Noise reduction (SPL) through various palms, shrubs, and bamboo

[Merlyn]

One of the big benefits of having a "jungle" yard is that it cuts down on noise from nearby traffic, lawnmower crews, schools, etc.  This became a big part of my backyard planting plan, as there's a big interstate a bit over 1/2 mile from my house.  When I bought it 20+ years ago you couldn't even hear the highway noise at night.  It was obscured by several dense oak and pine forest sections between my house and the highway.  Over the past 15ish years all the trees have been cut down to make way for office buildings, apartments and condos, leaving nothing to block or absorb any of the noise.  Of course daytime and especially nighttime traffic has increased a lot.  And then they decided to build a toll road about 1/3 of a mile from the house, essentially wrapping my entire backyard with traffic noise from all directions.  This meant my ambient noise level went from the early 2000s at barely whisper level (~30dBA) to constantly at average conversation level (~50dBA).  So as part of my yard layout I did an initial border of Viburnum hedges.  This choice was based on a couple of research papers:

• The Audible Landscape: A Manual for Highway Noise and Land Use: https://www.fhwa.dot.gov/environment/noise/noise_compatible_planning
• Green Buffers for Screening and Noise Reduction: https://gatrees.org/wp-content/uploads/2020/02/GreenBuffersforScreeningandNoiseReduction.pdf
• Bamboo plants as a noise barrier to reduce road traffic noise: https://www.researchgate.net/publication/312198747_Bamboo_plants_as_a_noise_barrier_to_reduce_road_traffic_noise
• The investigation of noise attenuation by plants and the corresponding noise reducing spectrum: https://www.researchgate.net/publication/43344334_The_Investigation_of_Noise_Attenuation_by_Plants_and_the_Corresponding_Noise-Reducing_Spectrum

The third paper lists reduction for Holly at 4-6dBA, Bamboo at 3.5-4dBA, Viburnum at 15dBA, and Willow at 30dBA.  I should have actually followed the footnote to read the referenced 4th study, which doesn't even come close to stating those numbers.  In fact they didn't study Willow or Holly at all, and Viburnum was 3-6dBA per meter of depth...not 15dBA.  However, they did have an interesting chart:

 

This is the projected noise loss for a 6 meter (20 feet) thick grove of bamboo, and it's pretty high!  They measured the loss in an existing grove and extrapolated it out to a thicker clump.  At the time I wasn't interested in bamboo, because all I'd heard was horror stories of neighborhood-eating unkillable running bamboos.  So I found a couple of Viburnum shrubberies near roads and did a quick listen test...standing behind the Viburnum and then off to the side and trying to decide if it worked well enough.  I decided it was "good enough" and planted a complete hedgeline of Viburnum Odoratissum, probably 75 or so spaced at 5' intervals.  This worked fairly well for a few years, but I could still hear a lot of noise coming through the hedgeline.  The problems were:

• Once the Viburnum gets tall enough to block the neighbor's 2nd story view, the bottom 6-10 feet is mostly leafless.  No amount of selective pruning would convince the shrubs to bush out, there just wasn't enough sun low down to make it grow dense foliage.  
• The SPL (Sound Pressure Level) reduction wasn't anywhere close to the 15dBA listed in the paper.  I could tell this by ear, without even pulling out an SPL meter.
• Viburnum are susceptible to Verticillium Wilt, a soilborne fungus that can kill them.  I'd have 2-4 shrubs per year just randomly die, leaving a giant gap in the hedge.

With the problems and poor performance of Viburnum I started looking for something else.  

[Merlyn]

This time I decided to apply a little bit of my own science to the question.  I used a handheld Nady Audio DSM-1X SPL meter and ran some actual tests on plants in my yard, around the neighborhood, and at a local nursery.  I used a Klipsch Groove portable Bluetooth speaker as my noise source, and made a test track using an EIA RS-426A speaker noise spectrum with a 600Hz 6dB/octave highpass filter to simulate traffic noise.  I enlisted my wife to help measure noise loss through the plants by having her stand on one side with the speaker and me on the other side with the SPL meter.  We'd measure through the plant and then step to the side at the same 8' distance, and remeasure with no plants in the way:

• Arenga Engleri - 4.7 dBA - Dense cluster in the SW corner, about 8' tall and 3-5' thick
• Caryota Mitis - 9.4 dBA - Dense cluster on the East side pathway, about 15' tall and 5' thick with lots of solid trunks
• Chrysalidocarpus/Dypsis Lutescens - 4.7 dBA - NE dense cluster, also about 8' tall and 3-4' thick
• Rhapis Excelsa - 4.6 dBA - 5' tall cluster with low and medium height leaves and trunks

Bamboo:

