Loudspeaker（2）

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(SONY VGP-BPS8 battery).

Other enclosure types alter the rear sound radiation so it can add constructively to the output from the front of the cone. Designs that do this (including bass reflex, passive radiator, transmission line, etc.) are often used to extend the effective low-frequency response and increase low-frequency output of the driver(SONY VAIO VGN-FZ Battery).

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 driver mounting locations forward or back so that the acoustic center of each driver is in the same vertical plane(SONY VAIO VGN-FZ11S Battery). This may also involve tilting the face speaker back, providing a separate enclosure mounting for each driver, or (less commonly) using electronic techniques to achieve the same effect. These attempts have resulted in some unusual cabinet designs(SONY vgp-bps9 battery).

The speaker mounting scheme (including cabinets) can also cause diffraction, resulting in peaks and dips in the frequency response. The problem is usually greatest at higher frequencies, where wavelengths are similar to, or smaller than(sony vgp-bpl9 battery), cabinet dimensions. The effect can be minimized by rounding the front edges of the cabinet, curving the cabinet itself(SONY VAIO VGN-FZ32 Battery), using a smaller or narrower enclosure, choosing a strategic driver arrangement, using absorptive material around a driver, or some combination of these and other schemes(SONY VAIO VGN-FZ440N Battery).

Wiring connections

Two-way binding posts on a loudspeaker, connected using banana plugs.

A 4-ohm loudspeaker with two pairs of binding posts capable of accepting bi-wiring after the removal of two metal straps(SONY VAIO VGN-FZ280E Battery).

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(SONY VAIO VGN-FZ460E Battery). If the wires for the 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(SONY VAIO VGN-FZ190 Battery). 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(SONY VAIO VGN-FZ150E Battery), 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(SONY VAIO VGN-FZ260E Battery).

Specifications

Specifications label on a loudspeaker.

Speaker specifications generally include:

Speaker or driver type (individual units only) – Full-range, woofer, tweeter, or mid-range(SONY VAIO VGN-FZ140E Battery).

Size of individual drivers. For cone drivers, the quoted size is generally the outside diameter of the basket. However, it may less commonly also be the diameter of the cone surround, measured apex to apex(SONY VAIO VGN-FZ11l Battery), or 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(SONY VAIO VGN-FZ11z Battery).

Rated Power – Nominal (or even continuous) power, and peak (or maximum short-term) power a loudspeaker can handle (i.e., maximum input power before destroying the loudspeaker; it is never the sound output the loudspeaker produces) (SONY VAIO VGN-FZ11m Battery). A driver may be damaged at much less than its rated power if driven past its mechanical limits at lower frequencies. Tweeters can also be damaged by amplifier clipping (amplifier circuits produce large amounts of energy at high frequencies in such cases) or by music or sine wave input at high frequencies(SONY VAIO VGN-FZ18m Battery). Each of these situations passes more energy to a tweeter than it can survive without damage. In some jurisdictions, power handling has a legal meaning allowing comparisons between loudspeakers under consideration(SONY VAIO VGN-FZ21m Battery). Elsewhere, the variety of meanings for power handling capacity can be quite confusing.

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) – two-way, three-way, etc.

and optionally(SONY VAIO VGN-FZ410 Battery):

Crossover frequency(ies) (multi-driver systems only) – The nominal frequency boundaries of the 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 sometimes includes a variance limit, such as within "± 2.5 dB"(SONY VAIO VGN-FZ21 Battery).

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(SONY VAIO VGN-FZ160 Battery).

Sensitivity – The sound pressure level produced by a loudspeaker in a non-reverberant environment, often specified in dB and measured at 1 meter with an input of 1 watt (2.83 rms volts into 8 Ω), typically at one or more specified frequencies. This rating is often specified by manufacturers to be impressive(SONY VAIO VGN-FZ210CE Battery).

Maximum SPL – The highest output the loudspeaker can manage, short of damage or not exceeding a particular distortion level. This rating is often specified by manufacturers to be impressive, and is commonly given without reference to frequency range or distortion level(SONY VAIO VGN-FZ38M Battery).

