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Introduction

A loudspeaker is commonly mounted in an enclosure (or cabinet). The major role of the enclosure is to prevent the out-of-phase sound waves from the rear of the speaker combining with the positive phase sound waves from the front of the speaker, which would result in interference patterns and cancellation causing the efficiency of the speaker to be compromised, particularly in the low frequencies where the wavelengths are large enough that interference will affect the entire listening area.

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

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

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

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Closed box enclosures

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

Acoustic suspension

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

Bass reflex enclosure

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

Compound enclosure

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

Passive radiator enclosure

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

Transmission line enclosure

The transmission line system is a waveguide system in which the guide reverses the phase of the driver's rear output, thereby reinforcing the frequencies near the driver's Fs. Transmission lines tend to be larger than the other systems, due to the size and length of the line required by the design. The payoff is an extended low end response and a characteristic sound that's appealing to many.

Dipole enclosure

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

Horn enclosure

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

Tapered Quarter Wave pipe enclosure

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

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