Loudspeaker enclosure
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A loudspeaker enclosure is a cabinet designed to transmit sound to the listener via mounted loudspeaker drive units. The major role of the enclosure is to prevent the out of phase sound waves from the rear of the speaker from combining with the in-phase sound waves from the front of the speaker. Such mixing 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 (at that time tube equipment) could be placed inside and cooled by convection in the open enclosure. 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 sound 180 degrees out of phase from the front, there will be constructive and destructive interference for loudspeakers without enclosures, and below frequencies related to the baffle dimensions in open baffled loudspeakers. This causes loss of bass and comb filtering (i.e. 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 one 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 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, practically of s size comparable to the lowest 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 enclosure, so few drivers are suitable for this kind of mounting.
Since infinite baffles are impractical and finite baffles tend to suffer poor response, 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 always play a significant role in sound production in addition to the intended design effects, adding 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 cabinets (one inside the other) 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 sound wave 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 designer must balance bass response, flatness of frequency response, efficiency, and size of enclosure. The most common enclosure types are listed below.
[edit] Closed-box enclosures
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. When a speaker is mounted in a closed box, the air in the box acts as a spring. This raises and enhances the resonance frequency. The enclosure is usually filled loosely with foam, pillow stuffing, long fibre wool, fiberglass, or other wadding, absorbing internal reflections which has the effect of converting some of the enclosed air volume's thermodynamic properties from adiabatic to isothermal, relaxing the spring effect and making the enclosure seem larger. The enclosure or driver 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.
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[edit] Infinite baffle
A variation on the 'open baffle' is to mount the loudspeaker in a very large sealed enclosure. The idea behind this enclosure is that the restoring force of the enclosed air spring is minimal. This minimizes the change in resonant frequency. Some infinite baffle enclosures use an adjoining room, or a closet or attic, as an enclosure.
[edit] Acoustic suspension
The closed-box or "acoustic suspension" enclosure, rather than using a large enclosure to avoid the change in resonance, uses a smaller sealed enclosure to exploit the more linear air spring. The "spring" suspension that restores the cone to a neutral position is a combination of a relatively soft mechanical suspension of the low frequency driver and mostly of the air inside the enclosure. At frequencies below resonance, the air pressure caused by the cone motion is the dominant force. An important advantage of a proper acoustic suspension design is that air is a more linear spring than is any practical mechanical cone suspension (i.e. 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.
[edit] Reflex enclosures
[edit] Bass-reflex
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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 (e.g., 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 area and length, 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.
However, bass reflex designs are more sensitive to the match of the precise behavior of the driver and the cabinet and port than are some other enclosure types. For drivers with variations in performance, this requires more quality control effort to ensure performance in each loudspeaker produced.
[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 radiator are usually much easier. And with the disadvantages that a passive radiator requires precision construction quite like driver design, thus increasing costs, and has excursion limitations, unlike a tube full of air.
[edit] Compound or band-pass
A 4th order bandpass filter can be simulated by a vented box in which the contribution from the driver is trapped in a sealed box which modifies the resonance of the driver.[clarify] 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 sensitive to the characteristics of the driver. As in other reflex enclosures, the ports may be replaced by passive radiators if desired.
[edit] Aperiodic enclosures
This design falls between acoustic suspension and bass reflex enclosures. It can be thought of as either a leaky sealed box or a ported box with large amounts of port damping. 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 a primary producer of these enclosures for many years. The design remains uncommon among commercial designs currently available. A reason for this may be that adding damping material reduces the efficiency of the system; the same alignment can be achieved by simply choosing a loudspeaker driver with the appropriate parameters and tuning the enclosure and port for the desired response.
A similar technique has been used in aftermarket car audio; it is called "aperiodic membrane" (AP). A resistive mat is placed in front of or directly behind the loudspeaker driver (usually mounted on the rear deck of the car in order to use the trunk as an enclosure). The loudspeaker driver is sealed to the mat so that all acoustic output in one direction must pass through the mat. This increases mechanical damping, and the resulting decrease in the impedance magnitude at resonance is generally the desired effect, though there is no perceived or objective benefit to this. Again, this technique reduces efficiency and the same result can be achieved through selection of a loudspeaker driver with a lower Q factor, or even equalization. This is reinforced by the purveyors of AP membranes; they are often sold with an electronic processor which, via equalization, restores the bass output lost through the mechanical damping. The effect of the equalization is opposite to that of the AP membrane, resulting in a loss of damping and an effective response similar to that of the loudspeaker without the aperiodic membrane and electronic processor.
[edit] Other enclosure types
[edit] Dipole
A dipole enclosure in its simplest form is a driver located on a flat baffle panel. The baffle's edges are sometimes folded back to reduce its apparent size, creating a sort of open-backed box. 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 and the air. 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 300Hz to 2KHz) much larger, perhaps 1 or 2 feet, and for low frequencies (under 300Hz) very large, dozens of feet. Various speaker manufacturers have produced folded low frequency horns which are much smaller (e.g., Altec Lansing, JBL, Klipsch, Lowther, Tannoy) and actually fit in practical living rooms. These are necessarily compromises, and because they are physically complex, they are expensive. In earlier times, some users built horns whose mouths were built into the walls of listening rooms.
[edit] Multiple horn drivers
These horns use several drivers in a single horn structure and so all but one of the drivers is not precisely matched to the horn. The result is increased distortion, especially at high output levels from the drivers which are not impedance matched by the horn, since they are, in effect, mounted in a tapered hole.
[edit] Multiple entry horn
The multiple entry horn (also known as a coentrant horn, unity horn or synergy horn) uses several drivers mounted on the horn at stepped distances from the horn's apex, where the high frequency driver is placed. Depending on implementation, this design offers an improvement in transient response as each of the drivers is aligned in phase and time and exits the same horn mouth. A more uniform radiation pattern throughout the frequency range is also possible.[1] A uniform pattern allows smooth arraying of multiple enclosures.[2]
[edit] Tapped horn
Both sides of a long-excursion high-power driver in a tapped horn enclosure are ported into the horn itself, with one path length long and the other short. These two paths combine in phase at the horn's mouth within the frequency range of interest. This new design is especially effective at subwoofer frequencies and offers reductions in enclosure size along with more output.[2]
[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 enhance 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. A variation on the transmission line enclosure utilizes a tapered tube, with the terminus (opening/port) having a smaller area than the throat. 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.
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 driver 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. The lower part of the pipe acts as a horn while the top can be visualised as an extended compression chamber. The entire pipe can also be seen as a tapered transmission line in inverted form. (A traditional tapered transmission line, confusingly also sometimes referred to as a TQWP, has a smaller mouth area than throat area.) 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 TQWP is also known as a Voigt pipe and was designed by P.G.A.H. Voigt, the main Lowther driver designer.
[edit] See also
[edit] Notes
[edit] External links
- Quarter-Wave - details about Transmission Line
- Humble Homemade Hifi - DIY site with examples & plans of several speaker enclosure types
