the dynamic loudspeaker
In general, a sound source will consist of one of 4 forms, corresponding to [0..3] (zero to three) dimensional geometry. These are: point source, line source, planar source, 3D source. An infinitesimally small point source is often considered the ideal, because it radiates all frequencies equally in all directions in a spherical radiation pattern. Line sources may be finite or infinite, and they radiate sound in a cylindrical pattern. These first two sources are not practical, although real sound sources may approximate them at some frequencies. Several manufacturers have attempted to approximate a point source by approximating a pulsating sphere. In actuality, most sound sources (i.e. loudspeakers) are actually complex 3D shapes such as cones and domes, but most can be approximated (at least at low frequencies) as a planar form that creates a soundwave that becomes more directional as frequency increases, because the wavelength of the soundwave becomes small compared to the size of the diaphragm. That is, the intensity of the sound produced varies depending on the listener's angle relative to the central axis of the speaker. Specifically, the high frequency output decreases when the observer is located further off axis, and the off axis attenuation increases as the frequency is increased. So for high frequencies where the wavelength of sound is smaller than the diameter of the transducer, the size and shape of the radiating surface has a lot to do with the radiating pattern.
A common variation on the dynamic loudspeaker design uses a small dome as the moving part instead of an inverted cone. Perhaps contrary to intuition, making the moving component in the form of a dome rather than an inverted cone does not help to direct sound evenly in all directions. The dome is used because it is an easily manufactured stiff structure - as anyone who has attempted to crush an egg the long way can attest to. The stiffness also moves self resonances upward in frequency. The typical one inch dome tweeter starts to become directive at about 8000 Hz, below this frequency it approximates a point source, above this frequency, it becomes increasingly directive off axis. At distances that are more than ~7 times the diameter of the cone or dome, the response is essentially that of a flat plane, at closer distances, the exact shape of the diaphragm becomes increasingly important.
2D and 3D sources can be monopolar or dipolar in nature. Most planar sources are dipolar, which means that the sound from the rear of the diaphragm is out of phase with the sound from the front. When the rear radiation is absorbed or trapped in a box, the diaphragm becomes a monopole radiator.
Bipolar speakers, made by mounting in-phase monopoles on opposite sides of a box, are essentially a method of approximating a point source or pulsating sphere. This design is essentially omnidirectional.
Various manufacturers employ geometry and the resulting radiation patterns to more closely simulate the way sound is produced by real instruments, or to mimic one of the ideal sound source types, or simply to create an energy distribution that mimics real soundfields.
The Manger bending wave transducer relies on the principle of bending waves, which start from the centre of the speaker and travel to the outside. The rigidity of the transducer material increases from the centre to the outside. High frequencies therefore come to an end in the inner area, while long waves reach the edge of the speaker. To prevent reflections, long waves are absorbed by a damper. The Manger transducer covers the frequency range from 80 Hz to 35,000 Hz, and is close to an ideal point sound source.
Coaxial speakers have been around since the 1930's. These approximate a point source by moving the radiating axes of the various drivers closer to the same point, with benefits in polar response. Coaxial mounting eliminates crossover lobing (interference between drivers caused by non coincident placement) by bringing the drivers radiating centers to the same point. The tweeter response tends to suffer somewhat and the woofer must act as a horn in most cases.
The technique of using concentric radiating elements for a multiway system has been used by several previous manufacturers, notably Technics. Cabasse recently published a paper showcasing the development of 3-way and even 4-way coaxial speakers using concentric ring-shaped radiators.
Several manufacturers (Tannoy, Eminence...) still build 2-way coaxials where the tweeter fires through a horn that passes through the woofer pole piece.
Several manufacturers (KEF, SEAS...) build coaxial units where the tweeter is mounted on the woofer pole piece. The small form factor was made possible by recent developments in rare earth magnets.
One school of thought is to approximate a pulsating sphere, below are some of the techniques used.
