Controlling Noise Pollution

Sound is transmitted by shock waves in the air or shock and vibration in solid and liquid. Our ears detect the most common noise or sound that is transmitted via air impulses. Rooms are made "quieter" with acoustical tiles and other usually porous or soft materials (with a high void volume) that prevent much of the noise impulses from being bounced back. Dead air between panes of glass is good insulation and works for sound also. The short answer is yes. Look for key words acoustic and insulation.

Yes, you buy a good pair of ear plugs or you can kill whatever's making the noise.

What, exactly, do you want to know?

Do you want to keep outside sound from entering a space? or do you want to make a test space with no outside sound AND no echoes(anechoic chamber)?

An anechoic chamber keeps outside sound out and mimics a space with no walls to reflect sound. Those who have been up in a balloon, while the flame is off at 1000 feet will know the non echoing effect I speak of.

They usually cover the walls with foam cones of graded density so that any sound ends up being infintely baffled and absorbed. They also make these for microwave testing.

To sound proof a room from exterior sound requires the room space be decoupled from the outside in such a way that low freqencies cannot excite a wall, and through the studs make the inner wall vibrate. This takes a dual wall and roof and floor to do a good job. The outer wall can be absorbent as well, depends if it is exposed to the weather or not. It can be hard skinned as long as it is decoupled from the outer wall by an absorbent layer. Often lead foil is used in several layers as you make these walls. In between the inner and outer walls you require a graded density absorbent, say 6 inches. This can be foam or fiberglass of the density needed. You grade the density for max attenuation. You can buy the walls built, but it gets costly.

These links to seaches will give you some ideas if you drill down

http://www.google.com/search?hl=en&q=%22anechoic+chamber%22+%2Bconstruction&btnG=Google+Search

http://www.google.com/search?hl=en&q=%22sound+proof%22+%2Bconstruction&btnG=Search

Guest did not say what sort of noise he is trying to reduce, so for all you noise experts out there, I will add my request for advice on reducing motorway - highway - traffic noise.

We live about 3/4 mile or 1 Km from a major motorway and the road / tyre generated noise is continuous. Unfortunately the road is raised on an embankment over the flood plain so we have direct line of sight to the vehicles.

New resurfacing of one lane has helped a great deal but is there anything I can do to reduce the noise transmission from the tyre and heavy truck - we allow up to 44 tonnes here - gearbox noise.

I am thinking of planting trees, but that is a long process, so what about a kind of fencing - any ideas?

Hugh Mattos

Chartered Engineer

Build your own embankment at the end of your garden? even 1 floor high will cut most of the sound from downstairs.

Those links that aurizon gave earlier should give you some ideas. Double walls, sound absorbing materials are all feasible. You could also try active sound cancelation but that would be a bit difficult to fine tune.

Planting trees, while a good green idea, won't provide enough attenuation unless you can make them real thick. Besides, when the trees shed their leaves, during winter, you're back to zero.

"Planting trees, while a good green idea, won't provide enough attenuation unless you can make them real thick. Besides, when the trees shed their leaves, during winter, you're back to zero."

http://en.wikipedia.org/wiki/Leylandii would appear to be a solution.

PWSlack,

Yes that would be my choice for a quicker dense attenuation, followed by other species which grow more slowly, but which can replace the dreaded Leylandii in due course

The article says:

In Britain it has been the source of a number of high profile disputes between neighbours, even leading to violence (and in one recent case, murder)

The sap can cause skin irritation in susceptible individuals.

'Sounds like a dangerous plant to have around .

Then plant some fast growing shrubs on the embankment, the denser you can make them the better your insulation will be.

2 or 3 layers of solid lightweight bamboos- close spaced -4 meter high, will block 10 dB more than 1 meter thick live foliage. Foliage may never reach high enough.And what do you do in Winter/Fall when leaves are gone.

Treat the bamboos with your local permitted anti termite Chemicals at bottom for many years' life.

noise can be cancelled by subjecting the opposite wave forms? true?

Yes it can and I recall that Lotus Engineering did a great deal of work on this for silencing the interior of cars by delivering an "anti noise" wave to counter the existing noise.

Not successful because of the "need" for a sports car to sound sporting not to be silent.

However, the chances of using this anti noise to counter the noise of passing traffic some 1 km away seem remote.

Hugh Mattos

Chartered Engineer.

