Loudspeaker（1）

A loudspeaker (or "speaker") is an electroacoustic transducer that produces sound in response to an electrical audio signal input(Sony Vaio VGN-FZ battery).

Terminology

The term "loudspeaker" may refer to individual transducers (known as "drivers") or to complete speaker systems consisting of an enclosure including one or more drivers. To adequately reproduce a wide range of frequencies(Sony VGP-BPS8 battery), most loudspeaker systems employ more than one driver, particularly for higher sound pressure level or maximum accuracy(Sony VGP-BPL9 battery). Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers (for very low frequencies); woofers (low frequencies); mid-range speakers (middle frequencies); tweeters (high frequencies) (Sony VGP-BPS9 battery); and sometimes supertweeters, optimized for the highest audible frequencies. The terms for different speaker drivers differ, depending on the application. In two-way systems there is no mid-range driver, so the task of reproducing the mid-range sounds falls upon the woofer and tweeter(Sony VGP-BPL11 battery). Home stereos use the designation "tweeter" for the high frequency driver, while professional concert systems may designate them as "HF" or "highs". When multiple drivers are used in a system, a "filter network"(Sony VGP-BPL15 battery), called a crossover, separates the incoming signal into different frequency ranges and routes them to the appropriate driver. A loudspeaker system with n separate frequency bands is described as "n-way speakers"(Sony VGN-FZ460E battery): a two-way system will have a woofer and a tweeter; a three-way system employs a woofer, a mid-range, and a tweeter.

History

Johann Philipp Reis installed an electric loudspeaker in his telephone in 1861; it was capable of reproducing pure tones, but also could reproduce speech(Sony VGP-BPS11 battery). Alexander Graham Bell patented his first electric loudspeaker (capable of reproducing intelligible speech) as part of his telephone in 1876, which was followed in 1877 by an improved version from Ernst Siemens(SONY VAIO VGN-FZ4000 Battery). Nikola Tesla reportedly made a similar device in 1881, but he was not issued a patent. During this time, Thomas Edison was issued a British patent for a system using compressed air as an amplifying mechanism for his early cylinder phonographs(Sony VGP-BPS10 battery), 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(Sony VGP-BPS13 battery); he then sold the rights to Charles Parsons, who was issued several additional British patents before 1910. A few companies, including the Victor Talking Machine Company and Pathé, produced record players using compressed-air loudspeakers( Sony VGP-BPS3 battery). However, these designs were significantly limited by their poor sound quality and their inability to reproduce sound at low volume(Sony VGP-BPS2 battery). 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 and vibration levels that the launching of rockets produces(Sony Vaio VGN-FZ21M battery ).

The modern design of moving-coil (also called dynamic) drivers was established by Oliver Lodge in 1898. The first practical application of moving-coil loudspeakers was established by Peter L. Jensen and Edwin Pridham, in Napa, California(Sony VGN-FZ150E battery). Jensen was denied patents. Being unsuccessful in selling their product to telephone companies, in 1915 they changed strategy to public address, and named their product Magnavox(Sony VGN-FZ15 battery). Jensen was, for years after the invention of the loudspeaker, a part owner of The Magnavox Company.

The moving-coil principle as commonly used today in direct radiators was patented in 1924 by Chester W. Rice and Edward W. Kellogg(Sony VGN-FZ15L battery). The key difference between previous attempts and the patent by Rice and Kellogg was the adjustment of mechanical parameters so that the fundamental resonance of the moving system took place at a lower frequency than that at which the cone's radiation impedance had become uniform. See the original patent for details(Sony Vaio VGN-FZ18M battery).

About this same period, Walter H. Schottky invented the first ribbon loudspeaker.

These first loudspeakers used electromagnets, because large, powerful permanent magnets were generally not available at a reasonable price(Sony VGN-FZ15T battery). The coil of an electromagnet, called a field coil, was energized by current through a second pair of connections to the driver(Sony VGN-FZ480E battery). 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(Sony VGN-FZ61B battery). 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(Sony VGN-FZ31E battery).

