Loudspeaker enclosure
Encyclopedia
A loudspeaker enclosure is a purpose-engineered cabinet in which speaker driver
s and associated electronic hardware, such as crossover circuits
and amplifiers, are mounted. Enclosures may range in design from simple, rectangular particle-board boxes to very complex cabinets that incorporate composite materials, internal baffles, ports and acoustic insulation.
The primary role of the enclosure is to prevent sound waves generated by the rearward-facing surface of the diaphragm
of an open driver interacting with sound waves generated at the front of the driver. Because the forward- and rearward-generated sound is 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. Additionally, because they would travel different paths through the listening space, the sound waves would arrive at the listener's position at slightly different times, introducing echo
and reverberation
effects not part of the original sound.
The enclosure also plays a role in managing vibration induced by the driver frame and moving airmass 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.
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.
is mounted on a panel, with dimensions 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 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 from each other because they are generated through 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 a 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 resonance
s, 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. Research experiments from the 1930s by Dr. Harry F. Olson
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..
s and subwoofer
s can be adequately modeled in the low-frequency region (approximately 100–200 Hz and below) 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.
(stiffness of the suspension) determines the driver's resonant frequency
. 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. Output falls below the system's resonant frequency (Fs), defined as the frequency of peak impedance. In a closed-box, 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 sealed-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, and not to the popular misconception of a change in operating conditions from adiabatic
to isothermal. The enclosure or driver must have a small leak so internal and external pressures can equalise over time, to compensate for barometric pressure or altitude - the porous nature of paper cones or an imperfectly sealed enclosure is normally sufficient to provide this slow pressure equalisation.
installations as they are intended to go to frequencies lower than 20 Hertz and displace large volumes of air. “Infinite baffle” or simply “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.
or air suspension is a variation of the closed-box enclosure, using a smaller box to exploit the almost linear air spring which results. The "spring" suspension that restores the cone to a neutral position is a combination of an exceptionally compliant (soft) woofer suspension, and the air inside the enclosure. At frequencies below system resonance, the air pressure caused by the cone motion is the dominant force. Developed by Edgar Villchur
in 1954, this technique was used in the very successful Acoustic Research
line of "bookshelf" speakers in the 1960s-70s. Although no longer popular in commercial designs, the acoustic suspension principle takes advantage of this relatively linear spring. The enhanced suspension linearity of this type of system is off-set by rather low efficiency. Drivers for these designs rely more upon the enclosure characteristics than typical drivers, and most modern woofers are not well suited to acoustic suspension use.
pioneered the use of these designs in a live event context in the early 1970s. Vented system design using computer modeling has been practiced since about 1985, when researchers Thiele and Small
first systematically applied electrical filter theory to the acoustic behavior of loudspeakers in enclosures. While ported loudspeakers had been produced for many years before computer modeling, achieving optimum performance was challenging, as it is a complex sum of the properties of the specific driver, the enclosure and port, because of imperfect understanding of the assorted interactions. These enclosures are sensitive to small variations in driver characteristics and require special quality control concern for uniform performance across a production run.
speaker uses a second "passive" driver, or drone, to produce similar low-frequency extension, or efficiency increase, or enclosure size reduction, similar to ported enclosures. The passive driver is not wired to an amplifier; instead, it moves in response to changing enclosure pressures. In theory, such designs 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. Tuning adjustments for a passive radiator are usually accomplished more quickly than with a bass reflex design since such corrections can be as simple as mass adjustments to the drone. The disadvantages are that a passive radiator requires precision construction quite like a driver, thus increasing costs, and has excursion limitations.
If the enclosure on each side of the woofer has a port in it then the enclosure yields a 6th order band-pass response. These are considerably harder to design and tend to be very sensitive to driver characteristics. As in other reflex enclosures, the ports may generally be replaced by passive radiators if desired.
An eighth order bandpass box is another variation which also has a narrow frequency range. They are often used to achieve sound pressure levels in which case a bass tone of a specific frequency would be used versus anything musical. They are complicated to build and must be done quite precisely in order to perform nearly as intended.
