Warmth & Clarity

A speaker's output off the listening axis has a significant effect on what we hear. Because of frequency dependent directivity the direct, reflected and reverberant sounds in a room all have different tonal balances. Even if a conventional loudspeaker had an absolutely flat on-axis response and was entirely free of resonance - a tall order - it would still sound colored and introduce imaging aberrations because of this alone.

So loudspeaker designers are generally forced to compromise and deploy multiple drive units of progressively decreasing diaphragm size. Large diaphragms provide the volume displacement necessary to reproduce low frequencies; small diaphragms take over at higher frequencies before the output of the larger units becomes too directional. Even so the speaker's directivity still varies significantly with frequency, and the use of crossovers to divide up the frequency range brings with it a host of unwelcome side effects: phase distortion, further disruption of off-axis output, more reactive elements in the loudspeaker load, and sound quality issues related to capacitor performance and the saturation behavior of inductor cores.

The NlightN Flat Panel Speaker is based Distributed-mode (DM) operation. Essentially this involves encouraging the diaphragm to produce the maximum number of bending resonances, evenly distributed in frequency. The resulting vibration is so complex that it approximates random motion - it is impossible, for example, to identify the points of excitation on a snapshot of the panel motion - thereby providing the freedom from directivity related problems described above. A panel, which is vibrating in a complex, pseudo-random way, can radiate sound evenly in all directions, without mutual cancellation.

A diaphragm vibrating randomly across its surface rather than coherently. Each small area of the panel vibrates, in effect, independently of its neighbors, rather than in the fixed, coordinated fashion of a pistonic diaphragm. Think of it as an array of very small drive units, each radiating a different, uncorrelated signal but summing to produce the desired output.

Such a randomly vibrating diaphragm behaves quite differently because power is delivered to the mechanical resistance of the panel, which is constant with frequency. The radiation resistance is now insignificant because the air close to the panel also moves in a random fashion, reducing the effective air load.

The sound wave from a vibrating surface is almost random in nature. It is incoherent, not in phase. It is not at all like the pebble in the pond. Technically, this is known as a diffuse and disperse source similar to light energy from a fluorescent light. Two bursts of random energy simply add together to create twice the power. There is no interference when speakers are close to each other or when sound is reflected from a room boundary. In fact, the more reflective the environment, the better the panels work, providing crystal clear sound at very even levels throughout the room.

Another major benefit is that the panel's acoustic output from both sides of the NXT panel is useful. In applications where the panel is not required to be baffled, as in high-end free-space loudspeakers, the power radiated from the back face sums up constructively with radiated power from the front face of the panel. Cabinet resonances are no longer an issue because there is no need for a cabinet.

Because one panel delivers all the audible frequencies (except for the lowest bass), there's no need for multiple drivers, crossovers and other parts necessary to conventional speaker designs. In particular, getting rid of the crossover eliminates electronic circuitry and its attendant acoustic vagaries right in the midrange frequencies, where human hearing is most sensitive.

Technical Articles: Warmth and Clarity

Distortion Mechanisms of Distributed Mode (DM) Panel Loudspeakers (Pre-Print #4757)

Martin Colloms (Consultant, London, UK), Joerg Panzer (Panzer & Partner, Munich, Germany), Vladimir Gontcharov and Valerie Taylor (New Transducers Ltd, Huntingdon, UK)

Abstract

Acoustic radiation from a Distributed Mode Loudspeaker (DML) results from low amplitude bending waves. Compared with the motor system of a pistonic driver, the DML exciter is of subtly different design and equivalent circuit with a different relationship to the radiating diaphragm. In this paper loudspeaker distortions are reviewed, the equivalent circuits modelled and compared with the DML case and the results for comparative measurements are presented.

Presented at the 104th Convention, Audio Engineering Society, 1998 May 16-19, Amsterdam, The Netherlands

The study report can be downloaded from the internet ($10) at the AES website. Search by Pre-Print number or author name. http://www.aes.org/publications/preprints/search.html

A New Approach to Speaker / Room Equalization (Pre-Print #5221)

K. Chiao and and C. Kyriakakis (Integrated Media Systems Center, University of Southern California); N. Harris (New Transducers Ltd.)

Abstract

In this paper we examine methods for digital room correction and loudspeaker equalization as they apply to Distributed Mode flat-panel loudspeakers. We present a method that combines LPC inverse filtering and tunable DWT octave-band equalization. Furthermore, we discuss frequency response and time-domain smearing/spread issues and considerations for real-time implementation.

Presented at the 109th Convention, Audio Engineering Society, 2000 September 22-25, Los Angeles

The study report can be downloaded from the internet ($10) at the AES website. Search by Pre-Print number or author name. http://www.aes.org/publications/preprints/search.html

The Influence of Loudspeaker Type on Timbre Perception (Pre-Print #5226)

Sheila Flanagan and Brian C J Moore (Department of Experimental Psychology, University of Cambridge)

Abstract

Perceived timbre depends strongly on spectral shape. We compared spectral shape discrimination for a Distributed Mode Loudspeaker (DML) and a cone loudspeaker. Subjects were asked to distinguish between a tone complex with a flat spectral envelope, and a tone complex with a ripple in its spectral envelope, for each type of loudspeaker. The ripple depth was varied to determine threshold. Performance was significantly better for the DML than for the cone loudspeaker.

Presented at the 109th Convention, Audio Engineering Society, 2000 September 22-25, Los Angeles

The study report can be downloaded from the internet ($10) at the AES website. Search by Pre-Print number or author name. http://www.aes.org/publications/preprints/search.html

The NXT Technical Review 01, January 2002

Available for internet download at: http://www.nxtsound.com/technology/techReview.php