"..All these articles were freely available on Google but Sound Right would like to take the opportunity to thank the authors for their research and for advancing our knowledge and understanding on aspects of sound reproduction and an insight into the phenomenon that is DML technology..."

Read David's SOS article here

DML Arrays

NXT Chaos

Active LivingRoom Simulator part 1

Active LivingRoom Simulator part 2

The Beneficial Coupling of Cardioids

The loudspeaker, as we know it, has been around for some 70 years. In that time, it has undergone a myriad of refinements. Today we have pretty much the best that can be achieved. That is not to say that improvements are still not being made, particularly as new computer-based measurement and mathematical modelling tools also develop.

These techniques are allowing new insights into loudspeaker behaviour, both in terms of the acoustic radiation itself and the mechanical/air load interactions at the radiating surface using finite element analysis. This technique also allows other underlying aspects of loudspeaker design to be investigated, such as the complex magnetic field right inside the pole pieces and motor assembly, parameters that cannot be measured, but can now be visualized with powerful computer analysis.


Advanced FFT techniques, correlation measures, time windowing, acoustic intensity, interference spectrometry and laser scanning, to name but a few of the newer measurement techniques are enabling corresponding and unprecedented advances to be made in the complexity and innovation of the measured data. Despite this, there have been few new major breakthroughs or radical changes to the way we that think about or use loudspeakers for a considerable number of years.

This is about to change.

NXT (New Transducers) was established just three years ago to exploit a novel discovery made in noise control materials technology by a UK government research agency, and the distributed-mode loudspeaker was born. As we shall see this new class of device has acoustic properties radically different from conventional loudspeakers and are such as to set many preconceptions and approaches to loudspeaker system design on their head.


NXT scientists are involved in a huge research program not only into the basic mechanics and acoustics of distributed-mode technology, but also into the corresponding psychoacoustic and measurement aspects. In the past two years more than 20 papers have been presented at Audio Engineering Society Conventions and US/UK acoustic conferences on these topics, indicating the potential significance of this new technology.


NXT itself does not make loudspeakers. It operates purely as a research, development and licensing company. Distributed-mode loudspeaker products or loudspeaker modules will instead be made by the growing worldwide network of 100 plus companies who have already taken out manufacturing and user licenses.

The concept.

The fundamental difference between distributed-mode loudspeakers and conventional cones or pistonic devices relates to the way in which sound is generated and radiated. The distributed-mode loudspeaker essentially consists of a thin, stiff panel that is set into vibration by means of a special electro-acoustic exciter. The exciter is normally a moving coil device, but piezo-electric and other forms of excitation can be used equally. The panel is excited by an exciter (or multiple exciters), carefully positioned and designed to excite the natural resonant modal structure of the panel optimally. Although it would seem counterintuitive and in direct opposition to the cumulative wisdom of 70 years of transducer design to excite a resonant structure, when appropriately effected in a controlled manner, it can lead to some interesting effects.


The point is that unlike air, the panel material is dispersive and a dense modal structure soon builds up. Studying the panel vibration by means of a scanning laser and time windowing the measurement shows the modal vibration to become rapidly complex.


The panel can be considered as a pseudo-randomly vibrating object, for at any given instant different areas are excited with different amplitudes and phases. The panel can be thought of as a whole series of individual radiators, each radiating sound effectively independently of its neighbour but summing in the far field to give the desired response.


The greater the modal density, the greater the number of these radiators and the greater the random nature of the vibration and the greater lack of correlation between them. It is this lack of correlation of the radiation over the surface that is one of the primary keys to the successful operation of a distributed-mode loudspeaker. In a conventional cone or pistonic device, the resonant modes would lead to coloration and other detrimental effects; the complete reverse is the case with the distributed-mode loudspeaker.


This is because unlike a pistonic cone loudspeaker where the objective is to move or accelerate the complete radiating surface as a whole, thus leading to coherent phase-related radiation across the entire surface, the distributed-mode loudspeaker is the complete reverse of this. Different parts of a distributed-mode loudspeaker panel radiate at different times and are not directly correlated with each other, thus creating a diffuse radiation characteristic.


Because the resultant wave front is not phase coherent, it will not produce the strong coloration effects associated with resonances in conventional loudspeakers, or the local boundary specular reflection effects. Furthermore, the surface motional vibration of the distributed-mode loudspeaker is orders of magnitude lower (micron scale) than a pistonic cone driver because it does not act like a piston in order to move the air and thus, radiate sound.


This matching to the air load impedance is also different. In an ideal DML, the radiation resistance is insignificant and is constant with frequency. As a result, the diaphragm dimensions no longer control directivity. This means that you can make the radiating area as large as you like without the high-frequency output becoming confined to a narrow solid angle about the forward axis, as is the case with conventional drivers.


By acting as a piston the diaphragm of a conventional driver moves as a rigid whole or at least that is the designer's aim. In acoustic terms such a loudspeaker is mass-controlled over most of its pass band. For a given input voltage, the motor generates a force that is constant with frequency, and the diaphragm resists with a mass (its own moving mass plus that of the air load). By Newton's second law of motion (F = ma), the acceleration of the diaphragm is constant with frequency. As a consequence its displacement decreases as the signal frequency rises at a rate of 12 dB/octave.

At low frequencies, where the wavelength in air is large compared to the dimensions of the diaphragm, this is as desired. The real part of the diaphragm's radiation resistance, into which the driver dissipates acoustic power increases with frequency at exactly the same rate as the diaphragm's displacement decreases with the result that the acoustic power output is constant.