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 (and driver manufacturers). Dynaco
was a primary producer of these enclosures for many years, using designs developed by a Danish driver maker. The design remains uncommon among commercial designs currently available. A reason for this may be that designing a perfectly resistive aperiodic vent is virtually impossible, and not a large number of commercially made vents were produced.
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. Again, this technique reduces efficiency and the same result can be achieved through selection of a driver with a lower Q factor
, or even via electronic 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 result is better damping and transient response for a similar frequency response.
to match the driver cone to the air. The horn structure 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; in effect the driver appears to have higher efficiency. 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, though implementation is sometimes difficult. Properly designed horns for high frequencies are small (above say 3 kHz or so, a few centimetres (a few inches)), those for mid-range frequencies (perhaps 300 Hz to 2 kHz) much larger, perhaps 30 to 60 cm (1 or 2 feet), and for low frequencies (under 300 Hz) very large, a few metres (dozens of feet). In the 1950s, a few high fidelity enthusiasts actually built full sized horns whose structures were built into a house wall or basement. With the coming of stereo (two speakers) and surround sound (four or more), plain horns became even more impractical. 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 rooms. These are necessarily compromises, and because they are physically complex, they are expensive.
The multiple entry horn (also known as a coentrant horn, unity horn or synergy horn) uses several different 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. A uniform pattern allows smooth arraying of multiple enclosures.
Consequent to the above, practical Transmission Line loudspeakers are not true Transmission Lines, as there is generally output from the vent at the lowest frequencies. They can be thought of as 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 of approximately comparable performance, due to the size and length of the guide required (typically 1/4 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, and the commercial designs of the now defunct IMF Electronics which received critical acclaim at about the same time.
A variation on the transmission line enclosure uses 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 system, resulting in a seashell like appearance. Bose uses similar patented technology on their Wave and Acoustic Waveguide music systems. Bowers & Wilkins
have used this approach in their flagship Nautilus speaker as well as smaller straight tapering tubes in many of their other lines.
Numerical simulations by George L. Augspurger and Martin J. King have helped refine the theory and practical design of these systems.
and was introduced in 1934 by Paul G. A. H. Voigt, Lowther's original driver designer.
Speaker driver
A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as...
s and associated electronic hardware, such as crossover circuits
Audio crossover
Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi-fi speaker systems use a...
and amplifiers, are mounted. Enclosures may range in design from simple, rectangular particle-board boxes to very complex cabinets that incorporate composite materials, internal baffles, ports and acoustic insulation.
The primary role of the enclosure is to prevent sound waves generated by the rearward-facing surface of the diaphragm
Diaphragm (acoustics)
In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...
of an open driver interacting with sound waves generated at the front of the driver. Because the forward- and rearward-generated sound is 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. Additionally, because they would travel different paths through the listening space, the sound waves would arrive at the listener's position at slightly different times, introducing echo
Echo (phenomenon)
In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...
and reverberation
Reverberation
Reverberation is the persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air...
effects not part of the original sound.
The enclosure also plays a role in managing vibration induced by the driver frame and moving airmass 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.
History
Early on, cabinets were made for radios for the sake of appearance, and the loudspeaker would be mounted into 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 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).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 driverTransducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
is mounted on a panel, with dimensions comparable to the lowest wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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 from each other because they are generated through 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 a 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 resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
s, diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
from cabinet edges and standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
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
Wharfedale (company)
Wharfedale is the name of a prominent audio equipment manufacturer in the UK, best known for its loudspeakers. It is currently part of the International Audio Group.Wharfedale also used to manufacture televisions, DVD players, set-top boxes and Hi-Fi players...
, in some designs, reduced panel resonance by using two wooden cabinets (one inside the other) with the space between filled with sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
. Home experimenters have even designed speakers built from concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
, granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
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. Research experiments from the 1930s by Dr. Harry F. Olson
Harry F. Olson
Harry Ferdinand Olson was a prominent engineer at RCA Victor.Harry F. Olson, a pioneer in the field of 20th century acoustical engineering, was born in Mount Pleasant, Iowa to Swedish immigrant parents...
