Our first conversations about DML usually start with descriptions of some of the many benefits of DML technology, like higher gain before feedback, no echo, and 165 degrees (effectively 180 degrees) from a 3-1/2" thick loudspeaker. Right about that point, if our conversation is with someone that has audio experience and a deep understanding of sound waves, we can see their eyes start to gloss over, and if we could read the text bubble above their head, it would read "snake oil."

This is understandable, especially since the only loudspeaker technology that has existed since the dynamic loudspeaker was made practical in 1925 is the "point source" loudspeaker. And whether you are dealing with the 4" speaker in your car's dashboard, or the 16 element line array at a Metallica concert, the loudspeakers are all point-source (either physically, or effectively.) And boy has this technology come a long way! 

It's natural for our hypothetical listener to have doubts about our claims, as many folks that we speak with have made their professional life around loudspeakers, which up until this point, meant point-source loudspeakers, which have been studied for 100+ years (even longer if you count the experimentation back in the late 1800s.) So when we come along with DML technology, which is NOT a point-source, it's a little difficult to comprehend, since it follows a different set of rules. Not breaking any laws of physics, but creating instead a different kind of sound wave - that does not suffer from some of the issue point-source experiences when it comes in contact with things like walls.

Before we dive into what those rules are that DMLs comply with, let's talk a little bit about the physics of point-source. A point-source loudspeaker (and this can be an 18" woofer or a titanium compression driver/horn) operates on a principal of "pistonic" motion. The entire diaphragm moves forward and backward as a single, rigid unit (until it doesn't, and that's distortion, but that's a topic for another note). This pistonic motion creates what physicists call "correlated sound waves" since the diaphragm moves as a single unit, every part of the cone is "in phase" with every other part, creating waves that are identical in timing across the frequency response of the driver.

Sounds good, right? The sound waves leave the speaker in a very predictable spherical way. Unfortunately it leads to both constructive and destructive interference (hot spots and dead spots) as you move off-axis, the distance between your ears and different parts of the cone changes - and you have "comb filtering." And when you add in boundary interaction (walls and ceilings) you have a ton of correlated energy reflecting off the walls in the room (see this article on DML and SPL) that can quickly make a mess out of clarity. You could almost say that "point-source loudspeakers don't work well in rooms" (without the help of skilled audio professionals..)

What's different about the sound from a DML?

Unlike our friend the point-source, the DML does not generate a correlated sound wave. The sound coming off of a DML is not a point-source at all. DMLs use a large lightweight but very rigid panel (carbon fiber over a nomex core in our case) and instead of being "pumped" like a piston, the DML is "excited" creating a complex pattern of random vibrations across the surface. In other words, frequencies don't all emanate from a single point-source. They come from different locations on the panel in a random pattern. Physicists call this an uncorrelated sound source. At any moment, one part of the panel may be moving forward, while another moves backward, and this happens all over the panel at very slightly different times and phases.

Because these uncorrelated waves are randomized, they don't interfere with each other the way correlated waves do. There are no "sweet spots", "dead zones" and comb filtering is non-existent. Think of it like the way a fluorescent light fills a room with sound, versus a theatrical spot light, or even a laser. The fluorescent fixture gives you very even, diffuse sound throughout the room.

So how does this help us?

Think for a moment about what we often do in a room to improve its acoustics. Often we will install "diffuser" panels around the space that scatter the sound or attempt to "randomize" the correlated nature of the audio coming from the point-source loudspeakers. This can be very effective in reducing the amount of variations in sound around the room, minimizing comb filtering and echo, etc. The DML, however, is a diffuse sound source from the very beginning, so the randomization occurs as the sound is being generated. 

What this means is that DMLs are much less concerned about boundaries and listener position - the whole room effectively becomes the sweet spot, and sound is much more even throughout the space, in both frequency response and SPL. You could say that DMLs work with the room. There's no interference between reflections off of walls and ceilings. In a highly reverberant space, there will be a reverberation tail, but clarity is not lost, because there is very minimal "echo". Echo being distinct repeating sound events versus reverb which is more of a dense collection of thousands of reflections (definitely a deep topic for another article.)

A simple analogy

Think of it this way - if you take an acoustic guitar into a highly reverberant space, like a large traditional church with hard surfaces, or a historic train station and you play this acoustic guitar, it will usually sound very beautiful and musical. Yet if you take a well done audio recording of that acoustic guitar, and play it back - even through the best point-source Loudspeaker that you can find, you are going to find that very close to the point-source loudspeaker, where the listener is in primarily the "direct" space - hearing mostly the sound directly from the loudspeaker, it will sound as you would expect a well recorded source to sound. However as you move away from the loudspeaker, the listener will enter the reverberant space - where all of those thousands of correlated arrivals are coming at the listener "out of time" creating an overwhelming "mess" to the brain, and will significantly affect intelligibility.

Play that same acoustic guitar recording back through a FlatPanel Audio DML, and you may be hard pressed to discern much of a difference between the live acoustic guitar, and the recording. The uncorrelated sound from the DML enters the room as a diffuse sound mostly unaffected by the time variations in the thousands of arrivals.  The brain simply hears it as a reverberant tail, but because the reflections are uncorrelated, they are not destructive. As the listener moves away from the DML into the reverberant space, there is little to no change in the sound and in fact, the sound will be much more even throughout the room (no beaming like a point-source.)

This drastic difference in playback quality has taken place because the acoustic guitar is more of an uncorrelated sound source than a correlated one - because of this, the sound played back through the DML is of similar characteristic as the original acoustic guitar played naturally in the room. Playing the recording back through a point-source loudspeaker changed the sound energy from mostly uncorrelated (natural acoustic guitar) to correlated (point source loudspeaker) and brought along all of the challenges that we've stated thus far. In a way, the DML (like the acoustic guitar) being an uncorrelated sound source, works with the room and does not battle the destructive interaction of a point source correlated loudspeaker.

The DML is not breaking any laws of physics - but it IS using a DIFFERENT set of laws, The magic is in the uncorrelated nature of the wavefront coming from the DML transducer - just like sound that exits a diffuser on a wall, the sound from a DML is diffuse the moment it leaves the loudspeaker. When it hits a wall or other boundary in a room, the reflections do not negatively react with the direct sound and the other reflections in the room - and when they combine, they are non-destructive - in fact, they are often additive. 

We call this "reclaiming the reflected space" - Point-source audio tries it's best to get the listener in a position to where they are, whenever possible, hearing direct sound from the loudspeaker, not the reflected sound (where clarity suffers.) DML sound in the reflective space is completely usable and intelligible. This is how SeaTac airport was able to replace 104 ceiling speakers with 6 DML panels:

What about localization?

Often in this discussion, the question comes up "If a DML is diffuse, how does my brain determine the direction of the sound source?" This is a deep subject, however in simple terms, this has to do with the Haas effect, and the fact that the initial impulse of a DML is "pistonic enough" to provide the directional information needed by the brain and this turns out to not be an issue at all. Please see the articles referenced below in the "Related Articles" section for more information, including the above mentioned topic of localization.

 

If you have not experienced a DML in person, please contact us - we'd love to show you how this technology can solve audio problems in challenging rooms, and make well behaved rooms sound like concert halls.