Loudspeaker enclosure

A loudspeaker enclosure is a cabinet designed for mounting of loudspeaker drive units. 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. This 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.

History

Before the 1950's many manufacturers did not fully enclose their loudspeaker cabinets, the back of the cabinet was typically left open. This was done for a variety of reasons, not least the fact that electronics could be placed inside and would cool by convection. Early on it was observed that the enclosure had a strong effect on the bass response of the speaker. Since the rear of the loudspeaker radiates soundwaves 180 degrees out of phase from the front, there is constructive and destructive interference from unhoused or open baffle loudspeakers which causes loss of bass and comb filtering (response peaks and dips). Most of the enclosure types in this article were invented to either wall off the out of phase sound from the rear of the driver, or modify it so that it could be used to enhance the sound from the front side of the driver.

Explanation

In some respects, the ideal mounting for a low frequency loudspeaker driver would be a flat board of infinite size with infinite space behind it because this would entirely prevent the rear soundwaves from cancelling the soundwaves from the front. An 'open baffle' loudspeaker is an approximation to this as 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. In either case, the driver would have to have a relatively 'stiff' suspension to provide the restoring force which might have been provided at low frequencies by a smaller enclosure, so only some drivers are suitable for this kind of mounting.

Most loudspeaker cabinets use some sort of container (usually a box) to contain the out of phase sound energy, and the walls of the box are usually made of wood or, more recently, plastic.

Loudspeaker cabinets sometimes are "sealed" and sometimes "ported". Ported cabinets allow some of the sound energy inside the cabinet to be released with proper attention to phase relationships, in order to increase bass response. Many other engineering variations on the design exist, such as acoustic transmission lines.

Enclosures always play a significant role in the sound production in addition to the intended design effects, adding unfortunate resonances, diffraction, and other unwanted phenomena. Problems with resonance are usually reduced by increasing enclosure mass and/or rigidity, by increased damping of the walls of the enclosure, or by adding absorption internally. In the past, one speaker manufacturer (Wharfedale) 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 pipe, and other exotic materials for similar reasons.

Diffraction problems, above low frequencies, can be addressed by the shape of the enclosure, avoiding sharp corners on the front of the enclosure, for instance. Experimental research from the 1930's by Dr. H.F. Olsen showed that curved loudspeaker baffles reduce response deviations due to soundwave diffraction, although his research did not show that proper placement of a speaker on even a sharp rectangular or circular baffle can minimize response changes. This has been demonstrated rather later. Sometimes the differences in phase response of the different drivers is addressed by adjusting the vertical location of the smaller drivers (usually backwards), by leaning or stepping the front baffle, so that the wavefront from all drivers is coherent at and aroudn the crossover frequencies when it leaves the cabinet. The acoustic center of the driver, the physical position of the front of each driver's voice coil, dictates the amount of rearward offset to time-align the drivers.

'Woofer' and 'subwoofer' enclosures

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.

Closed-box enclosures

Infinite baffle

A variation on the 'open baffle' is to place the loudspeaker in a very large sealed enclosure. 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 below the cabinet resonant frequency (F_s), which can be determined by finding the peak impedance. The designer trades off bass response for flatness of frequency response; the larger the resonant peak in the bass, the lower the speaker will reproduce its input evenly. But many feel that the resulting low frequency performance of such speakers is over-emphasized. Such enclosures must be large enough that the internal reflections and resonances caused when the driver cone moves backwards into the cabinet does not rise too high. The enclosure is usually filled loosely with foam, pillow stuffing, long fibre wool, fiberglass, or other wadding, converting the speaker's thermodynamic properties from adiabatic to isothermal.

Infinite baffle subwoofers are mostly custom-designed and built components. See page 38 of the February 2007 issue of affordableaudio.org http://www.affordableaudio.org/ e-zine for an example or see the website for the cult of the infinitely baffled.

