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Innovation In Studio Loudspeaker Technology

Key Milestones In History
By Phil Ward

Moving Air

Monitors are better than they've ever been — but they're still the weakest link in most studios. We trace the technology from its roots in the 1930s, and find out whether there's a DSP revolution on the way.

Far be it from me to deploy a cliché in the first sentence, but music recording technology has come a long way in the last decade or two. Who'd have thought, 20 years ago, that in 2019 we'd be able to create complex music on a small handheld touchscreen device, share it instantly with somebody on the other side of the world and then call them — with video — from the same device to enthuse over just how cool we both are? It is truly extraordinary, and if I think back to when I first started making and recording music with friends back in the early '80s, what we have now would have fallen then into Arthur C Clarke's 'magic' category (see Note 1).

The Glacial Pace Of Change

Can you tell where this is going? Despite the 'magic' we now take for granted, there's one piece of recording hardware that, in comparison to all the DAW, network, and plug-in malarkey, often looks as if it has spent the last 50 years standing still, never mind the last few years. I'm talking about monitor speakers, of course. In the context of extraordinary change elsewhere, doesn't it strike you as odd that the monitor that's still the one most often seen in commercial studios, the Yamaha NS10, was designed in 1978 — well before I first slipped an extra high-output chrome cassette tape into my Fostex 250 multitracker? So, having been asked initially to write about "developments in studio monitor technology", I'm going to begin by explaining why that phrase, these digital days, sometimes looks as if it might be a contradiction in terms.

The iconic, white‑paper-cone driver of the Yamaha NS10.The iconic, white‑paper-cone driver of the Yamaha NS10.If you consider a music production 'workflow', the elements between microphone (see Note 2) and monitor really do nothing more than process, store and transfer information. This was true even back in the analogue days. Developments in electronic data processing and storage since perhaps the 1980s, but at an extraordinary rate in the more recent past, have simply increased our ability to store and process musical information. The hardware at either end of the workflow, however — monitors and microphones — are different. They don't store information, and if they process information at all, they do so only done as an adjunct to their primary role of transduction. Microphones take information in the acoustic domain and translate it, via the electro-mechanical domain, into the electronic; monitor speakers do the opposite. And thanks to making a living simultaneously in three domains, monitors and microphones are subject to three sub-sets of the laws of physics...

Now, there's still likely to be a long way to go in electronic data processing and storage before the fundamental physical laws of that domain call a halt. We've become blasé about technological leaps of an order of magnitude every few years and the pace shows no sign of slowing. In the electro-mechanical and acoustic domains where monitors live, however, we've been pretty much toe to toe with the laws of physics since Rice and Kellogg patented the moving-coil speaker in 1925.

Solid Air

It's the density of air relative to the density of viable diaphragm materials that is the fundamental limiting factor on a monitor's moving-coil driver technology. When you move a driver diaphragm backwards and forwards in the hope of creating an acoustic wave, there's an enormous impedance mismatch between the diaphragm and the air, so very little of the diaphragm energy makes it into the acoustic domain. This is why a typical moving-coil driver is typically just a couple of percent efficient — it's for the same reason that doing breast stroke in air doesn't move you forward quite so well as it does in water!

Imagine a diaphragm made from a material with the same density as air — air itself, for instance (see Note 3). It would couple to the air around it almost perfectly, and energy transfer would be close to 100 percent. We can't use diaphragms made of air, of course, because along with being light, a diaphragm has to retain its shape when vigorously accelerated, so it has to be rigid too. And the lightest viably rigid diaphragm material? Well it's probably still the one Rice and Kellogg used: paper. (Or, as I've seen it described in a speaker brochure, "natural cellulose-based fibre composite"!) So, while in the domain of musical data processing and storage, technology races ahead at a giddy rate, in the electro-mechanical-acoustic domain, unless somebody makes an order-of-magnitude breakthrough in materials science (in which case they'd be well advised to apply their discovery in a few other fields before bothering with studio monitors!), moving-coil monitors are, I suspect, destined to remain pretty much 'as is' for a while yet.

Getting A Wriggle On

However, even the laws of physics allow for a bit of wriggle room, and there's no shortage of engineers striving to find innovative ways to exploit it. The wriggle room arises, ironically, thanks to monitors straddling those three domains, because this multiplies the number of degrees of freedom within which monitor designers can wriggle. Almost every month in SOS, a nearfield monitor review provides evidence of monitor designers wriggling and coming up with something slightly different in terms of the way the basic electro-acoustics are addressed.

