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BLAK HOLL BASS TRAP

By

1 September 2009

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It’s a black hole disguised as a white box that’s sympathetic to bottom-end.

Text: Andy Stewart

Unlike some of the acoustic treatments I’ve tried that have promised to solve all my sonic maladies with foam squares cut into fancy shapes by relatives of Edward Scissorhands, Blak Holl ‘bass traps’ have had an immediate effect on the bass response of every room I’ve hung them in thus far. For the purposes of this unusual product review I’ve had the standard ‘four pack of Holls’ at my disposal – apparently these room treatments are only supplied in groups of four for some reason – and to test them the units have been placed in three conventional domestic home studio environments as well as in my own studio, which is somewhat larger. In each case they’ve been positioned at points along the room boundaries where bass build-up has been most evident and tested using my favourite tool of trade – my ears.
Particularly suited to small studio environments of the home or garden shed variety, Black Holl bass traps consist of a lightweight (9kg) painted three-ply box measuring 1200mm x 600mm that houses a rigid fibreglass interior with a modest air gap, which act as the damping ‘shock absorbent’ material for the resonating surface. The panels are designed to be hung on walls just like a picture to control low-end bass anomalies that often manifest in the corners of rooms where standing waves terminate. Actually, when I say ‘just like a picture’ that’s not strictly true of the process required to hang them safely. Though they can be hung in several ways – or simply leant against walls and floors as an alternative – Blak Holls are most effective when hung across the 90º corners of a conventional room on a roughly 45º angle rather than flat against the wall like the Mona Lisa. To do this effectively is tricky and best done with a second pair of hands at the ready… accidentally dropping one to the floor from any height isn’t something I’d advise.
There are two straps on the back of each panel for mounting duties and to get both of these engaged onto your hooks of choice (none are supplied) is hard to manage, and what’s particularly difficult is getting both straps to take the weight of the panels evenly. I soon gave up trying to use both and eventually hung each of the four units by one hook and strap per panel.
From the moment the Blak Holls were hung at boundary positions where bass frequencies appeared exaggerated the difference in the evenness of that room’s frequency response was clearly audible. The panels achieve this by resonating 90º out of phase with the standing waves in the room and applying a damping force to that oscillation (in the case of the Blak Holls, this damping is performed by internal 50mm fiberglass sheeting). Whether or not Blak Holl panels will suit each and every listening environment obviously depends entirely on where they’re placed and what a room’s ‘balance issues’ are to begin with. If your room doesn’t have standing waves within the working limitations of the Blak Holls then they’re obviously going to be ineffective, but for the majority of small rooms out there whose operators typically deal with acoustic issues like an ostrich does threats to safety, chances are these room treatments will improve the situation. If they don’t, you can return them for a full refund.
According to the Queensland designer, Justin Douglas, who is resolutely tight-lipped about the specifics of the product’s test results – presumably out of a concern that someone might try and mock up an equivalent in the back shed and be sucked into oblivion – the Blak Holls can be used “in audio environments large or small”, but there are no independent test result to confirm this yet – something AT may pursue separately in a future issue. Obviously larger rooms would require greater numbers of panels in the same way a larger room requires a bigger PA, but whether or not the room modes requiring treatment would still fall within the region of effectiveness of the panels is impossible to say in the absence of measurements.

EVENT HORIZON – THE PHYSICS OF BLAK HOLLS

Blak Holl acoustic panels are essentially passive resonators, or more accurately, membrane absorbers. The design principles, benefits and limitations of this type of reactive room treatment have been well documented over the years, and there is disagreement amongst acousticians about their effectiveness, but to reduce it down to a few sentences the concept runs something like this: for a membrane absorber to work effectively they’re typically employed to act on a fairly narrow band of frequencies (equivalent to a narrow Q on an equaliser) to counter the anomalies caused by a standing wave (otherwise known as a ‘room mode’). To do this the treatments must be lightly damped and custom-tuned to match the modes they aim to control. To work on a broad Q they must be more heavily damped, which has the side-effect of rendering the panels less effective as their absorption coefficient drops, but this also allows them to cater to a wider range of modes, albeit less efficiently. Eventually, when the Q is made too wide by adding further amounts of dampening to the device, a membrane absorber becomes impractical for the simple reason that, to be effective, you’d need more of them than the room could physically accommodate. The Blak Holl therefore aims to reside somewhere in the middle of this sliding scale of effectiveness, working on the concept of a reasonably broad Q (which is still known only to the manufacturer) and a meaningful yet modest absorption coefficient (which also remain unpublished).
Blak Holls work by reducing the low-end peak and dip frequency response errors caused by standing waves present in all conventionally constructed rooms, like houses, sheds, offices etc. These standing waves are like the reflections off the side walls of a swimming pool that induce ‘chop’ back into the water. The aim of the Blak Holls is to reduce the strength of these returning waves by acting like a sponge on the edge of the pool. If left untreated, these standing waves have the potential to phase cancel and/or amplify certain frequencies, depending on where you are in the room, creating what’s commonly known as a ‘lumpy’ response: lots of bass in one part of the room, bugger all in another. If you find yourself in a lumpy room, judging which parts offer the flattest, truest indicator of the sound emanating from the speakers is tricky, and a guessing game worth avoiding wherever possible.

CAUSE & EFFECT

Though they’re not the most scientific solution to the problem of low-end room corruptions, nor are they necessarily the most effective, Blak Holl bass traps are a good option if you’re after a clean, simple, non-invasive and convenient treatment that might just have very positive effects on your room’s bottom-end response – whether this improvement is large or small will again depend on the inherent shortcomings of your room. In the end what must be made abundantly clear is that this sort of purchase isn’t like buying a new microphone; there’s simply no guarantee they will work.
My concern with any treatments of this nature is that the one-size-fits-all manufacturing principle isn’t always effective. What is certain, however, is that this sort of room treatment is always very difficult to accurately quantify without specialised equipment and someone qualified enough to interpret the results. Worse still, the whole concept of off-the-shelf acoustic treatments has the potential to over simplify acoustic issues and infer that they’re easily solved with prefabricated treatments irrespective of whether you know how to apply them, which isn’t the case. Regardless of this general warning, however, Blak Holl bass traps seem to do what they do well… they’re a no-brainer if you’re working in a rectangular box with no budget for an acoustician or the stomach for the physical revolution he or she might demand you subject your room to. The point is they work, seem unlikely to cause any harm and if they don’t do it for you, they can always go back. But I suspect if you’re reading this at home, and home also happens to be the studio, chances are these membrane absorbers would improve the sound of your space considerably.

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