The size of the box: the 1/2 wavelength but really who know?
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DIY Tube Bass Trap and frequency tuning
- Thread starter Razorhelm
- Start date
I posted a calculator earlier in this thread. Please check it.
A Helmholtz resonator works with resonating a volume of air. It can only be tuned to a single frequency. This is similar (but not exactly) the reason why you have a separate pipe for each frequency in a church pipe organ.
Please check the calculator I posted earlier in this thread.
These were pretty much my questions too.
If my post #62 haven't answered them please ask further in detail. I am here to help as much as I can. However, I urge you to check the calculator I posted on post #24 first to understand the workings of a Helmholtz Resonator.
MRC01
Major Contributor
I would imagine that, like an organ pipe, the length determines the resonant frequency and the diameter determines how much volume or impact it makes?
Not exactly. The pipe you mention only affects the volume of the air in a chamber. The chamber cam be a cube, sphere, a very, very long and thin box; as long as it has the same volume there will be no change in the resonance. The resonance is controlled by the port/opening that is attached to that volume of air. The port length, cross section area, and the volume of air in the main chamber controls the resonance.
Do please check the calculator I posted on post #24. I keep repeating the same thing over and over again and it is getting tedious. If you play around that calculator you will have the general idea. We can then take it from there...
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MRC01
Major Contributor
The Y scale is 1 dB per division. After writing the page, I changed the graph from 2 dB to 1 dB to amplify the curves. Then never went back and updated the text.
Yes dipoles can be a pain to set up. I'm happy with the response I achieved in the end, as seen in the final graph (2 dB per division), applying a few more room treatments & finally some parametric EQ.
Thanks for the suggestions. What I did wasn't much different I didn't touch 30 Hz because that was supported by a room mode. I tackled 68 Hz with some flat membrate resonators (GIK Scopus T70s) right behind the listener because through experimentation I discovered it was a LBIR with the back wall. Then I added some bass traps (RealTraps MegaTraps) in the corners to smooth the bass response further. Then used some parametric EQ for the rest. But after all the room treatments, the EQ was simple and gentle.
What about the standing wave at 50Hz. They improved 9dB. What was the helper there? Was it a (good) side-effect of the 68Hz resonator?
I’m still not sure whether it is more effective to have 4 smaller resonators tuned to frequency X around the room or whether 1 big resonator tuned to the same frequency X is more effective.
Neither am I, but I can clearly see the issues. Tuning a resonator at low frequencies is difficult because measuring low frequencies in a room is very difficult. Doing the same for more than one resonator is tricky to say the least. Besides, in acoustics, it is generally not a good idea for the same signal to be emanated from multiple locations. It makes calculations too complex or even unsolvable.
Meanwhile, may I ask what is your definition of small or big when it comes to resonating air below 100Hz? And, what size range of rooms you are talking about?
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I don’t see that answering my questions. It just confirms what I said: I can basically tune any size box to any frequency. It doesn’t tell me how large the box should really be?
You can have double tubes bass-reflex enclosures. It’s based on the same principle. Why would it not be possible?
They are all connected to the same volume of air.. I don’t see a contradiction. In any case, the pipe of the organ determines the tuning, there is no additional volume. It’s not exactly the same principle.
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I refrain to debate further because complex mathematical calculations are required to explain my point. I am neither prepared to do that in a public forum nor the forum editor is capable of entering such calculations.
If anyone wants to ask any more questions on how and why I built the resonators that I use today, please ask. If you do not agree with me please refrain from aiming your posts to me. I will not reply as I do not want to argue.
If I had been any help so far that makes me happy.
I’m pretty sure one could model these things with something like akkabak?
Still, I’m less interested in the multiple tuning. It will probably be just as complex as with a box with a speaker in it.
But determining how big the resonator should be seems like something that a rule of thumb would be useful for. There must be something there surely.
I did, why not just try to really answer them? I don’t want pages of math. Just the basic principles are fine.
There is nothing to agree or not, you haven’t given answers yet.
