Back in November I told you I hadn't seen a single DIY Helmholtz resonator work in a home studio. In all the years I've been doing this, not one.

Well, now I have. And it came with measurements.

It belongs to Mike Senior, a mixing engineer who writes the Mix Rescue and Studio SOS pieces for Sound On Sound (he also wrote the book Mixing Secrets, which shaped a lot of how I learned to mix).

He built the Helmholtz resonator in his new studio, tuned it, measured it and wrote the whole thing up. So I got him on a call to walk me through it.

What he found explains why almost every other DIY attempt out there fails. And it has very little to do with the maths.

A 50 quid gamble on a 100 Hz dip 

Mike's room is about 4.5 by 3.5 metres. He's moved studios five or six times in 30 years, and he treats every one in the same pragmatic way. Lots of broadband absorption, then work around whatever's left.

This time what was left was a big dip around 100 hertz at his listening position. He could hear that frequency come and go as he walked around the room, so a room mode was the likely suspect. And he'd already done everything he could with absorbers.

So he opened F. Alton Everest's classic acoustics handbook, found the Helmholtz equation, and figured a box with holes in it can't be that hard. Thick MDF (medium-density fibreboard), cut to size at his local DIY store, glued together, holes drilled with a big Forstner bit. About £50 in materials. "What have I got to lose?"

Then he tested it by ear. He played a staircase of sine tones a semitone apart, from 20 Hz up to about 250 Hz, and sat with his head right next to the box. When a tone hits the resonant frequency, you can feel the box vibrate and hear it boom, just like a bottle booms when you blow across the top of it.

It boomed at 100 Hz. Exactly where the maths said it would.

As Mike put it, that was the end of the easy bit.

Where the formula falls apart

An empty box resonates, but it barely damps anything. To actually take energy out of your room, it needs insulation inside. So Mike half-filled it with loft insulation and tested again.

The resonance had dropped by about 30 hertz.

Now some drop was expected. Insulation makes the cavity acoustically larger, and a larger cavity resonates lower. What nobody tells you is how much.

At that point the only thing Mike could still change was the number of holes (more hole area pushes the resonance back up). He'd started with 35, each about an inch and a half across.

So he set up his saw horses outside and started drilling. Test, drill, test, drill.

It took about 150 holes to get back near 100 Hz.

More than FOUR times what the formula asked for.

Think about what that means if you build one yourself. You notice it's detuned, so you add maybe 20 or 30% more holes. The frequency barely moves. And almost anybody would conclude they'd messed something up and the whole idea is broken.

In other words, the formula designs the one version of the box you'd never actually use: the empty one.

Everest does warn you, by the way. Mike found it in a caption under one of the diagrams, right at the end of the section:

"The presence of the mineral wool shifts the frequency of the resonance considerably from the theoretical values."

That's it. One sentence, in a caption, with nothing about how considerably or what it takes to get back on target. Mike also found a paper from students at Belmont University who built speaker stands as resonators from that same book, filled them, watched the frequency drop, and concluded they didn't work. They were one step (and a lot of sawdust) away from a working absorber.

It also explains all those YouTube builds where somebody follows the formula, stuffs the box, hangs it on the wall and says "that's better". No measurement. Their box is almost certainly tuned somewhere else entirely.

So what I said in November about tuning and damping being a constant ping pong still holds. But it is fixable! You just need a drill and some patience.

Where it ended up (and what it's worth)

For the first couple of months the box sat in a floor-to-wall corner, doubling as a shelf. That's a sensible first guess, because boundaries are the most reliable place to find high pressure.

Then Mike decided to measure instead of trusting his impression. Box in, box out. It made a difference. And since the room was cleared anyway, he tried other spots. About a foot out from the wall worked significantly better, which happened to be exactly where his keyboard stand used to go. So now it IS his keyboard stand.

At the deepest point of the dip, the resonator got him a 7 dB improvement.

And here's how Mike rates that himself. "Fundamentally, it's a small difference." Take out his broadband bass trapping and you'd hear a huge difference. Without that trapping, he says, there would have been no point building the resonator at all. It made judging the low end from his listening position a little easier, and he's honest that some of that could just be him getting used to the room.

That's the answer I trust most in this whole story. It's also the one a "that's better" video never gives you.

Where a resonator fits in your room

After Mike's box, my answer to "should I build one?" has moved from no to not yet.

Not yet, because the order stays the same. Speaker and listening position first, then thick broadband absorption, then speaker calibration if you need it. Before all that is done, a resonator doesn't make a big enough difference to be worth the money. If your room still has space for deep porous absorbers, that's where your next £50 should go.

Once that's done and there's still one stubborn dip at your listening position, which is actually caused by a room mode AND you have physical space left over, a resonator can be a real option. You won't break anything by trying. Acoustics isn't life or death, and the worst case is a pile of firewood (about £25 worth, by Mike's estimate) and a much better idea of how these things behave. Mike's article has his step-by-step guide and calculator. I've linked it here if you want to check it out.

Just go in knowing the formula tunes the empty box, and you'll be tuning the real one with a drill.