Benwin Speaker Research & Development

A revolution in the shape of sound.
Much of the past 40-plus years of loudspeaker development have revolved around identifying, understanding and then suppressing diaphragm resonances and their resulting coloration and 'smear'. So a new drive unit technology that, rather than attempting to eliminate diaphragm resonance, encourages and exploits it genuinely, merits the over-used description 'revolutionary'. In a sentence, that's precisely what NXT does. It tears up the loudspeaker rulebook, as we know it.

Audiophiles and audio professionals are understandably suspicious of an idea which so comprehensively inverts the status quo. But suspend disbelief for the course of this article and I will explain why such a crazy-sounding idea actually brings major benefits both to loudspeaker users and loudspeaker designers - benefits which have already encouraged major industry players to take out licenses to use the technology.

The first question to answer is: why should we need a new loudspeaker paradigm when so much academic and design effort has been expended on perfecting current technology? To answer that you need go back to the basic principles of how conventional loudspeakers operate and identify the fundamental restrictions on performance that they impose.

Conventional loudspeakers, whatever method of transduction they use (electromagnetic, electrostatic, piezoelectric etc), aim at achieving pistonic motion of the diaphragm, at least over the lower portion of their operating range. By pistonic we mean that the diaphragm moves as a rigid whole. In acoustic terms, such loudspeakers are 'mass controlled' over most of their passband. The motor provides a force which is constant with frequency, the diaphragm resists with a mass (its own moving mass plus that of the air load), and so by Newton's second law of motion the diaphragm undergoes constant acceleration with frequency. As a corollary, its displacement reduces at 12dB per octave with increasing frequency (i.e. quarters with every doubling of frequency).

At low frequencies, where the wavelength in air is large compared with the diaphragm dimensions, this is just what we want. The real part of the diaphragm's radiation resistance (Figure 1), 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 acoustic power output is constant.

Figure 1. Real part of radiation resistance vs. normalized frequency.

As frequency continues to rise, though, and the wavelength in air reduces to the point where it becomes comparable with the diaphragm dimensions, a major change occurs. Instead of continuing to rise, the real part of the radiation resistance reaches a limiting value and essentially becomes a constant for all higher frequencies. Because of this the diaphragm's acoustic power output now begins to fall at a rate of 12dB per octave. This doesn't mean that the on-axis pressure response falls away: what generally happens is that the diaphragm's acoustic output becomes restricted to progressively narrower solid angles. In other words, it becomes directional; it begins to beam.

Variation in directivity with frequency is one of the great bugbears of loudspeaker design. If we listened to reproduced sound in anechoic environments it wouldn't matter: we would hear the diaphragm's on-axis output and nothing else. But conventional listening rooms are far from anechoic, so a loudspeaker'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 coloured and introduce imaging aberrations because of this alone.

An obvious solution would be to use a diaphragm sufficiently small that the 'knee' in the radiation resistance curve was forced above the audible frequency range. But such a diaphragm would have to undergo enormous, impractical excursions to produce the volume displacements necessary at low frequencies. 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 behaviour of inductor cores.

A full-range drive unit, covering the entire audible frequency range with constant directivity, would banish these problems but, for the reasons outlined, simply isn't achievable using conventional wisdom.

We appear to have reached an impasse, but some wonderful things happen if you abandon the concept of pistonic motion and consider instead a diaphragm vibrating randomly across its surface rather than coherently. Each small area of the panel vibrates, in effect, independently of its neighbours, rather than in the fixed, co-ordinated 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. This means that diaphragm dimensions no longer control directivity: you can make the radiating area as large as you want without high frequency output becoming confined to a narrow solid angle about the forward axis. Such diaphragm behaviour clearly opens up the possibility of a full-range driver freed from the familiar restraints and compromises. Nice trick if you can do it: but how can you make a diaphragm vibrate randomly?

Snapshot of panel motion, after excitation with an impulse

Figure 2. Panel motion, after excitation with an impulse.

Actually you can't, but you can get very close to it by using what we term distributed-mode (DM) operation, on which NXT is based. 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 point(s) of excitation on a snapshot of the panel motion (Figure 2) - thereby providing the freedom from directivity related problems described above.

See the Benwin BW2000 Flat Panel Speaker by clicking here.