My work reviewing nearfield monitors for Sound On Sound occasionally provokes thoughts that are perhaps a little too far off-piste for the relatively simple needs of a magazine review, and my recent review of the newly introduced Neumann KH150 was prime example. I gave the KH150 a very positive review in the magazine, but having had a few more week’s experience of the monitor (Neumann have kindly let me keep hold of the review samples for a while) and on re-reading the review, I genuinely wonder if I could have been yet more positive: the KH150 really is THAT good. It’s so good that my expectations of the performance achievable from a modestly dimensioned and not unfeasibly expensive active monitor have been somewhat re-calibrated.
But what is it that makes the KH150 work so well? What’s the secret ingredient? This to me is a fascinating question because at first, and even second, glance the KH150 looks a relatively conventional loudspeaker that would be conceptually comprehensible by the very first speaker engineers: Edward Kellog and Chester Rice, who patented what we know know as a moving-coil driver in 1929. By comparison, for example, if you were to show Alexander Graham Bell (who, Wikipedia tells me, “is credited with patenting the first practical telephone”) a contemporary smartphone, he’d not really have the foggiest idea how it works (welcome to the club Alexander). The difference here is that while telephone development has progressed through conceptual leaps: from wired analogue via wireless analogue and wireless digital to wireless IP smartphone, loudspeakers development over its hundred or so years of existence has generally been a process of incremental development.
Having said that however, one element of the KH150 (and, it’s fair to say, of other notable contemporary monitors from manufacturers such as Genelec, Kii Audio, PMC, Dutch & Dutch, Dynaudio, ADAM Audio, etc) that would have Edward and Chester scratching their heads is its use of digital signal processing to manage and optimise the inherent electro-acoustics of its drivers and enclosure. DSP equalisation is partly responsible for the KH150s extraordinarily flat axial frequency response (I measured the frequency response as part of my Sound On Sound review and I’ve included the resulting response curve below). But DSP also works more subtly to help the KH150 achieve harmonic distortion numbers that are closer to those of audio electronics than some more traditional loudspeakers. And I strongly suspect that the exceptionally low distortion of the KH150, just as much as its exceptionally flat frequency response, contributes very significantly to its subjective performance.
The KH150 however isn’t just about DSP. It’s a monitor of the DSP age, but it’s also defined by some remarkable electro-acoustic design, in particular within its bass/mid driver. The second fifty-years of driver design have typically been dominated by considerations and developments of diaphragm materials. Conceptually, focussing on diaphragm performance makes intuitive sense: the loudspeaker diaphragm constitutes the final link in the audio reproduction chain and it is well known to be generally compromised. We don’t even, for example, have a diaphragm material that’s rigid and light enough to cover the full audio band without resonance. Consequently, since thermo-plastic diaphragms arrived (driven primarily by BBC research) in the mid 1970s and began to succeed paper (the original diaphragm material), a whole range of options; aluminium, woven fibres (carbon, kevlar, etc.), beryllium, graphene, ceramics, wood, and countless composites and sandwiches, have been tried – some of them with success, but most often, it has to be said, without the fanfare of a major breakthrough.
The KH150 bass/mid driver however employs a broadly traditional paper diaphragm (Edward and Chester’s diaphragm was paper too) and that reflects, I think, that in truth the traditionally accepted wisdom of diaphragm material being the primary arbiter of driver performance doesn’t really stand up to much scrutiny. What does increasingly stand up to scrutiny however, to my ears anyway, is that drivers designed with an emphasis on dynamic linearity (by which I mean the ability accurately to follow the input signal) and minimal distortion, offer significant subjective benefits over drivers that prioritise amplitude response linearity and bandwidth. The implication of this for driver design is that the focus moves away from diaphragm material to concentrate on the motor system (magnetic circuit components and voice-coil), roll-surround and suspension, for those locations are where the dynamic and distortion performance of a driver is predominantly defined. And that’s also where the electro-acoustic design effort that resulted in the KH150 bass/mid driver was spent (Bruno Putzey’s remarkable Purifi drivers tread a not dissimilar path). The driver combines extraordinarily low distortion combined with an ability to remain dynamically linear over a huge (in driver terms) ±12mm diaphragm displacement range. And I think it really shows in the way the KH150 performs.
So the secret ingredient of the KH150 isn’t really a secret, and it isn’t any single element. It’s a combination of maximising the bass/mid driver dynamic and distortion performance, and then employing DSP to optimise the system amplitude response. Audio DSP has been around for a while and, obviously, electro-acoustics has been around for much longer, but only now I think are a few loudspeaker manufacturers, Neumann obviously among them, really beginning to understand how best to combine the two disciplines in the interests of subjective audio performance.