Mitigate Low Frequency Vibration
prof_braino
Posts: 4,313
Mitigate Low Frequency vibration
Here's a weird one:
I've encountered a phenomenon similar to the Hum: http://en.wikipedia.org/wiki/The_Hum
"The Hum is a ... collection of phenomena, .... of a persistent and invasive low-frequency humming... noise not audible to all people... not been traced to a specific mechanical source."
In my case the source has been traced to an air handler 1 km from the reporting site. The dominant frequencies appear to be 252.8-253.1 hz, and/or 124.8 hz, depending on measurement location. If I measured correctly the sound is 60 dB at about 50 meters from the source.
Also, when the sound is properly identified individual people, all do in fact hear it. (Where as minutes before they responded they could not hear anything, and the original complaint was crazy or halucination). Furthermore, once an individual correctly identifies the hum, they continue hearing it, and notice it in more frequently in more locations.
The audio guy told me the rule of thumb is 600 Hz is middle, above is high frequency, below is low frequency. High frequencies tend to behave like light, they can be blocked by a barrier; low frequencies tend to behave like wind or water, they flow around obsticales and need a much larger barrier to block them. The effect is greater as we get farther from 600 Hz.
So we are talking low audio frequency like a sharp B2 or B3 on piano. In the daytime, the ambient noise covers this hum, and it is not noticed by most folks. In the still of the night, it becomes the dominant noise in the environment, but is still very low level and usually not noticed.
However, in a very quiet room, most or all amibent noises of other frequencies are blocked by the structure. In this case the hum is becomes significant. Due to the wavelength, distance and environmental factors like wind and geometry, the hum can fade in an out over the course or one to several minutes, from dull sine wave to raspy triangle wave. The affect can be like the "chineese water torture".
The room in question is in the center of a long wall with one window. It appears one or more external walls of the building is/are acting like an open air column and resonating with this hum, resulting in an antinode at this room.
Two inch solid foam was applied to this room's window. While the hum was dampened by about 25% (not accurately measured), other noises were damped more, and the result was the hum became MORE noticable.
After limited success with passive noise cancelation, I was considering using a prop to to do active noise cancelation; but the the audio folks said low frequency tends to be problematic.
So. How would one go about mitigating this vibration?
Thanks for all input and ideas!
Here's a weird one:
I've encountered a phenomenon similar to the Hum: http://en.wikipedia.org/wiki/The_Hum
"The Hum is a ... collection of phenomena, .... of a persistent and invasive low-frequency humming... noise not audible to all people... not been traced to a specific mechanical source."
In my case the source has been traced to an air handler 1 km from the reporting site. The dominant frequencies appear to be 252.8-253.1 hz, and/or 124.8 hz, depending on measurement location. If I measured correctly the sound is 60 dB at about 50 meters from the source.
Also, when the sound is properly identified individual people, all do in fact hear it. (Where as minutes before they responded they could not hear anything, and the original complaint was crazy or halucination). Furthermore, once an individual correctly identifies the hum, they continue hearing it, and notice it in more frequently in more locations.
The audio guy told me the rule of thumb is 600 Hz is middle, above is high frequency, below is low frequency. High frequencies tend to behave like light, they can be blocked by a barrier; low frequencies tend to behave like wind or water, they flow around obsticales and need a much larger barrier to block them. The effect is greater as we get farther from 600 Hz.
So we are talking low audio frequency like a sharp B2 or B3 on piano. In the daytime, the ambient noise covers this hum, and it is not noticed by most folks. In the still of the night, it becomes the dominant noise in the environment, but is still very low level and usually not noticed.
However, in a very quiet room, most or all amibent noises of other frequencies are blocked by the structure. In this case the hum is becomes significant. Due to the wavelength, distance and environmental factors like wind and geometry, the hum can fade in an out over the course or one to several minutes, from dull sine wave to raspy triangle wave. The affect can be like the "chineese water torture".
The room in question is in the center of a long wall with one window. It appears one or more external walls of the building is/are acting like an open air column and resonating with this hum, resulting in an antinode at this room.
Two inch solid foam was applied to this room's window. While the hum was dampened by about 25% (not accurately measured), other noises were damped more, and the result was the hum became MORE noticable.
After limited success with passive noise cancelation, I was considering using a prop to to do active noise cancelation; but the the audio folks said low frequency tends to be problematic.
So. How would one go about mitigating this vibration?
Thanks for all input and ideas!
