Ribbon speakers diaphragm
The diaphragm of a ribbon loudspeaker is conductive and it is positioned in a transverse magnetic field. It is also supported at each end. Current flowing along the diaphragm creates a force perpendicular to it. The mechanism produces an even trust on the diaphragm which can, therefore, be extremely thin and light.
Comparison with Electrostatic speakers
There are some parallels with the electrostatic speaker that evenly creates the driving force over the entire surface of the light diaphragm. The two systems have in common the fact that the transduction mechanism is basically linear and has a desirable minimum phase characteristic. However, there is a great deal of difference between the two when you try to put the principle into practice. The electrostatic speaker produces the driving force in the same direction as the applied (electrostatic) field so that the diaphragm size can be as large as required and the electrostatic speaker can be very efficient.
Magnetic field lines
Ribbon loudspeakers produce a driving force to the applied (magnetic) field. This means that a diaphragm is a plane of the magnetic field lines. As the diaphragm is very thin, an only a small part of the magnetic field passes through it. A further difficulty is that a very wide air gap severely limits the width of the diaphragm. As the gap has high reluctance, leakage is a serious problem. In the Decca/Kelly “London” ribbon speaker, the gap efficiency was less than 3%. These difficulties mean that a full-frequency range ribbon loudspeaker is impossible whereas it is possible with electrostatic technology.
Direct radiating and Horn-loaded units
Ribbon loudspeakers require separate low-frequency units. They may be direct radiating or horn-loaded. Direct radiating ribbon speakers must be very heavy owing to the size of the magnet system and the uniform drive over the diaphragm causes directivity problems as “ka” becomes too large. Horn-loaded ribbon loudspeakers avoid this difficulty and can give very good directivity. The horn-loading also brings the efficiency in line with direct radiating moving-coil transducers. However, unless the design is very well, the coloration of the horn may lose some of the inherent clarity of the ribbon transduction system. For these reasons, this alternative to the moving coil has not enjoyed as much success as the electrostatic speaker.
The choice of the ribbon material reflects the need for efficiency. If you double the thickness, twice as much current would flow, you would double the power and the force created. But in this case, the mass also doubles, making the amplitude the same. The efficiency would be halved. Thus the ideal diaphragm is one which is as thin as structural requirements allow. Practical diaphragms would be corrugated. This increases the transverse rigidity and reduces the resonant frequency. The amplitude achieved goes as the ration of the current mass, but the power required goes as the product of the current and the resistivity. Consequently, the greatest efficiency comes with a diaphragm material having the smallest product of resistivity and density.