Loudspeaker enclosure
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A loudspeaker enclosure is a cabinet designed for mounting loudspeaker drive units. The major role of the enclosure is to prevent the negative phase sound waves from the rear of the speaker from combining with the positive phase sound waves from the front of the speaker. This results in interference patterns and cancellation, causing the efficiency of the speaker to be reduced, particularly in the low frequencies where the wavelengths are large enough that interference will affect the entire listening area.
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[edit] History
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 (then always hot tube equipment) could be placed inside and, if the enclosure were open, could be cooled by convection. Early on, it was observed that the enclosure had a strong effect on the bass response of the speaker. An engineer at Bell Labs even patented a primitive ported enclosure design. Since the rear of the loudspeaker radiates soundwaves 180 degrees out of phase from the front, there will be constructive and destructive interference for loudspeakers without enclosures, and at below some frequencies, in open baffled loudspeakers. This causes loss of bass and comb filtering (ie, response peaks and dips in power regardless of the signal meant to be reproduced). Most of the enclosure types discussed in this article were invented to either wall off the out of phase sound from thone side of the driver, or to modify it so that it could be used to enhance the sound produced from the other side.
[edit] Background
In some respects, the ideal mounting for a low frequency loudspeaker driver would be an inert flat panel of infinite size with infinite space behind it. This would entirely prevent the rear soundwaves from interference (ie, comb filter cancellations) with the soundwaves from the front. An "open baffle" loudspeaker is an approximation of this, since the driver is mounted on a panel, hopefully of size comparable to the lowest wavelength to be reproduced. 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. However, for most purposes this is impractical and enclosures must use other techniques.
Most loudspeaker cabinets use some sort of structure (usually a box) to contain the out of phase sound energy. the box is characteristically made of wood or, more recently, plastic, both for reasons of ease of construction and appearance.
Loudspeaker cabinets are sometimes "sealed" and sometimes "ported". Ported cabinets allow some of the sound energy inside the cabinet to be released, and if designed correctly with proper attention to phase relationships, both increase bass response and reduce driver excursion. Many other engineering variations on the basic box design exist, such as acoustic transmission lines.
Enclosures always play a significant role in 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 enclosure walls, or by adding absorption internally. In the past, one speaker manufacturer (Wharfedale) even addressed the problem of cabinet resonance by using two wooden panels with the space between filled with sand. Home experimenters have designed speakers built from concrete sewer pipe, and other exotic materials for similar reasons.
Many diffraction problems, above the lowest 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 1930s by Dr. H.F. Olsen showed that curved loudspeaker baffles reduce response deviations due to soundwave diffraction, although his research did not show that careful placement of a speaker on even a sharp-edged baffle can minimize diffraction-caused response changes; this was discovered later. 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, when sound 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 needed to "time-align" the drivers.
[edit] Woofer and subwoofer enclosures
Enclosures used for woofers and subwoofers can be adequately modelled in the low frequency region (approximately 100–200 Hz and below) using acoustics and the lumped component model. Electrical filter theory has been used with considerable success (see Thiele/Small for details). For the purposes of this type of analysis, each enclosure has a loudspeaker topology. The most common enclosure types are listed below.
[edit] Closed-box enclosures
[edit] Infinite baffle
A variation on the 'open baffle' is to mount the loudspeaker in a very large sealed enclosure. The loudspeaker driver's mass and compliance, (i.e., the stiffness of the cone suspension) determines the driver's resonant frequency, and the damping properties of the system, both affect the low-frequency response of the speaker system. Output falls off below the cabinet resonant frequency (Fs), which can be determined by finding the peak impedance. The designer must balance bass response, flatness of frequency response, efficiency, and size of enclosure. 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 pressure reflections and resonances caused when the driver cone moves backwards into the cabinet does not rise too high and affect the cone's motion. The enclosure is usually filled loosely with foam, pillow stuffing, long fibre wool, fiberglass, or other wadding, converting some of the speaker's thermodynamic properties from adiabatic to isothermal.
Today, 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.
[edit] Acoustic suspension
The closed-box or "acoustic suspension" enclosure, rather than using a large enclosure to avoid the effects of internal air pressure changes caused by cone motion, uses a smaller sealed enclosure, generally much smaller. The enclosure must have a very small leak so 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 a neutral position is a combination of a relatively soft mechanical suspension of the low frequncy driver and mostly of the air inside the enclosure. At audible frequencies, the air pressure caused by the cone motion is the dominant force. Damping materials such as fiberglass are typically added to the enclosure to shape system performance (ie, damp) the driver/air volume resonance, and to absorb output (especially in the midrange) from the rear of the diaphragm. An important advantage of a proper acoustic suspension design is that air is a more linear spring than is any practical mechanical cone suspension (ie, cone surround and spider together) -- they are inherently non-linear in many respects. This improved linearity giving acoustic suspension designs lower distortion than infinite baffle designs, particularly at the 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.
