AskDefine | Define loudspeaker

Dictionary Definition

loudspeaker n : electro-acoustic transducer that converts electrical signals into sounds loud enough to be heard at a distance [syn: speaker, speaker unit, loudspeaker system, speaker system]

User Contributed Dictionary

English

Noun

loudspeaker (plural: loudspeakers)
  1. An electromechanical transducer that converts an electrical signal into audible sound.
  2. An encasing containing one or more loudspeaker devices and usually other electrical equipment such as a driver.

Synonyms

Translations

Loudspeaker:

Extensive Definition

'' For the Marty Friedman album, see Loudspeaker (album)
A loudspeaker, speaker, or speaker system is an electromechanical transducer that converts an electrical signal to sound. The term loudspeaker can refer to individual devices (otherwise known as drivers), or to complete systems consisting of an enclosure incorporating one or more drivers and additional electronic components. Loudspeakers, as with other electro-acoustic transducers, are the most variable elements in an audio system and are responsible for the greatest degree of audible differences between sound systems.
To reproduce a wide range of frequencies, most loudspeaker systems require more than one driver, particularly for high sound pressure level or high fidelity applications. Individual drivers are used to cover different frequency ranges. The drivers are named subwoofers, for very low frequencies; woofers, for low frequencies; mid-range speakers, for middle frequencies; tweeters, for high frequencies; and, also, the so-called supertweeters, which are basically tweeters optimized for higher frequencies than a normal tweeter.
These terms for different speaker applications/ranges can differ widely depending on the application. Home stereos use the designation "tweeter" for high frequencies whereas professional audio systems for concerts typically designate all types of high frequency drivers simply as HF or "highs". High frequency compression driver units are also called "horns" in cases where the professional loudspeaker's lower frequency drivers are front-loaded. There is also a distinct difference in terminology between that used in the U.S. versus the U.K.
A "filter network", called a crossover separates the incoming signal into different frequency bands appropriate for each driver. A loudspeaker system with 'N' separate frequency bands is described as "N-way speakers": a 2-way system will have woofer and tweeter speakers; a 3-way system is a combination of a set of woofers, mid-range speakers, and tweeters(HF drivers).

History

Alexander Graham Bell patented the first electrical loudspeaker as part of his telephone in 1876, which was followed in 1878 by an improved version from Ernst Siemens. Nikola Tesla reportedly created a similar device in 1881, but was not issued as a patent. During this time, Thomas Edison issued a British patent for a system using compressed air as an amplifying mechanism for his early cylinder phonographs, but he ultimately settled for the familiar metal horn driven by a membrane attached to the stylus. In 1898, Horace Short patented a design for a loudspeaker driven by compressed air, then sold the rights to Charles Parsons, who issued several additional British patents before 1910. Several companies, including Victor Talking Machine Company and Pathe, produced record players using compressed-air loudspeakers. However, these designs were significantly limited by their poor sound quality and their inability to reproduce sound at low-volume. Variants of the system were used for public address applications, and more recently other variations have been used to test space equipment resistance to the very loud sound levels that launching rockets produce (ca, 165 dB SPL).
The modern design of moving-coil drivers was established by Oliver Lodge in (1898). The moving coil principle was patented in 1924 by Chester W. Rice and Edward W. Kellogg.
These first loudspeakers used electromagnets because large, powerful permanent magnets were generally not available at a reasonable price. The coil of an electromagnet, called a field coil, was energized by current through a second pair of connections to the driver. This winding usually served a dual role, acting also as a choke coil filtering the power supply of the amplifier to which the loudspeaker was connected. AC ripple in the current was attenuated by the action of passing through the choke coil; however, AC line frequencies tended to modulate the audio signal being sent to the voice coil and added to the audible hum of a powered-up sound reproduction device.
The quality of loudspeaker systems until the 1950s was poor. Continuous developments in enclosure design and materials have led to significant audible improvements. The most notable improvements in modern speakers are improvements in cone materials, the introduction of higher temperature adhesives, improved permanent magnet materials, improved measurement techniques, computer aided design and finite element analysis.