The Kii Audio Three is directional, right down to the low end.The Kii Audio Three is directional, right down to the low end.For example, at around the same time I began writing what would become this article, I was listening to Kii Audio's remarkable Three monitors (reviewed in SOS January 2017: www.soundonsound.com/reviews/kii-audio-three). In terms of their drivers and even, despite appearances, their enclosure, the Kii monitors break little truly new ground. They are unusual, however, in the way contemporary DSP techniques have been employed to control dispersion, and in the way their mid-range amplifiers have been configured to enhance the control they have over the driver diaphragms through feedback of voice-coil current. Having said that, while the concept of DSP-based dispersion control is a recent one, that of diaphragm feedback and correction is not: Philips launched a range of active hi‑fi speakers in the 1980s featuring 'motional feedback'. The Kii Three also incorporates compensation to correct the group delays inherent in the crossover filtering, and similar delay compensation is claimed by PSI to endow their monitors with more linear time-domain characteristics than is typical of moving-coil speakers. However, as with any speaker incorporating multiple non-coincident drivers, time-domain characteristics are in practice significantly influenced by the drivers themselves and their differing path lengths to the listener's ears. So I wonder if the unusually neat looking square wave Illustrated on PSI's promotional material as evidence of linear time-domain characteristics might look rather less neat if the measuring microphone were moved a little...

The PSI monitors represent something of a speaker-technology curate's egg, in that they're advanced in some respects, yet traditional in others. Thanks to the multi-disciplinary nature of speaker design, such 'curate's eggery' isn't unusual. Speaker manufacturers, in the grand scheme of things, tend to be relatively small organisations, with limited development resources. So innovating simultaneously across more than one discipline is tough (especially tough when a Sales Director with nothing new to sell at the next trade show strides purposefully into the R&D office. I speak from experience!).

Circling The Square

Spherical or ovoid speakers are the best shape for minimising edge diffraction, and there have been a number of commercial products exploiting this over the years: pictured here are the Cabasse La Sphère, the Munro Sonic Egg, and the Minipod. But speakers this shape must be bigger than rectangular boxes if they're to provide the same bandwidth.Spherical or ovoid speakers are the best shape for minimising edge diffraction, and there have been a number of commercial products exploiting this over the years: pictured here are the Cabasse La Sphère, the Munro Sonic Egg, and the Minipod. But speakers this shape must be bigger than rectangular boxes if they're to provide the same bandwidth.As if conveniently to illustrate my point about the multi-disciplinary nature of speaker design, a product launched a few years ago by sE Electronics but later coming under the Munro Sonic brand, offered something in complete contrast to the PSI or Kii approach. Andy Munro, the designer, chose to wriggle in a different domain. It can't have been too difficult to come up with a name for the Egg — it's form is unmistakably ovoid. The shape arises thanks to one of the acoustic fundamentals of a diaphragm radiating from an enclosure: the shape of the enclosure affects the acoustic radiation. American audio engineering pioneer Harry F Olsen published a paper in 1969 describing the influence of enclosure shape on acoustic radiation. He showed that rectilinear enclosures have the greatest influence and spherical enclosures have the least. The effect arises due to the diaphragm radiation, at frequencies where the wavelength is less than the dimensions of the enclosure, diffracting and re-radiating from the enclosure edges. Thanks to the physical distance between the diaphragm and enclosure edges, the re-radiation happens a short time after the direct radiation so the two interfere and frequency-response anomalies result. A spherical, or egg‑shaped, enclosure has no edges so is all but immune to the phenomenon.

So why, you might reasonably ask, are all monitor enclosures not egg-shaped (see Note 4)? One answer illustrates a fundamental law of electro-acoustics; a law that comes into play when an enclosure is used to contain the rear radiation of a driver and stop it destructively interfering with the forward radiation: for a given driver there's a fixed relationship between low-frequency bandwidth, enclosure internal volume, and baseline efficiency (electrical power in compared with acoustic power out). Reduce the enclosure volume and either the efficiency must fall or the low-frequency cutoff must rise. For the same height, depth and width, an egg-shaped enclosure can't help but enclose much less air than a rectilinear enclosure, so if, as a speaker designer, you take the egg-shaped road, you have to accept either that your speaker will require significantly more powerful amplification and drivers that are better engineered to resist thermo-mechanical stress and compression — which...

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Published June 2019