[edit] Reflex enclosures
[edit] Bass-reflex
Other types of enclosures attempt to improve the low frequency response, or overall efficiency of the loudspeaker, or reduce the size of an enclosure, 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 referred to as vented, ported or bass reflex enclosures. Their interiors are typically lined with matting (eg, fiberglass) for some of the same reasons as the sealed box speakers above, however, the entire volume is not stuffed with absorbent for two reasons. Air must flow into and out of the port, and carrying bits of stuffing out the port is rarely acceptable. 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 and so the behavior of the driver and the sound the system produces. This enclosure type is 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 who applied electrical filter theory to the acoustic behavior of speakers in enclosures.
[edit] Passive radiator
A passive radiator speaker uses a second "passive" driver, or drone, to produce similar low frequency extension or efficiency increase or enclosure size reduction 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. As well, tuning adjustments for a passive readiator are usually much easier. And with the disadvantage that a passive radiator requires precision construction quite like driver design, thus increasing costs.
[edit] Compound or Band-pass
A 4th order bandpass is very similar to 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 it (or the chamber into which the driver faces) is ported.
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 very senstiive to the characteristics of the driver. As in other reflex enclosures, the ports may be replaced by passive radiators if desired.
[edit] Other enclosure types
[edit] Dipole
A dipole enclosure in its simplest form is a driver located on a flat baffle panel. The baffle is sometimes folded to reduce its apparent size. A rectangular cross-section is more common than a circular one since it is easier to fabricate in a folded form than a circular one. The baffle dimensions are typically chosen to obtain a particular low frequency response, with larger dimensions giving a lower frequency before the front and rear waves interfere with each other. 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 useful if it can be used to prevent the sound from being as loud in some places than in others.
[edit] Horn
A horn speaker is a speaker system using a "horn" to match the driver cone to the air. The horn itself does not amplify, 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). The motional characteristics of air and speaker cones (whatever the material) are quite different; properly designed horns have the effect of making the speaker cone transfer more of the electrical energy in the voice coil into the air. The driver in effect appears to have a large surface area. In addition, horns can help control dispersion at higher frequencies which is useful in some applications such as sound reinforcement.
The mathematical theory of horn coupling is well developed and understood, even if implementation is difficult in the real world. Proper horns for high frequencies are small (above say 3kHz or so, a few inches), those for midrange frequencies (perhaps 300H to 2KHz much larger, perhaps 1 or 2 feet), and for low frequencies (under 300Hz very large, 10's of feet). Various speaker manufactures have produced folded low frequency horns which are much smaller (eg, Altec Lansing, JBL, Klipsch, Lowther, Tannoy, ...) and can actually hope to fit in real rooms. These are necessarily compromises, and because they are physically complex, they are expensive. Some of these speakers are very well thought of. In earlier times, some users built horns whoe mouths were built into the walls of listening rooms.
[edit] 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 Fs. Transmission lines tend to be larger than ported enclosures, due to the size and length of the guide required (typically 1/4th the longest wavelength of interest). The design is often described as non-resonant, and some designs are sufficiently stuffed with absorbent material that there is indeed not much output from the line's port. But it is the inherent resonance (typically at 1/4 wavelength) that can enhances the bass response in this type of enclosure, albeit with less absorbent stuffing. Among the first examples of this enclosure design approach were the projects published in Wireless World by Bailey in the early 1970s.
Recently, numerical simulations by several researchers (eg, George Augspurger and Martin J. King) have brought a degree of order to the theory and practical design of these systems.[citation needed]
[edit] Tapered quarter-wave pipe
The tapered quarter-wave pipe (TQWP) is an example of a combination of transmission line and horn effects. It is highly regarded by some speaker designers. The concept is that the sound emitted from the rear of the loudspeaker is progressively reflected and absorbed along the length of the tapering tube, almost completely preventing internally reflected sound being retransmitted through the cone of the loudspeaker. In essence it is a horn in reverse.[citation needed] Its relatively low adoption in commercial speakers can mostly be attributed to the large resulting dimensions of the speaker produced and the expense of manufacturing a rigid tapering tube. The tapering tube can be coiled for lower frequency driver enclosures to reduce the dimensions of the speaker resulting in a seashell like appearance. Most notably Bowers & Wilkins have used this approach in their flagship Nautilus speaker as well as the use of smaller straight tapering tubes in many of their other lines.
[edit] Aperiodic enclosures
There is another design which falls between transmission lines and bass reflex enclosres. By setting up a port, and then blocking it precisely with sufficiently tightly packed fiber filling, it's possible to adjust the damping in the port as desired. The result is control of the resonance behavior of the system which improves low frequency reproduction, according to some designers. Dynaco was the primary producer of these enclosures for many years. The design remains uncommon among commercial designs currently available.
[edit] See also
[edit] External links
- Quarter-Wave - details about Transmission Line