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
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
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..
Woofer and subwoofer enclosures
Enclosures used for wooferWoofer
Woofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
s and subwoofer
Subwoofer
A subwoofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below...
s can be adequately modeled in the low-frequency region (approximately 100–200 Hz and below) using acoustics
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of 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.
Sealed (or closed) enclosures
The loudspeaker driver's moving mass and complianceStiffness
Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
(stiffness of the suspension) determines the driver's resonant frequency
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
. In combination with the damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
properties of the system (both mechanical and electrical) all these factors affect the low-frequency response of sealed-box systems. Output falls below the system's resonant frequency (Fs), defined as the frequency of peak impedance. In a closed-box, 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 sealed-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, and not to the popular misconception of a change in operating conditions from adiabatic
Adiabatic process
In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
to isothermal. The enclosure or driver must have a small leak so internal and external pressures can equalise over time, to compensate for 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 resonant frequency caused by the enclosure. Some infinite baffle 'enclosures' have used an adjoining room, basement, or a closet or attic. This is often the case with exotic rotary wooferRotary Woofer
The Rotary Woofer is a subwoofer-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil's motion to change the pitch of a set of fan blades rotating at a constant speed...
installations as they are intended to go to frequencies lower than 20 Hertz and displace large volumes of air. “Infinite baffle” or simply “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.
Acoustic suspension
Acoustic suspensionAcoustic suspension
The acoustic suspension woofer is a type of loudspeaker that reduces bass distortion caused by non-linear, stiff mechanical suspensions in conventional loudspeakers...
or air suspension is a variation of the closed-box enclosure, using a smaller box to exploit the almost linear air spring which results. The "spring" suspension that restores the cone to a neutral position is a combination of an exceptionally compliant (soft) woofer suspension, and the air inside the enclosure. At frequencies below system resonance, the air pressure caused by the cone motion is the dominant force. Developed by Edgar Villchur
Edgar Villchur
Edgar Marion Villchur was an American inventor, educator, and writer widely known for his 1954 invention of the acoustic suspension loudspeaker which revolutionized the field of high-fidelity equipment...
in 1954, this technique was used in the very successful Acoustic Research
Acoustic Research
Acoustic Research was a Cambridge, Massachusetts-based company that manufactured high-end audio equipment. The brand is now owned by Audiovox. Acoustic Research was well known for the AR-3 series of speaker systems, which used the 12-inch acoustic suspension woofer of the AR-1 with newly designed...
line of "bookshelf" speakers in the 1960s-70s. Although no longer popular in commercial designs, the acoustic suspension principle takes advantage of this relatively linear spring. The enhanced suspension linearity of this type of system is off-set by rather low efficiency. Drivers for these designs rely more upon the enclosure characteristics than typical drivers, and most modern woofers are not well suited to acoustic suspension use.
Bass-reflex
Also known as vented (or ported) systems, these enclosures improve low-frequency output, increase efficiency, or reduce the size of an enclosure, using cabinet openings or passive radiating elements to transform and transmit low-frequency energy from the rear of the speaker to the listener. They deliberately and successfully exploit the principles of the Helmholtz resonator. As with sealed enclosures, they may be empty, lined, filled or (rarely) stuffed with damping materials. Port tuning frequency is a function of cross-section and length. This enclosure type is very common, and provides the maximum deep-bass output for a given enclosure volume. Malcolm HillMalcolm Hill (audio engineer)
Malcolm Hill is one of the pioneers of the modern PA System, and an expert in Large scale event PAHe has been instrumental in the development of many modern items of audio equipment:Amplifiers: DX and Chameleon Range...
pioneered the use of these designs in a live event context in the early 1970s. Vented system design using computer modeling has been practiced since about 1985, when researchers Thiele and Small
Thiele/Small
"Thiele/Small" commonly refers to a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for...
first systematically applied electrical filter theory to the acoustic behavior of loudspeakers in enclosures. While ported loudspeakers had been produced for many years before computer modeling, achieving optimum performance was challenging, as it is a complex sum of the properties of the specific driver, the enclosure and port, because of imperfect understanding of the assorted interactions. These enclosures are sensitive to small variations in driver characteristics and require special quality control concern for uniform performance across a production run.