In the 1930s, loudspeaker manufacturers began to combine two and three bandpasses' worth of drivers in order to increase frequency response and sound pressure level. In 1937, the first film industry-standard loudspeaker system(Sony VGN-FZ340E battery), "The Shearer Horn System for Theatres" (a two-way system), was introduced by Metro-Goldwyn-Mayer(Sony VGN-FZ180E battery). It used four 15″ low-frequency drivers, a crossover network set for 375 Hz, and a single multi-cellular horn with two compression drivers providing the high frequencies(Sony VGN-FZ18E battery). John Kenneth Hilliard, James Bullough Lansing, and Douglas Shearer all played roles in creating the system. At the 1939 New York World's Fair, a very large two-way public address system was mounted on a tower at Flushing Meadows(Sony VGP-BPL4 battery). The eight 27″ low-frequency drivers were designed by Rudy Bozak in his role as chief engineer for Cinaudagraph. High-frequency drivers were likely made by Western Electric(Sony VGN-FZ160E battery).

Altec Lansing introduced the '604', which was to become their most famous coaxial Duplex driver, in 1943, incorporating a high-frequency horn sending sound through the middle of a 15-inch woofer for near-point-source performance(Sony VGN-FZ15M battery). Altec's "Voice of the Theatre" loudspeaker system arrived in the marketplace in 1945, offering better coherence and clarity at the high output levels necessary in movie theaters(Sony Vaio VGN-FZ18G battery). The Academy of Motion Picture Arts and Sciences immediately began testing its sonic characteristics; they made it the film house industry standard in 1955. Subsequently, continuous developments in enclosure design and materials led to significant audible improvements(Sony VGN-FZ345E battery). The most notable improvements in modern speakers are improvements in cone materials, the introduction of higher-temperature adhesives(Sony VGP-BPS14 battery), improved permanent magnet materials, improved measurement techniques, computer-aided design, and finite element analysis.

Driver design

Cutaway view of a dynamic loudspeaker.

A stamped steel loudspeaker basket frame is clearly visible (here, blue-grey) (Sony PCGA-BP2E battery).

The most common type of driver uses a lightweight diaphragm, or cone, connected to a rigid basket, or frame, via a flexible suspension that constrains a coil of fine wire to move axially through a cylindrical magnetic gap(Sony VGN-FZ11S battery). When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it a variable electromagnet(Sony VGN-FZ17L battery). The coil and the driver's magnetic system interact, generating a mechanical force that causes the coil (and thus, the attached cone) to move back and forth(Sony VGN-FZ17G battery), thereby reproducing 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(Sony VGN-FZ17 battery).

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 1) rigid, to prevent uncontrolled cone motions(Sony VGP-BPS18 battery); 2) have low mass, to minimize starting force requirements and energy storage issues; 3) be well damped, to reduce vibrations continuing after the signal has stopped with little or no audible ringing due to its resonance frequency as determined by its usage(Sony VGP-BPS15 battery). In practice, all three of these criteria cannot be met simultaneously using existing materials; thus, driver design involves trade-offs. For example, paper is light and typically well damped, but is not stiff; metal may be stiff and light, but it usually has poor damping(Sony VGP-BPS12 battery); plastic can be light, but typically, the stiffer it is made, the poorer the damping. As a result, many cones are made of some sort of composite material. For example, a cone might be made of cellulose paper, into which some carbon fiber, Kevlar(Sony VGP-BPL12 battery), glass, hemp or bamboo fibers have been added; or it might use a honeycomb sandwich construction; or a coating might be applied to it so as to provide additional stiffening or damping(Sony VGP-BPL7 battery).

The chassis, frame, or basket, is designed to be rigid, avoiding deformation which would change critical alignments with the magnet gap, perhaps causing the voice coil to rub against the sides of the gap(Sony VGN-FZ290 battery). Chassis are typically cast from aluminum alloy, or stamped from thin steel sheet, although molded plastic and damped plastic compound baskets are becoming common, especially for inexpensive(Sony VGN-FZ29VN battery), low-mass drivers. Metallic chassis can play an important role in conducting heat away from the voice coil; heating during operation changes resistance, causing physical dimensional changes, and if extreme, may even demagnetize permanent magnets(Sony VGN-SZ56 battery).