Driver design

The most common type of driver uses a lightweight diaphragm connected to a rigid basket, or frame, via flexible suspension that constrains a coil of fine wire to move axially through a cylindrical magnetic gap. When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current in the coil which thus becomes an electromagnet. The coil and the driver's magnetic system interact, generating a mechanical force which causes the coil, and so the attached cone, to move back and forth and so reproduce sound under the control of the applied electrical signal coming from the amplifier. The following is a description of the individual components of this type of loudspeaker.
The diaphragm is usually manufactured with a cone or dome shaped profile. A variety of different materials may be used, but the most common are paper, plastic and metal. The ideal material would be stiff, light and well damped. In practice, all three of these criteria cannot be met simultaneously, and thus driver design involves tradeoffs. For example, paper is light and well damped, but not stiff; metal can be made stiff and light, but it is not well damped; plastic can be light, but typically the stiffer it is made, the less well-damped it is. As a result, many cones are made of some sort of composite. This can either be a sandwich construction or simply a coating to stiffen or damp a cone.
The basket or frame must be designed for rigidity to avoid deformation, which will change the magnetic conditions in the magnet gap, and could even cause the voice coil to rub against the walls of the magnetic gap. Baskets are typically cast or stamped metal, although molded plastic baskets are becoming common, especially for inexpensive drivers. The frame also plays a considerable role in conducting heat away from the coil.
The suspension system keeps the coil centered in the gap and provides a restoring force to make the speaker cone return to a neutral position after moving. A typical suspension system consists of two parts: the "spider", which connects the diaphragm or voice coil to the frame and provides the majority of the restoring force; and the "surround", which helps center the coil/cone assembly and allows free movement aligned with the magnetic gap. The spider is usually made of a corrugated fabric disk. The surround can be a roll of rubber or foam or a ring of corrugated fabric, attached to the outer circumference of the cone and to the frame.
The voice coil wire is usually made of copper, though aluminum, and rarely silver, may be used. Voice coil wire cross sections can be circular, rectangular, or hexagonal, giving varying amounts of wire volume coverage in the magnetic gap space. The coil is oriented coaxially inside the gap, a small circular volume (a hole, slot, or groove) in the magnetic structure within which it can move back and forth. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet; the outside of the gap being one pole and the center post (a.k.a., the pole-piece) being the other. The center post and back-plate are sometimes a single piece called the yoke.
Modern driver magnets are almost always permanent and made of ceramic, ferrite, Alnico, or, more recently, neodymium magnet. The size and type of magnet and the magnetic circuit differ depending on design goals. A current trend in design, due to increases in transportation costs and a desire for smaller, lighter devices (as in many home theater multi-speaker installations), is the use of neodymium magnet instead of ferrite types.
Driver design, and the combination of one or more drivers into an enclosure to make a speaker system, is both an art and science. Adjusting a design to improve performance is done using magnetic, acoustic, mechanical, electrical, and material science theory, high precision measurements, and the observations of experienced listeners. Designers can use an anechoic chamber to ensure the speaker can be measured independently of room effects, or any of several electronic techniques. Some developers eschew anechoic chambers in favor of specific standardized room setups intended to simulate real-life listening conditions. Some of the issues speaker designers must confront are distortion, lobing, phase effects, off axis response and crossover complications.
Most loudspeaker drivers are currently manufactured in China. The fabrication of finished loudspeaker systems is segmented, depending largely on price, shipping costs, and weight limitations. High-end speaker systems, which are heavier (and often larger) than economic shipping allows outside local regions, are usually made in their target market area and can cost $10,000 or more per pair. The lowest-priced speaker systems are mostly manufactured in China or other low-cost manufacturing locations. Although the manufacture of drivers has become essentially commoditized, the fabrication and subsequent sale of finished speaker systems still carries high profits. Partly for this reason, manufacturers are increasingly combining power amplifier electronics (a typically lower profit item) with finished speaker systems to create powered speakers with an overall higher market value.

Driver types

An audio engineering rule of thumb is that individual electrodynamic drivers provide quality performance only over about 3 octaves. Specialized drivers (i.e., subwoofers, woofers, mid-range drivers, tweeters) are used to evade the practical effects of this limitation, though in some cases, a woofer can work high enough to reach a tweeter's low, to allow a high quality two-way system.

Full range drivers

A full-range driver is designed to have the widest frequency response possible. These drivers are small, typically 2 to 6 inches (5 to 16 cm) in diameter to permit reasonable high frequency response, and carefully designed to give low distortion output at low frequencies, though none have sufficient output at low frequencies for many purposes (eg, due to small maximum cone excursion) and limited power handling capacity (eg, due to a small voice coil). Those favoring this approach claim a coherence of sound (said to be due to the single source and resulting lack of phase interference, and likely to the lack of electrical crossover components). Disadvantages include a requirement for elaborate cabinets (i.e., transmission lines, horns, etc) to increase efficiency at low frequencies by better matching the driver to the air at those frequencies, thus increasing the output level at low frequencies.
Full range drivers often employ an additional cone called a whizzer: a small, light cone attached to the joint between the voice coil and the primary cone. The whizzer cone extends the high frequency response and broadens the high frequency directivity, which would otherwise be greatly reduced due to cone material breakup at higher frequencies (the cone area away from the coil fails to follow the area near the coil at higher frequencies in larger cones). The main cone is built so as to flex more in this region at high frequencies than the rest of the cone. The result is that the main cone delivers low frequencies and the whizzer cone contributes most of the higher frequencies. Since the whizzer cone is smaller than the main diaphragm, output dispersion at high frequencies is improved relative to a single larger diaphragm with no whizzer. Full range drivers are one approach to avoiding the possible effects of multiple driver systems, caused by non-coincident driver location and crossover issues.

