Question
I live 60 feet off a highway. In the effort to mitigate vehicle noise, I want to build a earth berm with evergreens on top.  How high and wide should the berm be? I live in Minnesota and this berm does NOT receive sun.
Thank you in advance, Joe B.

Answer
Place the berm as close to the noise source as possible, as this will have a "reflecting" property to it, and throw some of the sound back at the problem.  Also, the highway department may have concerns with large berms next to their roadways, even if it is on your property ?sight distances at intersections, launching stray vehicles, visual appeal, etc.

If area taked up by this sound berm is not a big concern, then pushing earth is the cheapest solution.  A combination of fence or wall & berm is going to get you a larger reflective structure to toss the sound back at the problem (at an increased cost).

Adequate research has not been conducted which clearly demonstrates the total effectiveness of plants in controlling sound pollution. A standard road will generate 90 decibels (Db) of noise, whereas a hedge will only attenuate about 7Db per 100 linear feet of hedge.  And, effective sound control is dependent on the interaction of several factors including: 1) direction, frequency and intensity of noise; 2) topography, wind direction and speed, temperature and humidity of the local area, and; 3) location, spacing, species and density of the plant screen. In many instances, these interrelationships are complex and unpredictable.

Plants have been shown to be most effective at screening the higher, more annoying frequencies of unwanted sound.

That said, you CAN combine a low wall or berm and/or plants to stop some of it.  Sound travels upward, so if you have a slope down to the road or a second story home, the closer to the road you get the berm/plants, the better.  Deciduous plants with thick, fleshy leaves have been shown to be most effective in absorbing, deflecting and reflecting noise. Evergreen trees, shrubs, and ground-covers can be mixed with deciduous plants to create a solid, year-round buffer. Wide bands of such plantings are not practical on medium- to small-sized lots.  A six-foot solid wall covered in Ivy and a 5-foot wide hedge in front will also absorb a lot more sound than a simple hedge of boxwood.   The wider the hedge, the more sound it reduces, as its the twigs, leaves and branches that "absorb" or reflect the sound back at the source - in this case, traffic.

Specifically for your location (Minnesota & in the shade to part-shade),here are some fleshy, deciduous plant that may work: Peking Cotoneaster (C. acutifolia),  Dogwood, Winged Euonymus, False Spirea (Sorbaria sobifolia), Honeysuckle,  Barberry, Ninebark, or Privet? They will get 搇eggy?or sparsely branched in the shade situation.  

As far as evergreens, there are very few (none?) that grow in shade/heavy shade; check with your local nursery to see if they have some local ideas.  A local, cold-hardy fern, possibly?  I have a Minnesota-specific Cold Climate Plants booklet, and there are none in there.  Really, it抯 the berm that is doing all of the hard work.

Also consider "masking" the noise with "white" sounds, like a trickling fountain or maybe attract some birds to your backyard with seeds or bird-friendly plants.  

Unfortunately, at night, when everything is quiet, the noise seems to amplify and resonate off of every hard surface.  We live about two miles from a freeway, and at night we can hear it's rumblings - strangely, not during the day.  I think part of it is that "white noise" sound effect at work.

All this, and a bit more, and this may help the situation.