Comments
Yes! Applying it to the window would lower your noise floor as that sound is being emitted right along with the world outside. You might try walling off a bigger area of the room. There could be reflections and resonances based on the dimensions of the room and the wavelength. On that note, you could make the room smaller by enclosing part of it too, using blankets, or something. Maybe a 2 meter box around you would work well. Do the floor, ceiling, sides, etc...
Too bad it isn't lower. My answer would be to insulate yourself. Put your chair on dampening materials, etc... and get rid of the lower body / bone conduction.
But this is high enough that probably won't work well. Try it though.
Because of the longer wavelength, you get partials all over the place. Your room could be resonant actually. If it's overall dimensions are a multiple of that wavelength, you've got an amplifier in the room itself.
To understand this, go into your shower. Hum softly, increasing pitch, until you find a point of resonance. At that point, your soft hum will boom quite loud. You can't miss it. Hang a big blanket in part of the shower and do it again. Note the change. Move that blanket, and note that. Apply to your room problem, taking the wavelength into account.
This sound is roughly 1.5 meter wavelength. http://www.hottconsultants.com/techtips/freq-wavelength.html
That's long, and the dampening won't work well, though it did probably attenuate the source, being the window as an emitter. It probably did not do much to eliminate secondary resonances from your overall room dimensions.
Try moving around in the room. You might have quiet spots. Do this with and without your dampening in place, or somewhere else.
Try a couple of large baffles near you. Use cardboard. You might be able to make a quiet spot by causing the sound to resonate and reflect off the cardboard and cancel itself out too. Try them a few meters apart, and close the gap. This won't be very big though.
At that frequency, there are standing waves in the room. Imagine a grid of dots about a meter apart in all dimensions. That's sort of where the standing waves are, ignoring lots of little details. This is why noise cancellation is a problem. You might be able to combine a baffle and an out of phase sound you generate to make a quiet spot of sorts, but the cancellation needs to phase match the sound at your ear, and the large distances make that problematic.
Personally, I would just raise the noise floor 10db at a time and not worry so much about it. Does a small fan help? Hey, maybe you can enjoy some great electronics related podcasts? I'm sure listening to http://www.theamphour.com/ can do some damage on this. If your player has EQ, bump the 100Hz a little to make sure the content and it's noise is nicely centered on the thing you don't want to hear.
Make people aware. Complain. Resort to the law if you can.
Problem is the significant volume of air in motion. It's not loud, but there is a lot of it. Has to be to travel 1Km at 60db source. Sheesh! Traffic in the street can exceed 60db for a reference.
What he's getting is in the maybe 20db range. That's pretty quiet. And again, the law is tough. They call out some basic metrics, and a single source like that just stick out, but won't violate ordinances and such. Some natural noise floors are higher than that.
There are a plethora of speakers that attach to windows. Maybe you could use one of those with a Prop for sound canceling. Using the entire window as a speaker should answer the problem of the frequency being too low for traditional sound-canceling techniques.
-Phil
An electronic solution of noise cancellation is likely to require you to wear earphones while sleeping. So the earplugs are a better solution. There are earphones that do cancel ambient noise, so that music comes through more cleanly. But the lower the frequency, the harder it is suppose to be to cancle noise (more energy in the low frequecies).
If you are really desparate and can't find regular ear plugs -- grab someone's pack of filtered cigarettes and break off two clean filters. They will work nicely in a pinch, even if you are intending to spend the afternoon working with a jack hammer.
Dampening the sound in the walls and the windows can get expensive. Foam is the wrong material, you need mass and lots of it. So triple glaze the windows and re-sheetrockober over the existing walls with 1/4" of lead foil under the new sheet rock. Things will quite down, but you will be out of pocket quite a bit. This is the kind of stuff that recording studios do to shut out the world.
Higher frequency sound can be dampened by absorption into foam. The foam is a minute lattice that is mostly open formed when manufactured by releasing billions of tiny air bubbles into the melted foam. As the material cools the bubble breaks and forms a skeleton like structure. Sound, a type of energy, goes into the material causing random vibrations flexing the material converting the sound into heat.
Lower frequencies need heavy, semi-solid sound dampening material that requires more energy to make it move. An example is the thick matting in your car attached to the firewall, then covered with carpet.
Low frequency sound can easily go through a wall. The outside wall has a hard connection to the studs, which are then connected to the inside wall. Sound can be stopped by offsetting studs such that theres no hard connection between outside and inside walls. 253 Hz or 124 Hz might easily go through a wall. The resilient channel is another possibility but would require the inside sheet-rock removed and reinstalled on the channel. Your outside walls act as a woofer.