The suspension system keeps the coil centered in the gap and provides a restoring (centering) force that returns the cone to a neutral position after moving. A typical suspension system consists of two parts: the "spider"(Sony VGN-SZ55 battery), 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 pistonic motion aligned with the magnetic gap(Sony VGN-FZ220E battery). The spider is usually made of a corrugated fabric disk, impregnated with a stiffening resin. The name comes from the shape of early suspensions(Sony VGN-FZ11Z battery), which were two concentric rings of Bakelite material, joined by six or eight curved "legs". Variations of this topology included the addition of a felt disc to provide a barrier to particles that might otherwise cause the voice coil to rub(Sony VGN-FZ11M battery). The German firm Rulik still offers drivers with uncommon spiders made of wood.

The cone surround can be rubber or polyester foam, or a ring of corrugated, resin coated fabric; it is attached to both the outer diaphragm circumference and to the frame(Sony Vaio VGN-FZ31M battery). These different surround materials, their shape and treatment can dramatically affect the acoustic output of a driver; each class and implementation having advantages and disadvantages(Sony Vaio VGN-FZ31Z battery). Polyester foam, for example, is lightweight and economical, but is degraded by exposure to ozone, UV light, humidity and elevated temperatures, limiting its useful life to about 15 years(Sony VGN-FZ19VN battery).

The wire in a voice coil is usually made of copper, though aluminum—and, rarely, silver—may be used. The advantage of aluminum is its light weight, which raises the resonant frequency of the voice coil and allows it to respond more easily to higher frequencies(Sony Vaio VGN-FZ38M battery). A disadvantage of aluminum is that it is not easily soldered, and so connections are instead often crimped together and sealed. These connections can corrode and fail in time(Sony Vaio VGN-FZ31S battery). 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 co-axially inside the gap(Sony VGN-FZ11L battery); it moves back and forth within a small circular volume (a hole, slot, or groove) in the magnetic structure. 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 (called the pole piece) being the other. The pole piece and backplate are often a single piece, called the poleplate or yoke(Sony VGN-FZ15G battery).

Modern driver magnets are almost always permanent and made of ceramic, ferrite, Alnico, or, more recently, rare earth such as neodymium and Samarium cobalt. A trend in design—due to increases in transportation costs and a desire for smaller(Sony VGP-BPS9/B battery), lighter devices (as in many home theater multi-speaker installations)—is the use of the last instead of heavier ferrite types. Very few manufacturers still use electrically powered field coils, as was common in the earliest designs (one such is French) (Sony VGN-FZ190 battery). When high field-strength permanent magnets became available, Alnico, an alloy of aluminum, nickel, and cobalt became popular, since it dispensed with the power supply issues of field-coil drivers(Sony VGN-FZ190E battery). Alnico was used for almost exclusively until about 1980. Alnico magnets can be partially degaussed (i.e., demagnetized) by accidental 'pops' or 'clicks' caused by loose connections, especially if used with a high power amplifier(Sony VGN-FZ18 battery). This damage can be reversed by "recharging" the magnet.

After 1980, most (but not quite all) driver manufacturers switched from Alnico to ferrite magnets(Sony Vaio VGN-FZ21E battery ), which are made from a mix of ceramic clay and fine particles of barium or strontium ferrite. Although the energy per kilogram of these ceramic magnets is lower than Alnico, it is substantially less expensive, allowing designers to use larger yet more economical magnets to achieve a given performance(Sony Vaio VGN-FZ21S battery).

The size and type of magnet and details of the magnetic circuit differ, depending on design goals. For instance, the shape of the pole piece affects the magnetic interaction between the voice coil and the magnetic field(Sony VGN-FZ230E battery), and is sometimes used to modify a driver's behavior. A "shorting ring", or Faraday loop, may be included as a thin copper cap fitted over the pole tip or as a heavy ring situated within the magnet-pole cavity(Sony Vaio VGN-FZ210CE battery). The benefits of this complication is reduced impedance at high frequencies, providing extended treble output, reduced harmonic distortion, and a reduction in the inductance modulation that typically accompanies large voice coil excursions(Sony Vaio VGN-FZ18S battery). On the other hand, the copper cap requires a wider voice-coil gap, with increased magnetic reluctance; this reduces available flux, requiring a larger magnet for equivalent performance(Sony VGN-FW11 battery).

