hello halfbakery, long time browser, first time poster here :) note i am from a small town in scandinavia so my english migtht not be the best, but i will try to get my point through ;) plus ofcourse i do enjoy the occasional herb, so i might be rambling a bit sometimes..

so, i am going to spend alot of time this summer in a caravan next to a barn (yes i am redneck..) to give me time to work on my two audis that are in the garage and need "maintanance" over the summer :) in this caravans front-tent i have braught my tv. to power this tv, and a microvawe and phone charger and so on i have a gas-powered(yees mr prius, i know i should have a solar powered polarbear-hugging set-up but they don't provide 220volts at a reaosnable cost.. plus i pay 2dollarsthirty for a quarter gallon of gas in my country so i feel the sting, be shure of that...) 2.2KW 220vAC 8.3A 12v generator. but this thing is very loud, so i have to keep it about 30m(100feet) away. i also modified the mufler and intake to make less noise, but i still have to turn my tv up loud just to hear myself think. i also put it a three-wall shed and put some wooden boards and some insulation on the wall facing the caravan, but it had only some effect. also tryed hooking up my 5.1 to the tv but then the generator revved up and made more noise. i turned the 5.1 up, it revved up etc etc... was never going to en up good that situation-.-

so my idea is basicly; can i just wire a car subwoofer using a car amplifier on the 8.3A 12v outlet on the generator and use the subwoofer to "cansel out" the vibrations of the generator? i could spare the watts if it would cannsell out some noise.. i was thinking of replicating the rpm of the engine with the subwoofer, so when the engine is revving 2000rpm the subwoofer could do the same(can a pretty normal svc 700w 10" do that??) and the subwoofer is mounted without a sub enclosure (or maybe with a port facing the exhaust/intake at a reasonable distance) thaught of trying it out tomorrow, if the weather picks up, but i still have no idea of what to use as a source of signal. i don't have a oscillometer or anything very fancy to do it with.. how could i get the subwoofer to replicate specific frequences? loop the signal wire around the spark wires? plug in a radio and use radio distortion caused the radio? the signal IN on the amplifier is a standard 2Xwhite-redRCA's, with the same engenious solution used as preouts

my idea being that a open-basket subwoofer mounted paralell to the gas flow of the generator(witch has intake one end->exhaust out the other) matching the RPM(or maybe double, maybe it will "break" the soundwaves?) with Hz will cancell out the soundwaves produced by the exhaust and intake by moving air around it in a confined space.
-- remzy, Jun 28 2011

Active muffler http://www.eetimes....d-to-diesel-exhaust
[mouseposture, Jun 29 2011]

Silent Mic Silent_20Mic
Less ambitious, but still flawed [marklar, Aug 25 2011]

Welcome to the Bakery/2, remzy.
Just how occasional is that herb?
Noise cancellation has been looked at hereabouts but maybe not quite like this!
-- gnomethang, Jun 28 2011


For a particular fixed location, it might be possible to cancel some frequencies of noise at the source, using a microphone and some inversion and delay circuitry, but likely you'd be far better off with a pair of noise-cancelling headphones.
-- csea, Jun 28 2011


//matching the RPM// This will only cancel the fundamental, not the harmonics, of course. So it won't do a perfect job of noise cancellation. But it might do a pretty good job -- who knows? Would be really interesting to try it & find out. But include 1. an audio equalizer (so you can fiddle with the frequency spectrum) and 2. some sort of variable delay (so you can fiddle with the phase spectrum) since you'll probably have to hand-tune the thing empirically for best results.

//How could i get the subwoofer to replicate specific frequences// Let the input be a microphone, placed near the exhaust; that should catch the harmonics, too. In addition to fiddling with the audio equalizer and variable delay, you'll also need to fiddle with the microphone location.

If what you want is peace & quiet, rather than an interesting project, then: what [csea] said.
-- mouseposture, Jun 28 2011


Ah, that's the life, rambling the meadows to pick wild thyme.

