I've got a very compromised room to work in for the next year or so with a terrible 48Hz mode.

After reading the thread on DIY Helmholz resonators I decided to try an experiment to see what the effect would be of using one of these devices.

How bad is this room? I'm still waiting on broadband bass traps so with no bass trapping at all, this is the response:



The resonance at 48Hz and the null between 80-90Hz are on full display. What's really shocking is the decay in this room. Play a tight snappy kick drum, and it sounds like there's a competition subwoofer booming an 808 kick behind you:



Hopefully the broadband absorbers will help soften the peak, the null, and especially shorten that awful decay time.

As an experiment I decided to build a Helmholz resonator to see how it's done. I've never built one before so I loosely followed the advice in this thread to build a 60x40x100cm box with 32 22mm holes for an expected resonance around 50Hz.

I used MDF melamine board from a local hardware store because I don't currently have access to a good shop or quality tools with which to build. For damping material I used just a small amount of rockwool. I really had no idea how much to use, but my understanding is that the more absorption you use, the lower the "peak" but wider the "Q" of the box. Since my room's peak is really severe, I guessed that I wanted a very high peak resonance and narrow Q, so I used only a little absorption. First mistake.

Once I built the cabinet and drilled the holes my second mistake was obvious. Having used low grade MDF without crossbracing, the box was clearly resonant in a bunch of midrange frequencies just exactly like you'd expect from a cheap MDF box without enough bracing. Unfortunately, being Very Smart I knew that vibration and air leaks were my enemies so when I built the box I used plenty of glue all around. Therefore I wouldn't be able to open the box to correct the bracing problem or add absorption. Third mistake. I've built subwoofers before, I know better than this. I felt like a right numpty. Oh well, it was a beta-test.

I put the box in the most likely position in the most likely corner and had a listen. The reduction at 48Hz in the listening position was immediately noticeable. Also noticeable was the expected awful ringing sound produced by the cabinet. Now, when I played back a tight kick drum, the 808 subwoofer was turned down, but it also sounded like someone playing a snare drum from the other room at the same time. No bueno.

I fired up some measurements which really told an interesting story. The box's Q is definitely too narrow. It needs more absorption. It made a nifty narrow notch right in the middle of my ugly peak frequency:





Here's the overlay of the FR charts:



So you can see that even though this box is a throwaway, I was able to achieve a real improvement in my critical frequency (a reduction of ~20 dB!!), as well as a significant improvement in the related 80-90Hz notch.

One thing that wasn't mentioned in the tutorial (or if it was, I missed it) is that you can play with the tuning of the box by covering and uncovering some of the holes. When I first drilled out the holes, I drilled 33. After the box was in place I experimented with covering and opening a few, and my ear told me that the best results were with 32 open holes. Once I did the measurements it was confirmed. You can run the tests with a little duck tape over the holes, and the tuning shifts up and down by a few % as you open / close the holes. That's a remarkable feature if you ask me.

My broadband bass traps should arrive soon. I'll find out how the room sounds with those. If they don't sufficiently address the problem, then the next step will be Helmholz #2 , this time with better MDF, crossbracing, more absorption, and one removable panel for adjustments after the fact.

So my broadband panel bass traps arrived unexpectedly early today and I decided to take some measurements.

There are 4 columns total, one for each corner. As soon as I put the rear traps in I could hear a difference in the decay time of the nasty 48Hz mode. Once all four were in I took some measurements. Here's the original for reference:



And here is the waterfall with the 4 panels:



When I added the front-wall panels, everything (the speakers and the listening position) got shifted forward, which is why the two waterfalls look so dramatically different.

You can see that while the peak was "only" dropped about 10dB the most significant change is the decay. Decay times are much less problematic now. And the 105Hz peak is also lower, with far far less decay time.

Unfortunately, the panels also exacerbated the hole at ~83Hz.

Here's the comparison of no traps vs broadband traps:



By adjusting the bass extension settings of my monitors, I was able to arrive at this result:



I think my next step should be to try another Helmholz resonator.....

As an experiment I decided to put the (bad) Helmholz resonator back in the room. This was a very interesting result. First, the "before" (room treated with 4 bass panel absorbers):



And then with the resonator:



So, there's the really narrow notch frequency. But what's crazy is how much decay the poorly-damped box added.

It seems that this box just needs more absorption. That should reduce it's ringing, and broaden its Q. More to come.....

There are 2 ways to damp the resonator and they both have different effects on how well it works, and how it sounds. The first way is to cover the port with a velocity-based absorber (instead of duct tape) and the second way is to add velocity-based absorption inside the resonant cavity.

The second way just requires to add something into the box. The first way can be easily done with some fabric or something more heavy duty over the port.

What works / sounds better? Honest question.

Edit: It's a success!! Read on:

I figured I'd try @ OpusOfTrolls 's advice and experiment with using some acoustic fabric to damp my experimental Helmholz resonator. I'll be using Guilford of Maine FR701.

