speakers usually also include a suspension system to provide lateral stability and make the speaker components return to a neutral point after moving. A typical suspension system includes the 'spider', which is at the apex of the cone, often of 'concertina' form; and the 'surround', which is at the base of the cone. The parts are held together by a chassis or basket.

When an electrical signal is applied, a magnetic field is induced by the electric current in the coil which becomes an electromagnet. The coil and the permanent magnet interact with magnetic force which causes the coil and a semi-rigid cone (diaphragm) to vibrate and reproduce sound at the frequency of the applied electrical signal. When a multi-frequency signal is applied, the complex vibration results in reproduction of the applied signal as an audio signal.

Image:Loudspeaker.arp.500pix.jpg Driver cones may be constructed of a variety of materials, including paper, metal, various polypropylenes, and kevlar. Baskets must be designed in order to preserve rigidity and are typically cast or stamped metal, although injection-molded plastic baskets are becoming much more common in recent years. The size and type of magnets can also differ. Sometimes, larger and more powerful magnets are associated with higher quality speakers. Tweeters are subject to a unique set of variables and parameters; their design and construction is extremely variable.

Despite marketing claims, lighter and more rigid cones do not always sound better. The weight and damping of the cone in a dynamic speaker should be appropriate for the characteristics of the rest of the driver and enclosure in order to produce accurate sound.

The cross-section image shows the construction of a speaker that uses an overhung coil. This term is used when the coil's height is greater than the gap's, and this construction is most commonly used. This construction attempts to keep the number of windings within the gap (and hence the force experienced by the coil) to be constant, as the coil moves back and forth. The other construction is the underhung construction. Here, the voice coil's height is smaller than the gap's. This construction attempts to keep the magnetic flux constant across the coil as it moves back and forth (this also results in the coil experiencing a constant force). Both methods try to achieve the same thing — a linear force on the coil, and each has its pros and cons. The overhung design offers softer or gradual non-linearity (compression) when the coil starts exceeding the X_max limits. Speakers using overhung coils have better efficiency and power handling. The underhung design offers greater linearity within the X_max limits but as the coil starts exceeding X_max limits the non-linearity (and hence distortion) rapidly increases. This design also requires a more powerful magnet, and so speakers using underhung coils tend to be less efficient [3]. X_max is the maximum linear peak-to-peak travel of the voice coil. This means that the displacement of the voice coil is a linear function of the input voltage within the limits specified by X_max.

Electrical characteristics of a dynamic loudspeaker

Template:Main A dynamic loudspeaker presents a complex load to the amplifier as opposed to a pure resistance. It is a combination of resistive, capacitive, inductive as well as mechanical effects. A typical amplifier (amp) is most usually quoted for a given power into a resistive load. However a loudspeaker of say, a rated impedance of 8Ω/100W can easily overload an amp designed with a purely resistive load of 8Ω/100W as a target. [4]

Loudspeaker types

Woofers

Template:Main A woofer is a loudspeaker capable of reproducing the bass frequencies. The frequency range varies widely according to design and hence while some woofers can cover the audio band from 50 Hz to 3 kHz, yet others may only work up to 1 kHz.

Mid-ranges

Template:Main A mid-range loudspeaker, also known as a squawker is designed to cover the middle of the audio spectrum, typically from about 200 Hz to about 4-5 kHz. The distinction between woofers and mid-ranges is blurred however since many woofers can operate up to 3 kHz. These are used when the bass driver (or woofer) is incapable of covering the mid audio range. Mid-ranges typically appear where large (>16 cm or 8") woofers are used for the bass end of the audio spectrum.

Tweeters

Template:Main A tweeter is a loudspeaker capable of reproducing the higher end of the audio spectrum, usually from about 1 kHz to 20 or 35 kHz.

Full-ranges

Main article: Full-range

A full-range speaker is designed to have as wide a frequency response as possible. These employ an additional cone called a whizzer to extend the high frequency response. A whizzer is a small, light cone attached to the woofer's apex around the dust cap. However, the whizzer might not be necessary if the diaphragm is stiff and light enough. There exist full-range drivers with pure titanium diaphragm which are capable of reproducing a frequency range from 50 Hz to 20 kHz and higher without the whizzer cone, and also capable of avoiding the break-ups in the highest frequency range. These drivers are often quite small. Typically 2" to 5" in diameter.

Subwoofers

Template:Main A subwoofer driver is a woofer optimised for the lowest range of the audio spectrum. Modern speaker systems often include a single speaker dedicated to reproducing the very lowest bass frequencies. This speaker (and its enclosure) is referred to as a subwoofer. A typical subwoofer only reproduces sounds below 120 Hz (although some subwoofers allow a choice of the cross-over frequency). Because the range of frequencies that must be reproduced is quite limited, the design of the subwoofer is usually quite simple, often consisting of a single, large, down-firing woofer enclosed in a cubical "bass-reflex" cabinet. Subwoofers often contain integrated power amplifiers that may incorporate sophisticated feedback mechanisms to assure the least distortion of the reproduced bass acoustic waveform.

