Simplest way to reduce low frequency noise from windows

Question

Could you please advise the simplest method to reduce low frequency noise from windows?

The problem:

1. Somewhere outside there is a source of noise with frequency of 166 Hz. I can’t neither find nor remove it. This could be ventilation hoods on other houses, factories or something else. Let’s take this as granted.
2. I have windows which consist of packets of 3 glasses each 4 mm of width which I can’t change.
3. As the result there is a buzzing noise in the room in 166 Hz frequency (and I see 49 Hz as well). The noise on these frequencies about 20-25 dB higher than on other surrounding frequencies.

What I tried to do:

1. I put shields made of expanded polyethylene about 2 cm thick and this didn’t help at all, low frequency sound passes through easily. Again, these are still windows, not good idea to cover them.
2. I put suction cups towel holders (made of plastic) on windows, to made it harder for glass to vibrate and it helped a little bit. They are not so massive, so effect is low.

Ideas:

1. Of course, I can buy weighty glass cubes and glue them to windows instead of suction cups towel holders, this could help bit more.
2. I can put extra pressure on these suction cups towel holders by planks attached to window frames.

But are there any better ideas how to cancel the noise from window on the specific low frequency?

Answer 1

Some thoughts on your problem: have a look at the physics behind "Chladni plates" - your window pane will act as a passive speaker at 166Hz but not in a "pumping" fashion like a passive membrane in loudspeaker. Most likely, there will be a symmetrical pattern of high amplitude areas. You want to take energy away from those areas.

This is how I would go about it: use a sharpie (or washi tape) to divide the pane into a regular grid of about 8x8 in squares. Label them: A1, A2, etc. ... have a 2nd person press their finger in the center of those squares with light but equal pressure while you have your eyes closed. Note the areas where you experience the highest noise reduction. Hang your mass from the top of the window frame and then use a dampening = absorbing polymer (PUR/Sorbothane, not silicone or rubber) and wedge/glue it between the mass an the high-amplitude zone(s). Alternatively, wedge a pole-dancing pole (or a background paper holder for photo studios) between floor & ceiling in front of the window and use this as an anchor point instead of the "mass"... don't forget to seal the gaps between window & frame...

Answer 2

From your troubleshooting it does sound like the window is resonating. Below is a good visualization of one potential vibration mode from the Vibration of plates Wikipedia article. The mode for your given geometry will be different but it gives you and idea of how the window can resonate.

Increasing the mass or stiffness at the current anti-nodes (the peaks that are moving up and down) will reduce motion in that area and change the mode of oscillation; typically to a higher frequency. If your window resonates at a higher frequency than the outside source it will reduce the transfer of that frequency into the room. Note that adding mass or stiffness at the nodes (the points which do not move) will make very little change to the resonance and not improve the situation.

One thing you could try would be to get some lengths of 5/8" aluminum towel bar that you could secure vertically along the window surface. It is a common product in many hardware stores and you can get it in longer lengths. Any stiff material will work, but condensation at the window requires low moisture absorption and corrosion resistance. Running the support(s) all the way from the top of the window to the bottom will greatly increase the stiffness in that area; essentially turning the bar into a node. To adhere it to the window I recommend using 1/2" double sided tape all the way along the length. It will be difficult to get on and off, so for initial testing, just remove a bit of the tape backer from each end to temporarily hold it in place. Then you can play around with different numbers of them and different positions. Then when you like the result, remove all the backer and stick them in place.

Good luck! Let us know how you ended up solving it.