There are a few other rarer methods used beyond the dynamic transducer, briefly described here:
Electrostatic loudspeakers use a thin metallised plastic sheet suspended near a fixed grid. When a high voltage audio signal is applied to the grid the electrostatic charge pulls and pushes the sheet making sound. This full range speaker is often considered superior to dynamic as the diaphragm is lighter and moved across it’s whole surface.
Ribbon loudspeakers use a thin ribbon of metal in front of a strong magnet. A high current audio signal is fed across the ribbon and it moves with the changing magnetic field of the ribbon, making sound. This is most often used as a tweeter, sometimes midrange, and is considered superior to dynamic as again the ribbon can be very light and the whole ribbon is moved by the magnetic field.
Magneplanar (or ortho-dynamic) uses a conductive trace drawn on the surface of a thin plastic film. Strong magnets are placed along the trace and in a similar way to a ribbon speaker the film is moved by the audio signal fed across the trace that creates a changing magnetic field. These are often only used in headphones due to their limited range of movement (therefore unable to move a large mass of air) and are again considered better than dynamic due to the lighter diaphragm.
AMT or air motion transformer technology is again related to ribbon and magneplanar where the diaphragm is suspended in a magnetic field with traces deposited on its surface. But in this case the diaphragm is corrugated or folded like a bellows which introduces a different mode of sound production that requires less movement from the membrane. So you have the light diaphragm with less movement.
All of the above have a common issue - they use a solid mass to drive the air. As a result of this there will always be some distortion present as the solid mass acts as a spring-mass-damper system and never truly follows the original signal. Of course this distortion is of little consequence for a wide variety of uses, however if we are trying to approach perfect sound reproduction then it is an area that needs to be addressed.
Dynamic speakers produce quite a lot of distortion because of this, however good or expensive they get. Using servo assist, a variety of acoustic methods in the boxes and many different driver arrangements all help but do not solve the problem. The electrostatic, magneplanar and ribbon designs have lower distortion as the mass is lighter and the audio signal is driven across the whole diaphragm, but it is still there. There is a trend in describing higher end magneplanar and electrostatic headphones as massless in their marketing. They are still a solid mass driving a gas and the spring-mass-damper system is still there. With a solid driving a gas the impedance mismatch also introduces inefficiency and distortion.
In an ideal world we would want to drive the air with the lightest medium possible and that is where the "massless" speaker comes in - it makes sound using only the air itself, either converted to a plasma or driven directly by electrons/ions from high voltage sources. Yes there is still some mass but it is equivalent and made from the medium being driven (air) and therefore does not add mass to the system, overall massless.
Other advantages of having no speaker diaphragm include: the lack of resonant frequencies due to the dimensions of the solid diaphragm; rear output reflections have no diaphragm to interact with reducing distortions from that mechanism; no mass from the driving system such as a copper coil or metal coating.
These are actually one of the simplest possible forms of speaker, you need no more than a separated pair of wires with some correctly modulated high voltage across them to produce the sound. Making them safely perform to Hi-Fi standards is the challenge.
Briefly, the most common (if that word can be used for any of these speakers!) type is known as a plasma tweeter, and that is what you will usually see in videos and on DIY websites. A glow discharge plasma arc is created between two electrodes or in a quartz tube and a basic radio or switching circuit is used to create an audio modulated signal via a high voltage transformer to drive it. As the plasma arc is quite small, it is difficult to produce low frequency sounds with any power and so these designs are usually made as tweeters - and very effective they can be as most commercial designs have been based on this method.
There is also the ion or corona wind method using a corona discharge that can potentially create a wider range of frequencies as it usually uses a large number of thin electrodes or wires to generate ions that collide with and drive air particles (creating a constant wind). When this is modulated in a similar way to the plasma speaker the wind will oscillate and produce sound. There have been no commercial designs of this type, but there have been many built and documented - some very near to commercialisation.
A few rarer and earlier methods have been experimented with including modulation of flame or gases directly.
Hopefully you now know why you might want a speaker of this type, take a look below for more information on safety, different types and the methods have been used to create them.