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[John Roberts {JR}]

I believe they require mufflers on drag race tracks in Oz. I suspect the noise is part of the attraction.

A physical sound barrier that reflects the noise back toward the race track and slightly down, rather than up should help. You don't mind the noise at the track.

Adding turbo chargers would reduce the exhaust SPL.

JR 

[Patrick Tracy]

record the engines while they are racing. play back at 180deg opposite and see if it works.

Recording them wouldn't work, you'd have to have a live mic going to the speaker. The waveforms have to be exactly correlated which is pretty much impossible given the distances and directions involved. If the source of the noise weren't so large and the area in which you wanted to reduce the levels was all in one direction it just might be possible.

[John Roberts {JR}]

Noise canceling mufflers "could" mitigate the noise at the individual sources without the horsepower robbing back pressure, but this is a lot of high technology expense that would not likely be embraced by the competitors.

JR 

[Jerome Malsack]

I Grew up in Slinger Wi.  If you google Slinger Super Speedway you will find that they are 1/4 mile track on the edge of the town and the town is growing around it. 
There is a ski hill next door followed by a city park. 

Distance is a good thing with a ski hill to block some. 

The track actually sits down in a bowl of hills allowing them to have banked straights and steep turns for the fast 9 second 1/4 mile track. 

Adding the trailers is not as good as the concrete highway barriers because these go from ground up and side walls are joined leaving no air gaps except above. 

What would the city and the track share the cost of the highway sound barriers. 

[Art Welter]

The semi trailer are in place on the side of the track closest to town,, They are also in the process of erecting new stands.. They are metal but solid with no Holes in them..

So this is my question as the rumble of motors with mufflers is low pitched,,,is there a way if can measure the primary freq the motors operate at to turn these large boxes (semi Trailers) into some kind of low freq traps?? They could be placed two high if needed,,,  They problem is reduced after the trees have all their leaves and it is hot enough to keep the air on,,  Also with the new stands could they be treat to "absorb" sound if they are not full of people?  I think they are being placed with the idea to divert sound and not that they would be full..

Joe,

A 40 Hz wave is 28 feet long, to divert it requires height.
Double stacking the trailers would help, as would skirting the trailers or removing the wheels and placing them directly on the ground.

The trailers could be turned in to bass traps by cutting port holes in the sides facing the track.
Helmholtz bass traps work over a relatively narrow band, the trailers individually could be tuned to a variety of frequencies.
There are a number of port calculators on line, just plug in the volume of the particular trailer and the frequency you want it tuned to (Fb), the calculator will tell you the size hole required to ruin the resale value of the trailer  .

The bass traps would only be absorbing a small portion of the radiated low frequency energy, making sure the sound wall is basically air tight would be the primary concern.

Art

[George Friedman-Jimenez]

Art makes some good points. I especially like his idea of a wall of double stacked trailers, only a subwoofer maven would have come up with that one! Unfortunately, that would only approach 1 wavelength in height for 40 Hz, kind of a puny sound shield for low frequencies.

Regarding the OP's question about using trailers as bass traps, bass traps are typically used indoors to reduce the resonant modes in the bass frequencies that occur when parallel walls reflect the sound waves back and forth and create standing waves. Outdoors, the observed sound is either direct or reflected, but there are generally no standing waves set up. So as bass traps the trailers would not have much impact.

[John Roberts {JR}]

One observation, we don't need to disappear the noise, just direct it away from neighbors. Physical barrier large enough and solid enough to steer it in a benign direction. Of course this is not trivial.

Back in the day when we had empty ocean freight containers coming out of ears, they could be repurposed, but these days not really a cheap option. Just think dirt, lots of dirt.

 JR

[Brad Weber]

The effectiveness of a barrier depends on a number of factors including the transmission loss of the barrier, the frequencies in question, the barrier height relative to the sources and listeners and the distance the barrier is located from the sources and listeners.  The effects of a 20' high barrier with a noise source 10' to one side and a listener 10' on the other side is much different than the same barrier but with the source and listener both 100' away or with the listener almost at the top of the barrier.
 
For environmental noise you also get into the effects of topography, thermal conditions, wind and so on.  Basically, it can get rather complicated, especially when looking at not only multiple listener locations but also multiple moving sources with constantly varying locations, levels and frequency content.
 
I know that Road Atlanta used to limit the activities based on the time of day.  And while the low frequency noise of big blocks can be problematic, I watched one of the AMS Winston Cup races from the pits and was amazed at how loud they were, probably the most annoyingly loud cars I've heard are unmuffled or lightly muffled rotaries, see http://www.torquenews.com/1084/forza-4-records-loudest-racecar-planet-video and http://www.topspeed.com/cars/car-news/video-the-mazda-787b-is-the-loudest-car-in-forza-4-ar127395.html.  I was at Road Atlanta for one track day where you could easily hear the scream of one 2nd gen RX-7 over any of the Porsches, Corvettes, BMWs, Mustangs, etc.

[Doug Fowler]

I have heard the technique of playing the noise at 180 degrees to cancel it out but would guess it needs to be exact with virtually no delay. Guessing being this is low frequency it may be pretty expensive to get enough live microphones playing back to subwoofers around the perimeter. But boy that is an experiment I would love to see.

"180 degrees" at what frequency?

[Steve Alves]

"180 degrees" at what frequency?

It would have to be a live microphone. If you can duplicate the sound exactly 180 degrees out of phase, it would cancel out. (Theoretically but probably not possible)

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