Subwoofer

A subwoofer is a woofer driver used only for the lowest part of the audio spectrum: typically below 120 Hz. Because the intended range of frequencies is limited, subwoofer design is usually simpler than for conventional loudspeakers, often consisting of a single subwoofer driver enclosed in a suitable cabinet. To accurately reproduce very low bass notes without unwanted resonances (from cabinet panels, for instance), subwoofer systems must be solidly constructed and properly braced; good ones are typically heavy. Many subwoofers are designed to include power amplifiers and electronic filters, with additional controls relevant to low frequency reproduction. These variants are known as "active subwoofers". Passive subwoofers require external amplification.

Loudspeaker system design

Crossover

Most loudspeaker systems consist of drivers mounted in an enclosure, or cabinet. The role of the enclosure is to provide a place to mount the drivers and to prevent sound waves from the back of a driver from interfering destructively with those from the front -- doing so typically causes cancellations (eg, comb filtering) and significantly alters the level and quality sound at low frequencies.
The simplest driver mount is a flat panel (ie, baffle) with the drivers mounted to it. However, in this design, frequencies with a wavelength longer than the baffle dimensions are canceled out because the antiphase radiation from the rear of the cone interferes with the radiation from the front. With an infinitely large panel interference could be entirely prevented. A sufficiently large sealed box can approach this behavior..
Since panels of infinite dimensions are impractical, most enclosures function by containing the rear radiation from the cone. A sealed enclosure prevents transmission of the sound emitted from the rear of the loudspeaker by confining the sound in a rigid and airtight box. Techniques used to reduce transmission of sound through the walls of the cabinet include thicker cabinet walls, lossy wall material, internal bracing, curved cabinet walls or more rarely visco-elastic materials (eg, mineral loaded bitumen), or thin lead sheeting applied to interior enclosure walls.
However, a rigid enclosure internally reflects sound which can then be transmitted through the loudspeaker cone, again resulting in degradation of sound quality. This can be reduced by internal absorption using absorptive materials (often called "damping") such as fiberglass, wool, or synthetic fiber batting within the enclosure. The internal shape of the enclosure can also be designed to reduce this by reflecting sounds away from the loudspeaker diaphragm where they may then be absorbed.
Other enclosure types alter the rear radiation so it can add constructively to the output from the front of the cone. Designs that do this (including bass reflex, passive radiators, transmission line, etc) are often used to extend the effective low frequency response, and increased low frequency output, of the driver.
To make the transition between drivers as seamless as possible, system designers have attempted to time-align (or phase adjust) the drivers by moving one or more drivers forward or back, so that the acoustic center of each driver is in the same vertical plane. This may also involve tilting the face speaker back, or providing separate enclosure mounting for each driver, or, less commonly, using electronic techniques to achieve the same effect. These attempts account for some unusual cabinet designs.
Speaker cabinets cause diffraction, causing peaks and dips in the frequency response. This is usually a problem at higher frequencies where wavelengths are similar to, or smaller than, cabinet dimensions. The effect can be minimized by rounding the front edges of the cabinet, rounding the cabinet itself, using a smaller or narrower enclosure, choosing a strategic driver arrangement, or using absorptive material around a driver.

Wiring connections

Most loudspeakers use two wiring points to connect to the source of the signal (for example, to the audio amplifier or receiver). This is usually done using binding posts, or spring clips on the back of the enclosure. If the wires for left and right speakers (in a stereo setup) are not connected 'in phase' with each other (the + and - connections on the speaker and amplifier should be connected + to + and - to -) the loudspeakers will be out of polarity. Given identical signals, motion in one cone will be in the opposite direction of the other. This will typically cause monophonic material within a stereo recording to be canceled out, reduced in level and made more difficult to localize, all due to destructive interference of the sound waves. The cancellation effect is most noticeable at frequencies where the speakers are separated by a quarter wavelength or less; low frequencies are affected the most. This type of wiring error doesn't damage speakers but isn't optimal.

