Pace, Rhythm, & Dynamics Page 4

The designers who cared about rhythm felt---and may still feel---that they possessed some basis for a common understanding of the lifelike reproduction of recorded music. That understanding was not intentionally distanced from the main high-end thrust. However, whenever dialogue was initiated between the two camps, it transpired that they were often not on the same wavelength. Pieces of equipment from the two design schools even sounded uncomfortable when used with each other. At times insults were traded between the various proponents, while both took comfort from their allegiances to different kinds of music. The tonal purists felt more comfortable and less threatened by classical material, while the rhythm practitioners became more closely identified with modern rock and jazz, where the best points of their systems would be more readily revealed. In the UK, the split has persisted for a decade and a half.

It doesn't have to be like this.

There is no good reason why neutral, transparent systems cannot also reproduce dynamic and rhythmic elements well, particularly if designed to do so from the start. However, such design specifications do not yet exist in a quantifiable sense. At present, the specifications must, of necessity, be subjective, based on careful listening and the use of comparative references. Ultimately, the reference is live music.

The technology of rhythm is not in the textbook
If an audio designer is truly aware of these properties typically found in good live music, he cannot help but try, to the best of his ability, to preserve them in his product designs. Such designs are painstakingly interactive processes which must be continuously supported by critical listening. This auditioning must cover preliminary concepts, prototype studies, passive components, and finally working models.

Such knowledge is hard-won by designers over many years of development, their findings often contradicting the conventional wisdoms found in electronic engineering textbooks. The latter, generally bound up with the classical engineering concepts of value and efficiency, do not recognize sound quality per se. In fact, the over-engineering often seen in top-quality audio gear is generally scorned by the electronic engineering fraternity who just cannot see that audiophiles value sound quality so highly that they are willing to pay an extravagant engineering price to achieve it. If, for example, at present the only way of significantly advancing the sound quality of a top-grade power amplifier is to double the size of its power transformers, so be it. If, later in our understanding of the relationships between the engineering, technical performance, and sound quality we find a better, more cost-effective way, that's fine too.

A loudspeaker example
A given moving-coil speaker is heard to suffer from a lack of dynamics---not in the global compressive sense, but in the expressive, inner-dynamics sense. Likely causes are poor transient definition resulting from some combination of unwanted energy storage in the driver and enclosure system, and/or major flaws in the energy arriving at the listener; for example, poor frequency and phase responses. Tightening up the driver integration and frequency response can work wonders here. Attention paid to decay problems will also be helpful. In addition, problems in dynamics, rhythm, and timing are often tied to hysteresis effects in the mechanical suspensions and/or the cabinet acoustic loading.

Hysteresis in a speaker drive-unit is a phenomenon wherein recovery from a transient is artificially delayed due to some viscoelastic or memory effect, the suspensions failing to act as pure, linear springs. That delayed recovery distorts subsequent waveforms and seems to weaken the impression of good timing. Specifically, the bass sounds sluggish, even "half a beat" behind, despite the extra damping afforded by "lossy" low-Q suspension systems.

A neat example of bass rhythm differences is provided by the lovely little BBC-designed LS3/5a. Two or so years ago, this near-field monitor was revised to accommodate a later, more consistent bass unit, while the system's basic, closely toleranced sound was largely preserved. While the new system was generally a little cleaner in the midrange than typical examples of the earlier production, (Footnote 4) rock enthusiasts determined that one other difference had emerged: the older version was found to "time" better, to have more "rhythm."

Searching analysis revealed that the design feature most accountable was the choice of the type of front surround for the bass driver. This had been high-Q, springy Neoprene in the old type, but low-Q, absorptive vinyl composite for the new. The latter had been specifically chosen for its lossiness to improve the cone's midrange termination, successfully linearizing the response, but at the significant expense of the speed and fluidity of bass lines. The latter quality appears to rely on the principle of a linear spring, with simple pistonic motion uncomplicated by delay or hysteresis effects.

Footnote 4: See Stereophile, February & March 1989, Vol.12 Nos.2 & 3.---JA