Dipole speaker
Encyclopedia
This article is about loudspeaker systems designed such that one approximate point-source emits sound equally in two opposing directions, 180 degrees out of phase. For discussion of "stereo dipole" speakers, see stereo dipole
and ambiophonics
.
A dipole speaker enclosure in its simplest form is constructed by mounting a loudspeaker
driver on a flat panel. The panel may be folded to conserve space.
The term dipole
derives from the fact that the polar response consists of two lobes, with equal radiation
forwards and backwards, and none perpendicular to the axis. This can be useful in reducing the stimulation of resonant room modes
at low frequencies. It also results in high frequencies being reflected from any rear wall, which can enhance the naturalness of the sound in typical listening rooms by creating more diffuse reverberation
, though in theory it could detract from stereo
localization. For this reason dipole speakers are often used as surround channel speakers, where a diffuse sound is desired to create ambience.
A dipole speaker works by creating air movement (as sound pressure waves) directly from the front and back surfaces of the driver, rather than by impedance matching one or both outputs to the air. As a result, diaphragm motion is constrained primarily by the driver's restoring force (e.g., diaphragm suspension) and not by acoustic loading from an enclosure. This implies that cone motion will be larger at the same output level than in a more usual enclosure, and that power handling will be accordingly limited. Especially at lower frequencies, dipole drivers tend to be large and flat, and necessarily open at both front and back. Common examples include electrostatic or ribbon drivers, though a conventional cone driver mounted in an open baffle also works as a dipole loudspeaker. All of these variations are characterised by a "figure-of-eight" radiation pattern in which the loudness falls towards the sides of the enclosure where interference between front and back waves is maximized. Sometimes the enclosure is modified into an "H-frame" with the driver located on a wall dividing two open compartments. Such enclosures require some control over the radiated sound from the rear of the enclosure to achieve the desired response. This is usually done by mounting two drivers, one over the other, in a push-pull configuration.
Some of the speaker cabinets referred to as dipolar arrange the two poles at an angle of about 90 degrees rather than 180 degrees, especially for wall-mounted rear speakers (e.g. Jamo C-80-SUR , Castle dipoles, and Theophany S4).
Dipole speakers are said to be better for rear speakers in surround-sound systems, although this is disputed (or at least success depends on factors such as room acoustics, type of music, and so on).
Stereo dipole
A Stereo dipole is a sound source in an Ambiophonic system, made by two closely spaced loudspeakers that ideally span 10-30 degrees. Thanks to the cross-talk cancellation method, a stereo dipole can render an acoustic stereo image nearly 180° wide or 360° ....
and ambiophonics
Ambiophonics
Ambiophonics is a method in the public domain that employs digital signal processing and two loudspeakers directly in front of the listener in order to improve reproduction of stereophonic and 5.1 surround sound for music, movies, and games in home theaters, gaming PCs, workstations, or studio...
.
A dipole speaker enclosure in its simplest form is constructed by mounting a loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
driver on a flat panel. The panel may be folded to conserve space.
The term dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
derives from the fact that the polar response consists of two lobes, with equal radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
forwards and backwards, and none perpendicular to the axis. This can be useful in reducing the stimulation of resonant room modes
Resonant room modes
Room modes are the collection of resonances that exist in a room when the room is excited by an acoustic source such as a loudspeaker. Most rooms have their fundamental resonances in the 20 Hz to 200 Hz region, each frequency being related to one or more of the room's dimension's or a...
at low frequencies. It also results in high frequencies being reflected from any rear wall, which can enhance the naturalness of the sound in typical listening rooms by creating more diffuse 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...
, though in theory it could detract from stereo
STEREO
STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...
localization. For this reason dipole speakers are often used as surround channel speakers, where a diffuse sound is desired to create ambience.
A dipole speaker works by creating air movement (as sound pressure waves) directly from the front and back surfaces of the driver, rather than by impedance matching one or both outputs to the air. As a result, diaphragm motion is constrained primarily by the driver's restoring force (e.g., diaphragm suspension) and not by acoustic loading from an enclosure. This implies that cone motion will be larger at the same output level than in a more usual enclosure, and that power handling will be accordingly limited. Especially at lower frequencies, dipole drivers tend to be large and flat, and necessarily open at both front and back. Common examples include electrostatic or ribbon drivers, though a conventional cone driver mounted in an open baffle also works as a dipole loudspeaker. All of these variations are characterised by a "figure-of-eight" radiation pattern in which the loudness falls towards the sides of the enclosure where interference between front and back waves is maximized. Sometimes the enclosure is modified into an "H-frame" with the driver located on a wall dividing two open compartments. Such enclosures require some control over the radiated sound from the rear of the enclosure to achieve the desired response. This is usually done by mounting two drivers, one over the other, in a push-pull configuration.
Some of the speaker cabinets referred to as dipolar arrange the two poles at an angle of about 90 degrees rather than 180 degrees, especially for wall-mounted rear speakers (e.g. Jamo C-80-SUR , Castle dipoles, and Theophany S4).
Advantages
An advantage of dipoles is that the sound is concentrated in the listening area due to the figure-of-eight polar response. This means that for a given output loudness, locations falling within the "dead-zone" of the speaker do not perceive as loud a sound as they would with more traditional enclosures.Dipole speakers are said to be better for rear speakers in surround-sound systems, although this is disputed (or at least success depends on factors such as room acoustics, type of music, and so on).