Let me ask it differently: for a reflex box, the size of the box and tuning are depending on the parameters of the driver. Since a driver is lacking, I would guess that interaction with the room will takes it’s place. Question is in what way?
The answers are in the calculator as that is working the formulae.
You are on the wrong track comparing the HR to a vented speaker where the air in the box is interacting with the speaker. In a Helmholtz resonator there is no speaker, no Vas, no Cms, etc. just a volume of air. The area and length of the opening, and the volume of the chamber are the only parameters. There is no interaction with the room either. It is a free-standing resonator.
An HR cancels standing waves in a room. I really cannot know what else I can say about the principal as it is as simple as that. That is why I urge everyone to please enter some meaningful dimensions relating to your own use case and see what you can build to tame your room's low frequency anomalies. Without knowing anything about your requirement how can I offer guidance? All I can do is to offer you a tool.
PS. What is akkabak?
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No, they are not. The calculator only shows you how to get to a specific tuning frequency for a given box size, not why the box should have that size.
Okay. Then what determines how big of a box I need? Why do I need a hypothetical 200l box, and not a 70l box? One can tune both to the same frequency. As per usual, I guess bigger is probably better, but given that big empty boxes in living rooms have a limited acceptability threshold with the better half, one usually wants the smallest acceptable box
. So how do I get to that? What are the trade-offs of a smaller box vs bigger box?Hopefully with the above, it becomes more clear what I’m after.
That's because the chamber volume is irrelevant, but see below.
The chamber is only used to trap the resonance frequency and dissipate it via internal reflections. In theory any chamber size can be used for any frequency as long as the resonance criteria is satisfied. However, as with anything in physics there are other limits imposed. The most obvious one is that a small chamber (compared to the wavelength of the resonance frequency) will not be able to dissipate the sound energy well.Okay. Then what determines how big of a box I need? Why do I need a hypothetical 200l box, and not a 70l box? One can tune both to the same frequency. As per usual, I guess bigger is probably better, but given that big empty boxes in living rooms have a limited acceptability threshold with the better half, one usually wants the smallest acceptable box
The wavelength of 50Hz is around 6m, hence say, a chamber with the largest dimension of 60cm will be too small to dissipate the sound that has ten times the wavelength. I can also surmise that larger chambers will be more efficient (high Q) as they can dissipate the sound energy easily. I have not built many resonators to be sure about that but general physics points to that direction.
I choose the longest dimension of my resonator chambers to be 5m, where 1/2 wavelength is 38Hz, as I wasn't expecting to go lower.
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Thanks @sarumbear, that makes things a bit clearer. None of this info can be found in that calculator you posted. Is there a rule of thumb for the minimal internal dimensions? You mentioned half wavelength.. that is still quite massive
If we were to take those 5m, and fold that thing into a boxy shape (like a folded transmission line), how would that compare to the same volume without the line in there?
If we were to take those 5m, and fold that thing into a boxy shape (like a folded transmission line), how would that compare to the same volume without the line in there?
I was wrong to assume that you (and possibly others as well) wouldn't think that a HR works with dissipating the sound energy within the chamber. I am sorry.
In order for a good dissipation to happen the sound waves must bounce within the chamber many times. And for that to happen at least one of the dimensions of the chamber should be near (at most 1/4) the wavelength of the resonance. I have not heard of a folded HR but why not? I cannot see any problem. Not many exists now but there had been folded pipe church organs in the past.Is there a rule of thumb for the minimal internal dimensions? You mentioned half wavelength.. that is still quite massive
If we were to take those 5m, and fold that thing into a boxy shape (like a folded transmission line), how would that compare to the same volume without the line in there?
If I were you I will also look at using the entire width of one of the walls and use a box that is wall to wall, relatively thin and medium tall. Like a heavy duty skirting board. Or a false wooden beam in the ceiling! Don't laugh: That was one of my ideas until I realised the large cavity above the false ceiling in my rooms allows more freedom.
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