I agree with Heater, the best fix is at the source. It may be a simple mechanical problem, the air-handler manufacture may have some insight.
A local company that manufactures microprocessors had a problem with a sick room. There was a certain room in one of their large production buildings that, when occupied, people would get headaches and nausea. As soon as they left the symptoms went away. Eventually it was traced to a roof-top HVAC unit with a broken mount. The extremely low frequency HVAC vibration turned the roof into a giant speaker with little ants inside.
The foam earplugs were my first experiment. They significantly reduce the ambient noise, but only slight affect the "hum". The net affect is that the hum is more noticable.
Management did not accept this experiment.
One long experiment was to lay on a matress in various orientations. No affect.
This was observed. There were loud spots, and louder spots. But not necessarily where I would expect a node, and no quiet spots.
QUOTE]Try a couple of large baffles near you. Use cardboard. You might be able to make a quiet spot by causing the sound to resonate and reflect off the cardboard and cancel itself out too. Try them a few meters apart, and close the gap. This won't be very big though. [/QUOTE]
This was on the list, but I didn't get to it as the result would be a little messy for the room.
I used a small radio tuned to static. The hum remained clearly audible, it cuts through. At the level needed to drown out the hum, it was too loud for the purpose (sleep).
There were a couple other in-room passive experiements. The one that appeared to have the best affect was 2 inch foam on the inside AND outside of the window; and leaving the door open to allow in the ambient noises of the rest of the house - computer fan, refrigerator, clocks, etc. But the hum remained "too annoying for sleep".
This is the direction I was heading. I will continue to look for something off the shelf as a solution or a starting point. Now I have a bit of breathing room before the eventual "next time".
So, this was the avenue with the best result. It took more care and effort that the other experiments combined, but was quickest and least expensive dollar wise. Its kind of a long story, I'll try to be brief...
If this is the kind of situation you have, it may require seeking support of local governments to remove the problem. Studying what San Francisco did may help. Seattle is trying to now dig a huge tunnel to similarly get rid fo the above ground din, but they have fallen way behind due to the vast scale of the project and failure of the drilling head.
So I can understand the desire to overcome such a nusance sound. People are burdened by such background noise and flee such areas. In many cases, the residents in an area have gotten so used to coping with the problem that they are not even aware it is there. They just tend to dislike being outside, and seek the indoors to get relief from the din.
+++++++
Other than getting rid of the source, all the alternatives are somewhat complex and dubious, unless you can simply build a concrete bunker from the ground up.
You might actually consider adding white or pink noise just to mask it. That could very well be the cheapest and most reliable solution. And having music playing 24/7 at a low volume might be acceptible.
The active audio solution is to sample the noise from a mike on the outside of the building and to produce an audio that is 180 degrees out of phase with the offending noise. This is NOT digital technology -- it is analog. You merely need to have a pre-amp that can reverse the phase of the noise, and an audio power amp driving appropriate speakers.
I am not sure putting speakers on windows will produce good results. Yes, windows tend to be where a greater volume of the offending sound enters from, so do open doorways. But I actually rented a house in Kenewick, Washington that was situated in a high noise problem (18 wheel trucks down-shifting and they entered town), and the landlord has solved the worst of the problem by installing heavy glass storm windows over that side of the house.
Domanik is right about creating a special staggered stud wall. This is often done in building duplex rentals on the common wall so that the neighbors don't hear each other. The common practise is to have the walls actually be two separate diaphrams so that sound moving one side never gets to the other. Also, it is common to cut a slot in the flooring in the gap between the two walls so that footsteps will not be heard from one unit to the next. In some case, the gap must be filled with fire retardant foam to serve as fire block -- otherwise a fire in the floor below might travel quickly to the floor above the slot.
Called police non-emergency at next event, about 3 AM. Car came out, officer could hear the sound. I pointed out that the sound had not be noticed before, something may have changed in the last month, perhaps the unit needs service. Officer said another squad car in front of factory could NOT detect sound. I clarified that the machine was in back, on the side of building facing our location.
Next evening, called called in next event, about 4 am. Desk officer refered me to "officer's advice from last night" (appearantly to Smile-off). I requested clarification as to what this "advice" was, as officer had supplied none. Desk office said he would send another car. Car came by, smiling officer said that while he could hear the noise, there was nothing he could do, and no-one else has ever complained in 20 years. He indicated the police handle "dogs and parties", and this was not that. I asked about the ordinance specifically stating commercial & industrial noise beyond property line 7PM to 6AM, how this situation did not apply. Officer advised to contact Village Office of Community Development.