The very long wavelength of the very low frequency bass sounds reproduced by the subwoofer usually makes it impossible for the listener to localize the source of these sounds. Localization starts to happen above the 60Hz point. Because of this phenomenon, it is usually satisfactory to provide just a single subwoofer no matter how many individual channels are being used for the full-spectrum sound. For the same reason, the subwoofer does not need a special placement in the sound field (for example, centered between the Left Front and Right Front speakers). It can instead be hidden out of sight. Placing it in the corner of a room may produce louder bass sounds. A subwoofer's powerful bass can often cause items in the room or even the structure of the room itself to vibrate or buzz. Extended periods of high volume bass can cause items throughout a room to "walk" on a flat surface until they fall off.

Amplified subwoofers frequently accept both speaker-level and line-level audio signals. When teamed with a modern surround sound receiver and full range speakers, they are typically driven with the specific LFE (low frequency enhancement) output channel (the ".1" in 5.1, 6.1, or 7.1 specifications) provided by the receiver. This is because most full-range speakers are incapable of delivering the acoustic power required by the LFE in movies or in some cases, music. When used with speakers that do not reproduce low frequencies well, a subwoofer will often be configured to reproduce both the LFE channel and all other bass in the system, the latter being referred to as "bass management".

Enclosures

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A loudspeaker is commonly mounted in an enclosure (or cabinet). The major role of the enclosure is to prevent the out-of-phase sound waves from the rear of the speaker combining with the positive phase sound waves from the front of the speaker, which would result in interference patterns and cancellation causing the efficiency of the speaker to be compromised, particularly in the low frequencies where the wavelengths are large enough that interference will affect the entire listening area.

Phase or polarity

Image:Banana plugs speaker.jpg All speakers have two wires, and in a multi-speaker system these must be connected from the source of the signal (the amplifier or receiver) to the speaker's input terminals with the correct polarity, or phase. If both sets of wires for left and right (in a stereo setup) are not connected in phase, the speakers will be out of phase from each other. In this case, any motion one cone makes will be opposite to the other. This type of wiring error creates inverse sound waves that partially cancel out the sound of the other speaker. This does not cause silence, because reflections from surfaces diminish the effect somewhat, but results in a major loss of sound quality. The most prominent effect to the untrained ear is a loss of bass response. The second most noticed is an unsettling feeling.

A similar effect is used in sound-cancelling headphones. The headphones produce the inverse sound waves of the external noise. The inverse sound waves and external noise cancel each other out and produce near silence.

Construction and testing

Speaker design is considered both an art and science. Adjusting a design is done with instruments and with the ear. Speaker designers use an anechoic chamber (essentially a room with soundproofing that inhibits any reverberation or echo) to ensure the speaker will perform the way it is intended to. Some developers (such as Bose) eschew the sole use of anechoic chambers in favor of specific standardized room set-ups designed to replicate likely real-life listening conditions. Some of the issues in speaker design are lobing, phase effects, off axis response and time coherence.

Care of speakers

Loudspeakers are rugged devices and can take some amount of abuse. However they do have limits and exceeding them by a large factor almost always causes permanent damage. The tweeters are usually the first to go under circumstances of abuse, since they have the lightest voice coil made of thin wire which easily melts if the temperature rises excessively. Tweeters are usually designed (and rated) keeping in mind that a typical music signal doesn't contain a lot of power or energy at the higher end of the audio spectrum. Thus a tweeter rated for 50 W is meant to be used with a 50 W amplifier only if the signals below the tweeter's lower operating frequency are filtered out. Thus, feeding a low frequency (or a DC) signal to a tweeter even though electrically it may be within the tweeter's specification may cause permanent damage to the tweeter. A badly clipping amplifier may also damage the tweeter despite a crossover, since a clipped waveform generates high-frequency harmonics which can contain sufficient power to heat up the tweeter's voice coil. Most woofers (and mid-ranges) can easily take up to 1.5 times or more power than what they are rated for - however this is dependent on the particular driver and the duration of the abuse or overload. Woofers will usually take a lot of power before burning out or suffering damage to their moving systems. Physical damage occurs if the signal causes the woofer's cone displacement to exceed the safe X_mech limits for prolonged periods. In rare cases, a very loud signal may cause the coupling between the parts of the woofer to simply give way. A large DC fed to the woofer may cause twisting or deformation of the voice coil such that it rubs against the pole-pieces or magnet. Electrical damage occurs when the voice coil burns out. The latter two typically happen when the amplifier dumps a large DC current into the speaker - a condition typical of a failing (or failed) amplifier. In all cases, replacement or full repair of the driver are the only options.

