With almost all of the tools we use in audio production now having moved into software, we are in the position of having a choice about whether or not to take the hardware or the software route, the only pieces of the production process that still necessarily involve hardware are the transducers - microphones and speakers. While software has made inroads into our transducers courtesy of speaker correction and mic modelling, is is still true that all speakers and microphones rely on the pistonic motion of a surface backwards and forwards to capture or reproduce sounds in air, with one exception that I can think of - the AMT tweeter.
Actually I can think of two non pistonic ways of creating a speaker, the AMT tweeter and a plasm or “blue flame” tweeter. However unlike the plasma tweeter, which is exotic, impractical and frankly dangerous, the AMT tweeter is every bit as practical as a conventional tweeter but works differently and every bit as well and some would say better than a conventional hard or soft dome tweeter.
How Do Conventional Drivers Work?
While there are outliers like electrostatics, the overwhelming majority of loudspeakers are powered by electrodynamic drivers. Operating like a dynamic microphone in reverse, they use a coil of wire suspended in a magnetic field as a linear electric motor so that when an alternating current is passed through the coil, the driver moves backwards and forwards, compressing and rarefying the air in front of it, creating sound waves.
In the case of tweeters the mass is kept to a minimum to reduce inertia and help extend the frequency response upwards. The choice of material might be hard or soft but it will be light to enable the rapid changes of direction necessary to reproduce high frequencies. To preserve lightness, tweeters are kept small and there is an upper limit to how loud a conventional tweeter can be made to go, as illustrated by the use of horn loaded tweeters in high power applications like PA systems.
AMT or Air Motion Transformers, often misleadingly referred to as “ribbon tweeters”, are based on the same electrodynamic principles as conventional tweeters but their clever design means they move the air in a very different way, perform differently and importantly, sound different to conventional pistonic tweeters.
How Do AMT Tweeters Work?
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 can be a little 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.
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.
It’s easy to see why people compare the AMT to accordion bellows but 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.
OK, So It Works Differently And Is Pretty Clever But Why Should I Hear Some?
There are a few answers we could give; Because It’s more efficient, because it can have lower distortion. we could go on but the short answer is because you might like it. Three of the Production Expert team have tried AMT loaded monitors, each from a different manufacturer and here is what they thought:
Julian - HEDD Type 05
Although I’m going to reference the HEDD Type 05 because it was the first monitor I spent any significant amount of time with that had an AMT tweeter. But I’m going to illustrate what it is I like about them using the HEDD Tower Mains as an example.
When I heard the Tower Mains I was struck by the scale of the presentation. Two subs a side, each with four drivers and two 1200W amplifiers and four more bass and midrange drivers in the main unit. The solitary AMT tweeter in each stack looked very small by comparison but even when driven hard there wasn’t the slightest hint of distortion. When things get loud with “normal” tweeters I almost take it for granted that the top end will get a bit edgy but these little AMT tweeters give the impression of unlimited headroom - I’m sure it isn’t unlimited but I’ll be giving up long before they give up from what I’ve heard.
Looking back over the articles I’ve written on HEDD monitors I’m struck by how little I have to say about the sound of the AMT tweeter, it’s all there, it doesn’t run out of headroom, transient response is spot on and the HF response goes all the way up to frequencies none of us need ever worry about. They get the job done without any fuss. The best monitors should be the ones you don’t even think about the sound of shouldn’t they? Here’s a quote:
The AMT ribbon tweeters have great transient response, they really catch every corner in the top end. The sound is towards the bright end of things, I’d characterise it as between a soft and a hard dome but with really great transient response.
So there you have it, these tweeters don’t distract, they don’t change things but they do to their job really well. That’s why you owe it to yourself to check them out.
Eli - EVE Audio SC207
One of the most significant aspects of air motion transformer technology is that it uses a folded ribbon, as opposed to the flat unfolded ribbon found in conventional ribbon tweeters. All of the mid-size EVE Audio speakers use a proprietary version of this technology called RS2. RS2 stands for Ribbon System 2, meaning it has a bigger front plate and a larger magnet driving the motion with the utmost efficiency. What this means ultimately, is that it can deliver a higher output level, due to the bigger magnet system inside. The larger models incorporate their proprietary AMT RS3 tweeter, which uses an even bigger magnet and front plate, for even greater efficiency.
EVE Audio’s larger monitors utilise RS6 AMT technology. It is 2.5 times bigger than the RS1 (found on their smaller SC 203 and SC204 models,) RS2, and RS3 technology. They set the crossover for these at 1800 Hz, so they play a role in the upper mids as well.
So what does all this mean in actual use? Well, in addition to great sound, these tweeters deliver a higher output level with virtually no distortion, even at high output levels. I’m personally using the SC207 model, with the RS2 version of EVE Audio’s AMT technology. I routinely monitor at high volumes without giving a moment’s thought to how hard I am driving the speakers.
EVE Audio includes a magnetic protective grill that snaps right into place over the tweeters. So, I am not worried about them getting damaged physically by any protruding objects. It’s a nice touch that, not unimportantly, adds to your peace of mind.
The AMT RS2 tweeters sound crisp and clean, without any hint of the harsh, strident response I used to get from my old Mackie HR824 speakers. I really wouldn’t want to go back to mixing on conventional tweeters at this point. You can also learn more about Air Motion Transformer technology in my interview with EVE Audio CEO Roland Stenz.
Dan - ADAM Audio Range
I’ve used my set of dinosaur ADAM Audio P22A monitors for over 12 years in my studio. These have what ADAM Audio call ART tweeters and these were one of the many aspects that sold these monitors to me. At the time some close friends, in the know, warned me that these could get damaged easily. I didn’t care if these were fragile or not, as the exceptional sound quality of the high end overshadowed any potential problems I could experience down the road. As I said, these monitors have been in service for 12 years now with the tweeters not missing a beat and still going strong. Bear in mind that these monitors have been used hard and at times irresponsibly but yet they never failed on me, which others thought they would.
Scroll on a decade or so and this style of tweeter still catches my attention, especially in ADAM Audio’s newer product ranges being the affordable T-Series which feature U-ART tweeters and their flagship S-Series monitors, which feature high performance S-ART tweeters. The ART tweeters play one of the biggest parts in the ADAM Audio sound and has been in its DNA for a couple of decades. I’m just very accustomed to the sound of a monitor with ribbon tweeters, especially ADAM monitors. Some users report they find ADAM monitors too harsh sounding, I don’t. I feel the beautiful top end, which these deliver, helps me to quickly spot if I’m overcooking the top end at all in a mix.
As for my current P22A monitors I’m currently considering changing these for an ADAM Audio S Series monitors. I’m still deliberating over the two way S2V or three way S3V… but more on that for another day.