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Mechanical noise - A/C and radon fan standing wave hum

  1. Jun 8, 2015 #1

    I am moving into a new house, remodeling, etc. & am installing a new A/C compressor and radon fan. Being that low frequency noise highly bothers me & affects my insomnia quite a bit, I am trying to look at all considerations regarding acoustics, mechanical sound propagation properties, standing wave issues, etc. with regards to the fan locations & models.

    I will share the work I've done so far with my physics major friend - it has been fairly easy to determine the fundamental frequencies that these two systems generate. I'm not sure how to figure out the amplitude in dBLs or otherwise, nor the more complex mid range and high frequency harmonics that create rattle, etc., however it is really the fundamental frequency (the low freqs) that I'm primarily concerned with.

    What is a major mystery to me is how to determine with any degree of accuracy how these low frequencies will resonate within the walls, creating unique standing wave patterns that I can't predict.

    I will be living in a house with brick veneer, but of course the guts are still wood, and wood has this resonant quality to it, like a tuning fork being constantly struck by mechanical hums - so I wonder about mitigation of this tendency.. (sound moves much faster through wood and concrete than air). How do I determine how it will propagate through my house? (through wood, concrete, sheetrock, ceilings, floors, attics, closets, defined spaces, etc?) I also don't know how the decibels fall off over distance because of this resonance problem; it adds a bit of a wildcard to the mix. Are there other considerations I haven't thought about here? I do know that parallel surfaces are the worst acoustic problem, that sound waves move in a sphere, and orthogonal angles should be considered.

    This stuff isn't installed yet obviously, so I can't just bring a microphone and try to determine these things practically, it's all theoretical at this point.

    The main concern is the location of the A/C compressor - if I put it in the most aesthetically pleasing and logical location, it will be the farthest from my bedrooms, however it will be in a prime place to propagate LF sound through parallel surfaces and resonate into said bedrooms (it will be located on a concrete platform on the opposite side of the house). If I instead install it on the side that is perpendicular to the orientation of the bedrooms and aforementioned parallel surfaces, technically the compressor will be closer to one of the bedrooms, but it will not be making noise through parallel surfaces, nor will I likely be in the 'line of fire' of the standing wave it generates. The standing wave would more likely fall in that case to somewhere in the front of my house, the living room or garage. It has a relatively low RPM speed, which I suppose is why air conditioner compressors make so much annoying low frequency noise and can propagate such a long distance.

    If I install it in the location where it will be oriented in a parallel fashion, I have thought about putting old mattresses in the garage on the walls to mitigate A/C noise, as a sort of baffling, or maybe even using rubber or another type of foam since sound moves the slowest through it.

    The radon fan looks as though it is higher frequency than the A/C compressor, and will be far enough away from me on the other side of the house to not experience the LF standing wave frequency that it creates, in my bedroom anyway.

    We determined the fundamental frequency both of these generate from looking at the fan RPMs. I bet fan radius and blade shape also determine plenty about the sound of the fans, as do other factors I've probably forgotten about, but wouldn't the most annoying aspect of the mechanical noise (IMO the low frequencies) probably be determined by the RPMs? I'm hoping we're right on this one..

    Trane XR13 A/C compressor outdoor fan

    fan RPM = 825

    13.75 Hz = fundamental tone

    wavelength of sound propagation =

    81 ft (halve this to get harmonics)

    40.92 ft - distance of 2nd harmonic

    27.28 ft - 3rd

    20.46 ft - 4th

    16.37 ft - 5th

    13.64 ft - 6th

    11.69 ft - 7th

    10.23 ft - 8th

    9.09 ft - 9th harmonic

    Fantech HP 190 radon fan

    RPM = 2541

    42.35Hz = fundamental tone

    wavelength = 8.1m

    26.57 ft

    13.28 ft - 2nd harmonic

    8.86 ft - 3rd

    6.64 ft - 4th

    5.31 ft - 5th

    4.43 ft - 6th

    These were determined in Wolfram Mathematica via wave equations.

    Much thanks in advance for your help!
  2. jcsd
  3. Jun 9, 2015 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    It is impossible to determine something with any degree of accuracy - instead you have to say how accurate you want to be.
    Without this information we cannot help much.

    You want to explore the relationship between resonance and damping.
    You need to isolate the vibrating machine from the objects you don't want it to vibrate and engineer the dimensions to avoid resonance.
    Bear in mind: you will always get some noise.

    Big fans are quieter than small ones - but make more noise when they are off-balance: it's the wobble that does it.
    All fans will develop a wobble over time. In the end it's a matter of specifying "quiet" to the vendor (unless you want to build one to spec) and hope for the best.
  4. Jun 16, 2015 #3
    isn't there a way to negate harmonics with another sound source aimed at the original source creating the harmonics? like a white noise being projected at the offending sound. I assume the wobble effect is not the cause but the natural sound a fan makes cutting through air. if not then simply cleaning the blades and oiling the axle should significantly reduce noise.
  5. Jun 16, 2015 #4


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    Staff: Mentor

    Welcome to the PF.

    I helped a friend with some construction work on a new house he had built for him and his wife a few years ago. He wanted to add some extra soundproofing to several rooms, so we installed some "sound board" (or something with a similar name) material before the sheetrock was put up. You might consider doing this as you remodel the bedrooms and maybe even the living room:

  6. Jun 16, 2015 #5
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