Amar Bose of Bose (also a professor at MIT) spent many years trying to reproduce this spherical wavefront by constructing a one-eighth sphere covered in small drivers that would be situated in the corner of a room, thus mimicking one-eighth of a spherical wavefront emanating from that corner; in practice this idea never became workable, but Bose's experience with combining multiple small drivers in one loudspeaker cabinet gave rise to the popular speakers which use multiple small drivers pointed in various directions to help create the balance of direct and reflected sound that Amar Bose determined is usual in a concert hall. The technique is actually somewhat controversial.
Conical bending wave transducers
The Ohm model "F" speakers invented by Lincoln Walsh feature a single driver mounted vertically as though it were firing downwards into the top of the cabinet, but instead of the normal almost flat cone, having a very-much extended cone entirely exposed at the top of the speaker.
The usual problem with designing a driver is how to keep the cone as stiff as possible (without adding mass), so that it moves as a unit and does not become subject to traveling waves on its surface. The Ohm drivers were designed so that the entire purpose of the electromagnetic driver was to generate traveling waves that traversed the cone from the electromagnet at the top downwards to the bottom. As the waves moved down the truncated cone, the effect was to reproduce the omnidirectional soundwave, as with a cylinder that changed diameter. This created a very effective omnidirectional radiator (although it suffered the same "planarity" effect as ribbon tweeters for higher-frequency sounds) and eliminated all problems of multiple drivers, such as crossover design, phase anomalies between drivers, etc. However, in practice it was found necessary to use a very complex cone made up of various materials at different points along its length, in order to maintain the waveform traveling evenly.
The ribbon speaker consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon which vibrates creating the sound. The advantage of the ribbon loudspeaker is that the ribbon has very little mass; as such, it can accelerate very quickly, yielding good high-frequency response. Ribbon loudspeakers can be very fragile, thin ones can be torn by a strong puff of air. Ribbon tweeters emit sound that exits the speaker in a roughly cylinder-shaped pattern. Above and below the ends of the ribbon there is often less treble sound, but the precise amount of directivity depends on the length of the ribbon. Ribbon designs generally require powerful magnets which make them costly to manufacture. Ribbons have a very low resistance that most amplifiers cannot drive directly. A step down transformer is therefore used to increase the current through the ribbon. The amplifier "sees" a load that is the ribbon resistance times the transformer turns ratio squared. The transformer must be carefully designed so that its frequency response and parasitic losses do not degrade the sound, further increasing cost relative to conventional designs.
Planar Magnetic speakers (having printed conductors on a diaphragm - see below) are sometimes described as "ribbons", and they are related, but are not truly ribbon speakers.
While most loudspeakers essentially have planar diaphragms, the term planar is generally reserved for speakers which have roughly rectangular shaped planar radiating surfaces. Below are some of the methods of making speakers having flat plane-shaped diaphragms.
Flat panel speakers
There have also been many attempts to reduce the size of loudspeakers, or alternatively to make them less obvious. One such attempt is the development of voice coils mounted to flat panels to act as sound sources. These can then be either made in a neutral colour and hung on walls where they will be less noticeable, or can be deliberately painted with patterns in which case they can function decoratively. There are two, related problems with flat panel technology; firstly, that the flat panel is more flexible than the cone shape and therefore fails to move as a solid unit, and secondly that resonances in the panels are difficult to control, leading to considerable distortion in the reproduced sound. Some progress has been made using such rigid yet damped material as styrofoam, and there have been several flat panel systems demonstrated in recent years.
These consist of a flexible membrane with a voice coil printed on them. The current flowing through the coil interacts with the magnetic field of strategically-placed magnets, causing the membrane to vibrate. The driving force covers a larger percentage of the membrane and reduces the resonant problems inherent in coil-driven planar diaphragms. Many designs touted as "ribbons" are in fact planar magnetic designs. The long planar tweeters that are used in some designs have a small cavity in front of the diaphragm that is used to accomodate the magnets. This is not ideal and it sometimes creates a "cavity resonance" response peak that requires corrective filtration. Failure to correct this cavity resonance is sometimes the cause of the steely or shrill sound often attributed to these designs.
A newer implementation of the Flat Panel involves an intentionally flexible panel and an "exciter", mounted off-center in a location such that it excites the panel to vibrate. Speakers using NXT design methods can reproduce sound with a wide directivity pattern (paradoxically somewhat like a point source) with adequate quality.
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