Efficiency

The sound pressure level (SPL) that a loudspeaker produces is measured in decibels (dBSPL). The efficiency is measured as dB/(W·m)—decibels output for an input of one nominal watt measured at one metre from the loudspeaker usually on the axis of the speaker. This is called the "sensitivity" rating. Loudspeakers are very inefficient transducers. Only about 1% of the electrical energy put into the speaker is converted to acoustic energy. The remainder is converted to heat. The main reason for this low efficiency is the difficulty of achieving proper impedance matching between the acoustic impedance of the drive unit and that of the air. This is especially difficult at lower frequencies. The better the matching, the higher the efficiency. Large horn loudspeakers that used to be used in cinemas, were very efficient by today's hi-fi speaker standards. From a technical standpoint "sensitivity" is not the absolute reference of efficiency. As an example, a simple cheerleader's horn makes more sound output in the direction it is pointed than the cheerleader could by herself, but the horn did not improve or increase the cheerleader's total efficiency. True or absolute efficiency is the ratio of "desired" output power divided by total input power.

• Normal loudspeakers have a sensitivity of 85 to 95 dB/(W·m).
• Nightclub speakers have a sensitivity of 95 to 102 dB/(W·m).
• Rock concert, stadium speakers have a sensitivity of 103 to 110 dB/(W·m).

Current state-of-the-art loudspeakers can approach efficiencies of 70% or higher. This is partly due to a very high magnetic field and partly to a high amplitude displacement (speaker cone pumping in and out). The ratio of the sound output to the mass of the cone/coil combination grows significantly at high sound pressure levels i.e. above 140 decibels. In closed or small environments (such as cars or bedrooms) it is far more important to have a speaker with a high X_max (cone eXcursion maximum) as opposed to high (dB/(W·m)) rating. A higher X_max indicates that the driver can move a larger volume of air as power increases. A few top of the line woofers have a very low "sensitivity" rating i.e. 80 to 86 dB/(W·m) (sensitivity efficiency of 0.01%). However at full power may achieve 160+ decibels at 20% to 40% "true" efficiency.

• video of 158 dB woofer at 80 millimeter amplitude 450 kB mpeg

As shown in this example, sometimes the speaker with the lower sensitivity rating outputs a far higher amount of acoustic watt output.

In general a higher quality speaker will have a higher sensitivity rating, larger and or heavier magnet, and a higher X_max.

It should be noted that a higher power driver will not necessarily be louder than a lower power one. For example a speaker that is 3 dB more efficient than another will produce double the SPL (or loudness) for the same power than the other. Thus a 100W speaker (A) rated at 92 dB/(W.m) efficiency will be double as loud as a 200W speaker (B) rated at 89 dB/(W.m) when both are driven with 100W of input power. For this particular example, when driven at 100W, speaker A will produce the same SPL or loudness that speaker B would produce with 200W input. Thus a 3 dB increase in efficiency of the speaker means that it will need half the power to achieve a given SPL, and this translates into a smaller power amplifier and some cost savings.

Specifications

Speaker specifications generally include:

Image:Eltax Silverstone 200 loudspeaker label.jpg

• Speaker or driver type (Individual units only) – Full-range, woofer, tweeter or mid-range.
• Rated Power – Nominal or continuous or RMS power and peak or maximum short-term power.
• Impedance – 4 Ω, 8 Ω, etc.
• Baffle or enclosure type (Finished systems only) – Sealed, bass reflex, etc.
• Number of drivers (Finished systems only) – 2-way, 3-way, etc.

and optionally,

• Crossover frequency(ies) (Finished systems only) – The frequency or frequencies where electrical filtering occurs.
• Frequency response – The measured or specified variance in sound pressure level over a range of frequencies.
• Thiele/Small parameters (Individual units only) – These include the driver's F_s (resonance frequency), Q_ts (the driver's Q or damping factor at resonance), and V_as (the equivalent air compliance volume of the driver).

Interaction with listening environments

A complication is the interaction of the speaker with the listening environment. This interaction affects the speaker's electromechanical behavior and thus the load it represents to the amplifier, making it difficult to predict the sound a given system will produce in its intended environment without listening tests. It has been theorized by some of the audiophile world that the perceived differences in sound between amplifier/loudspeaker combinations are in fact only differences in their interaction with their environment, rather than absolute differences in sound quality; and similarly, that any perceived differences in speaker cables, past a minimum set of specifications regarding resistance, inductance, capacitance, etc. are mainly due to advantageous interactions with a particular speaker-room combination.