Later that morning, 6 AM, desk sergeant offered to stop by and clarify the situtation. He stopped by the factory, determined which machine was creating it, to the extent of which switch controled that machine. Verified that flipping that switched cause the noise to start and stop at my location. Reported that factory official would contact management. Officer requested that I please delay contacting Community Development. Repeated that in 16 years, out of hundreds of diverse noise calls, he had never experienced any complaint on this factory.
Stated I could not afford wait indefinately for some action. We agreed that we would give them two weeks to access the situation and determine a course of action. I added that my next step would be to organize the residents, as it was unlikely that my hearing and situation could be exceptionally sensative.
Seargant then asked if I had previously reported this disturbance six months ago. I said no, noticed it about 4 to 6 weeks ago. Seargant stated a similar call had come in six months ago. (Obviously statment "no calls on this factory in 16 years" should be revised to "no calls correctly attributed to this source in 16 years. Some percentage of the 'hundreds of sound calls' were exactly related to this factory".)
At 7 AM that morning (presumably when management arrived for work) the hum stopped. The sound levels returned to those experienced prior to first notice of the hum six weeks ago. It seems the unit need not run 24/7 for the succesful operation of the business. My guess is this is a well known issue, somebody forgot to switch it off after a run, but nobody notice due to the normal machine noise present in the factory.
I have also learned that there has been at least one previous attempt to organize the residents on this issue. I am contacting those folks to better organize action when needed.
Summary: While arguing with large men carrying guns is risky and uncomfortable, in this case it may have paid off. Preparation and persistance were the key factors to success. Future preparation includes work on detection and active noise cancelation for low audio frequencies.
Well, done.
Recently I was at an international Traffic conference. One presentation given there was on the subject of "Big Data" the "Internet of Things" with relation to vehicle traffic. An example was an organization building a sensor board for regular people to keep outside the houses, constantly measuring pollution and noise among other things. This had already been used successfully to support residents complaints about noise due to an airport runway change.
I'm glad you got a good result. My personal experiences have been less than good in the past.
When I last lived in San Francisco, it seemed that all noise complaints went to a message recording machine that was staffed by one person that worked 8 to 5, 5 days a week. Calling the regular police would get you a referal to that number. In other words, noise complaints for the entire city were pretty much ignored unless you knew somebody or had a knack for pushing to the top of the list.
It would seem to me that if an industrial zoned area was the source, it might be very difficult to seek noise reduction that is not really a hazard to anyone.
Taiwan is noisy 24/7 and I live next to a major hospital, so I get the sirens at all sorts of hours.
Unfortunately, the police are too busy even in my tiny town, and they cannot give proper attention to a call unless the complaint has lots of supporting evidence, and the complainer is peristant. Obviously mine was not the first complaint even though they claimed otherwise, the office did not put 2 & 2 together (or did not admit prior calls) until we began discussing my options for escalation. And he specifically got me to agree to wait until things can blow over (and hopefully I forget about it) before further action is taken.
I was careful NOT to contact the factory directly, this has to wait until its "their idea" to take action and/or talk to me. They don't know me from Adam, and I did not want to give any reason for them to claim some sort of harrasement or tresspass. Also, people often take any type of inquiry or attempt at change as a personal attack (as it gives evidence they are not perfect already). So now comes the interesting part.
I think I have to wait for two weeks, contact Community Development, and ask for follow-up on how this is being resolved to close the loop. As long as all parties conclude that its cheaper and less effort to prevent future recurences, they will drive improvements from their end(s) instead of blocking.
Rather than complain go to Mr Factory-Owner with a plan. "I can save you X dollars per year by shutting down <whatever noisy machine> when it is not required". All I want is 10 percent of the saving.
I doubt that my complaint was the only one. It seems more likely that my complain was the only one correctly associated with this factory. Three separate police officers tried to convince me there was no issue. Only when I told them the exact name and address of the factory and the identified the specific machine, and asked about options for escalation, did they begin to address the situation.
I intend to demonstrate that calling the factory ("Hey, check if you left the machine on again, and please turn it off") at the first call would be cheaper and easier than sending three squad cars to my house.
I'm going to look into de-coupling the inside wall. This looks right.