Which sound barrier will be most effective if costs are equal? Two-lane residential (with many pick-up trucks) street is 65' from the listening point. The street is 1' above grade. The fence/wall will be built 12' from the edge of the street. The choices are A) a combination of an 8' high treated wood fence constructed of nominal 6" x 1" boards that overlap the adjacent board by 1/2," and 'floating' 6" above grade (for an effective height of 8'6"), combined with a low, dry-laid stone/masonry wall that is 12" deep and 24" high built on the outside perimeter of the wood fence; or B) a 6' high masonry wall. - There will be lots of plantings on the inside of the sound barrier and there is an 8' high holly hedge on the outside of it. Please suggest any other StackExchange forum that may have sound engineers lurking.
I'm a sound guy as well as a garden guy. Your answers are hidden in the graphics you posted. Go with the block wall. I had a somewhat similar situation with nearby freeway noise and after building a block wall along one side of my yard the sound was significantly (but not totally) reduced.
If possible and your wall has an adequate footings for the extra weight you could also fill the blocks with sand to give yourself a few more dBs. Plants on both sides of the wall will also help. Sound has a harder time propagating through different materials so if you create a barrier that is essentially plant-concrete-sand-concrete-plant you're providing several layers of sound insulation. Different materials also conduct sound differently so the layers add another bonus. Finally, the surface area of the plants (leaves, branches, etc.) increase surface area and create sound traps where sound that bounce off the block wall get absorbed by the leaves.
All in all I think you be happiest with a block wall, but note, you will still get noise. The wall won't stop everything and sound can still be transmitted through the ground, reflected off buildings, rooftops, even clouds. Temperature and humidity can even play a part. Like the graphic says, it's difficult to reduce noise by more than 10 dB with a barrier wall, but 10dB is still an order of magnitude so even hitting a good portion of that can make a difference.
One year later, my 8 foot high sound-amelioration-privacy fence is completed, save for putting some paving-type blocks between the fence boards and the ground to prevent small dogs escaping. The completed project is the result of much research and many compromises. I did not go with a masonry wall because I did not want to disturb the roots of the large tree more than necessary. Because of the hot, humid, weather here in northern Alabama, I opted to allow air through. Although I researched the shrinkage of the wood product, the gaps are larger than I would like. Even so, I find that I am not bothered by the traffic and lawn tractor noise as much as I was before the fence was installed. There will be further actual and perceived sound reduction after the landscaping, with many tall bushes, is installed and matures. I could not find a contractor to install the steel posts that I specified, so I rented a truck and drove the 3 hour round trip to purchase the posts from the nearest distributer. After that chore was taken out of the equation, I found a small contractor that was willing to do something different, including using screws for the fasteners. The galvanized steel posts were set 6' apart and in 30" of concrete. The pressure and stabilizer treated boards are finished on 4 sides and are an actual 3/4" thick. The fence is coated with a UV-resistant siding stain, including the end-grain on the bottom of the fence boards. I have my "30 year fence."