Specifications

Speaker specifications generally include:
  • Speaker or driver type (individual units only) – Full-range, woofer, tweeter or mid-range.
  • Size of individual drivers. For cone drivers, this number may be the outside diameter of the frame, the diameter of the surround, or the diameter of the cone. It may also be the distance from the center of one mounting hole to its opposite. Voice coil diameter may also be specified. If the loudspeaker has a compression horn driver, the diameter of the horn throat may be given.
  • Rated Power – Nominal (or even continuous) power, and peak (or maximum short-term) power a loudspeaker can handle (i.e., maximum input power before thermally destroying the loudspeaker. It is never the sound output the loudspeaker produces). A driver may be damaged at much less than its rated power if driven past its mechanical limits at lower frequencies (eg, by bass heavy electronica or theatre organ music). Tweeters can also be damaged by amplifier clipping (lots of high frequency energy in such cases) or by music, or sine wave input at high frequencies. Each of these situations pass more energy to a tweeter than it can survive without damage.
  • Impedance – typically 4 Ω (ohms), 8 Ω, etc.
  • Baffle or enclosure type (enclosed systems only) – Sealed, bass reflex, etc.
  • Number of drivers (complete speaker systems only) – 2-way, 3-way, etc.
and optionally:
  • Crossover frequency(ies) (multi-driver systems only) – The nominal frequency boundaries of the signal division between drivers.
  • Frequency response – The measured, or specified, output over a specified range of frequencies for a constant input level varied across those frequencies. It often includes a variance limit such as within "+/- 2.5 dB".
  • Thiele/Small parameters (individual drivers only) – these include the driver's Fs (resonance frequency), Qts (a driver's Q (more or less, its damping factor) at resonant frequency), Vas (the equivalent air compliance volume of the driver), etc.
  • Sensitivity – The sound pressure level produced by a loudspeaker in a non-reverberant environment, usually specified in dB, and measured at 1 meter with an input of 1 watt or 2.83 volts, typically at one or more specified frequencies. This rating is often inflated by manufacturers.
  • Maximum SPL – The highest output the loudspeaker can manage, short of damage or not exceeding a particular distortion level. This rating is often inflated by manufacturers and is commonly given without reference to frequency range or distortion level.

Electrical characteristics of a dynamic loudspeaker

The load a driver presents to an amplifier consists of a complex electrical impedance -- a combination of resistance, and both capacitive and inductive reactance, which combines properties of the driver, its mechanical motion, effects of crossover components (if any are in the signal path between amplifier and driver), and effects of air loading on the driver as modified by the enclosure and its environment. Most amplifiers output specifications are given at a specific power into an ideal resistive load. However, a loudspeaker does not really have a constant resistance across its frequency range. Instead, the voice coil is inductive, the driver has mechanical resonances, the enclosure changes the driver's electrical and mechanical characteristics, and a passive crossover between the drivers and the amplifier contributes its own variations. The result is a load resistance which varies fairly widely with frequency, and usually a varying phase relationship between voltage and current as well, also changing with frequency.

Electromechanical measurements

Fully characterizing the sound output quality of a loudspeaker driver or system in words is essentially impossible. Objective measurements provide information about several aspects of performance, so informed comparisons and improvements can be made. Examples of typical measurements are: amplitude and phase characteristics vs. frequency; impulse response under one or more conditions (eg, square waves, sine wave bursts, ...); directivity vs. frequency (eg, horizontally, vertically, spherically, ...); harmonic and intermodulation distortion vs. SPL output using any of several test signals; stored energy (ie, 'ringing') at various frequencies; impedance vs. frequency and small signal vs. large signal performance. Most of these measurements require relatively expensive equipment to perform and good judgement, but the raw sound pressure level output is rather easier to report and so is often the only specified value, sometimes in misleadingly exact terms. The sound pressure level (SPL) a loudspeaker produces is measured in decibels (dBspl).

