Misplaced Pages

This is an old revision of this page, as edited by 125.190.137.42 (talk) at 06:48, 28 February 2006 (→External links). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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.

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 device that might fit this description was created in 1881.

Alexander 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).

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. 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

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.

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. Generally, 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 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 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 . 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

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.

Loudspeaker types

Woofers

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

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

A tweeter is a loudpeaker 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

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 effects) 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

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.

The ideal mount for a loudspeaker would be a flat board of infinite size with infinite space behind it. Thus the rear soundwaves cannot cancel the front soundwaves. An 'open baffle' loudspeaker is an approximation to this - the transducer is mounted on a simple board of size comparable to the lowest wavelength to be reproduced. However, for many purposes this is impractical and the enclosures must use other techniques to maximize the output of the loudspeaker (called loading).

Enclosures play a significant role in the sound production, adding resonances, diffraction, and other unwanted effects. Problems with resonance are usually reduced by increasing enclosure rigidity, added internal damping and increasing the enclosure mass. The speaker manufacturer Wharfedale has addressed the problem of cabinet resonance by using two layers of wood with the space between filled with sand. Home experimenters have designed speakers built from concrete sewer pipes for similar reasons. Diffraction problems are addressed in the shape of the enclosure; avoiding sharp corners on the front of the enclosure for instance. Sometimes the differences in reaction time of the different size drivers is addressed by setting the smaller drivers further back, by leaning or stepping the front baffle, so that the resulting wavefront from all drivers is coherent when it reaches the listener. The Acoustic Center of the driver, or physical position of each driver's voice coil, dictates the amount of rearward offset to time-align the drivers.

Enclosures used for woofer and subwoofer are applications that can be adequately modelled in the low frequency range (approximately 100–200 Hz and below) using acoustics and the lumped component model. For the purposes of this type of analysis, each enclosure has a loudspeaker topology. The most common enclosure types are listed below.

Closed-box enclosures

Infinite baffle

A variation on the 'open baffle' is to place the loudspeaker in a very large sealed box. The loudspeaker driver's mass and compliance, i.e. the stiffness of the suspension of the cone, determines the resonant frequency and damping properties of the system, which affect the low-frequency response of the speaker; the response falls off very sharply below the cabinet resonant frequency (F_cb). The designer trades off bass response for flatness; the larger the resonant peak in the bass, the lower the speaker will seem to reproduce, but the more over-emphasized the resonant frequency will be. The box must be large enough that the internal pressure caused when the driver cone moves backwards into the cabinet does not rise high enough to affect this. The box is usually filled loosely with foam, pillow stuffing, fiberglass, or other wadding, converting the speaker's thermodynamic properties from adiabatic to isothermal, and giving the effect of a larger cabinet.

Acoustic suspension

The closed-box or 'acoustic suspension' enclosure, rather than using a large box to avoid the effect of the internal air pressure, uses a smaller, tightly sealed box. The box is typically designed with a very small rate of leakage so that internal and external pressures can slowly equilibrate over time, allowing the speaker to adjust to changes in barometric pressure or altitude. In this case, the true suspension of the driver's cone is the air trapped inside the box which acts as a spring with very close to ideal behavior rather than the mechanical suspension of the speaker driver, which for this application must be very weak, just strong enough to keep the cone centered in the absence of any signal. The drawback of these speakers is their low efficiency, due to the loss of the power absorbed inside the cabinet.

Reflexed enclosures

Bass-reflex

Other types of enclosures attempt to improve the low frequency response or overall efficiency of the loudspeaker by using various combinations of reflex ports or passive radiating elements to transmit the energy from the rear of the speaker to the listener; these enclosures may also be referred to as vented/ported enclosures, bass reflex, transmission lines (see below). The interiors of such enclosures are also often lined with fiberglass matting for absorption. Reflex ports are tuned by amount of mass within the vent, using appropriate diameter and length to reach this point. This enclosure is the most common as it lends itself to small size and reasonable bass.