Electrical characteristics of a dynamic loudspeaker

Main article: Electrical characteristics of a dynamic loudspeaker

The load that 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(SONY VAIO VGN-FZ31z Battery), the effects of crossover components (if any are in the signal path between amplifier and driver), and the 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(Sony Vaio VGN-FZ31S battery); however, a loudspeaker does not 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(SONY VAIO VGN-FZ31E Battery), and a passive crossover between the drivers and the amplifier contributes its own variations. The result is a load resistance that varies fairly widely with frequency(SONY VAIO VGN-FZ31J Battery), and usually a varying phase relationship between voltage and current as well, also changing with frequency. Some amplifiers can cope with the variation better than others can(SONY VAIO VGN-FZ31M Battery).

To make sound, a loudspeaker is driven by modulated electrical current (produced by an amplifier) that pass through a "speaker coil" (a coil of copper wire), which then (through resistance and other forces) magnetizes the coil, creating a magnetic field(SONY VAIO VGN-FZ31B Battery). The electrical current variations that pass through the speaker are thus converted to varying magnetic forces, which move the speaker diaphraghm, which thus forces the driver to produce air motion that is similar to the original signal from the amplifier(HP PAVILION DV6000 battery).

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 that informed comparisons and improvements can be made(SONY VGP-BPS13 Battery), but no combination of measurements summarizes the performance of a loudspeaker system in use, if only because the test signals used are neither music nor speech. Examples of typical measurements are(SONY VGP-BPS13B/B Battery): amplitude and phase characteristics vs. frequency; impulse response under one or more conditions (e.g., square waves, sine wave bursts, etc.) (Dell INSPIRON 1525 battery); directivity vs. frequency (e.g., horizontally, vertically, spherically, etc.); harmonic and intermodulation distortion vs. SPL output, using any of several test signals; stored energy (i.e., ringing) at various frequencies; impedance vs. frequency; and small-signal vs. large-signal performance(Dell INSPIRON E1505 battery). Most of these measurements require sophisticated and often expensive equipment to perform, and also good judgment by the operator, 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(Dell INSPIRON 1420 battery). 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(Dell INSPIRON 1520 battery); only about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to acoustic energy. The remainder is converted to heat, mostly in the voice coil and magnet assembly(Dell INSPIRON 1464 battery). 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 (at low frequencies improving this match is the main purpose of speaker enclosure designs) (Dell INSPIRON 1564 battery). The efficiency of loudspeaker drivers varies with frequency as well. For instance, the output of a woofer driver decreases as the input frequency decreases because of the increasingly poor match between air and the driver(Dell INSPIRON 1764 battery).

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 (except for headphones) (Dell N3010 Battery), 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(SONY VGP-BPS13/B Battery).

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 (directly in front of it), under the condition that the loudspeaker is radiating into an infinitely large space and mounted on an infinite baffle(Sony VAIO VGN-FZ15T Battery). 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(Sony VAIO VGN-FZ15G Battery). 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, thus "focusing" them(SONY VGP-BPS13A/S Battery). The horn also improves impedance matching between the voice and the air, which produces more acoustic power for a given speaker power. In some cases(SONY VGP-BPS13AS Battery), improved impedance matching (via careful enclosure design) will allow the speaker to produce more acoustic power(SONY VGP-BPS13S Battery).

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%(SONY VGP-BPS13B/S Battery).

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%(SONY VAIO PCG-5G2L battery).

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(SONY VAIO PCG-5G3L battery). In the examples that follow, assume (for simplicity) that the drivers being compared have the same electrical impedance; are operated at the same frequency within both driver's respective pass bands; and that power compression and distortion are low. For the first example(SONY VAIO PCG-5J1L battery), a speaker 3 dB more sensitive than another will produce double the sound power (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 put out 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. In this particular example(SONY VAIO PCG-5K2L battery), when driven at 100 W, speaker A will produce the same SPL, or loudness, as 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(SONY VAIO PCG-5L1L battery). This translates into a smaller, less complex power amplifier—and often, to reduced overall system cost(SONY VAIO PCG-5J2L battery).