Efficiency vs. sensitivity

Loudspeaker efficiency is defined as the sound power output divided by the electrical power input. Most loudspeakers are actually very inefficient transducers; about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to the acoustic energy we can hear. The remainder is converted to heat, mostly in the voice coil and magnet assembly. The main reason for this is the difficulty of achieving proper impedance matching between the acoustic impedance of the drive unit and that of the air into which it is radiating. The efficiency of loudspeaker drivers varies with frequency as well. For instance, the output of a woofer driver decreases as the input frequency decreases.
Driver ratings based on the SPL for a given input are called sensitivity ratings and are notionally similar to efficiency. Sensitivity is usually defined as so many decibels at 1 W electrical input, measured at 1 meter, often at a single frequency. The voltage used is often 2.83 VRMS, which is 1 watt into an 8 Ω (nominal) speaker impedance (approximately true for many speaker systems). Measurements taken with this reference are quoted as dB with 2.83 V @ 1 m.
The sound pressure output is measured at (or mathematically scaled to be equivalent to a measurement taken at) one meter from the loudspeaker and on-axis or directly in front of it under the condition that the loudspeaker is radiating into an infinitely large space and mounted on an infinite baffle. Clearly then, sensitivity does not correlate precisely with efficiency, as it also depends on the directivity of the driver being tested and the acoustic environment in front of the actual loudspeaker. For example, a cheerleader's horn produces more sound output in the direction it is pointed, by concentrating sound waves from the cheerleader in one direction, and thus "focusing" them. The horn also improves the impedance matching between voice and the air, which produces more acoustic power for a given speaker power. In some cases, impedance matching (via careful enclosure design) will allow the speaker to produce more power.
  • Typical home loudspeakers have sensitivities of about 85 to 95 dB for 1 W @ 1 m - an efficiency of 0.5-4%.
  • Sound reinforcement and public address loudspeakers have sensitivities of perhaps 95 to 102 dB for 1 W @ 1 m - an efficiency of 4-10%.
  • Rock concert, stadium PA, marine hailing, etc speakers generally have higher sensitivities of 103 to 110 dB for 1 W @ 1 m - an efficiency of 10-20%.
A driver with a higher maximum power rating cannot necessarily be driven to louder levels than a lower rated one, since sensitivity and power handling are largely independent properties. In the examples that follow, assume for simplicity that the drivers being compared have the same electrical impedance, are operated at the same frequency which is within both driver's respective pass bands, and that power compression and distortion are low. For the first example, a speaker 3 dB more sensitive than another will produce double the sound pressure level (or be 3 dB louder) for the same power input. Thus a 100 W driver ("A") rated at 92 dB for 1 W @ 1 m sensitivity will output twice as much acoustic power as a 200 W driver ("B") rated at 89 dB for 1 W @ 1 m when both are driven with 100 W of input power. For this particular example, when driven at 100 W, speaker A will produce the same SPL, or loudness, speaker B would produce with 200 W input. Thus a 3 dB increase in sensitivity of the speaker means that it will need half the amplifier power to achieve a given SPL. This translates into a smaller, less complex power amplifier and often to reduced overall cost.
It is not possible to combine high efficiency, especially at low frequencies, with compact enclosure size, and adequate low frequency response. One can, more or less, only choose two of the three parameters when designing a speaker system. So, for example, if extended low frequency performance and a small box size are important, one must accept low efficiency. This rule of thumb is sometimes called Hoffman's Iron Law (after J. A. Hoffman, the H in KLH).

Listening environment

The interaction of a loudspeaker system with its environment is complex and is largely out of the loudspeaker designer's control. Most listening rooms present a more or less reflective environment, depending on size, shape, volume, and furnishings. This means the sound reaching a listener's ears consists not only of sound directly from the speaker system, but also the same sound delayed by traveling to and from (and being modified by) one or more surfaces. These reflected sound waves, when added to the direct sound, cause cancellation and addition at assorted frequencies (eg, from resonant room modes), thus changing the timbre and character of the sound at the listener's ears. Our brains are very sensitive to small variations, including some of these, and this is part of the reason why a loudspeaker system sounds different at different listening positions or in different rooms.
A significant factor in the sound of a loudspeaker system is the amount of absorption and diffusion present in the environment. Clapping one's hands in a typical empty room, without draperies or carpet, will produce a zippy, fluttery echo which is due both to a lack of absorption and to reverberation (that is, repeated echoes) from flat reflective walls, floor, and ceiling. The addition of hard surfaced furniture, wall hangings, shelving and even baroque plaster ceiling decoration, will change the echoes, due primarily to the diffusion caused by somewhat reflective objects with shapes and surfaces having sizes on the order of the sound wavelengths being diffused. This somewhat breaks up the simple reflections otherwise caused by bare flat surfaces, and spreads the reflected energy of an incident wave over a larger angle on reflection.

Placement

In a typical rectangular listening room, this resonant phenomenon happens differently in each of the three dimensions, and there are even more complex interactions involving four or even all six boundary surfaces. It is primarily an issue for low frequencies which are not much affected by such things as furniture or its placement. In addition, the location of the loudspeakers, and the listener, with respect to room boundaries affect how strongly the resonances are excited. Many people are familiar with certain locations in some rooms, clubs, or buildings which have much more, or less, bass - most usually near room walls or corners. This is because standing wave patterns are most easily heard in these locations and at lower frequencies, below the Schroeder frequency - typically around 200-300 Hz, depending on room size.