Acoustic suspension

The closed-box or "acoustic suspension" enclosure, rather than using a large enclosure to avoid the effect of the internal air pressure, uses a smaller sealed enclosure. The enclosure is typically designed with a very small rate of leakage so that internal and external pressures can slowly equalise over time, allowing the speaker to adjust to changes in barometric pressure or altitude. The "spring" suspension that restores the cone to the neutral position is a combination of a relatively loose mechanical suspension of the low frequncy driver and the air inside the enclosure. At audible frequencies, the air is the dominant suspension. Damping materials such as fiberglass are typically added to the enclosure to damp the driver/air volume resonance and to absorb output (especially in the midrange) from the rear of the diaphragm. An important advantage of acoustic suspension design is that air is a more linear spring, as opposed to practical mechanical cone suspensions (surround and spider together) which are inherently non-linear. This improved linearity gives acoustic suspension designs lower distortion than infinite baffle designs, particularly at lower frequencies and higher power levels at which cone excursion is large. The drawback of these speakers is their low efficiency, due to the loss of the power absorbed inside the cabinet, combined with generally reduced transient response at low frequencies. .

Reflex enclosures

Bass-reflex

Other types of enclosures attempt to improve the low frequency response, or overall efficiency of the loudspeaker, by using various combinations of cabinet openings or passive radiating elements to transmit low frequency energy from the rear of the speaker to the listener; these enclosures are also be referred to as vented, ported or bass reflex enclosures. The interiors of such enclosures are typically lined with matting (eg, fiberglass) for the same reasons as the sealed box speakers above. Reflex ports are tuned by their diameter, length, and to some extent shape, all of which affect the mass and motion of the air within the vent. This enclosure type is the very commonly used as it lends itself to smaller size and reasonable bass when tuned properly. These enclosures are relatively well understood, at least for low frequency performance, based on work by Thiele and Small applying electrical filter theory to the acoustic behavior of these speakers.

Passive radiator

A passive radiator speaker uses a second "passive" driver, or drone, to produce similar low frequency extension or efficiency increase from a small enclosure as do ported enclosures. In theory, such enclosures are variations of the bass reflex type, but with the advantage of avoiding a relatively small port or tube through which air moves, sometimes noisily. And with the disadvantage that a passive radiator requires precision construction quite similar to driver design, thus increasing costs.

Compound or Band-pass

A 4^th order bandpass is really just the same as a vented box in which 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 6^th order band-pass response. This enclosure is considerably harder to design and tends to be driver-specific. As in other reflex enclosures, the ports may be replaced by passive radiators if desired.

Other enclosure types

Dipole

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. A dipole enclosure has a "figure-of-eight" radiation pattern, which means that there is a reduction in sound pressure or loudness at the sides as compared to the front and rear. This is very useful when it is desired to prevent the sound from being heard at places other than the listening room/venue.

Horn

A horn speaker is a speaker that uses a "horn" to get more sound (volume) from the driving loudspeaker. The horn itself does not amplify anything, but rather improves the coupling between the speaker driver (typically made of paper or, more recently, more exotic materials such as titanium) and the air (which has a very low density). Air is very light and speaker cones are relatively heavy, so horns make the speaker cone appear to be very light (more like air) and to have large surface area. In addition, horns can help control dispersion at higher frequencies which is useful in some applicaitons such as sound reinforcement. The mathematical theory of horn coupling is well developed and understood. Proper horns for high frequencies are small (above say 3kHz or so) (a few inches), those for midrange frequencies (perhaps 300H to 2KHz) zmuch larger (1 or 2 feet), and for low frequncies (under 300Hz) very large (10's of feet). Various speaker manufactures have produced folded low frequency horns which are much smaller (eg, JBL, Altec Lansing, Lowther, Klipsch). These are necessarily compromises, and because they are physically complex, expensive. Some of these speakers are very well thought of by some.

Transmission line

A transmission line enclosure is a waveguide in which the structure shifts the phase of the driver's rear output by at least 90°, thereby reinforcing the frequencies near the driver's F_s. Transmission lines tend to be larger than ported enclosures, due to the size and length of the guide required (typically 1/4th of longest wavelength). While often described as non-resonant, it is the inherent 1/4-wave resonance that enhances the bass response of this type of enclosure. In addition, the waveguide is often stuffed with absorbent material that reduces the effect of the transmission line on the motion of the driver. Recently, numerical simulations by several researchers (such as George Augspurger and Martin J. King[1]) have brought a degree of order to the design of these systems.^{[citation needed]}

Tapered Quarter Wave Pipe

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

External links

Quarter-Wave - details about Transmission Line