Passive radiator
A passive radiatorPassive radiator (speaker)
A speaker enclosure using a passive radiator usually contains an "active driver" , and a "passive cone" . The active driver is a regular driver, and the passive is typically the same or similar, but without a voice coil and magnet assembly. It is only a suspended cone, not attached to a voice coil...
speaker uses a second "passive" driver, or drone, to produce similar low-frequency extension, or efficiency increase, or enclosure size reduction, similar to ported enclosures. The passive driver is not wired to an amplifier; instead, it moves in response to changing enclosure pressures. In theory, such designs 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. Tuning adjustments for a passive radiator are usually accomplished more quickly than with a bass reflex design since such corrections can be as simple as mass adjustments to the drone. The disadvantages are that a passive radiator requires precision construction quite like a driver, thus increasing costs, and has excursion limitations.
Compound or band-pass
A 4th order electrical bandpass filter can be simulated by a vented box in which the contribution from the rear face of the driver cone is trapped in a sealed box, and the radiation from the front surface of the cone is into a ported chamber. This modifies the resonance of the driver. In its simplest form a compound enclosure has two chambers. The dividing wall between the chambers holds the driver; typically only one chamber 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. These are considerably harder to design and tend to be very sensitive to driver characteristics. As in other reflex enclosures, the ports may generally be replaced by passive radiators if desired.
An eighth order bandpass box is another variation which also has a narrow frequency range. They are often used to achieve sound pressure levels in which case a bass tone of a specific frequency would be used versus anything musical. They are complicated to build and must be done quite precisely in order to perform nearly as intended.
Aperiodic enclosures
Dynaco
Founded by David Hafler and Ed Laurent in Philadelphia, Pennsylvania in 1955, Dynaco was an American hi-fi audio system manufacturer popular in the 1960s and 1970s for its wide range of affordable, yet high quality audio components..-Early company history:...
was a primary producer of these enclosures for many years, using designs developed by a Danish driver maker. The design remains uncommon among commercial designs currently available. A reason for this may be that designing a perfectly resistive aperiodic vent is virtually impossible, and not a large number of commercially made vents were produced.
A similar technique has been used in aftermarket car audio
Car audio
Car audio/video , auto radio, mobile audio, 12-volt and other terms are used to describe the sound or video system fitted in an automobile. While 12-volt audio and video systems are also used, marketed, or manufactured for marine, aviation, and buses, this article focuses on cars as the most common...
; 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
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
is generally the desired effect. Again, this technique reduces efficiency and the same result can be achieved through selection of a driver with a lower Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
, or even via electronic equalization
Equalization
Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
. 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 result is better damping and transient response for a similar frequency response.
Dipole enclosures
A dipole enclosure in its simplest form is a driver located on a flat baffle panel, similar to older open back cabinet designs. 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 curved ones 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 as in others.Horn enclosures
A horn loudspeaker is a speaker system using a hornHorn (acoustic)
A horn is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air...
to match the driver cone to the air. The horn structure 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; in effect the driver appears to have higher efficiency. 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, though implementation is sometimes difficult. Properly designed horns for high frequencies are small (above say 3 kHz or so, a few centimetres (a few inches)), those for mid-range frequencies (perhaps 300 Hz to 2 kHz) much larger, perhaps 30 to 60 cm (1 or 2 feet), and for low frequencies (under 300 Hz) very large, a few metres (dozens of feet). In the 1950s, a few high fidelity enthusiasts actually built full sized horns whose structures were built into a house wall or basement. With the coming of stereo (two speakers) and surround sound (four or more), plain horns became even more impractical. 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 rooms. These are necessarily compromises, and because they are physically complex, they are expensive.