Directivity

Acousticians, in studying the radiation of sound sources have developed some concepts important to understanding how loudspeakers are perceived. The simplest possible radiating source is a point source, sometimes called a simple source. An ideal point source is an infinitesimally small point radiating sound. It may be easier to imagine a tiny pulsating sphere, uniformly increasing and decreasing in diameter, sending out sound waves in all directions equally, independent of frequency.
Any object radiating sound, including a loudspeaker system, can be thought of as being composed of combinations of such simple point sources. The radiation pattern of a combination of point sources will not be the same as for a single source, but rather will depend on the distance and orientation between the sources, the position relative to them from which the listener hears the combination, and the frequency of the sound involved. Using geometry and calculus, some simple combinations of sources are easily solved; others are not.
One simple combination is two simple sources separated by a distance and vibrating out of phase, one miniature sphere expanding while the other is contracting. The pair is known as a doublet, or dipole, and the radiation of this combination is similar to that of a very small dynamic loudspeaker operating without a baffle. The directivity of a dipole is a figure 8 shape with maximum output along a vector which connects the two sources and minimums to the sides when the observing point is equidistant from the two sources, where the sum of the positive and negative waves cancel each other. While most drivers are dipoles, depending on the enclosure to which they are attached, they may radiate as monopoles, dipoles (or bipoles). If mounted on a finite baffle, and these out of phase waves allowed to interact, dipole peaks and nulls in the frequency response result. When the rear radiation is absorbed or trapped in a box, the diaphragm becomes a monopole radiator. Bipolar speakers, made by mounting in-phase monopoles (both moving out of or into the box in unison) on opposite sides of a box, are a method of approaching omnidirectional radiation patterns.
In real life, individual drivers are actually complex 3D shapes such as cones and domes, and they are placed on a baffle for various reasons. A mathematical expression for the directivity of a complex shape, based on modeling combinations of point sources, is usually not possible, but in the farfield, the directivity of a loudspeaker with a circular diaphragm will be close to that of a flat circular piston, so it can be used as an illustrative simplification for discussion. As a simple example of the mathematical physics involved, consider the following: the formula for farfield directivity of a flat circular piston in an infinite baffle is p(\theta) = \frac where k_a=\frac, p_0 is the pressure on axis, a is the piston radius, \lambda is the wavelength (i.e. \lambda = \frac = \frac) \theta is the angle off axis and J_1 is the Bessel function of the first kind.
A planar source will radiate sound uniformly for low frequencies whose wavelength is shorter than the dimensions of the planar source, and as frequency increases, the sound from such a source will be focused into an increasingly narrower angle. The smaller the driver, the higher the frequency where this narrowing of directivity occurs. Even if the diaphragm is not perfectly circular, this effect occurs such that larger sources are more directive. Several loudspeaker designs have been built which have approximately this behavior. Most are electrostatic or planar magnetic designs.
Various manufacturers use different driver mounting arrangements to create a specific type of sound field in the space for which they are designed. The resulting radiation patterns may be intended to more closely simulate the way sound is produced by real instruments, or simply create a controlled energy distribution from the input signal (some using this approach are called monitors, as they are useful in checking the signal just recorded in a studio). An example of the first is a room corner system with many small drivers on the surface of a 1/8 sphere. A system design of this type was patented by, and actually produced commercially, by Professor Amar Bose -- the 1801. Later Bose models have deliberately emphasized production of both direct and reflected sound by the loudspeaker itself, regardless of its environment. The designs are controversial in high fidelity circles, but have proven commercially successful. Several other manufacturers' designs follow similar principles.
Directivity is an important issue because it affects the frequency balance of sound a listener hears, and also the interaction of the speaker system with the room and its contents. A speaker which is very directive (ie, on an axis perpendicular to the speaker face) may result in a reverberant field lacking in high frequencies, giving the impression the speaker is deficient in treble even though it measures well on axis (eg, "flat" across the entire frequency range). Speakers with very wide, or rapidly increasing directivity at high frequencies, can give the impression that there is too much treble (if the listener is on axis) or too little (if the listener is off axis). This is part of the reason why on-axis frequency response measurement is not a complete characterization of the sound of a given loudspeaker.

