Transducers are the most important part of the audio signal path, those devices which convert acoustic energy into electrical energy (microphones) and the converse, electrical energy into acoustic energy (loudspeakers) are the points in the journey our music takes from performance to playback where the most damage is done. The choice of transducer type is well known to all of us when choosing a microphone, dynamics, condensers and ribbons all have their respective strengths and these differences occasionally cause people like us to agonise over these differences and favour a ribbon for it’s high frequency characteristics or a dynamic for it’s midrange performance.
So why are almost all of our monitors electrodynamic? When teaching I’ve illustrated the reversibility of transducers many times in the past, easily done in the case of electrodynamic drivers. A speaker being used as a microphone on kick drums is a well known trick. I’ve had rooms full of students using headphones to record into Pro Tools many times but condenser and ribbon microphones also have their opposites in the loudspeaker world.
The speaker equivalent of a condenser microphone would be the electrostatic speaker. These enjoy a small but dedicated following but to really move air and deliver bass frequencies electrodynamic drivers are hard to beat.
What About Ribbons?
While the ribbon tweeter shares the same fundamental principle as a ribbon microphone, there are some important differences between the operation of these two transducers. In fact calling them “ribbon” tweeters at all is probably unhelpful. They do incorporate a ribbon of conductive material; which is suspended in a magnetic field but the operation isn’t quite the same as a ribbon mic in reverse.
The AMT Tweeter
The Air Motion Transformer is a transducer design which, because it is extremely light and because it has very little intrinsic stiffness, is extremely well suited to reproducing high frequencies. Because of this, they can accelerate extremely quickly. Electrodynamic tweeters do an amazingly good job of reproducing high frequencies considering they are comparatively stiff and heavy, both things which limit their ability to change direction quickly. Something a HF driver needs to be able to do.
Something which seems counterintuitive about the AMT is that unlike an electrodynamic driver it isn’t pistonic. Most drivers work by moving backwards and forwards, compressing and rarefying the air in front of it. To visualise how an AMT driver works it can be helpful to think of a pair of accordion bellows with the folds coming together and expelling the air in front of them (however this is misleading as the motion isn’t quite like this, see the diagrams below). The AMT is a folded diaphragm and as such has a much larger surface area than the equivalent flat driver. This offers what can be thought of as a mechanical advantage, with a small movement of a large surface, driving the air in front of it more efficiently than an unfolded surface would. In fact the AMT moves air four times faster than the diaphragm moves, conventional pistonic designs have a a 1:1 relationship between the air speed and the diaphragm speed.
HEDD Explain: The Air Motion Transformer is an electromagnetic driver, as it is based on the Lorentz force that moves the air in the single folds. The diaphragm itself has an aluminium circuit printed on it (violet arrows) and is surrounded by a strong magnetic field. The graphics in and around the small circles show the motion of the individual foils producing a sinusoidal waveform: from where it starts (black circle) through the positive (green circle) and negative (red circle) half-waves. The resulting air flow (blue arrows) is four times faster than speed in which the individual folds move, which is a big advantage when it comes to reproducing musical signals with fast transients (cymbals, plucked guitar strings, etc.).
AMT - Not Like An Accordion After All
So we can see that the accordion analogy isn’t quite right because with changes in polarity from negative to positive alternate folds in the ribbon come together causing the air in front of the diaphragm to be compressed setting up a pressure wave. In an accordion the edges of the bellows move causing the forward and rearward facing folds to stretch apart or come together. In the motion of the AMT the edges of the diaphragm don’t move but alternate folds are drawn together and repelled apart producing compression on one side of the diaphragm and rarefaction on the opposite side, just like a piston would but faster and more efficiently.
To hear the story of how HEDD were introduced to the AMT and to see how they are made watch the video below.