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, choose only two of the three parameters when designing a speaker system(SONY VAIO PCG-6S2L battery). So, for example, if extended low-frequency performance and 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) (SONY VAIO PCG-6S3L battery).

Listening environment

Main article: Room acoustics

Jay Pritzker Pavilion

At Jay Pritzker Pavilion, a LARES system is combined with a zoned sound reinforcement system, both suspended on an overhead steel trellis, to synthesize an indoor acoustic environment outdoors(SONY VAIO PCG-6V1L battery).

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(SONY VAIO PCG-6W1L battery). 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(SONY VAIO PCG-7111L battery). These reflected sound waves, when added to the direct sound, cause cancellation and addition at assorted frequencies (e.g., from resonant room modes), thus changing the timbre and character of the sound at the listener's ears(SONY VAIO PCG-6W3L battery). The human brain is 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(SONY VAIO PCG-7113L battery).

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(SONY VAIO PCG-7133L battery), 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(SONY VAIO PCG-7Z1L battery), 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(SONY VAIO PCG-7Z2L battery).

Placement

In a typical rectangular listening room, the hard, parallel surfaces of the walls, floor and ceiling cause primary acoustic resonance nodes in each of the three dimensions: left-right, up-down and forward-backward(SONY VAIO PCG-8Y1L battery). Furthermore, there are more complex resonance modes involving three, four, five and even all six boundary surfaces combining to create standing waves. Low frequencies excite these modes the most, since long wavelengths are not much affected by furniture compositions or placement(SONY VAIO PCG-8Y2L battery). The mode spacing is critical, especially in small and medium size rooms like recording studios, home theaters and broadcast studios. The proximity of the loudspeakers to room boundaries affects how strongly the resonances are excited as well as affecting the relative strength at each frequency(SONY VAIO PCG-8Z2L battery). The location of the listener is critical, too, as a position near a boundary can have a great effect on the perceived balance of frequencies. This is because standing wave patterns are most easily heard in these locations and at lower frequencies(SONY VAIO PCG-8Z1L battery), 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(SONY VAIO PCG-7112L battery). 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(SONY VAIO PCG-6W2L battery). 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(SONY VAIO PCG-5K1L battery).

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(SONY VGP-BPS21A/B Battery), 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(SONY VGP-BPS21B Battery). 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(SONY VGP-BPS21 Battery). 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(SONY VGP-BPS21/S Battery), 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) (SONY VGP-BPS13A/Q Battery). 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(SONY VGN NR11Z/T battery), 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(SONY VGN NR11Z/S battery).

Polar plots of a four-driver industrial columnar public address loudspeaker taken at six frequencies. Note how the pattern is nearly omnidirectional at low frequencies, converging to a wide fan-shaped pattern at 1 kHz, then separating into lobes and getting weaker at higher frequencies(SONY VGN NR11M/S battery).

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(SONY VGN NR11S/S battery), 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(SONY VGP-BPS13B/Q Battery). 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 where , p0 is the pressure on axis, a is the piston radius, λ is the wavelength (i.e. ) θ is the angle off axis and J1 is the Bessel function of the first kind(SONY VGP-BPS13Q Battery).

A planar source will radiate sound uniformly for low frequencies whose wavelength is longer 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(SONY Vaio VGN-FW21M Battery). 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(SONY Vaio VGN-FW21L Battery). Several loudspeaker designs have been built which have approximately this behavior. Most are electrostatic or planar magnetic designs(SONY Vaio VGN-FW21J Battery).

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(SONY Vaio VGN-FW21E Battery), 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(SONY Vaio VGN-FW11S Battery). A system design of this type was patented by, and actually produced commercially, by Professor Amar Bose—the 2201. Later Bose models have deliberately emphasized production of both direct and reflected sound by the loudspeaker itself, regardless of its environment(SONY Vaio VGN-FW11M Battery). The designs are controversial in high fidelity circles, but have proven commercially successful. Several other manufacturers' designs follow similar principles(SONY Vaio VGN-FW11 Battery).