As the frequency rises and the wavelength in air decreases to the point where it becomes comparable in dimension to the size of the diaphragm, a significant change occurs. Instead of continuing to rise, the real part of the radiation resistance reaches a limiting value and essentially becomes constant for all higher frequencies. Consequently, the diaphragm's acoustic power output begins to fall at 12 dB/octave. This does not necessarily mean that the on-axis pressure response drops; in practice, the diaphragm's acoustic output becomes restricted to progressively narrower solid angles. The radiation becomes directional, and the loudspeaker begins to beam.

Variation of directivity with frequency is one of the major problems of loudspeaker and sound system design.

Whereas the on axis response of a given loudspeaker may well be flat, the frequency dependent directivity and subsequent off-axis response will not be and so the direct early reflected and reverberant sound fields in a room or space will all have different tonal balances. In hi-fi and multimedia systems this can affect not only the overall sound quality and perceived coloration but also the stereo imaging. In larger spaces employing sound reinforcement or public address systems this inherent characteristic can also affect perceived speech clarity and intelligibility.


Consequently, most loudspeaker systems use multiple drive units (two - and three-way systems) of progressively decreasing diaphragm size in order to maintain a wide coverage angle.

This still generally means that the loudspeaker's directivity varies significantly with frequency. Although the acoustic behaviour of an NXT panel appears random, the design process is completely deterministic. The key parameters that affect a panel's performance are the size and shape of the panel (which may be curved), the position of the exciters, the bending stiffness, the surface density and the internal damping. Provided that these are known, it is then possible to predict the acoustic performance of a panel with a high degree of accuracy.


Distributed-mode loudspeaker panels using moving coil exciters offer remarkably benign amp loads being essentially resistive at low and mid frequencies. As the frequency rises, the inductance of the voice coil becomes significant and the impedance increases as the load becomes reactive.


Nowhere is there a low impedance and large phase angle in combination.

Unlike conventional diaphragms, where the moving mass determines the upper limit of the frequency response, with distributed mode loudspeaker panels there is no equivalent restriction. As a result, the technology is truly scalable. You can make a large panel without impairing dispersion or high-frequency response. In fact, the performance actually improves as the panel size increases because the frequency of the fundamental bending wave is lowered, which not only extends the bass response, but also increases the modal density at mid and high frequencies. That the loudspeaker is operating entirely in resonance seems entirely at odds with conventional loudspeaker design practices in which it is well known that resonances are to be avoided at all costs.


Provided that they are correctly designed, distributed-mode loudspeakers do not sound colored as might be expected. This is due to their complex radiation and its decorrelated nature.

Distributed-mode panels also exhibit an unusual impulse response that display a fast rise time or initial transient but incorporates a decaying resonant tail. This impulse response can lead to a flat frequency response and a flat acoustic power output. An interesting outcome of the unique sound-radiating properties of the distributed-mode loudspeaker is that it can be used un-baffled as a free-standing loudspeaker.


The acoustic power output radiated from the back sums non-destructively with the sound from the front instead of cancelling. This is attributable to the complexity of the distributed modal radiation and uncorrelated phase of the individual radiating elements as seen from the far field.

The uncorrelated nature of the radiation has some other interesting effects. First, the panels interact far less strongly with local boundaries and thereby exhibit significantly less comb filtering and coloration as compared to conventional loudspeakers.

This reduction in comb filtering and reduced interaction with reflecting surfaces has a number of implications. For example, it means that distributed-mode loudspeaker can be used in close proximity to room boundaries or reflecting surfaces with reduced effect on stereo imaging and timbre.


Furthermore, recent research has shown that the off axis and, in particular, the rear radiation from a distributed-mode loudspeaker are highly decorrelated with respect to the forward radiation. This suggests that the majority of early reflections within a typical listening room will also be decorrelated and therefore effectively act as diffused reflections – a highly desirable goal but without the need for acoustically diffusing wall treatments.

This property, coupled with the wide dispersion, results in a very uniform sound field within a listening room.

Examining the in-room impulse response it shows the distributed-mode panel to generate a significantly greater number of early reflections than even a wide-dispersion conventional two-way comparable unit. The listener becomes far more immersed in the sound field which should potentially offer a greater sense of spaciousness and is often reported as an increase in perceived loudness.

Although exhibiting wide dispersion, distributed-mode loudspeakers still appear to offer precise stable imaging.

In typical domestic conditions, the stereo sweet spot often extends further than that experienced with conventional designs primarily because the wide dispersion and the reduction of destructive boundary/room interaction effects.


These characteristics, together with the slower rate of in-room sound level, fall off with distance and greater diffuse reflection density suggest that DML panels should be particularly well suited to multi-channel home theatre systems. In addition to the wide dispersion and low visual impact, the diffuse nature of their sound radiation ensures the required surround channel diffusion, so that listeners are not conscious of the surround loudspeakers as distinct entities. Furthermore, in video projection systems, the screen itself can be a large-panel loudspeaker, thereby ensuring perfect synchronization of sound with picture.


DML loudspeakers, although inherently bipolar, can be enclosed. The enclosure can be shallow (approximately 2 inches or 51 mm), and so a low profile loudspeaker can still result. Enclosing a panel changes the bipolar directivity pattern to wide dispersion forward radiation, but the diffuse nature of the radiation is maintained. Also, by enclosing the panel in a known way, the response can be optimized, and the unit can then be wall or boundary mounted with only minimal acoustic effect.

Reproduced by kind permission