I started by baselining the room without the resonator (just broadband bass traps):



and with the undamped resonator creating the giant notch and lousy ringing:



Then I tried first covering one hole, then two, then four, then six. The first few tests with only one and two holes covered didn't seem to be producing a change. At 4 & 6 I could start to see an improvement:



The notch is becoming reduced. I realized that covering the holes also effectively reduced port size, which was dropping the resonant frequency. No problem, I added some more holes. After a lot of experimenting I had added 10 extra holes. At 10 extra holes, loosely covered with bits of fabric taped on, there was a substantial improvement in the performance of the box:



Now it's not producing a notch effect. More importantly the decay was improved:



For the purpose of experimenting, I was using small fabric squares that I would tape across the individual holes:



Once I had settled on a tuning, I intended to wrap the box more professionally with the fabric, covering the top row(s) of holes and leaving the bottom uncovered.

It occurred to me that the box might perform differently once the fabric was stretched tightly, and that my experiments using taped squares might not be valid. So I cut a rectangle of cloth and, using some spray adhesive, affixed it tightly across the desired holes:



I ran the sweep:



Bummer. The tightly stretched fabric functioned considerably less as damping, and became more acoustically invisible. The notch came back, as well as the long decay. Okay. The cloth will need to be loose.

Now I was curious to try something different. So I tossed a section of fabric over the box, like draping a tablecloth, all the way down to the floor. This way I covered all the holes, but not very tightly. The cloth is just resting on the surface of the holes:



Magic! It was the smoothest result yet. Here's the comparison with the original (no resonator) response:



While the peak reduction is now only a few dB, the real story is told in the waterfall plot:



WOW! Low frequency decay times were cut by more than half vs just using the broadband absorbers. At this point, in this room, the 48Hz bump is no longer a dealbreaker. At least it's no longer booming like an 808.

Now to do something about that horrible 75Hz null....

Wow this is a great writeup, and it's totally impressive how you went through the troubleshooting and tuning based on common sense after carefully reading through the other account.

As you've seen in my other thread, I built a slightly different box that failed to have an acoustic effect. I think this is because I chose a small hole diameter that led to turbulent effects that violate the models used.

Sorry if I missed it, but what is the thickness of the MDF you used? It seems common to use 18.5mm (3/4") but I didn't see in your post.

Thanks for taking the time to post all these details! I am interested in the physics of how these things work, so will definitely try and replicate.

Edit: one thing that's nice about your write-up is that you clearly identify when the plot comparisons are based on measurements with or without the Helmholtz box and no other changes. I've read through a number of reports where the comparisons are just before/after a multi-pronged treatment that combines porous and resonant absorbers.

Yep, it's 18mm

You're welcome! Since I learned from others here it made sense to share my learning curve for others. There isn't enough good information on how to get started.

Thanks!

There are a couple of other pieces of info that might also help further flesh out your account for the benefit of future readers (or reductionist nerds like me).

1. Can you give some basic info about the room dimensions, and where you placed the resonator for the measurements? Not that this really matters, as you have specified the modal frequency of interest, but it just helps give some context.
   
2. It would be interesting to see the result of an REW frequency sweep with your measurement mic placed a few millimetres in front of one of the holes (ideally for a damped and and undamped hole, if this is convenient). This gives a qualitative idea of how the resonator is interacting locally with the sound field, and can serve as a crude sort of "unit test" . This test, along with looking at candle flame deflection, is how I was able to establish that the small holes on my test box were undergoing "jet formation" (due to "vortex shedding") and that this was greatly altering the absorption behaviour. My holes were around 6mm, which appears to increase the effect of the sharp edges on the neck relative to your 22mm holes.
   

I hope it doesn't sound like I'm asking you to "prove it works" because that's not what my questions are about. I just think that some of these points can help to generalize your experience to readers with slightly different design targets.

Hi again,

Just for the record, had you added any rock wool when you made the measurements shown in your posts? Or do you mean that you had planned to, but forgot to put it in before gluing everything together (meaning that the outer cloth was the only damping added)?

Your demonstration that fabric over the neck is effective is quite significant, as this seems much more practical to implement than titrating mineral wool into a sealed box.

And just to confirm (sorry if this should have been obvious), all the measurements are done with the mic at the main listening position?

Yes, just a little bit, a few pieces lying around, enough to put 4" or so in the bottom. The article I was basing my build on didn't use any, which surprised me, so I really had no clue.

Yep! I think measuring the box seems somewhat pointless. I'm not sure what sort of behavior to expect at the box/room interface. Kind of like measuring a corner. Interesting, but not the area of effect we really care about. The only performance that matters to me is more or less where my head is, so that's where I took all my measurements.

Ok thanks for clarifying that!

I agree with you, particularly since you are replicating a somewhat proven design. For me the value of measuring the box is as sort of an "alpha test" before beta testing, which is probably more important with a new design in which the resonant parameters are calculated from scratch. It allowed me to capture some design flaws that were important.

