DMLs are Dipole Radiators

"Dipole" is not a term used in reference to loudspeakers very often. The term is more commonly seen referring to antenna radiation patterns, and microphone pickup patterns (usually termed "figure-eight".)  FlatPanel Audio Distributed Mode Loudspeakers (DMLs) are unique in that they are actually dipole loudspeakers, which means that they radiate sound nearly equally from both sides of the loudspeaker, generating sound waves in a figure eight pattern. We will address why we say "nearly" below in this article, and we will also share how to use this characteristic to your advantage when designing system.

Point-Source Considerations

Since traditional (pistonic/point-source) loudspeakers create correlated sound waves, most designers work to place loudspeakers so that the listener is in the direct-radiating sound field of the loudspeaker, where maximum clarity is maintained. As the listener gets farther away from a point-source loudspeaker and begins to hear a larger percentage of the reflected sound in the room, clarity and intelligibility can suffer greatly.

Most professional audio point-source loudspeakers have a predictable and measurable dispersion pattern that can be beneficial when designing sound reinforcement systems. Designers can select coverage patterns and accurately aim loudspeakers to help keep the majority of the sound aimed at the listener. This helps minimize putting sound energy into the room that would not end up being directly heard by the listener, and instead would only contribute to the reflected sound energy, further reducing intelligibility.

DML Behaviors

With FlatPanel DML's, resonant, uncorrelated "diffuse" sound is generated which has very different characteristics than traditional point-source loudspeakers, especially when considering boundary interaction.  FlatPanel DMLs are diffuse sound sources, that do not suffer from clarity and intelligibility issues as the listener transitions out of the loudspeaker's direct sound field into the reflected sound field. For the designer, this means that as the listener moves more into (or completely into) the reflected sound field, the listener experiences clear, intelligible sound. This releases the designer from the burden of choosing only loudspeakers with specific coverage patterns, and having to use software for precision aiming the loudspeaker for best behavior.

DML500 from FlatPanel has a nominal 165 degree coverage pattern out of both the front and the rear of the loudspeaker. Because this is uncorrelated energy, any sound that reflects off of walls, floors, and other boundaries ends up at the listener's ear is non-destructive, uncorrelated sound energy. Rather than being destructive to the listening experience, this reflective energy is, in most cases, additive - meaning the direct and reflected sound ends up at the listener's ear without any peaks and valleys in frequency response (comb filtering) and the two levels combine to simply make a cohesive, louder signal. This behavior is not infinite, we find that intelligibility is retained until RT (reverb time) reaches 7 seconds or more, but reverb times this great are extremely difficult to overcome with any loudspeaker. 

The sound at the rear of the DML500 has a slight variation in frequency response when compared to the forward sound due to the presence of the exciters in the rear center of the panel. Rear radiation is also attenuated by about -6dB with acoustic damping material.

Point-source Loudspeakers Struggle in Rooms.

I know that's a bold statement, but hear me out. Most modern professional, point-source loudspeakers sound fantastic when experienced in a well treated room - meaning a room with acoustic diffusers and sound-damping material appropriately applied. This makes sense, as all of the measurement data taken by the manufacturer was usually done in an anechoic chamber- a room completely without reflections.

When a point-source loudspeaker is introduced into a space with boundaries, the designer now has to take into consideration the direct sound field versus the reflected sound field. As correlated sound waves interact with room boundaries and end up at the listener, they combine together with the direct sound, and this results in dips and valleys in frequency response, called comb-filtering. This causes sound to degrade in clarity, and intelligibility drops rapidly when the listener hears less and less of the direct sound from the loudspeaker. In other words, even the best loudspeaker in a reflective room will struggle with intelligibility. To correct for this, designers will specify costly "acoustic treatments" that "diffuse" the sound as it interacts with the boundary, improving clarity. Unfortunately, these acoustic treatments can sometimes be more costly than the loudspeaker system, and often difficult to work into the space aesthetically.

With DML500, the sound coming from the loudspeaker is diffuse at the source, removing the concern for destructive boundary interaction. Simply stated, the DML500 "reclaims the reverberant space" - the reflected sound has clarity on par with the direct-radiated sound from the loudspeaker. Simply stated, DMLs work very well in rooms where point-source loudspeakers would struggle.

Back to the Dipole

Because the DML500 radiates sound as a dipole (figure-eight,) the DML500 sends energy throughout the room. If this energy was correlated sound from a point-source, this could end up being a disaster for the designer, as sound energy would be going everywhere in the room, resulting in clarity, intelligibility and even feedback issues, were it correlated pistonic energy. But since the energy from a DML is diffuse, uncorrelated sound, even the reflected energy becomes additive in nature combining with the direct sound to create a broadband increase in SPL. The sound does not degrade as it transition from direct to reflected sound at the listener.

Advantages to the Dipole:

With the rearward-facing sound being completely usable with the DML500, this opens up new possibilities to put this sound to work for us in the space. Some of the ways we have seen this be used are: 

  • Stage Fill: Rearward sound works as a stage fill, side fill,  or monitor for some smaller live venues. This is a very popular feature for churches and venues without a monitor engineer - while band members may still need individual monitor mixes, the rear sound from the DML provides a fantastic "base mix" for the band, choir, orchestra, etc.
  • Extreme side fill: Left and right "wing" seating in some smaller venues may be able to avoid additional fills at the extreme left and right of the stage.  
  • Talent Reinforcement: In larger venues, the speaker on the stage may need to hear themselves to aid in speaking confidence
  • Increases coverage: With DML500 in close proximity to a wall, some users can experience an effectively 180 degree coverage, as the rearward reflections fill in the last 15 degrees of coverage.

Of course, there may be cases where that rearward energy may not be desirable - in which case external attenuation material can be employed to limit the rearward energy. The purpose of this article is to remind the reader that uncorrelated rearward energy from a loudspeaker may be much more desirable and in some cases helpful/beneficial versus traditional correlated rearward energy that negatively effects clarity and frequency response.