A loudspeaker enclosure or loudspeaker cabinet is an enclosure (often rectangular box-shaped) in which speaker drivers (e.g., woofers and tweeters) and associated electronic hardware, such as crossover circuits and, in some cases, power amplifiers, are mounted. Enclosures may range in design from simple, homemade DIY rectangular particleboard boxes to very complex, expensive computer-designed hi-fi cabinets that incorporate composite materials, internal baffles, horns, bass reflex ports and acoustic insulation. Loudspeaker enclosures range in size from small "bookshelf" speaker cabinets with woofers and small tweeters designed for listening to music with a hi-fi system in a private home to huge, heavy subwoofer enclosures with multiple or even speakers in huge enclosures which are designed for use in stadium concert sound reinforcement systems for rock music concerts.
The primary role of an enclosure is to prevent sound waves generated by the rearward-facing surface of the diaphragm of an open speaker driver from interacting with sound waves generated at the front of the speaker driver. Because the forward- and rearward-generated sounds are out of phase with each other, any interaction between the two in the listening space creates a distortion of the original signal as it was intended to be reproduced. As such, a loudspeaker cannot be used without installing it in a baffle of some type, such as a closed box, vented box, open baffle, or a wall or ceiling (infinite baffle).
An enclosure also plays a role in managing vibration induced by the driver frame and moving air mass within the enclosure, as well as heat generated by driver voice coils and amplifiers (especially where woofers and subwoofers are concerned). Sometimes considered part of the enclosure, the base may include specially designed feet to decouple the speaker from the floor. Enclosures designed for use in PA systems, sound reinforcement systems and for use by electric musical instrument players (e.g., bass amp cabinets) have a number of features to make them easier to transport, such as carrying handles on the top or sides, metal or plastic corner protectors, and metal grilles to protect the speakers. Speaker enclosures designed for use in a home or recording studio typically do not have handles or corner protectors, although they do still usually have a cloth or mesh cover to protect the woofer and tweeter. These speaker grilles are a metallic or cloth mesh that are used to protect the speaker by forming a protective cover over the speaker's cone while allowing sound to pass through undistorted.
Speaker enclosures are used in homes in stereo systems, home cinema systems, televisions, boom boxes and many other audio appliances. Small speaker enclosures are used in car stereo systems. Speaker cabinets are key components of a number of commercial applications, including sound reinforcement systems, movie theatre sound systems and recording studios. Electric musical instruments invented in the 20th century, such as the electric guitar, electric bass and synthesizer, among others, are amplified using instrument amplifiers and speaker cabinets (e.g., guitar amplifier speaker cabinets).
History
Early on, radio loudspeakers consisted of horns, often sold separately from the radio itself (typically a small wooden box containing the radio's electronic circuits, so they were not usually housed in an enclosure.Illustrations , Retrieved May 5, 2024. When paper cone loudspeaker drivers were introduced in the mid 1920s, radio cabinets began to be made larger to enclose both the electronics and the loudspeaker.Illustrations Retrieved May 5, 2024. These cabinets were made largely for the sake of appearance, with the loudspeaker simply mounted behind a round hole in the cabinet. It was observed that the enclosure had a strong effect on the bass response of the speaker. Since the rear of the loudspeaker radiates sound out of phase from the front, there can be constructive and destructive interference for loudspeakers without enclosures, and below frequencies related to the baffle dimensions in open-baffled loudspeakers . This results in a loss of bass and in comb filtering, i.e., peaks and dips in the response power regardless of the signal that is meant to be reproduced. The resulting response is akin to two loudspeakers playing the same signal but at different distances from the listener, which is like adding a delayed version of the signal to itself, whereby both constructive and destructive interference occur.
Before the 1950s, many manufacturers did not fully enclose their loudspeaker cabinets; the back of the cabinet was typically left open. This was done for several reasons, not least because electronics (at that time, tube equipment) could be placed inside and cooled by convection in the open enclosure.
Most of the enclosure types discussed in this article were invented either to wall off the out-of-phase sound from one side of the driver or to modify it so that it could be used to enhance the sound produced from the other side.
Background
In some respects, the ideal mounting for a low-frequency loudspeaker driver would be a rigid flat panel of infinite size with infinite space behind it. This would entirely prevent the rear sound waves from interfering (i.e., comb filter cancellations) with the sound waves from the front. An open baffle loudspeaker is an approximation of this, since the driver is mounted on a panel, with dimensions comparable to the longest wavelength to be reproduced. In either case, the driver would need a relatively stiff suspension to provide the restoring force, which might have been provided at low frequencies by a smaller sealed or ported enclosure, so few drivers are suitable for this kind of mounting.
