Hello,
I am trying to design a device to reduce a 42Hz standing wave I get in my bedroom which I think is being set up from the fans in a neighbouring hotel. There is no audible noise at higher frequencies and so this frequency is intensely annoying and difficult to sleep in even though it is only just audible. I play brown noise through a ceiling speaker which helps to mask it but I am intending to build a Helmholtz device to absorb this 42Hz.........
The room is ~3.8m x 3.8m x 2.5m high. There are 2 large windows on 2 sides, a built in cupboard and door on the 3rd side and on the 4th side I'm thinking of building an absorbing perforated panel (rather than diaphragm panel type). I can build it about 3m in length along this side and full height. My understanding is that the greater the % of hole openings the better. The opening % seems to be dependent on the thickness of board and volume of the resonator only. Using practical dimensions and readily available wood I'm proposing the following. My main question is do I make each panel 1.2m x 2.5m high or should I divide this 2.5m height into smaller compartments, say 3 smaller ones ?! Say 3 and each one ~833mm height X 1.2m wide. I think I read somewhere that the "volume" should be << than the wavelength (42Hz~8m) (whatever that means). If a panel 2.5m high x 1.2mwides is OK then obviously easier and less work to do.
18mm thick plywood 1.2mW x 2.5mH (could also use mdf or chipboard but assume ply is better as it is stronger / less flexible ?!)
50mm fibreglass insulation on wall
Panel 250mm away from wall
900 x 4mm holes
~0.4% open area
Appreciate any help / advice before I start to invest time and money !!
Design of Helmholtz panel to reduce 42Hz
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Re: Design of Helmholtz panel to reduce 42Hz
Hi Spriter1. Welcome.
Please read the forum rules for posting (click here). You seem to be missing a couple of things!
Having said that, I'm not convinced that your problem is a standing wave inside your room, so much as astanding wave outside it, and a resonant issue inside. Your room has a natural modal resonance at 45.3 Hz (which could take the form of a standing wave), but it doesn't have anything at 42 Hz.
I suspect that the fan might well be setting up a standing wave, but it is doing it all over the neighborhood, not only in your room. Your room just happens to be located at the exact right distance from the fan where a resonant peak of that wave is located, and triggers an additional modal resonance within your room. If your entire room were a little bit closer distance closer or further away, then it would be in a null, and you would not hear the sound. It's just the unfortunate distance that is causing the issue. It might well be that the standing wave in question also happens to excite a specific modal resonance in your room, thus amplifying that tone inside the room.
However, I'm also not convinced that what you propose would make much difference. Firstly, resonators are notoriously hard to tune, and standing waves occur at very precise frequencies where accurate tuning is crucial. So it is quite probable that your device would not be tuned to the right frequency anyway, unless you can figure out a way to make it so that you can vary the tuning after it is built.
Second, tuned resonators are used to treat resonant issues that come from sources within the room, not ones that come from outside it, and all they do is damp the resonance, not eliminate the note. So even if you did tune your device perfectly, the note would not go away: it would just resonate less.
For example if a speaker inside the room happens to excite a mode at a specific frequency then a tuned resonator could damp the decay of that mode, so it fades faster than it would have other wise, but it does not remove that tone from the room, which is the problem you have. So if you did build such a device, and it did work perfectly, all that you would accomplish is a reduction in the decay of the modal resonance within the room, but you would not kill the sound itself, since it is due to a source outside the room. In other words, you might reduce the resonance a bit, but the tone would still be there.
What you could do to your room to fix it, is to either isolate the entire room such that the sound of that fan does not ever get inside in the first place, or you could modify the room in some way so that the incoming sound no longer coincides with a resonant mode of your room. In effect, you could "de-tune" your room such that the fan wave no linger excites any resonance in your room. But that would still leave the original tone unchanged.
Isolating your room for such a low frequency would not be easy, nor would it be cheap. Ditto for changing the shape enough to de-tune it.
A third and better option would be to talk to the ownesr of the fan, and get them to damp the sound at the source by putting their fan on proper vibration mounts, or by adding some other method for killing the standing wave, or by de-tuning it by changing the speed of the fan slightly. Just enough that the standing wave will have a different wavelength, and no longer have a peak that coincides with the location of your room. If they don't want to do that, you might also have legal recourse to get a court order to have the fan shut down entirely, depending on what your noise local regulations say.