Other driver designs

Other types of drivers which depart from the most commonly used direct radiating electro-dynamic driver mounted in an enclosure include:

Horn loudspeakers

Horn speakers are the oldest form of loudspeaker system, having been used from very early on for cylinder recording players. They use a shaped waveguide in front of or behind the driver to increase the directivity of the loudspeaker and to transform a small diameter, high pressure condition at the driver cone surface to a large diameter, low pressure condition at the mouth of the horn. This increases the sensitivity of the loudspeaker and focuses the sound over a narrower area. The size of the throat, mouth, the length of the horn, as well as the area expansion rate along it must be carefully chosen to match the drive to properly provide this transforming function over a range of frequencies (every horn performs poorly outside its acoustic limits, at both high and low frequencies). The length and cross-sectional mouth area required to create a bass or sub-bass horn require a horn many feet long. 'Folded' horns can reduce the total size, but compel designers to make compromises and accept increased complication such as cost and construction. Some horn designs not only fold the low frequency horn, but use the walls in a room corner as an extension of the horn mouth. In the late 1940s, horns whose mouths took up much of a room wall were not unknown amongst hi-fi fans. Room sized installations became much less acceptable when two or more were required.
A horn loaded speaker can have a sensitivity as high as 110 dB @ 2.83 volts (1 watt @ 8 ohms) @ 1 meter. This is a hundredfold increase in output compared to a speaker rated at 90 dB sensitivity, and is invaluable in applications where high sound levels are required or amplifier power is limited.

Piezoelectric speakers

Piezoelectric speakers are frequently used as beepers in watches and other electronic devices, and are sometimes used as tweeters in less-expensive speaker systems, such as computer speakers and portable radios. Piezoelectric speakers have several advantages over conventional loudspeakers: they are resistant to overloads which would normally destroy most high frequency drivers, and they can be used without a crossover due to their electrical properties. There are also disadvantages: some amplifiers can oscillate when driving capacitive loads like most piezoelectrics, which results in distortion or damage to the amplifier. Additionally, their frequency response, in most cases, is inferior to that of other technologies. This is why they are generally used in single frequency (beeper) or non-critical applications.
Piezoelectric speakers can have extended high frequency output, and this is useful in some specialized circumstances; for instance, sonar applications in which piezoelectric variants are used as both output devices (generating underwater sound) and as input devices (acting as the sensing components of underwater microphones). They have advantages in these applications, not the least of which is simple and solid state construction which resists the effects of seawater better than, say, a ribbon based device would.

Electrostatic loudspeakers

Electrostatic loudspeakers use a high voltage electric field (rather than a magnetic field) to drive a thin membrane between two perforated conductive plates called stators. Because they are driven over the entire membrane surface rather than from a small voice coil, they can provide a more linear and lower distortion response than dynamic drivers. They have the disadvantage that the diaphragm excursion is severely limited because of practical construction limitations. The further apart the stators are positioned, the higher the voltage must be to achieve acceptable efficiency, which increases the tendency for attracting dust and producing electrical arcs. For many years electrostatic loudspeakers had a reputation as a generally unreliable and occasionally dangerous product. Arcing remains a potential problem with current technologies, especially when the panels are allowed to get collect dust or dirt, or when driven with high signal levels.
Electrostatics are inherently dipole radiators and due to the thin flexible membrane cannot be used in enclosures to reduce low frequency cancellation as with common cone drivers. Due to this and the low excursion capability, full range electrostatic loudspeakers are large by nature, and even so are not outstanding performers at the lowest frequencies. To reduce the size of commercial products, they are often used as a high frequency driver in combination with a conventional dynamic driver which handles the bass frequencies.

Ribbon and planar magnetic loudspeakers

A ribbon speaker consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon which moves with it, thus creating the sound. The advantage of a ribbon driver is that the ribbon has very little mass; thus, it can accelerate very quickly, yielding very good high-frequency response. Ribbon loudspeakers are often very fragile -- some can be torn by a strong gust of air. Most ribbon tweeters emit sound in a dipole pattern; a very few have backings which limit the dipole radiation pattern. Above and below the ends of the more or less rectangular ribbon, there is less audible output due to phase cancellation, but the precise amount of directivity depends on ribbon length. Ribbon designs generally require exceptionally powerful magnets which make them costly to manufacture. Ribbons have a very low resistance that most amplifiers cannot drive directly. As a result, a step down transformer is typically used to increase the current through the ribbon. The amplifier "sees" a load that is the ribbon's resistance times the transformer turns ratio squared. The transformer must be carefully designed so that its frequency response and parasitic losses do not degrade the sound, further increasing cost and complication relative to conventional designs.
Planar magnetic speakers (having printed or embedded conductors on a flat diaphragm) are sometimes described as "ribbons", but are not truly ribbon speakers. The term planar is generally reserved for speakers which have roughly rectangular shaped flat surfaces that radiate in a bipolar (ie, front and back) manner. Planar magnetic speakers consist of a flexible membrane with a voice coil printed or mounted on it. The current flowing through the coil interacts with the magnetic field of carefully placed magnets on either side of the diaphragm, causing the membrane to vibrate more or less uniformly and without much bending or wrinkling. The driving force covers a large percentage of the membrane surface and reduces resonance problems inherent in coil-driven flat diaphragms.
Some planar magnetic designs have small cavities between the magnet structures and the diaphragm. This can cause a "cavity resonance" response peak that requires correction. Failure to correct this cavity resonance is likely the cause the steely or shrill sound sometimes attributed to these designs.