The forward- and rearward-generated sounds of a speaker driver appear out of phase with each other because they are generated through the opposite motion of the diaphragm and because they travel different paths before converging at the listener's position. A speaker driver mounted on a finite baffle will display a physical phenomenon known as interference, which can result in perceivable frequency-dependent sound attenuation. This phenomenon is particularly noticeable at low frequencies, where the wavelengths are large enough that interference will affect the entire listening area.
Since infinite baffles are impractical and finite baffles tend to suffer poor response as wavelengths approach the dimensions of the baffle (i.e., at lower frequencies), most loudspeaker cabinets use some sort of structure (usually a box) to contain the out-of-phase sound energy. The box is typically made of wood, wood composite, or, more recently, plastic, for reasons of ease of construction and appearance. Stone, concrete, plaster, and even building structures have also been used.
Enclosures can have a significant effect beyond what was intended, with panel resonances, diffraction from cabinet edges and standing wave energy from internal reflection/reinforcement modes being among the possible problems. Bothersome resonances can be reduced by increasing enclosure mass or rigidity, by increasing the damping of enclosure walls or wall/surface treatment combinations, by adding stiff cross bracing, or by adding internal absorption. Wharfedale, in some designs, reduced panel resonance by using two wooden cabinets (one inside the other) with the space between filled with sand. Home experimenters have even designed speakers built from concrete, granite and other exotic materials for similar reasons.
Many diffraction problems, above the lower frequencies, can be alleviated by the shape of the enclosure, such as by avoiding sharp corners on the front of the enclosure. A comprehensive study of the effect of cabinet configuration on the sound distribution pattern and overall response-frequency characteristics of loudspeakers was undertaken by Harry F. Olson. It involved a very wide number of different enclosure shapes, and it showed that curved loudspeaker baffles reduce some response deviations due to sound wave diffraction. It was discovered later that careful placement of a speaker on a sharp-edged baffle can reduce diffraction-caused response problems.
Sometimes the differences in phase response at frequencies shared by different drivers can be addressed by adjusting the vertical location of the smaller drivers (usually backwards), or by leaning or stepping the front baffle, so that the wavefront from all drivers is coherent at and around the crossover frequencies in the speaker's normal sound field. The acoustic center of the driver dictates the amount of rearward offset needed to time-align the drivers.
Types
Enclosures used for woofers and subwoofers can be adequately modeled in the low-frequency region using acoustics and the lumped component models. Electrical filter theory has been used with considerable success for some enclosure types. For the purposes of this type of analysis, each enclosure must be classified according to a specific topology. The designer must balance low bass extension, linear frequency response, efficiency, distortion, loudness and enclosure size, while simultaneously addressing issues higher in the audible frequency range such as diffraction from enclosure edges, the baffle step effect when wavelengths approach enclosure dimensions, crossovers, and driver blending.
Closed-box (sealed) enclosures
The loudspeaker driver's moving mass and compliance (slackness or reciprocal stiffness of the suspension) determine the driver's resonance frequency (Fs). In combination with the damping properties of the system (both mechanical and electrical), all these factors affect the low-frequency response of sealed-box systems. The response of closed-box loudspeaker systems has been extensively studied by Small and Benson, amongst many others. Output falls below the system's resonance frequency (Fc), defined as the frequency of peak impedance. In a closed-box loudspeaker, the air inside the box acts as a spring, returning the cone to the zero position in the absence of a signal. A significant increase in the effective volume of a closed-box loudspeaker can be achieved by a filling of fibrous material, typically fiberglass, bonded acetate fiber (BAF) or long-fiber wool. The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound propagation through the filler material as compared to air. The enclosure or driver must have a small leak so that the internal and external pressures can equalise over time, to compensate for changes in barometric pressure or altitude; the porous nature of paper cones, or an imperfectly sealed enclosure, is normally sufficient to provide this slow pressure equalisation.
Infinite baffle A variation on the open baffle approach is to mount the loudspeaker driver in a very large sealed enclosure, providing minimal air spring restoring force to the cone. This minimizes the change in the driver's resonance frequency caused by the enclosure. The low-frequency response of infinite baffle loudspeaker systems has been extensively analysed by Benson. Some infinite baffle enclosures have used an adjoining room, basement, or a closet or attic. This is often the case with exotic rotary woofer installations, as they are intended to go to frequencies lower than 20 Hz and displace large volumes of air. Infinite baffle (IB) is also used as a generic term for sealed enclosures of any size, the name being used because of the ability of a sealed enclosure to prevent any interaction between the forward and rear radiation of a driver at low frequencies.