Overall, it would be far better to get the problem fixed at the source. Trying to fix it inside your room is going to be expensive, no matter what you do, and might not be as effective as you'd like.
One more option that I thought of: The wavelength here is about 8 m, so there's a peak every 4m, and a null every other 4m. Have you tried moving your bed a couple of meters one way or the other, to see if you can find a location where your pillow is in a null, instead of a peak? That might be the easiest solution of all. If you can find a position for your bed where your head is in a null, then you would not hear the note at all. It might be as simple as just flipping your orientation: put your head where your feet normally are, and your feet where your hear normally is. That would very probably move your head into a null...
- Stuart -
Please read the forum rules for posting (click here). You seem to be missing a couple of things!
Having said that, I'm not convinced that your problem is a standing wave inside your room, so much as astanding wave outside it, and a resonant issue inside. Your room has a natural modal resonance at 45.3 Hz (which could take the form of a standing wave), but it doesn't have anything at 42 Hz.
I suspect that the fan might well be setting up a standing wave, but it is doing it all over the neighborhood, not only in your room. Your room just happens to be located at the exact right distance from the fan where a resonant peak of that wave is located, and triggers an additional modal resonance within your room. If your entire room were a little bit closer distance closer or further away, then it would be in a null, and you would not hear the sound. It's just the unfortunate distance that is causing the issue. It might well be that the standing wave in question also happens to excite a specific modal resonance in your room, thus amplifying that tone inside the room.
However, I'm also not convinced that what you propose would make much difference. Firstly, resonators are notoriously hard to tune, and standing waves occur at very precise frequencies where accurate tuning is crucial. So it is quite probable that your device would not be tuned to the right frequency anyway, unless you can figure out a way to make it so that you can vary the tuning after it is built.
Second, tuned resonators are used to treat resonant issues that come from sources within the room, not ones that come from outside it, and all they do is damp the resonance, not eliminate the note. So even if you did tune your device perfectly, the note would not go away: it would just resonate less.
For example if a speaker inside the room happens to excite a mode at a specific frequency then a tuned resonator could damp the decay of that mode, so it fades faster than it would have other wise, but it does not remove that tone from the room, which is the problem you have. So if you did build such a device, and it did work perfectly, all that you would accomplish is a reduction in the decay of the modal resonance within the room, but you would not kill the sound itself, since it is due to a source outside the room. In other words, you might reduce the resonance a bit, but the tone would still be there.
What you could do to your room to fix it, is to either isolate the entire room such that the sound of that fan does not ever get inside in the first place, or you could modify the room in some way so that the incoming sound no longer coincides with a resonant mode of your room. In effect, you could "de-tune" your room such that the fan wave no linger excites any resonance in your room. But that would still leave the original tone unchanged.
Isolating your room for such a low frequency would not be easy, nor would it be cheap. Ditto for changing the shape enough to de-tune it.
A third and better option would be to talk to the ownesr of the fan, and get them to damp the sound at the source by putting their fan on proper vibration mounts, or by adding some other method for killing the standing wave, or by de-tuning it by changing the speed of the fan slightly. Just enough that the standing wave will have a different wavelength, and no longer have a peak that coincides with the location of your room. If they don't want to do that, you might also have legal recourse to get a court order to have the fan shut down entirely, depending on what your noise local regulations say.
Overall, it would be far better to get the problem fixed at the source. Trying to fix it inside your room is going to be expensive, no matter what you do, and might not be as effective as you'd like.
One more option that I thought of: The wavelength here is about 8 m, so there's a peak every 4m, and a null every other 4m. Have you tried moving your bed a couple of meters one way or the other, to see if you can find a location where your pillow is in a null, instead of a peak? That might be the easiest solution of all. If you can find a position for your bed where your head is in a null, then you would not hear the note at all. It might be as simple as just flipping your orientation: put your head where your feet normally are, and your feet where your hear normally is. That would very probably move your head into a null...
- Stuart -