We often get this question from folks who are new to the world of Distributed Mode Loudspeakers (DMLs) - and are performing their due diligence research.

There is a plethora of DML information out in the forums of the internet - unfortunately we find that there are a handful of well-meaning keyboard warriors out there that are providing DML information based on having experienced either an outdated product, or, sadly, has never experienced our DML500 in person. In our experience, individuals who experience DMLs via a physical demo, usually end up selecting it as their loudspeaker choice. With a demo, it doesn't really matter what the internet says, in a live demo, the listener gets to hear it with their own ears, and judge it first hand.

For this particular question, assuming that the comment has come from someone who actually experienced a demo, we can with great confidence say that in some cases, this may have been an accurate experience.

A 2-Way Device

The PL-11 was a 2-way device. At the time of release, the DML drivers from Tectonic were only able to reach up to about 6Khz in frequency response. To combat this, the PL-11 was a 2-way loudspeaker, and utilized a high-end gold foil ribbon tweeter to deliver the content from 6Khz to about 24Khz.

While this resulted in a revolutionary product for the time, it did come with some compromises.

Mismatched Coverage Patterns

The PL-11's DML driver (the ancestor of the current DML500 driver)  was only able to reach up to about 6Khz - requiring an additional driver to cover the rest of the audio spectrum up to 20Khz. This was accomplished using a high quality gold-foil ribbon driver - which completed the frequency response up to 20Khz +.

The challenge was that the revolutionary DML driver had a conical coverage pattern of 165 degrees, and the ribbon driver was fixed at 120 degrees wide by 15 degrees in the vertical. This meant that the listener could easily slip out of the HF ribbon coverage, while still being well within the 165 degree coverage of the DML (content below 6Khz.) Unlike today's DML500, movement within a room could result in audible changes in frequency response.

Uncorrelated vs. Correlated

The second issue that could contribute to lackluster High Frequency response at the extremes in a room was that the two drivers of the PL-11 were each creating very different types of sound, with differing characteristics.

First, the DML500 driver in the PL-11 contained all of the characteristics and benefits of a DML, most specifically that a DML is an uncorrelated sound source. Uncorrelated sound, rather than being destructive when it interacts with boundaries, results in reflections that end up being additive. As the sound interacts with the room and reflects off of surfaces in the room, no matter where you stand, the frequency response is remarkably even,

The ribbon HF driver (6khz to 20Khz+) in the PL-11 is a traditional pistonic loudspeaker, generating "correlated" sound waves. One of the characteristics of correlated sound waves is that as they interact with boundaries - the tightly correlated signals result in significant changes in frequency response when all of the arrivals hit the listener's ears. This could result in inconsistencies in HF coverage as listeners move farther out of the "sweet spot" - the listener could still hear the DML generating sound up to 6Khz, but the 6Khz - 20Khz+ audio may be inconsistent around the room.

DML500 = 165 Degrees of Coverage, Full Range

The current flagship product from FlatPanel Audio is a full range DML, covering 8.5 octaves of sound, from around 85Hz up to 20Khz and beyond. This means uncorrelated, non-destructive reflections around the room, and 165 degrees of coverage of the direct sound from the loudspeaker. The final result is consistent sound across the whole frequency spectrum, largely independent of the listeners position in the room. In other words, the room becomes the sweet spot.