Variations on the dynamic loudspeaker

One problem with loudspeakers is that the essentially-planar form of most loudspeakers creates a soundwave that is somewhat directional, that is, the intensity of the sound produced varies depending on the listener's angle relative to the central axis of the speaker. This is especially a problem for high frequencies where the loudspeaker may be physically large compared to the wavelength of the sound being reproduced. A point source or a sphere that varies in size with the amplitude of the desired pressure wave would avoid this problem of beam-formation but is generally physically impossible or impractical. Several approaches have attempted to remedy this by approximating the sphere.

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.

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). Perhaps contrary to intuition, making the moving component in the form of a dome rather than an inverted cone does not help to direct sound evenly in all directions. The dome is used because it is an easily manufactured stiff structure - as anyone who has attempted to crush an egg the long way can attest to. The stiffness moves self resonances upward in frequency.

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 recently designed planar tweeters 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.

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.

Other technologies

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

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:

• 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 [5], once commercially sold as the Ionovac [6]. 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 [7], 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 [8], much as the advent of digital audio caused the marketing of a large number of "digital" headphones and speakers whose drive-units were analog in nature.

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 often, 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.

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.

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. A newer implementation of the Flat Panel involves the panel and an "exciter", such as the NXT technology.

Electrostatic loudspeakers (ESL)

Template:Main 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.

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.

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.

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.

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

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.

See also sound reproduction, electronics

Home cinema speakers

There are various different speaker set-ups for home cinema speaker systems. They include :

• 5.1 channel sound. This requires:
  • Left, center, and right front speakers
  • Left and right surround speakers
  • A subwoofer (which is counted as ".1" channel because of the narrow frequency band that it reproduces). This speaker can reproduce the bass frequency from all the main channels or may only do so for those speakers incapable of doing so. This is usually achieved by an amplifier setting of 'large' or 'small' defining the speaker type.
• 6.1 channel sound is similar to 5.1 but there is an added center rear channel
• 7.1 channel sound in home theater is identical to 6.1 except that it has left and right rear speakers. In SDDS, 7.1 is the same as 5.1 but adding center-left and center-right speakers in the front of the listener for better audio positioning.

It is important to note that the sound channels offered to the speakers may be original individual channels (normal 5.1) or they may decode additional channels from the surround channels (This distribution can be accomplished by a Dolby Digital EX decoder, a THX Surround EX decoder) or they may be simulated (where the two surround channels are spread to center rear or twin rear speakers.

See also: Home theater in a box

Wireless

So-called wireless loudspeakers are becoming popular in many applications, such as home theater, due to their convenience, removing the need to run speaker wire. Despite its name, however, the unit is really a wireless receiver, amplifier and loudspeaker in a single box.

Multi driver systems

Home cinema systems generally include multi-driver systems.

'Multi driver' refers to any speaker system that contains two or more separate drive units, including woofers, midranges, tweeters, and sometimes horns or supertweeters. Many multi driver systems use a bass reflex, or ported, design. These incorporate a small hole, (called a port), in the speaker cabinet to allow the low frequencies generated by the rear of the woofer cone to escape from the cabinet in phase with that radiated from the front of the cone. This improves the bass response of the system.

See also

Template:Commons

• Audio crossover
• Audiophile
• Computer speaker
• Ferrofluid
• Frequency response
• Headphone
• High-end audio
• Impedance
• Loudspeaker acoustics
• Loudspeaker measurement
• Music centre
• PMPO
• RMS
• Sensitivity
• Speaker wire
• Subwoofer
• Thiele/Small Parameters are used to determine the ideal enclosure size for any given transducer.

External links

• Cable Nonsense -This newsgroup message from a speaker manufacturer is very informative about issues of speaker cables.
• The Audio Circuit - An almost complete list of manufacturers of loudspeakers.

Manufacturers

• Altec Lansing (USA)
• Audiovox (USA)
• Bang & Olufsen (Denmark)
• Behringer (Germany)
• Blaupunkt (Germany)
• Bose (USA)
• Boston Acoustics (USA)
• Bowers & Wilkins (UK)
• Bravox (Brazil)
• Creative (Singapore)
• DALI (Denmark)
• Genelec (Finland)
• JBL (USA)
• KEF (UK)
• KLH (USA)
• Klipsch (USA)
• Krell (USA)
• Linn Products (UK)
• Logitech (USA)
• Mackie (USA)
• Martin-Logan
• Naim (UK)
• Paradigm Electronics (Canada)
• Philips (the Netherlands)
• PSB Speakers (Canada)
• Tannoy (UK)
• Teledyne (USA)
• Wharfedale Loudspeakers (UK)
• Yamaha (Japan)

ca:Altaveu cs:Reproduktor da:Højttaler de:Lautsprecherbox es:Altavoz eo:Laŭtparolilo fr:Haut-parleur hr:Zvučnik id:Speaker it:Altoparlante he:רמקול hu:Hangszóró nl:Luidspreker ja:スピーカー no:Høyttaler pl:Głośnik pt:Altifalante fi:Kaiutin tr:Hoparlör