I am not an engineer (if you are an engineer willing to help us pro-bono, please contact me), I’ve just crammed to figure out how to build an 8×10 safe room in our garage, at Sunset Beach, three miles away from the 12 existing 2.5 MW Kahuku Wind Farm turbines, so we have a place we can get REM and deep sleep at home rather than drive to sleep in our cars on the other side of the island when the turbines are on (most of the turbines have been off the past two weeks and Sunset Beach residents have noticed they are able to get quality sleep and dream (so even people who don’t have wristband sleep monitors know they’re getting good sleep – but how long will the turbines stay off… 1/26/2020 UPDATE: as of last night, Kahuku Wind Farm Turbines are back on in full force again). These calculations will work for Pupukea and Haleiwa – my guess is Kawailoa residents would need as much shielding as Kahuku residents. FEMA has a safe room grant program worth looking in to, if you are not in the tsunami flood zone. I am saving my receipts because I expect Kahuku Wind Farm to pay for our materials.
Below are the most important calculations I’ve found (sound transmission loss mass law and calculating resonance frequency of a double-wall construction). Additional considerations are that low-frequency sound easily bends/refracts around sharp corners, so all four sides, the bottom, the roof, and the door, need the same level of sound attenuation or you might as well not do the soundproofing. There can’t be a window; the door must have the same mass as the walls/ceiling and be air tight. Mechanical ventilation, like a fantech fan (exhaust fan in an air exhaust baffle with a a shaft for air to enter elsewhere, both with shafts that prevent the low-frequency sound from entering, with exhaust fan pulling the CO2-laden air from the bottom of the room – CO2 is heavier than oxygen, so the stale air will accumulate at the bottom of the room first) and a UPS power unit with an alarm if the power goes out, in addition to a battery-operated CO2 monitor/alarm or you will asphyxiate/die when you use up all the oxygen in the “confined space” of this type of sealed room – When you breath, your lungs take up oxygen and when you breath out, you breath out CO2, which is a waste product from your body. If you don’t push fresh air into your safe room and pull stale air out of your safe room, you will quickly use up all the oxygen in the room and when you breath in, you will be breathing in CO2 and you will die.) For obvious reasons, it is not legal for you to reside in this type of structure. Because the Kahuku Wind Farm will be decommissioned after its 20-year period (2031), our soundproof safe room will be temporary (my husband’s table saw is in the garage where the safe room will go – the sooner the wind turbines are removed, the better, as far as us getting back to our normal lives having full use of our garage is concerned).
See graph above and summary 1 or summary 2 online: Below the resonance frequencies of the various wall structures (each wall will probably be different, so you will have more than one resonance frequency where the structure will not block that particular frequency), stiffness of the wall is the most important factor affecting how much sound is blocked. To get a general idea what the ideal (mass-law region) sound reduction of your wall/ceiling would be, use the following math:
We may be able to build safe room walls/ceiling with ten layers of 2.3 lb/ft2 materials (five layers of Hardieboard siding on the outside and five layers of 5/8 sheet rock on the inside, all layers tightly bound to each other. We are caulking seams and pin holes with noiseproofing sealant to prevent air from moving through the walls. The total mass of the wall will be 23 lb/ft2 (which is 112 kg/m2). If 8 Hz transmission loss is controlled by mass-law, transmission loss would be 11-12 decibels at 8 Hz (so if you are at Sunset Beach or on Pupukea where the 8 Hz sound from the turbines is 50 dB, and you sleep soundly when 8 Hz is 40 dB, and you build this 10-layer safe room that should reduce 8 Hz sound pressure level to 38 dB, you are set (well, at least you won’t get dementia from the turbines).
In Kahuku Town, currently exposed to 63 dB of 8 Hz sound, they would need to block 23 dB of sound to achieve 40 dB interior sound pressure level. The 83 dB of 8 Hz sound Kahuku Town would be exposed to if the new Na Pua Makani Wind Farm 3.45 MW turbines are allowed to operate isn’t something I have any idea how to block (and I assume those will never operate because they would be too dangerous). Doubling the mass of the wall only reduces sound level by six dB because, in part, decibels is a log scale. To fit it in a reasonable area, you’d have to use much more expensive material than sheet rock – lead sheet 1/16 inch thick weighs the same per square foot as 5/8 sheet rock. I’ve read people living that close to a wind turbine go below ground to their basements to sleep. A few months ago I read somewhere that 1 meter of wet sand blocks 40 dB of 3 Hz (I can’t remember) – so that would have to be an external shed – I spoke to DPP about a 10×12 shed – they said the 10×12 is the inside floor space (which is important for this type of safe room structure with thick (1 m thick) walls. The DPP can look over your plans before you build if you want – you don’t need a permit for 120 sq foot shed, but they sounded like they could at least make a notation confirming the location (5-ft setback, I assume the sand bags can be in the “yard”) and size of the shed are legal.
If you decide to use double-wall construction with an air gap between two stud walls, there are a lot of important considerations including damping, filling the air space with fiberglass or Safe n Sound insulation, and calculating resonance frequency based on mass and air gap depth between the two faces of mass (never use three walls – Google triple-leaf effect). In this single-wall construction soundproofing project in our garage, we are damping between every other layer using Green Glue Noiseproofing Compound. The Green Gule Noiseproofing Compound is only important if any of the sections of wall or ceiling become a double-wall system (say if Na Pua Makani is allowed to operate). If you use a double-wall construction, carefully consider that if the wall is not damped (if the air gap and walls on each side resonate like a guitar) the possibility that sound pressure level may actually INCREASE above the level in your yard (at an annoying low frequency – listen to what 30, 50, and 60 Hz sound like on youtube if you decide to build a non-damped double-wall) by as much as 20 decibels, if your air space resonates at a low frequency (see resonance frequency calculation below, from Marshall Day Acoustics.