Multiple entry horn
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 design is especially effective at subwoofer frequencies and offers reductions in enclosure size along with more output.Transmission line
A perfect transmission line enclosure has an infinitely long line, stuffed with absorbent material such that all the rear radiation of the driver is fully absorbed, down to the lowest frequencies. Theoretically, the vent at the far end could be closed or open with no difference in performance. The density of and material used for the stuffing is critical, as too much stuffing will cause reflections due to back-pressure, whilst insufficient stuffing will allow sound to pass through to the vent. Stuffing often is of different materials and densities close to the cone, and changes as one gets further from the cone.Consequent to the above, practical Transmission Line loudspeakers are not true Transmission Lines, as there is generally output from the vent at the lowest frequencies. They can be thought of as 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
Thiele/Small
"Thiele/Small" commonly refers to a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for...
. Transmission lines tend to be larger than ported enclosures of approximately comparable performance, due to the size and length of the guide required (typically 1/4 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
Wireless World
Wireless World was the pre-eminent British magazine for radio and electronics enthusiasts. It was one of the very few "informal" journals which were tolerated as a professional expense.- History :...
by Bailey in the early 1970s, and the commercial designs of the now defunct IMF Electronics which received critical acclaim at about the same time.
A variation on the transmission line enclosure uses 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 system, resulting in a seashell like appearance. Bose uses similar patented technology on their Wave and Acoustic Waveguide music systems. Bowers & Wilkins
Bowers & Wilkins
Bowers & Wilkins is a British loudspeaker company that produces mid-range through reference quality hi-fi and home theater speakers. The company name is often abbreviated to just B&W.Joe Atkins is current owner and chairman...
have used this approach in their flagship Nautilus speaker as well as smaller straight tapering tubes in many of their other lines.
Numerical simulations by George L. Augspurger and Martin J. King have helped refine the theory and practical design of these systems.
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 pipeVoigt pipe
The Voigt pipe is a type of loudspeaker enclosure that embodies a combination of transmission line, ported enclosure and horn characteristics. It is highly regarded by some speaker designers, as evidenced by established manufacturers such as Castle, and . Due to its relatively high efficiency the...
and was introduced in 1934 by Paul G. A. H. Voigt, Lowther's original driver designer.
See also
- Full-rangeFull-rangeA full-range loudspeaker drive unit is defined as a driver which reproduces as much of the audible frequency range as possible, within the limitations imposed by the physical constraints of a specific design. Frequency range is of these drives is maximized through the use of a whizzer cone and...
- Guitar speaker cabinet
- Mid-range speakerMid-range speakerA loudspeaker driver that produces the frequency range from approximately 300–5000 hertz is known as a mid-range.Midrange drivers are usually cone types or, less commonly, dome types, or compression horn drivers...
- Powered speakersPowered speakersPowered speakers, also known as self-powered speakers and active speakers, are loudspeakers that have built-in amplifiers. They can be connected directly to a mixing console or other low-level audio signal source without the need for an external amplifier...
- SoundbarSoundbarA soundbar or sound bar is a special loudspeaker enclosure which creates a reasonable stereo or surround sound effect from a single cabinet. They are much wider than they are tall, partly for acoustical reasons, but also so that they can be mounted above or below a display device e.g...
- Speaker grilleSpeaker grilleA speaker grille is usually found in front of many consumer and industrial loudspeakers, and consists of either a hard or soft screen/grille mounted directly over the face of the speaker driver...
- TweeterTweeterA tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...
External links
- How a Hole-in-the-Box Works - information about bass reflex.
- Quarter-Wave - details about transmission line design
- Humble Homemade Hifi - DIY site with examples & plans of several speaker enclosure types
- IB Subwoofer FAQ Pages - Infinite Baffle Subwoofer FAQ page with illustrated guide and links to forum
- Free Speaker Plans - Community oriented DIY loudspeaker design plans, general resources and forum.