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 (i.e., 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 (e.g., "flat" across the entire frequency range) (SONY VAIO VGN-FZ21J Battery). 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(SONY VAIO VGN-FZ21Z Battery).

Other driver designs

Other types of drivers which depart from the most commonly used direct radiating electro-dynamic driver mounted in an enclosure include(SONY VAIO VGN-FZ21E Battery):

Horn loudspeakers

A three-way loudspeaker that uses horns in front of each of the three drivers: a shallow horn for the tweeter, a long, straight horn for mid frequencies and a folded horn for the woofer(SONY Vaio VGN-FW31M Battery)

Main article: Horn loudspeaker

Horn loudspeakers are the oldest form of loudspeaker system. The use of horns as voice-amplifying megaphones dates at least to the 17th century, and horns were used in mechanical gramophones as early as 1857(SONY Vaio VGN-FW465J Battery). Horn loudspeakers 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(SONY Vaio VGN-FW139E/H Battery), low pressure condition at the mouth of the horn. This increases the sensitivity of the loudspeaker and focuses the sound over a narrower area(SONY Vaio VGN-FW139E Battery). 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) (SONY Vaio VGN-FW31E Battery). 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(SONY Vaio VGN-FW17W Battery), 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(SONY Vaio VGN-FW32J Battery).

A horn loaded speaker can have a sensitivity as high as 110 dB at 2.83 volts (1 watt at 8 ohms) at 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(Dell INSPIRON 1420 Battery).

Piezoelectric speakers

See also: Piezo tweeter

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(Dell Inspiron E1505 Battery): 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(Dell Latitude D620 Battery). 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(Dell RM791 battery). 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) (Dell N3010 battery). 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(Dell INSPIRON 1525 Battery).

Magnetostrictive speakers

Magnetostrictive transducers, based on magnetostriction, have been predominantly used as sonar ultrasonic sound wave radiators, but their usage has spread also to audio speaker systems(Dell Inspiron 6000 battery). Magnetostrictive speaker drivers have some special advantages: they can provide greater force (with smaller excursions) than other technologies; low excursion can avoid distortions from large excursion as in other designs; the magnetizing coil is stationary and therefore more easily cooled(Dell Inspiron 6400 battery); they are robust because delicate suspensions and voice coils are not required. Magnetostrictive speaker modules have been produced by Fostex and FeONIC and subwoofer drivers have also been produced(Dell Inspiron 1501 battery).

Electrostatic loudspeakers

Main article: Electrostatic loudspeaker

Electrostatic loudspeakers use a high voltage electric field (rather than a magnetic field) to drive a thin statically charged membrane. Because they are driven over the entire membrane surface rather than from a small voice coil(Dell Vostro 1710 battery), they ordinarily provide a more linear and lower distortion motion 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(Dell Studio 1735 battery), the higher the voltage must be to achieve acceptable efficiency, which increases the tendency for electrical arcs as well as the increasing the speaker's attraction of dust particles. For many years electrostatic loudspeakers had a reputation as a generally unreliable and occasionally dangerous product(Dell Latitude E6400 battery). Arcing remains a potential problem with current technologies, especially when the panels are allowed to collect dust or dirt, or when driven with high signal levels(Dell Latitude E6500 battery).

Electrostatics are inherently dipole radiators and due to the thin flexible membrane are less suited for use in enclosures to reduce low frequency cancellation as with common cone drivers. Due to this and the low excursion capability(Dell Inspiron 1320 battery), full range electrostatic loudspeakers are large by nature, and the bass will roll off at a frequency corresponding to a quarter wavelength of the narrowest panel dimension. To reduce the size of commercial products(Dell Studio 1450 battery), they are sometimes used as a high frequency driver in combination with a conventional dynamic driver which handles the bass frequencies(Dell Inspiron 1464 battery).

