Outside my bedroom window, there is a store air conditioner which runs 24/7. It's noisy in general, but closing the window takes care of most higher frequencies. My issue is with a specific low frequency (around 50hz I think) which obviously does not yield to my windows or curtains.

Now, when sleeping with my head against the back wall of the bedroom, this low frequency noise is very noticeable and is generally an issue for sleeping well. However, if I turn around and sleep with my head towards the center of the room the noise is barely noticeable.

I accept my lack of options in regards to blocking the noise getting into the room, but I'm wondering if there is a way to mitigate it even when sleeping closer to the wall. Would a acoustic foam on the wall by the top of the bed help at all?

And in general, is what I'm dealing with here the waves reverb in the room? do reverb solutions (foam, soft surfaces, etc.) help at all with such low frequencies?


  • At all, yes. Much, well, not so much. Low frequency is difficult.
    – Ecnerwal
    Dec 19, 2014 at 23:47
  • 1
    Quarter wave baffle curtains hanging a foot or two down from ceiling? sengpielaudio.com/calculator-waves.htm That's about every 1.7 meters. Get a spectral analyzer App on your cellphone to tell you the exact frequencies you're dealing with. Low freq attenuation is hard. Dec 20, 2014 at 12:42
  • @WayfaringStranger Sorry, I don't get your meaning. What are "baffle curtains" and why quarter wave?
    – roded
    Dec 20, 2014 at 21:32
  • Example of ceiling mounted baffle curtains here: amcraftindustrialcurtainwall.com/products/acoustic-curtains/… Hanging them 1/4 wavelength of the frequency your trying to get rid of will make them more effective. Looks like lots of good info on these and wall panel attenuaters at Google here: google.com/… Dec 20, 2014 at 21:47
  • I had a grammar blackout w/ your sentence. Thanks.
    – roded
    Dec 20, 2014 at 22:05

2 Answers 2


Read about bass traps. There are all different types; you can build your own or buy one pre-made. It sounds like for your situation, a correctly tuned Helmholtz resonator will accomplish the task in a small package. It's supposed to be cheap and easy; try this calculator.

You can determine the frequency of your noise with this app or similar. Make sure that you see a clear low frequency peak because your phone might have a high-pass filter. If you don't see one, then try putting your phone against the wall or against the AC unit, or try another phone/tablet/etc.


The lower frequencies have wavelengths that are much longer than the thickness of the wall and any padding. To reflect these sound waves, you can cause an impedance mismatch by increasing the mass at the wall.

Can you put a bookcase there?

  • Yes, I can try.
    – roded
    Dec 20, 2014 at 6:45

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