Compound or Band-pass

A 4 order bandpass is really just the same as a vented box where the contribution from the driver is trapped in a sealed box which modifies the resonance of the driver. In its simplest form it has two chambers. The dividing wall between the chambers has the driver mounted on it and the panel opposite to it (or the chamber into which the driver faces) has a port. If the enclosure on each side of the woofer has a port in it then the enclosure yields a 6 order band-pass response. This enclosure is considerably harder to design and tends to be driver-specific.

Passive radiator

Sometimes a passive radiator (PR) or drone, similar to a speaker driver but without an electrically activated voice coil, is used instead of a reflex port. Passive radiators are used primarily to tune small volumes to low frequencies, where a port would need to be very long. They are also used to eliminate port turbulence and reduce power compression caused by high velocity airflow in ports. Passive radiators are tuned by their mass (Mmp) and the way their compliance interacts with the compliance of the air in the box. Passive radiators add a complication to vented systems which causes a notch in frequency response at the PR's free air resonant frequency and this causes a steeper rolloff below the drone's tuning frequency Fb and poorer transient response than standard vented loudspeakers. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PR's to reflex ports. PR's do add considerable cost to the system, however.

Other enclosure types

Transmission line

The transmission line system is a waveguide system in which the guide shifts the phase of the driver's rear output by at least 90°, thereby reinforcing the frequencies near the driver's F_s. Transmission lines tend to be larger than the other systems, due to the size and length of the line required by the design (1/4th of a wavelength). In 2004 there was a breakthrough in this design category. The coupler transmission line enclosure was designed in part by veteran audio design engineers Brad Judah and Andy Bartha of Nucore Electromagnetics. The new coupler transmission line speaker enclosure design features a non-resonant characteristic and flat frequency response from 14 Hz to 25 kHz combined with high efficiency.

Dipole

A dipole enclosure in its simplest form is a driver located on a flat baffle. The baffle may be folded in order to conserve space. A rectangular cross-section is more common than a circular one since it is much easier to fabricate in folded form than a circular cross-section. The baffle dimensions are chosen to get the desired response, with larger dimensions giving a lower frequency before the front and rear waves combine and cancel

This section needs expansion. You can help by adding to it.

Horn

A horn (like a cheerleader horn) is an enclosure which has a flare or cone shaped structure attached to the front of the driver (speaker). This type has a very high efficiency and reasonably small size for reproducing mid to high frequencies. For the bass or low-frequency region the size of the horn becomes exceedingly large and impractical (3ft x 2ft x 2ft, for example). Some low frequency horns employ a folded horn design to conserve space. Designs that use horn woofers occupy a large space, and are heavy. Despite this, they are used about 70 to 90 percent of the time in large stadiums or arenas. To minimize the size, some bass horns are designed as a "modified" or "cut" horn. This means instead of using the perfect large size length of say 10ft, they cut it off at a length of say 3.3ft. This reduces the theoretical output by 3, but it is still maybe 5 times the output of a simple speaker in a box (no horn). This compromise is extremely attractive and used 90 percent of the time in bass horns.

Tapered Quarter Wave Pipe

The Tapered Quarter Wave Pipe (TQWP) is an example of a combination of transmission line and horn effects.

This section needs expansion. You can help by adding to it.

Phase or polarity

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 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:

• 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 , once commercially sold as the Ionovac . 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 , 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 , 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)

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

• Audio crossover
• Audiophile
• Computer speaker
• Ferrofluid
• Frequency response
• Headphone
• High-end audio
• Impedance
• Loudspeaker acoustics
• Music centre
• PMPO
• RMS
• Sensitivity
• Speaker wire
• 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.
• Free IC DataSheet Search Site : http://www.Datasheet4U.com

Manufacturers

• Altec Lansing (USA)
• Audiovox (USA)
• Bang & Olufsen (Denmark)
• 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
• PSB Speakers (Canada)
• Paradigm Electronics (Canada)
• Tannoy (UK)
• Teledyne (USA)
• Wharfedale Loudspeakers (UK)
• Yamaha (Japan)

• Speakers
• Telecommunications history