Magnetostatic loudspeaker
The magnetostatic speaker is a speaker type.
Magnetostats are loudspeakers whose drive is not concentrated locally in the form of a voice coil, but is distributed over the entire membrane (foil magnetostats) or represents the membrane itself (classic ribbon). The term refers to the fact that the drive is flat, as with electrostats , but by magnetic fields (or by magnetostatic forces).
Magnetostats are mainly used in the upper frequency range as tweeters or sometimes as mid-range speakers (e.g. on some models from Elac ), but there are also full-range magnetostats (loudspeakers (housing)) the size of a cabinet for e.g. B. Magnepan or full-range magnetostats with an additional subwoofer for the very low frequencies.
Ribbon magnetostats
Aluminum is usually used as the membrane material for ribbon loudspeakers. It has (apart from some alkali and alkaline earth metals) the highest mass-specific electrical conductivity and, due to the formation of an oxide layer, has a certain self-protection against environmental influences. Additional coatings can still be useful. With ribbon magnetostats, significant partial oscillations can occur as soon as the wavelength of the sound becomes smaller than half the width of the ribbon. For 17 kHz, the width should therefore be no more than one centimeter. However, due to the relatively large membrane area and low mass, the partial vibrations are well damped by the ambient air (as with electrostats).
In order to achieve horizontal radiation, the ribbon is oriented vertically, while in order to achieve wider radiation while reducing floor and ceiling reflections, it is significantly higher than it is wide (e.g. tweeter 25 mm × 80 mm, mid-range speaker 60 mm × 200 mm) and often slightly convex. This curvature, as well as a light structure that is often encountered, give the very thin (approx. 10 µm, i.e. about aluminum foil ) and very sensitive membrane a certain mechanical stability.
Electric current flows through this film vertically and is located in a strong magnetic field (stator field) of a permanent magnet whose field lines run horizontally. The resulting Lorentz force moves the membrane back and forth and leads to sound radiation.
Due to the short conductor length, the impedance is very low (0.2 Ohm to max. 1 Ohm). Therefore, either special high current amplifiers or transformers are necessary. Increases in impedance are very limited due to the lack of degrees of freedom in the topology (there are no isolating membrane parts).
The aluminum ribbon, i.e. the membrane, is only clamped at the top and bottom, not on the sides. It swings freely between the magnets on the side.
A distinction is made between single-ended and push-pull structures. In the single-ended design, the stator field has large asymmetries, which lead to non-linearities even at medium oscillation amplitudes; however, in the push-pull design, the front noise must also be guided through the magnet, which leads to errors in the frequency response, especially at higher frequencies.
- Single-cycle construction (sectional view, view from above)
- Push-pull construction (sectional view, view from above)
Foil magnetostats
The membrane is a plastic film on which conductor tracks are applied. Here too, aluminum is common. The impedance is in the normal range between 4 and 8 ohms, since longer and thinner conductor tracks are possible with this technology. The conductor tracks run in a meandering shape on the membrane. The mostly rectangular membrane is attached to all sides in the loudspeaker chassis. Significantly more designs are possible than with ribbon magnetostats. In the case of foil magnetostats, significant partial oscillations occur as soon as the wavelength of the sound is less than half the distance between the conductor tracks. For 17 kHz, distances of no more than one centimeter are therefore permissible.
Foils are much more robust than tapes, which even the landing of a housefly can cause damage. However, there are often problems with the durability of the connection between the conductor tracks and the film.
The magnets are located behind the membrane, often also in front of the membrane. A distinction is made accordingly between single-ended and push-pull structures. In the single-ended design, the stator field has large asymmetries, which lead to non-linearities even at medium oscillation amplitudes; however, in the push-pull design, the front noise must also be guided through the magnet, which leads to errors in the frequency response, especially at higher frequencies.
- Single-stroke construction
NNN SS NN SS NN SS NN SS NN SS NN SS NN SS NN SS NNN Magnet mit akustischen Durchbrüchen ##-x--o--x--o--x--o--x--o--x--o--x--o--x--o--x--o-## Membran mit Alu-Mäander
The single-ended construction is used with Magnepan .