Driver design—including the particular way two or more drivers are combined in an enclosure to make a speaker system—is both an art and science. Adjusting a design to improve performance is done using some combination of magnetic(Sony VGN-FW11M battery), acoustic, mechanical, electrical, and material science theory; high precision measurements, generally with the observations of experienced listeners. A few of the issues speaker and driver designers must confront are distortion, lobing, phase effects, off-axis response, and crossover complications(Sony VGN-FZ18L battery). Designers can use an anechoic chamber to ensure the speaker can be measured independently of room effects, or any of several electronic techniques which can, to some extent, replace such chambers(Sony Vaio VGN-FZ31B battery). Some developers eschew anechoic chambers in favor of specific standardized room setups intended to simulate real-life listening conditions(Sony VGN-FZ145E battery).

The fabrication of finished loudspeaker systems has become segmented, depending largely on price, shipping costs, and weight limitations. High-end speaker systems, which are typically heavier (and often larger) than economic shipping allows outside local regions(Sony Vaio VGN-FZ31J battery ), are usually made in their target market region and can cost $140,000 or more per pair. The lowest-priced speaker systems and most drivers are manufactured in China or other low-cost manufacturing locations(Sony VGN-FZ72B battery).

Driver types

Individual electrodynamic drivers provide optimal performance within a limited pitch range. Multiple drivers (e.g., subwoofers, woofers, mid-range drivers, and tweeters) are generally combined into a complete loudspeaker system to provide performance beyond that constraint(Sony VGN-FZ140E battery).

Full-range drivers

Main article: Full-range

A full-range driver is designed to have the widest frequency response possible. These drivers are small, typically 3 to 8 inches (7.6 to 20 cm) in diameter to permit reasonable high frequency response, and carefully designed to give low-distortion output at low frequencies, though with reduced maximum output level(Sony Vaio VGN-FZ21J battery). Full-range (or more accurately, wide-range) drivers are most commonly heard in public address systems, in televisions (although some models are suitable for hi-fi listening), small radios, intercoms, some computer speakers, etc(Sony VGN-FZ32B battery). In hi-fi speaker systems, the use of wide-range drive units can avoid undesirable interactions between multiple drivers caused by non-coincident driver location or crossover network issues. Fans of wide-range driver hi-fi speaker systems claim a coherence of sound(Sony VGN-FZ35 battery), said to be due to the single source and a resulting lack of interference, and likely also to the lack of crossover components. Detractors typically cite wide-range drivers' limited frequency response and modest output abilities (most especially at low frequencies), together with their requirement for large(Sony VGN-FZ37 battery), elaborate, expensive enclosures—such as transmission lines, or horns—to approach optimum performance(Sony VGN-FZ38 battery).

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 of the driver and broadens its high frequency directivity(Sony VGN-FZ340E/B battery), which would otherwise be greatly narrowed due to the outer diameter cone material failing to keep up with the central voice coil at higher frequencies. The main cone in a whizzer design is manufactured so as to flex more in the outer diameter than in the center(Sony VGN-FZ440N battery). 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 an equivalent single larger diaphragm(Sony VGN-FZ32 battery).

Limited-range drivers, also used alone, are typically found in computers, toys, and clock radios. These drivers are less elaborate and less expensive than wide-range drivers, and they may be severely compromised to fit into very small mounting locations. In these applications, sound quality is a low priority(Sony VGN-FZ430E battery). The human ear is remarkably tolerant of poor sound quality, and the distortion inherent in limited-range drivers may enhance their output at high frequencies, increasing clarity when listening to spoken word material(Sony VGN-FZ11E battery).