Although there's not much benefit for you, it would be interesting to see what this "close mic" test looks like for a resonator that has been shown have an effect at the listening position. The potential benefit would be for future builds with new design parameters. No problem if it's too much hassle - depends on length of mic cord, distance between computer and box, etc. It was just a thought...

This thread is a good one that gets into details of HH
Panel and Helmholtz resonators for studio

Thanks for the detailed write up and test results on this!

I am getting ready to do a very similar experiment in my control room. I already have a ton of broadband absorption (too much in fact) multiple 6" bass traps, a few diffusers in a 12.5' x 9.5' room.

I have run baselines with REW with the default treatment in place (corner bass traps, broadband on the walls, etc) but plan to strip the room and start from scratch - taking measurements with each change.

I am going to sacrifice some real estate with some deep 18"+ pink fluffy traps, but will be looking at Helmholtz and/or tuned mass absorbers for narrow band and deep bass treatment.

your room is close in size to this so look at room layout and treatment depths in my post #10 here:
Room treatment suggestions

You can't really have too much absorption in a room but the wrong type.(too thin, too small in area and too dense absorption material usually)

What a wonderful read @ riprowan !

I first started reading @ PhaseRoll thread and learned so much.

The first reading material I found really seemed so easy to integrate but now that I have read both your experience, I'm rethinking my venture.

I have a few questions for both of you; ( or anyone that want to chime in)

1- As I was looking at PhaseRoll results, I was thinking to myself; '' is it possible that a thinner , more resonating material like his bottle for example, make a bigger difference in resonance absorption than a thick hard and none resonating box made out of MDF !?''

I'm wondering if I was to make an HR made out of 1/4'' plywood, would it yield greater resonance, therefor absorb my target even better?

2 - I was also wondering if fluffy absorption like SafenSound would be better to broaden the Q when placed inside the resonator vs using some rockwool/rockboard for example.

3 - I was also wondering how someone should put the textile ( inside or outside) the resonator. I've seen a video where the women use cheese cloth stapled inside the drilled panel. As Riprowan pointed, my main goal is to minimize the decay of my low frequency. My main target (the worst) is 89 hz but 62 hz and 160hz really resonate a lot as well. I can EQ the peaks with DSP but I cant dampen my room low end other then putting in A LOT of dampening material in each corners and given the price, my only way through this would be DIY and sofit mounted. Something I'd love to avoid as I'm in a rental in a basement of an apartment building and my dedicated studio room is super small : 9'2'' x 12'8'' x 8'

4 - Last question, does the size of the HR (HxWxD) has any incidence on how much damping it provides ?

I understand now that it's better to undershoot with the number of holes on the front panel to then add more as it's really impossible to accuratly calculate it with any calculator available online, is that statement correct ?


Thanks for your help !!

I was just looking at inexpensive and easy way of building an Helmholtz Resonator and came across a few studies that mention plastic bottles as test materials.

I used ChatGPT to check a few quick measurements and found out that a typical 5 Gallon Jug (the one used for water for example ) when empty, tend to be centered at 50 to 60 hz.

Here's the result of a few calculation I've submitted:

The resonance frequency of the jug with a neck diameter of 1.7 inches, a neck length of 2.4 inches, and a bottle height of 18.3 inches with a diameter of 10.4 inches is approximately 50.4450.44 Hz.

The resonance frequency of the jug with a neck diameter of 5 cm and length of 8 cm, and a bottle height of 48 cm with a diameter of 26.5 cm, is approximately 50.1550.15 Hz.

The resonance frequency of a typical 5-gallon water jug used as a Helmholtz resonator, assuming the neck has a diameter of about 5 cm, is approximately 57.857.8 Hz.

That seems to be a very cost effective way of absorbing that 50hz peak you got there.

In a different study I just read this morning, Sedigheh Basirjafari goes and touch a bit of different concept including unit neck shape and amount of units but more importantly the positioning to broaden the absorption coefficient and the frequency bandwidth.

Now, I'm following up on @ PhaseRoll idea of the Plastic bottle.

I'm having a massive 50 hz dip in my measurements and I think I'll try to buy 4 of those empty 5 gallon water jug and see where it lead me.

Now the part I find confusing is how much porous material should I fill them with ?

PhaseRoll suggest cotton balls but that's really not easy to understand as I don't know the density of the cotton balls material and I don't know the optimal amount per chamber size. The actual size of those cotton balls is also unknown.

I don't want to buy a pack of Owens Corning Safe N Sound just to do that experiment but if that's what's needed, I 'll definitely invest in it so I can have a better understanding of the actual need for those to work.

I'm also wondering if 4 of those 5 Gallon jugs will be enough for my very small room (9.2 x 12.8 x 8ft high (280cm x 386cm x 244cm)). I wouldn't have any issue going up to 8 or maximum 12 of those units but if a lot more is needed, that would become a real estate issue.

I also noticed your loosely textile that you put up front and I remember seeing someone using cotton cloth in a different study. I'm wondering how to fix those to the bottle neck so it is actually effective.

There's a lot of uncertainty but I'll start with 2 of those jugs and experiment placing them in each front corners.