The Rountree OmniMon Loudspeaker

New loudspeakers debut at every hi-fi show, but nearly all of them are no more than fresh variations on well-worn and familiar themes. Encountering a new loudspeaker with even one genuinely original and radical idea is very rare indeed, so it came as quite a shock to encounter something called the OmniMon 1 which was simply stuffed with all sorts of fresh and unusual ideas, lurking in one of the smaller dem rooms at last Autumn’s Heathrow show. So what’s an OmniMon, and what’s so special about it? It’s the creation of Rountree Acoustics, which comprises the very experienced speaker designer Paul Burton, plus his partner Fiona Rountree, who handles the administration. Burton has a remarkable track record for coming up with very unusual speakers. His CV includes the Sumo Aria dynamic panel speaker and the on-the-wall Sequence. He also had an important hand in the Cyrus Icon NXT hybrid, and the large area Podium. The £6,600/pair OmniMon 1 is entirely different from any of these, and arguably even less conventional too; so much so that it’s very difficult to know just where to begin the description and explanation.

Let’s start by looking at the biggest bit – the enclosure. For a decade, from 1996 until fairly recently, flat panel speaker developer and licenser NXT was a hot bed of British loudspeaker design talent – a veritable think-tank employing a number of the best known and most respected people in the field. Burton spent some years there, and got chatting to Graham Bank in the canteen one day. At Celestion in the 1980s, Bank came up with the classic SL600 compact standmount, famous for using a super-light enclosure made from Aerolam with the idea of avoiding the energy storage that is endemic with conventional high mass enclosures. Bank commented that it was a shame no-one had subsequently taken the concept any further. Burton started getting ideas, and twenty years after the SL600, he’s done just that.

The OmniMon 1 might be a floorstander, but its 44 litre enclosure actually weighs less than 2kg. That’s because it’s made from a rigid but super-lightweight structural foam, covered by a paper-like laminate. This enclosure reaches down quite close to the floor, and performs at least two important acoustic roles, but plays no part in the structural integrity of the speaker. Indeed it’s quite deliberately engineered to be excited by the 180mm ScanSpeak Revelator bass/ mid driver, but because it’s an ultra-light structure, it doesn’t store this energy for any appreciable time. It also includes no fewer than 16 carefully placed internal braces, partly to add strategic stiffening, but also to control and spread the inevitable break-up modes across a wide range of frequencies, the net result being that the whole enclosure acts a little like an NXT panel, adding an uncorrelated contribution to the total sound output.

Besides this sonic augmentation function, the well damped interior of the enclosure acts as a (rather short) transmission line, terminating an inch or so above the plinth, loading the front(!) side of the bass/mid driver (while also preventing front-to-back phase cancellation). It can do this in spite of its miniscule mass because it plays no part in supporting the drive units. Instead it’s actually ‘hung’ from the frame of the main driver, which itself is mounted on the end of a thick steel rod, rising up from a similarly massive steel plinth. This combination, totalling something like 40kg, is literally the backbone and foundation of the design: the threaded top of the rod passes right through the central polepiece and back plate of the up/downward facing main driver, which is secured by a large nut, hidden beneath a polished decorative boss. This of course means that the bass/mid diaphragm moves vertically rather than horizontally, so the direct output heard by listeners is actually radiated omni-directionally from the back of the cone. It also means that mechanical forces generated by the main driver operate vertically, so they don’t risk modulating the tweeter output, and are ‘grounded’ in the high mass plinth, with no need for spikes: Definitely very unconventional.

So where does the tweeter go? This is not an omni-directional device – indeed rather the reverse, as it’s a high quality ribbon with a 63x9mm diaphragm, based on the Aurum Cantus unit and modified by Rountree. This forward-firing device is mounted on a perforated metal collar that fits snugly against a rubber decoupling gasket fixed around the main driver’s exposed magnet, giving a measure of timealignment between the two drivers. That classy Scandinavian main driver has a slit’n’fill paper cone diaphragm, the better to control resonances, and natural directivity means that any high frequency break-up modes are automatically directed upwards, rather than towards listeners. Faraday rings on the voice-coil former and top of the mounting post reduce distortion and inhibit over-excursion. The crossover network, operating with fairly gentle slopes at 2.2kHz (acoustically), is housed in an external metal box, which is sand-filled to prevent or damp any vibration effects. It’s equipped with two pairs high quality five-way terminals, permitting bi-wiring or bi-amping, and links to the speaker enclosure by a single Neutrik Speakon terminated umbilical. Although not in our preproduction samples, regular production will have high quality LFD internal wiring.