Building A Helmholtz Panel

Mike’s £40/$50 perforated‑panel Helmholtz resonator. All it needs now is a lick of paint...

Affordable and effective, broadband absorbers are the first port of call for acoustic treatment with good reason. But sometimes you’ll hit on an acoustics challenge that requires greater precision...

When you’re acoustically treating a project studio for mixing purposes, there are lots of straightforward acoustics fixes you can try. It doesn’t take more than a few strategically placed patches of acoustic foam, for instance, to scotch flutter echoes at high frequencies or to stop early reflections off nearby hard surfaces from comb‑filtering your midrange monitoring. And judicious broadband bass trapping, typically in the form of large fabric‑covered mineral‑fibre slabs near the room boundaries, is usually pretty effective at damping most low‑frequency room resonances. Furthermore, all these tactics can be very cost‑effective, since acoustic foam is fairly inexpensive and you can build broadband bass traps for yourself at a fraction of the cost of off‑the‑shelf products.

Occasionally, though, there may be an acoustics problem that’s just too challenging for general‑purpose absorptive treatments, especially if inconvenient realities such as doors, windows, and furniture restrict your placement options. In smaller mix rooms in particular, it’s not uncommon for one or two troublesome low‑frequency room resonances to survive even quite a lot of broadband bass trapping. That’s when it’s useful to have a bass trap that can be ‘tuned’ to target more specific frequency ranges. One of the simplest such designs is the perforated‑panel Helmholtz absorber, and in this article I’d like to explain how you can build one of these for yourself.

Ready To DIY

Everyone’s heard someone blowing over the top of an empty bottle to create a musical note — a note that results from the inherent resonance of the bottle’s contained body of air. If you placed that bottle in your mix room while playing back music, it would also resonate in sympathy with the soundwaves hitting it and, crucially, would therefore remove some of that frequency energy from the room. It’s this fundamental principle that Helmholtz absorbers take advantage of, the idea being that you tune them to match your troublesome room mode, and they then absorb some of that frequency — or, to be more accurate, a narrow band of frequencies centred on the absorber’s resonant frequency. In practice, though, a Helmholtz trap is usually a lot larger than a bottle, so that it absorbs more energy, and it usually has some damping material added to its resonant cavity to make its resonant peak broader, and therefore widen the band of frequencies it absorbs.

You can build Helmholtz absorbers in several different ways, but the perforated‑panel variant is tempting for the DIY‑minded home‑studio owner, simply because it’s so easy and cheap to construct. Basically, it’s just a large, sealed wooden box with holes drilled in the front, the box typically being half‑filled with low‑density mineral‑wool loft insulation. For example, I put together the one pictured in this article in a single morning, with just a cordless drill, a staple gun, and maybe $50 £40 worth of materials from my local hardware shop. (For detailed instructions, see the ‘Building The Trap: Step By Step’ box.) The biggest potential pitfall, in fact, isn’t the construction itself: it’s making sure that you’ve tuned the trap to the correct resonant frequency. I say this because, as I discovered while...