Subwoofer

Main article: Subwoofer

A subwoofer is a woofer driver used only for the lowest part of the audio spectrum: typically below 200 Hz for consumer systems, below 100 Hz for professional live sound, and below 80 Hz in THX-approved systems(Sony VGP-BPS13B/B battery). Because the intended range of frequencies is limited, subwoofer system design is usually simpler in many respects than for conventional loudspeakers, often consisting of a single driver enclosed in a suitable box or enclosure(Sony VGP-BPS13A/B battery).

To accurately reproduce very low bass notes without unwanted resonances (typically from cabinet panels), subwoofer systems must be solidly constructed and properly braced; good speakers are typically quite heavy(Sony VGP-BPS13/S battery). Many subwoofer systems include power amplifiers and electronic sub-filters, with additional controls relevant to low-frequency reproduction. These variants are known as "active subwoofers". In contrast, "passive" subwoofers require external amplification(Sony VGP-BPS13/B battery).

Woofer

Main article: Woofer

A woofer is a driver that reproduces low frequencies. The driver combines with the enclosure design to produce suitable low frequencies (see speaker enclosure for the design choices available) (Sony PCGA-BP2EA battery). Some loudspeaker systems use a woofer for the lowest frequencies, sometimes well enough that a subwoofer is not needed. Additionally, some loudspeakers use the woofer to handle middle frequencies, eliminating the mid-range driver(Sony VGP-BPS13AS battery). This can be accomplished with the selection of a tweeter that can work low enough that, combined with a woofer that responds high enough, the two drivers add coherently in the middle frequencies(Sony VGP-BPS13S battery).

Mid-range driver

Main article: Mid-range speaker

A mid-range speaker is a loudspeaker driver that reproduces middle frequencies. Mid-range driver diaphragms can be made of paper or composite materials, and can be direct radiation drivers (rather like smaller woofers) or they can be compression drivers (rather like some tweeter designs) (Sony VGP-BPS13B/S battery). If the mid-range driver is a direct radiator, it can be mounted on the front baffle of a loudspeaker enclosure, or, if a compression driver, mounted at the throat of a horn for added output level and control of radiation pattern(Sony VGP-BPS13A/S battery).

Tweeter

Exploded view of a dome tweeter.

Main article: Tweeter

A tweeter is a high-frequency driver that reproduces the highest frequencies in a speaker system. Many varieties of tweeter design exist, each with differing abilities with regard to frequency response, output fidelity, power handling(Sony VGP-BPS21 battery), maximum output level, etc. Soft-dome tweeters are widely found in home stereo systems, and horn-loaded compression drivers are common in professional sound reinforcement. Ribbon tweeters have gained popularity in recent years(Sony VGP-BPS21B battery), as their output power has been increased to levels useful for professional sound reinforcement, and their output pattern is wide in the horizontal plane, a pattern that has convenient applications in concert sound(Sony VGP-BPS21/S battery).

Coaxial drivers

A coaxial driver is a loudspeaker driver with two or several combined concentric drivers. Coaxial drivers have been produced by many companies, such as Altec, Tannoy, Pioneer, KEF, BMS, Cabasse and Genelec(Sony VGP-BPS21A/B battery).

Loudspeaker system design

Crossover

Main article: Audio crossover

A passive crossover.

Bi-amped.

Used in multi-driver speaker systems, the crossover is a subsystem that separates the input signal into different frequency ranges suited to each driver. The drivers receive only the power in their usable frequency range (the range they were designed for), thereby reducing distortion in the drivers and interference between them(Sony VAIO PCG-5G2L battery).

Crossovers can be passive or active. A passive crossover is an electronic circuit that uses a combination of one or more resistors, inductors, or non-polar capacitors. These parts are formed into carefully designed networks and are most often placed between the power amplifier and the loudspeaker drivers to divide the amplifier's signal into the necessary frequency bands before being delivered to the individual drivers(Sony VAIO PCG-5G3L battery). Passive crossover circuits need no external power beyond the audio signal itself, but do cause overall signal loss and a significant reduction in damping factor between the voice coil and the crossover(Sony VAIO PCG-5J1L battery). An active crossover is an electronic filter circuit that divides the signal into individual frequency bands before power amplification, thus requiring at least one power amplifier for each bandpass(Sony VAIO PCG-5J2L battery) (Sony VAIO PCG-5K2L battery). Passive filtering may also be used in this way before power amplification, but it is an uncommon solution, due to inflexibility compared to active filtering(Sony VAIO PCG-5L1L battery). Any technique that uses crossover filtering followed by amplification is commonly known as bi-amping, tri-amping, quad-amping, and so on, depending on the minimum number of amplifier channels(Sony VAIO PCG-6S2L battery). Some loudspeaker designs use a combination of passive and active crossover filtering, such as a passive crossover between the mid- and high-frequency drivers and an active crossover between the low-frequency driver and the combined mid- and high frequencies(Sony VAIO PCG-6S3L battery).