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(Dell Inspiron 1564 battery); 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(Dell Inspiron 1764 battery); 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(Dell Studio 1457 battery). 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(Sony Vaio VGN-FZ battery). 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(Sony Vaio VGN-FZ21M battery ), 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"(Sony VGN-FZ150E battery), 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 (i.e., front and back) manner(Sony VGN-FZ15 battery). 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(Sony VGN-FZ220E battery), 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(Sony VGN-FZ11Z battery).

Bending wave loudspeakers

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Bending wave transducers use a diaphragm that is intentionally flexible(Sony VGN-FZ11M battery). The rigidity of the material increases from the center to the outside. Short wavelengths radiate primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections from the outside back into the center, long waves are absorbed by a surrounding damper(Sony Vaio VGN-FZ31M battery). 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. This uncommon approach is being taken by only a very few manufacturers, in very different arrangements. The line of Ohm Walsh speakers use a unique driver designed by Lincoln Walsh(Sony Vaio VGN-FZ31Z battery). Lincoln Walsh was a brilliant engineer who was part of the engineering team that developed radar during World War II. He later designed audio amplifiers, and his final project was a unique, one-way speaker with one driver. It was a large cone that faced down into a sealed, airtight enclosure(Sony VGN-FZ19VN battery). Rather than move back-and-forth as conventional speakers do, the cone rippled and created sound using a principle known as “transmission line”. The new speaker created a single, perfectly rendered sound wave of remarkable clarity(Sony Vaio VGN-FZ38M battery). A new company, Ohm Acoustics, was formed to develop and market Walsh’s new speaker design. Lincoln Walsh died before his speaker was released to the public. After developing the Ohm A prototype, in 1973 Ohm introduced the Ohm F speaker to critical acclaim(Sony Vaio VGN-FZ31S battery).

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(Sony VGN-FZ11L battery). 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(Sony VGP-BPS9/B battery). 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(Sony VGN-FZ190 battery). Some progress has been made using such lightweight, rigid, materials such as Styrofoam, and there have been several flat panel systems commercially produced in recent years(Sony VGN-FZ190E battery).

Distributed mode loudspeakers

Main article: Distributed mode loudspeaker

A newer implementation of the flat panel speaker system involves an intentionally flexible panel and an "exciter", mounted off-center and located so as to excite the panel to vibrate with minimal resonances(Sony VGN-FZ18 battery). 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(Sony Vaio VGN-FZ21E battery ).

Heil air motion transducers

Main article: Air Motion Transformer

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(Sony Vaio VGN-FZ21S battery). 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(Sony Vaio VGN-FZ31B battery).

ESS, a California manufacturer, licensed the design, employed 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(Sony Vaio VGN-FZ31J battery ). At present, there are two manufacturers of these drivers in Germany, one of which produces a range of high end professional speakers using tweeters and mid-range drivers based on the technology. Martin Logan produces several AMT speakers in the US(Sony Vaio VGN-FZ21J battery).

Plasma arc speakers

Main article: Plasma speaker

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(Sony VGN-FZ11E battery). Problems of maintenance and reliability for this approach tend to make it unsuitable for mass market use. In 1978 Alan E. Hill of the Air Force Weapons Laboratory in Albuquerque, NM, designed the Plasmatronics Hill Type I(Sony VGP-BPS13B/B battery), a 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(Sony VGP-BPS13A/B battery), who produced the Ionovac (marketed as the Ionofane in the UK) during the 1950s. Currently, there remain a few manufacturers in Germany who use this design, and a do-it-yourself design has been published and has been available on the Internet(Sony VGP-BPS13/S battery).

A less expensive variation on this theme is the use of a flame for the driver, as flames contain ionized (electrically charged) gases(Sony VGP-BPS13/B battery).

Digital speakers

Main article: Digital speakers

Digital speakers have been the subject of experiments performed by Bell Labs as far back as the 1920s. The design is simple; each bit controls a driver, which is either fully 'on' or 'off'(Sony VGP-BPS13S battery).

There are 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 physical size of a speaker system becomes very large(Sony VGP-BPS13B/S battery). Secondly, due to inherent analog digital conversion problems, 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. No workable scheme has been found to adequately deal with this(Sony VGP-BPS13A/S battery).