The Distributed Mode Loudspeaker (DML) redefines acoustic radiation. Understanding DML technology requires a radical re-framing of how we understand loudspeakers.
For over 90 years, loudspeaker development revolved around identifying and suppressing
diaphragm resonances and their resulting coloration. DML technology turns this approach on its ear. The goal isn’t to eliminate diaphragm resonances. Rather, the goal is to encourage and exploit them
To appreciate the DML’s many benefits, the basic principles of how conventional loudspeakers operate should be explored.
Distributed modes vs. pistonic radiation
The conventional piston loudspeaker
Conventional loudspeakers aim to achieve pistonic motion of the diaphragm, meaning the diaphragm moves as a rigid whole to produce a steady acoustic power output. As frequency continues to rise, the wavelength in the air decreases to where it becomes comparable to the diaphragm’s dimensions
The rising radiation impedance compensates the falling diaphragm velocity above resonance up to this frequency. However, at this convergence point of the diaphragm’s size and radiated wavelength, the radiation impedance stops rising and flattens off. Thus, the radiated power falls off, limiting the pistonic diaphragm’s useable bandwidth.
This phenomenon, coupled with the need to provide sufficient volume velocity to reproduce frequencies at the lower extreme of its passband, limits a conventional driver’s power bandwidth to a four to five octave range. Even a perfect pistonic radiator—if one existed—couldn’t solve this issue
The distributed mode loudspeaker
The DML operates by exciting a light, rigid diaphragm into complex vibration patterns (modes). Excitation of these modes is achieved by coupling one or more electro-mechanical transducers (exciters) to the diaphragm’s rear side.
Of course, it’s possible to make a flat panel loudspeaker by attaching an exciter to a suitable diaphragm. However, this object is far from being classified as a DML.
To realize a DML, it’s necessary to distribute the vibrational modes of the diaphragm optimally across the required bandwidth. This requires consideration and careful analysis of:
- The diaphragm’s aspect ratio (length to width ratio)
- Material properties of the diaphragm
- The attachment point(s) for the exciter(s)
- Boundary conditions (how the diaphragm is supported)
The design is likely to perform as a DML once these key areas have been understood and optimized, as well as many other aspects, such as the properties of the exciter
Achieving even room coverage
A DML’s acoustic output behaves differently than that of a conventional loudspeaker. The diaphragm’s complex vibration patterns radiate with a largely uncorrelated nature, which is the essence of a DML’s diffuse output. Because of the acoustic radiation’s uncorrelated nature, a DML reduces interactions with local boundaries, exhibiting considerably less comb filtering and coloration compared to a conventional loudspeaker. A DML can be used near room boundaries with a reduced adverse effect on stereo imaging and timbre.
The decorrelated nature* of a DML’s radiation over adjacent angles extends to the rear side radiation, as well. Thus, early reflections within a typical environment will be substantially diffuse, eliminating the need for costly wall treatments. This diffuse nature, along with the wide dispersion, results in a very smooth and homogenous sound field within a space.
*Gontcharov, Vladimir. Hill, Nick. “Diffusivity Properties of Distributed Mode Loudspeakers.” 108th AES Convention. Feb. 2000
Intelligibility across eight octaves
The DML’s increased intelligibility is another obvious benefit. Most conventional loudspeakers are forced to crossover between a midrange and tweeter at around 2-4kHz, the most sensitive region of the human auditory system.
The mid-woofer and tweeter are often spaced further apart than the acoustic wavelength at this crossover frequency, which exacerbates the issue and makes smooth integration challenging or impossible.
A DML panel only requires a conventional subwoofer to cover the audible frequency range’s lowest octaves, making a seamless crossover transition at around 150Hz much easier to achieve. Because of the low off-axis correlation, a DML panel improves intelligibility more effectively than a conventional loudspeaker. Perhaps even more pertinent is that a DML operates with wide directivity over an entire eight octaves (~100Hz to >25kHz)—all from a single diaphragm. To our knowledge, no other speaker can deliver such a performance.
Temporal considerations and stereo imaging
The DML produces a well-defined and stable stereo image with a wide sweet spot, somewhat counterintuitively, given its highly decorrelated radiation. Why is this? The answer lies in the solutions to the 4th order partial differential equations that describe bending wave behavior in plates. These solutions have two forms: propagating waves and exponentially decaying nearfield disturbances.
Here’s how it works:
The propagating waves travel radially outward through the panel from the point of excitation. They are reflected at the panel boundaries and setup standing waves (modes) that produce the diffuse output.
The near-field disturbance is produced close to the excitation point and dissipates exponentially. It is this disturbance that is responsible for the high speed transient spike observed at the start of a DML’s impulse response.
The output from the near-field disturbance is highly correlated and provides all the critical timing (phase) information required to localize the source and setup a precise stereo image. The diffuse output that immediately follows contains the bulk of the radiated energy. Energy from both these mechanisms arrives within about 9ms. This falls well within the precedence window, ensuring that all this energy is perceived as a single event.
There are some beneficial consequences from this dual nature of a DML’s radiation:
A DML exhibits a predominantly resistive mechanical impedance with very little stored energy, providing exceptional speed and transient accuracy. Contrast this with a conventional cone speaker where, above resonance, the device is mass controlled, which implies energy is stored in the inertia of the moving components.
The diffuse radiation interacts so benignly with reflective surfaces, like walls, floor, and ceiling, that reflections do not disturb the perceived sound, guaranteeing a more consistent temporal and timbral accuracy.
Unlike conventional speakers, the DML’s stereo image and timbre is less perturbed by the perceptual distraction of correlated reflections. Because of this, the DML can deliver superior imaging and clear tonality in many real-world environments.
What the DML can do for you
After decades of research, the DML proves its high effectiveness in resolving the acoustic challenges that accompany modern architectural designs. It embodies a powerful solution for spaces requiring optimal audio performance.
The DML benefits:
- High vocal intelligibility
- Wide-directivity with predominantly diffuse radiation
- Advantageous room interaction
- Improved feedback resistance
To sum it up, the DML is the only loudspeaker that encourages resonance and provides clear intelligibility in reverberant spaces.