Passive crossovers are commonly installed inside speaker boxes and are by far the most usual type of crossover for home and low-power use. In car audio systems, passive crossovers may be in a separate box(Sony VAIO PCG-6V1L battery), necessary to accommodate the size of the components used. Passive crossovers may be simple for low-order filtering, or complex to allow steep slopes such as 18 or 24 dB per octave. Passive crossovers can also be designed to compensate for undesired characteristics of driver, horn, or enclosure resonances(Sony VAIO PCG-6W1L battery), and can be tricky to implement, due to component interaction. Passive crossovers, like the driver units that they feed, have power handling limits, have insertion losses (10% is often claimed), and change the load seen by the amplifier(Sony VAIO PCG-7111L battery). The changes are matters of concern for many in the hi-fi world.[26] When high output levels are required, active crossovers may be preferable. Active crossovers may be simple circuits that emulate the response of a passive network, or may be more complex(Sony VAIO PCG-6W3L battery), allowing extensive audio adjustments. Some active crossovers, usually digital loudspeaker management systems, may include facilities for precise alignment of phase and time between frequency bands, equalization, and dynamics (compression and limiting) control(Sony VAIO PCG-7113L battery).

Some hi-fi and professional loudspeaker systems now include an active crossover circuit as part of an onboard amplifier system. These speaker designs are identifiable by their need for AC power in addition to a signal cable from a pre-amplifier(Sony VAIO PCG-7133L battery). This active topology may include driver protection circuits and other features of a digital loudspeaker management system. Powered speaker systems are common in computer sound (for a single listener) and(Sony VAIO PCG-7Z1L battery), at the other end of the size spectrum, in modern concert sound systems, where their presence is significant and steadily increasing(Sony VAIO PCG-7Z2L battery).

Enclosures

Main article: Loudspeaker enclosure

An unusual three-way speaker system. The cabinet is narrow in order to raise the frequency at which a diffraction effect called the "baffle step" occurs(Sony VAIO PCG-8Y1L battery).

Most loudspeaker systems consist of drivers mounted in an enclosure, or cabinet. The role of the enclosure is to provide a place to physically mount the drivers, and to prevent sound waves emanating from the back of a driver from interfering destructively with those from the front; these typically cause cancellations (e.g., comb filtering) and significantly alter the level and quality of sound at low frequencies(Sony VAIO PCG-8Y2L battery).

The simplest driver mount is a flat panel (i.e., baffle) with the drivers mounted in holes in it. However, in this approach, sound frequencies with a wavelength longer than the baffle dimensions are canceled out(Sony VAIO PCG-8Z2L battery), because the antiphase radiation from the rear of the cone interferes with the radiation from the front. With an infinitely large panel, this interference could be entirely prevented. A sufficiently large sealed box can approach this behavior(Sony VAIO PCG-8Z1L battery).

Since panels of infinite dimensions are impractical, most enclosures function by containing the rear radiation from the moving diaphragm. 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(Sony VAIO PCG-7112L battery). 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 (e.g., mineral-loaded bitumen) or thin lead sheeting applied to the interior enclosure walls(Sony VAIO PCG-6W2L battery).

However, a rigid enclosure reflects sound internally, which can then be transmitted back through the loudspeaker diaphraghm—again resulting in degradation of sound quality. This can be reduced by internal absorption using absorptive materials (often called "damping"), such as glass wool, wool(Sony VAIO PCG-5K1L battery), or synthetic fiber batting, within the enclosure.