Towards a better tinyhouse

Inventing to freedom?

How to get fresh air without the noise

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An essential feature of a good home is that it provide adequate soundproofing from the exterior environment (given the context). This is one of the problems with RVs which is overcome with tinyhouses.

Another essential feature is adequate indoor air quality, which usually entails fresh air (although I once looked into air purifiers and plants which consume air pollutants, leaving clean air, maybe more on that in another post).

Obviously these are in conflict to some degree, which is why people often do not open the window as much as they would want to be able to, because it lets noise in. Plus blocking noise while allowing airflow is an issue when there is a ventilation fan too…

So the challenge is to get fresh air in from the outside without letting noise in somehow. In a normal house this is basically accomplished, to what degree it is (often inadequate) by having long ducts, in which the sound reflects around many times, each time reducing the volume of the sound.

So anyway, looking around for a product that would do this sort of thing I found the “silenceair” thing, various motorized vents (built in fans), some that are embedded in the walls (“acoustic vents” see also “acoustic ventilation”), and some made to be embedded in a window (“trickle vents”, some are acoustic some are not though).

I think the trickle vents are the most interesting. They are fairly cheap, about 10 pounds, which I think is very roughly $15 for a 8000 mm2 free area unit (free area is not the same as equivalent free area, it is an old measurement inherited from the days when building codes specified the free area, basically the cumulative area of all holes and cracks, in a building wall that would be expected to allow adequate ventilation. I tried to get the details but could not google it up, the standards or sections of the building code where the official measurement methodology is described, but you have to pay serious $$ to see them, however fortunately most reputable mfgrs such as greenwood airvac also seems to provide at least some flow rate vs. pressure information in the product documentation, for example). Two to four seems like it would suffice for an on-grid tinyhouse, more for off grid because you have lower pressure. Also, there are some models that have some provisions to try to automatically regulate the airflow when the pressure varies, as it does if you are trying to use the wind to provide all your pressure (which is cheaper than using an electric fan if you are off grid)…

Okay, and now for some stuff that I gave up on editing and writing because it was taking too long, so it’s just here as-is for anyone interested:

There are several different ways that I have been able to find to try to attenuate the sound signal while allowing air through, there is the side pipe thing in mufflers, there is the sabin attenuation method thing which uses fibrous material on the walls of the duct that helps increase the fraction of sound energy that gets converted into heat at each reflection off the side of the duct (and you can make a section of duct that is designed to force the sound wave to bounce around a lot too, increasing they amount of attenuation per unit duct length).

Actually if you think about it, if you have an sound attenuator that reduces, say we are only talking about a single frequency of sound, by a certain number of decibels, say 20 dB, you divide the sound wave energy by 100 of course. Okay, then if you put 2 such attenuators in series, ignoring any vibrations that managed to sneak around the attenuators due to real world realities etc. you would have 40 dB attenuation, an the resulting attenuator would be twice as big, twice as heavy and twice as expensive.

Then, if you want to have more airflow, same deal. Put 2 in parallel and, with the same air pressure differential, you get twice the airflow.

So you could, for any attenuator you could calculate a sort of specific performance which you can use to compare various units of different size etc. e.g. a liter-per-second/kilogram of attenuator, at several test pressures, like 20, 50, 100 pascals, at a variety of test frequencies, to achieve several dB reductions.

For the duct and fibrous stuff what is often done is you use flexible insulated duct, bent into a sinuous path and then add some reflective surfaces to increase the number of times the sound wave passes through the fibrous stuff between the input of the attenuator and the output . Flex duct not because it is a particularly good thing to use but because it’s performance is already known.

But my investigation of doing it this way, supposedly you get 3 dB(a) reduction per reflection with insulated flex duct, and if I wanted the equivalent of 3600mm2 (more on that later) Equivalent Free Area (27 lps at 50 pascals) and 60 dBa reduction it would cost about $320 worth of duct, be the size of a fridge, and weigh almost 2 metric craploads (~65 kg). Trickle vents seem to be way way better, maybe $15 or so.

Visco-elastic damping is another way to try to reduce the amount of sound reflected off of the duct surface at each reflection (see also “constrained layer damping”).

You can also force the air through the fibrous material (might also be able to use the fibrous material as regenerator media to do heat- (or energy-, which includes latent heat) recovery ventilation…).

You could have a bellows like mechanism that inhaled air from outside, closed a soundproof door/valve, opened a soundproof door to the indoors, then exhaled it, so at no point is there any continuous low-attenuation path for the sound waves to travel from outdoors to indoors, i.e. there is always one closed valve along the path from outdoors to indoors.

You could try to reflect it back from whence it came, using carefully shaped baffles.

The “silenceair” thing sounded like it used the muffler approach, in which a main pipe has another piece of pipe sticking off of it, sealed at the end, which causes the sound wave (of freq x) to be reflected back into the main pipe precisely out of phase, which works remarkably well at freq x and nearby freqs. So take several tuned to different frequencies and put em one after the other.


Written by gregor

April 7, 2011 at 05:37

Posted in Uncategorized

One Response

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  1. How to get fresh air without the noise | Towards a better tinyhouse

    September 28, 2013 at 18:41

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