Yes, there are a number of noise cancelling headsets in use by military and other pilots. It can be done for any space if you want to spend the time and money doing it, but it is easier with headsets. Some cars have it I hear??

In essence you have microphones around the zone you wish protected and in real time before the sound has propagated inwards you calculate a composite wave to a number of speakers that are closer in towards the zone you wish protected that match frequency, intensity and phase to cancel each peak with a trough 180 degrees out of phase. As you can imagine a complex real time task.

Cancelling traffic sound 1 KM away is a lot simpler than random sounds from all around.

The traffic will be mainly lower freqencies and you can set up a picket of microphones and 200 yards closer your picket of speakers. That 600 miliseconds will allow for a very good cancellation waveset to be computed and sent from the speakers.

http://www.google.com/search?hl=en&q=%22Noise+cancelling+technology%22&btnG=Google+Search

This ultra large sound cancelling array would be a great project, but maybe all the money and energy required for it may be better used elsewhere. I would rather persuade the farmer to plant 100 metres of trees and bushes - evergreen - especially as he can probably get an environmental grant for that

However, I can see a scheme to reduce the noise of landing planes through the transmission of anti noise could work well at some airports where the landing costs are charged on the noise footprint.

Hugh Mattos

Chartered Engineer

Well, air planes are 3D sound sources, versus a line on the ground that can be dealt with via a line of microphones, also on the ground.

It would be hard to create and maintain an array of microphones to directly intercept the sound path from air to ear in the same manner, as would be needed. So I doubt this could be done for takeofff and landing types of things. With an aerodrome further away, so the sound resembled a line source, it coukd be done.

We tried active noise cancellation once in a duct. It was letting sound in from the outside into a room. I don't know why they had a duct going outside the room but that wasn't the focus of our project.

We didn't need any computers or fancy circuits. We just had a microphone, a speaker, a power amplifier (low power) and a microphone pre-amp.

When we first tested it, the sound became louder! Oops! Then we switched the connections on the speakers, reversing the phase of the sound. Then we just adjusted the volume and position of the speaker in the duct. It cancelled the sound very effectively.

We didn't try experimenting on sound cancellation for a whole room. We figured that we'd need a lot of microphones and amplifiers and tuning the thing would be a long and difficult process.

Our acoustics engineer (who once worked for Bose, by the way), said that even if you could assemble a large number of microphones, amplifiers and speakers, in some areas of the room the sound waves would add up instead of cancel out because of the interactions of the speakers and reflections off the walls.

yes, That will work in a controlled space like that. In fact that is how some of the simpler ones work. Some use a mechanical coupler to a crystal pickup and you slide the pickup along the rod for a minimum and fiddle with the gain for a minimum.

The moving of the speaker in the duct was the equivalent of the time delay. That would work perfectly in a duct.

In 1983, lining the top half of a room containing pneumatically-operated can inversion equipment with marine acoustic insulation reduced the noise level from 93dBA to 86dBA.

Ear defenders and earplugs work as well.

Thanks for your help, but the main issue is outside as the closed windows in winter are good attenuators of the noise. However, I do not think that I can persuade all the guests at a garden BBQ to wear ear defenders!

Hugh Mattos Chartered Engineer

Build a glass enclosure around your garden BBQ? Like a green house?

Yes I like that idea, but since it is so cold and windy here today, that gardening is no fun, but if I covered the whole garden - about 1 hectare - with a glasshouse, it would be all nice and warm inside.

We are not far from the Eden Project which has the right sort of thing - just it cost 75 million pounds - which is above my budget!

Hugh Mattos - Chartered Engineer

Invite my niebours they can cancel out any noise. When they are drunk quite often 200db would be no trouble at all. Alternatively discover Japanese knot weed growng in the road surface. It will soon solve the problem. Grows any where and through any thing at all concrete ashfelt no bother.

It's been a LONG time since I've done any noise-control design but I remember a few basic rules. Vegetation has a minimal effect because the tree trunks are what absorb the noise. If you remove all the leaves and smaller branches, there's not a great percentage of solids left to do the actual attenuation.

Embankments work, but only if they're extremely close to the source. A retaining wall right beside the roadway works best. Noise attenuation hardware might work for this if you lay it out carefully. Uni-directional microphones aimed right at the source are used to feed a signal into an amplifier that inverts the phase. The amplifier's output is fed through speakers facing the viewer. This kind of system works best when the viewer is completely isolated, such as an airplane pilot who is listening through headphones.

Good luck!!