Bending wave loudspeakers

Bending wave transducers use a diaphragm that is intentionally flexible. The rigidity of the material increases from the center to the outside. Short wavelength sound therefore radiates primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections, long waves are absorbed by a surrounding damper. Such transducers can cover a wide frequency range (80 Hz to 35,000 Hz) and have been promoted as being close to an ideal point sound source.

Flat panel loudspeakers

There have been many attempts to reduce the size of speaker systems, or alternatively to make them less obvious. One such attempt was the development of voice coils mounted to flat panels to act as sound sources. These can then be made in a neutral color and hung on walls where they will be less noticeable than many speakers, or can be deliberately painted with patterns in which case they can function decoratively. There are two related problems with flat panel techniques: first, a flat panel is necessarily more flexible than a cone shape in the same material, and therefore will move as a single unit even less, and second, resonances in the panel are difficult to control, leading to considerable distortions. Some progress has been made using such lightweight, rigid, materials as Styrofoam, and there have been several flat panel systems commercially produced in recent years.

Distributed mode loudspeakers

A newer implementation of the flat panel speaker system involves an intentionally flexible panel and an "exciter", mounted off-center in a location such that it excites the panel to vibrate, but with minimal resonances. Speakers using such techniques can reproduce sound with a wide directivity pattern (paradoxically somewhat like a point source) and have been used in some computer speaker designs and bookshelf loudspeakers.

Heil air motion transducers

Dr. Oskar Heil invented the air motion transducer in the 1960s. In this approach, a pleated diaphragm is mounted in a magnetic field and forced to close and open under control of a music signal. Air is forced from between the pleats in accordance with the imposed signal, generating sound. The drivers are less fragile than ribbons and considerably more efficient (and able to produce higher absolute output levels) than ribbon, electrostatic, or planar magnetic tweeter designs.
ESS, a California manufacturer, licensed the design, employed Dr. Heil, and produced a range of speaker systems using his tweeters during the 1970s and 1980s. Radio Shack, a large US retail store chain, also sold speaker systems using such tweeters for a time. At present, there are two manufacturers of these drivers, both in Germany, one of which produces a range of high end professional speakers using tweeters and midrange drivers based on the technology.

Plasma arc speakers

Plasma arc loudspeakers use electrical plasma as a radiating element. 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. Problems of maintenance and reliability for this approach tend to make it unsuitable for mass market use. In 1978 Dr. Alan Hill of the Los Alamos National Laboratory designed the Hill Plasmatronics, an $8000 tweeter whose plasma was generated from helium gas. This avoided the ozone and nitrous oxide produced by RF decomposition of air in an earlier generation of plasma tweeters made by the pioneering DuKane Corporation, who produced the Ionovac (marketed as the Ionofane in the UK) during the 1950s. Currently, there remain a few manufacturers in Germany, and a do it yourself design has been published.
A less expensive variation on this theme is the use of a flame for the driver, as flames contain ionized (electrically charged) gases.

Digital speakers

Digital speakers have been the subject of experiments by Bell Labs as far back as the 1920s. The design is simple; each bit drives an independent speaker driver. Increasingly significant bits drive speakers of twice the area of the previous (often in a ring around the previous driver). A value of "1" causes that driver to be driven to full amplitude; a value of "0" causes it to be completely shut off.
There are two problems with this design which have led to it being abandoned as impractical for the present. First, for a reasonable number of bits (required for adequate sound reproduction quality), the size of the system becomes very large. Secondly, due to analog digital conversion, 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, causing an unacceptably high level of ultrasonics to accompany the desired output.
The term "digital" or "digital-ready" is often used for marketing purposes on speakers or headphones, but these systems are not digital in the sense described above. Rather, this is a somewhat deceptive marketing tactic, in which the manufacturer is trying to capitalize on the popularity of digital sound recordings and equipment.

External links

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Synonyms, Antonyms and Related Words

acoustical network, capacitor speaker, coaxial speaker, cone, crossover network, diaphragm, dynamic speaker, earphone, electrodynamic speaker, electromagnetic speaker, electrostatic speaker, excited-field speaker, full-fidelity speaker, headphone, headset, high-fidelity speaker, high-frequency speaker, horn, low-frequency speaker, midrange speaker, monorange speaker, moving-coil speaker, permanent magnet speaker, speaker, speaker system, speaker unit, triaxial speaker, tweeter, voice coil, woofer
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