The magnetic field shows strong inhomogeneities. Even with moderate diaphragm deflections, strong distortions occur. A large membrane area is therefore provided in order to keep the deflections small. A
SS NN SS NN SS NN SS NN SS NN SS Magnet mit akustischen Durchbrüchen NN-x-SS-o-NN-x-SS-o-NN-x-SS-o-NN-x-SS-o-NN-x-SS-o-NN Membran mit Alu-Mäander
Although this reduces these inhomogeneities, the now large distance between the individual tracks leads in practice to strong partial oscillations in the presence area.
- Push-pull construction
NNN SS NN SS NN SS NN SS NN SS NN SS NN SS NN SS NNN Magnet mit akustischen Durchbrüchen ##-x--o--x--o--x--o--x--o--x--o--x--o--x--o--x--o-## Membran mit Alu-Mäander NNN SS NN SS NN SS NN SS NN SS NN SS NN SS NN SS NNN Magnet mit akustischen Durchbrüchen
The push-pull structure shows fewer inhomogeneities and distortions. It allows larger deflections and / or smaller membrane areas. It was introduced by Infinity in 1976 (EMIT, electromagnetic induction tweeter).
The foil magnetostat was invented in 1968 by Jim Winey and introduced in 1969 by his company Magnepan .
Air Motion Transformer (AMT)
It is also called JET emitter in a special form and was invented by the German physicist Oskar Heil (patent 1969). It is still used today in different versions by some manufacturers, due to its design complexity, rather in the high-priced area. The membrane is folded like an accordion. This has the advantage that by contracting the membrane with a comparatively small membrane movement, a multiple of the speed of the air pressed out is achieved.
View from above (# magnetic material, N north pole, S south pole, + - + membrane, ox conductor tracks)
- Single-ended construction AMT
- Push-pull structure Heil-AMT
- Push-pull design of the JET heater
### S N S N S N S N S N S N S N S N S N S N S N S ### Magnet mit akustischen Durchbrüchen ### +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-### ### o x o x o x o x o x o x o x o x o x o x o x o ### gefaltete Membran mit Alu-Mäander ###-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ ### ### N S N S N S N S N S N S N S N S N S N S N S N ### Magnet mit akustischen Durchbrüchen
In the JET push-pull structure of the AMT, the magnets and conductor tracks are arranged differently to one another. The JET heater was introduced in the early 1990s by the ARES company by Klaus Heinz (founder of Arcus) and has been further developed by ELAC since 1993 .
4-pi emitter
This is an all- round radiator in which a circular aluminum ribbon is used as a membrane.
Rotationally symmetrical, vertical section through the axis (# magnetic material, N north pole, S south pole, + - + membrane, ox conductor tracks)
Symmetrieachse
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The film is slightly ribbed vertically so that the membrane can breathe.
The 4Pi loudspeaker was introduced by ELAC in 1985 .
literature
- Thomas Görne: Sound engineering. 1st edition, Carl Hanser Verlag, Leipzig, 2006, ISBN 3-446-40198-9
- Berndt Stark: Loudspeaker manual. 7th edition, Richard Pflaum Verlag GmbH & Co.KG, Munich, 1999, ISBN 3-7905-0807-1
- Wolfgang-Josef Tenbusch: Basics of the loudspeakers. 1st edition, Michael E. Brieden Verlag, Oberhausen, 1989, ISBN 3-980-18510-9
Individual evidence
- ^ Infinity Classics, Klaus Pohlig 2005 .
- ↑ Magnepan (Wikipedia article).
- ↑ Patent US3636278 : Acoustic Transducer With A Diaphragm Forming A Plurality Of Adjacent Narrow Air Spaces Open Only At One Side With The Open Sides Of Adjacent Air Spaces Alternatingly Facing In Opposite Directions. Published on January 18, 1972 , inventor: Oskar Heil.
- ^ German Hifi Museum: ARES (Berlin) (information there from Audio 10/1988).
- ↑ ELAC (Wikipedia article).
- ↑ ELAC (Wikipedia article).