Grant any audiophile one hi-fi-related wish (world peace is beyond the reach of this particular fantasy) and he, or she, after some thought, will most probably alight upon a singular desire... ‘a listening room of my own, free of compromise’.

Let’s be honest, our personal musical spaces are very often the ‘elephant in the room’ – they have unfavourable proportions with doors, stairways and windows positioned for living, not listening. Then there’s the occasional furniture, the TV, myriad ornaments, trendy pendant lighting, metal radiators and other manifestations of domesticity, all singing along to the best efforts of our loudspeakers. We have suspended ceilings and floors – huge diaphragms eager to grasp those low-frequency room modes – while wall coverings shred and shape the response of crucial first reflections.

But that’s real life. Compromises are inevitable, and that has never stopped us enjoying the thrill of our systems. We know there’s a limitation on loudness, on the firmness of bass or the brilliance of treble that might be achieved in our rooms. But what if describing the sound of a speaker or amplifier, etc, is not a hobby but your profession?

It is fair to say that the top TV and movie journalists do not wear tinted spectacles and neither are Formula One cars tested on dirt tracks... We needed a neutral listening facility, engineered to accommodate every scale of hi-fi component and allow it to perform at its very best. The ambition: our description of sound, free of random ‘room’ colouration, would allow our readers to ‘map’ those subjective qualities onto the knowledge of their own listening environment. The result: better, more reliable advice for ‘real world’ listening situations.

Barnstorming

A house and office move provided the opportunity – a substantial stone barn long since abandoned by livestock but still frequented by several species of bats. Planning, conservation and ecology added two years to the project but at least the flying fur balls now have a separate, purpose-built hotel complete with a pool table and coffee machine. But I digress...

We had a big space with solid foundations, no neighbours and, as luck would have it, a utility pole with an available three-phase AC supply about 20ft from the building. And perhaps the bats were fortuitous, for the delay allowed our project to coincide with the fruition of KEF’s ‘Experience’ centres in London, New Jersey, Tokyo, Hong Kong and Beijing [see https://uk.kef.com/pages/kef-experience-locations].

Above: Early design drafts including room treatments (bass traps, length mode mitigation, side and ceiling diffusion and absorption)

Without great fanfare, KEF had assembled a team of acousticians and programmers to develop a bespoke modelling solution able to predict the behaviour of complex spaces populated, in some instances, with dozens of architectural loudspeakers. When offered by KEF’s VP of Technology, Jack Oclee-Brown, the opportunity to leverage the power of that research in the design of our own room was perfectly timed. Within the existing stone walls we could build a room-within-a-room of idealised proportions.

KEF’s Prathmesh Thakkar, R&D engineer par excellence, set to work. Almost immediately he called with a request, ‘can you build another wall at the end of the stone barn?’. Sure... ‘We need to set the length to 8.5m or we’ll have large dips in the bass response that we’ll never be able to treat’. Another silver lining – the space behind this foot-thick blockwork partition turned out to be perfect for our plant room, the hub for incoming AC, the satellite service, server and NAS drives, and the 45 individual network connections running through our listening and bespoke lab facility.

Music by numbers

Calculating idealised dimensions for the room was fundamental, because this is still a ‘room’ – not an anechoic chamber – and so it will still have axial, tangential and oblique resonant modes.

Jack Oclee-Brown explains, ‘By first optimising dimensions you can basically control the room modes in a way that’s impossible if you take a fixed [random living room] space and try to introduce absorption to deal with it'. Prathmesh Thakkar concurs, ‘If you don’t have optimised room dimensions with tailored room treatments, then you can quite easily have big 20dB holes in the bass region’.

Above: KEF R&D engineer, Prathmesh, during the early stages of our room optimisation and RT60 validation. Note the dodecahed- ron speaker on its tripod!

Setting the room dimensions allowed us to control the balance of modes that would occur and ensure that the absolute minimum were coincident, ‘degenerate modes’ as these are particularly difficult to treat. The reverberation time was also plugged into the calculations. The length of time it takes for sound to die out in a given space is pivotal\ to the useful application of the room volume. A common measure, the RT60, defines the time taken for sound pressure to diminish by 60dB. ‘For example’, says Prathmesh, ‘for a performance space, the room plays a big part in what’s reaching the listener, so you want a reverberation time that’s reasonably long. But for listening [reviewing], you want a reverberation time that’s quite short. This ensures that spatial cues captured in music recordings are faithfully represented by the speakers without being contaminated by the room’.

Above: RT60 curve for the Listening Room averaged over ten different positions. Room reverberation time may be increased or decreased by adjusting the treatment [note: this is not frequency response]

We also addressed flutter echo in the room design. ‘This is essentially sound bouncing between parallel walls’, says Prathmesh, ‘and it’s very easy for two speakers in an untreated room to create a lot of flutter’. Balancing modes, and commissioning specific treatment, was key in developing our inherently ‘neutral’ sounding room, as were the materials chosen for the physical structure itself [see boxout]. We’ll circle back to this a little later.

Creative bass

In the meantime, extending this inherently balanced sound down into the deepest bass was either going to involve a lot of invasive treatment or some leftfield thinking. In fact, the solution proposed by KEF’s engineers might also be tried by HFN readers in their own rooms...

‘Room dimensions tell us where the bass modes are going to be’, says Prathmesh, ‘but how much of an issue they’re going to be depends on where you put your speakers and where you sit. Most rooms are longer than wider, so the problematic modes will be down the length, but the lower you get in frequency, the harder it is to deal with these modes using traps, etc’.

In practice, we simply got creative about how that room mode would be excited, and where we would sit. The room plan [see above] puts the seating position just past halfway. Accounting for the position of the speakers this puts us at a pressure node for the longest room resonance. ‘By virtue of positioning’, explains Prathmesh, ‘we’re not coupling into that room mode. It’s a “natural” remedy that takes care of the primary resonance’.

Narrowband fix

Of course, this does land us at the apex of the harmonic mode which is where the 12 resonant absorbers (the Scopus tuned-membrane bass traps on the speaker wall) take the heat out of this secondary at ~40Hz. They are precisely tuned to the room and operate over a very tight bandwidth. In fact, this is the only band-restricted treatment in the entire room and its only job is to take care of that second room mode.

Above: Room control by GIK Acoustics includes custom corner Tri-Traps, banks of Scopus tuned bass traps with Grey Elm Alpha 2Da faceplates, and two ‘Monster’ bass traps with Flexrange technology on the mass-loaded doors to the plant room. All cables are routed through screened channels under the floor, emerging into the plant room and through ports into the listening room

By virtue of the symmetry of the room and the loudspeaker positioning, the side-to-side modes were readily accommodated into the computer modelling. The height modes also needed care. We had the option to go for an even higher ceiling – something of a fad in audiophile circles – but, in practice, this was not a good idea. Again, this boils down to room volume, the degree of treatment required and the balance of room modes.

If our ceiling was higher than 2.8m then its vertical mode would be pushed lower in frequency and would likely play in with the rest of the room modes that we were trying to deal with. We have opted for controlled absorption at the first reflection point, so we carefully manage the ceiling bounce back.

The room masters

All our treatments – the corner and Tri-Traps, absorption, diffusion and scattering panels – were built to specification by GIK Acoustics LLC in Leeds. GIK Acoustics was even able to colour match its fabrics to our interior decor – everything counts when cre...

Hi-Fi News

Message History

Delivery option(s)