2000 rpm is 33 and a third Hz; if that plus a few harmonics were the main source of sound then your generator would sound like an electricity substation on a dry day - just a deep hum that can be felt more than heard.

You could try connecting a microphone to the sub-woofer and messing around with their placement relative to the generator [edit] like [mouseposture] said. Also try reversing the sub-woofer's electrical polarity. I don't think you'll get a dramatic reduction in sound, though.

What you really need is a heavy bunker-like arrangement, with sound insulation on the inside. Low frequency sound isn't stopped very effectively by light building materials such as wooden boards, or by thin layers of insulation.
-- spidermother, Jun 29 2011


//Low frequency sound isn't stopped very effectively by light building materials// Hoffentlich that means [remzy] only needs to cancel out the 33 & 1/3 Hz fundamental plus a few harmonics. Perhaps the soundproofing could filter out the higher harmonics.

Another approach to noise abatement would be some form of energy storage that would let you run the generator only some of the time. Maybe pumped water storage? You'd need a turbine.
-- mouseposture, Jun 29 2011


You make a good point; homemade active sound cancellation is likely to work best on lower frequencies anyway, as the timing and positioning don't need to be as precise. Generators produce lots of non-harmonic sound though (hence the term 'noise') at both low and high frequencies.
-- spidermother, Jun 29 2011


Put rubber feet on the generator and put sandbags around it. Solved.

Blind active noise-cancellation will only work if your generator is exactly consistent, if it burps or slows for a second (or to be exact, 1/66.666.. of a second), you stand as much chance of having the noise doubled as cancelled.

But 33.3Hz isn't the fundamental: that would be a sine wave... it's just noise that happens 33'ish times a second.
-- FlyingToaster, Jun 29 2011


<Pedant> Noise that happens 33'ish times a second _does_ imply a (possibly low amplitude) 33'ish Hz sine wave as one component of the signal, but neither guarantees nor excludes the presence of harmonics of 33'ish hertz.
-- spidermother, Jun 29 2011


wanted: spectral analysis of a running engine.

(what you say makes sense, but my car engine running at 2,000 rpm does *not* have a main frequency of 33Hz... of course it has a muffler, which helps)
-- FlyingToaster, Jun 29 2011


I think you'd do better to build a sound-absorbing shed around your generator (with real sound-absorbing materials - not just wooden boards and insulation)

- or get another generator and run them both at the same rpm but out-of-phase...
-- hippo, Jun 29 2011


If you are going to put the generator in a box, make sure that the box is made of heavy stiff sound reflecting material (mettle), to increase the sound levels inside. Then put the sound absorbing material on the out side.

If you really want to use sound cancelling, then perhaps this trick will help.
Use two microphones .instead of one, because with only microphone it will pick up the sound from the speaker, as well as from the engine, and attempt to cancel the cancellation. Reducing the overall effectiveness of the system.
The set up would be, on a line between the generator and the caravan the speaker is placed far enough from the caravan that that the sound waves reaching the caravan are nearly plain waves. Then place the two microphones on the same line between the generator and the speaker set quarter of a wave length of the lowest troubling frequency apart. Add a delay line to the microphone closest to the generator, such that the sound from the generator arrives at the electronics a the time. The sound from the speaker will arrive back at the electronics out of phase, so the speaker will not be trying to cancel its own out put.
This system has a few problems with it, and one big advantage. By the time it is all set up correctly, the generator will be so far away that you will not hear it anyway.
-- j paul, Jun 30 2011


TV, microwave, phone charger. Batteries, shirley.
-- methinksnot, Aug 25 2011


33hz is completely irrelevant, you are cancelling the sound of a contained explosion, multiple times per second. To prove this, reduce the RPM to 1,000. This will take it to 16hz, which is below the range of human hearing.
-- marklar, Aug 25 2011


I'm with those that say "Bunker" & "Sand bags". Also I add: "Hole".

Eliminate the noise as close as possible to the source. Fancy stuff is just not going to do it.
-- Ling, Aug 25 2011



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