• "Jesse Durko" Bambusa - 6.9-7.7 dBA - about 3' thick with very dense foliage
• "Jesse Durko" Bambusa - 3.9 dBA - about 3' thick but in an area with very few leaves
• "Jesse Durko" Bambusa - 1.7 dBA - about 3' thick but in an area with very few leaves AND very few culms
• Bambusa Tuldoides "Swollen Internode" - 3.3 dBA - also 4' thick in an area with very few leaves
• Bambusa "New Guinea Black" - 4.9 dBA - very dense foliage
• Bambusa Textilis "Gracilis" - 5.2 dBA - at a spot with medium amount of trunks but very sparse leaves
• Bambusa Textilis "Gracilis" - 10.6 dBA - at a spot with very dense trunks but very sparse leaves

Others:

• Zamia Furfuracea "Cardboard Palm" - 3.7-4 dBA - solid huge clusters 8 feet thick
• Strelitzia Nicolai "White Bird of Paradise" - 6.9 dBA - through a section of dense trunk and lots of big leaves
• Viburnum Odoratissum - 3.2 dBA - through a 6' wide lower section with a few trunks but very few leaves
• Viburnum Odoratissum - 5.4 dBA - through a 6' wide section of very dense foliage, impossible to see through
• Philodendron Selloum - 6.5 dBA - dense 8' thick clump in the SW corner
• "Bordelon" bananas - 8.3 dBA - dense clump 8 feet thick with lots of trunks but not too many leaves
• Lattice 4x8' sheet - 4.1 dBA - white PVC for making a lattice fence
• Plywood 4x8' sheet - 11 dBA - probably picked up reflections from nearby cars.
• Podocarpus - 2.6-3.3 dBA - across relatively thin section
• Podocarpus - 3.7-5.4 dBA - across medium to high density >3' thick sections

[Phoenikakias]

55 minutes ago, Merlyn said:

One of the big benefits of having a "jungle" yard is that it cuts down on noise from nearby traffic, lawnmower crews, schools, etc.  This became a big part of my backyard planting plan, as there's a big interstate a bit over 1/2 mile from my house.  When I bought it 20+ years ago you couldn't even hear the highway noise at night.  It was obscured by several dense oak and pine forest sections between my house and the highway.  Over the past 15ish years all the trees have been cut down to make way for office buildings, apartments and condos, leaving nothing to block or absorb any of the noise.  Of course daytime and especially nighttime traffic has increased a lot.  And then they decided to build a toll road about 1/3 of a mile from the house, essentially wrapping my entire backyard with traffic noise from all directions.  This meant my ambient noise level went from the early 2000s at barely whisper level (~30dBA) to constantly at average conversation level (~50dBA).  So as part of my yard layout I did an initial border of Viburnum hedges.  This choice was based on a couple of research papers:

• The Audible Landscape: A Manual for Highway Noise and Land Use: https://www.fhwa.dot.gov/environment/noise/noise_compatible_planning
• Green Buffers for Screening and Noise Reduction: https://gatrees.org/wp-content/uploads/2020/02/GreenBuffersforScreeningandNoiseReduction.pdf
• Bamboo plants as a noise barrier to reduce road traffic noise: https://www.researchgate.net/publication/312198747_Bamboo_plants_as_a_noise_barrier_to_reduce_road_traffic_noise
• The investigation of noise attenuation by plants and the corresponding noise reducing spectrum: https://www.researchgate.net/publication/43344334_The_Investigation_of_Noise_Attenuation_by_Plants_and_the_Corresponding_Noise-Reducing_Spectrum

The third paper lists reduction for Holly at 4-6dBA, Bamboo at 3.5-4dBA, Viburnum at 15dBA, and Willow at 30dBA.  I should have actually followed the footnote to read the referenced 4th study, which doesn't even come close to stating those numbers.  In fact they didn't study Willow or Holly at all, and Viburnum was 3-6dBA per meter of depth...not 15dBA.  However, they did have an interesting chart:

 

This is the projected noise loss for a 6 meter (20 feet) thick grove of bamboo, and it's pretty high!  They measured the loss in an existing grove and extrapolated it out to a thicker clump.  At the time I wasn't interested in bamboo, because all I'd heard was horror stories of neighborhood-eating unkillable running bamboos.  So I found a couple of Viburnum shrubberies near roads and did a quick listen test...standing behind the Viburnum and then off to the side and trying to decide if it worked well enough.  I decided it was "good enough" and planted a complete hedgeline of Viburnum Odoratissum, probably 75 or so spaced at 5' intervals.  This worked fairly well for a few years, but I could still hear a lot of noise coming through the hedgeline.  The problems were:

• Once the Viburnum gets tall enough to block the neighbor's 2nd story view, the bottom 6-10 feet is mostly leafless.  No amount of selective pruning would convince the shrubs to bush out, there just wasn't enough sun low down to make it grow dense foliage.  
• The SPL (Sound Pressure Level) reduction wasn't anywhere close to the 15dBA listed in the paper.  I could tell this by ear, without even pulling out an SPL meter.
• Viburnum are susceptible to Verticillium Wilt, a soilborne fungus that can kill them.  I'd have 2-4 shrubs per year just randomly die, leaving a giant gap in the hedge.

With the problems and poor performance of Viburnum I started looking for something else.  

Try Rhamnus alaternus instead of Viburnum. With age it can develop to a green wall!

[Merlyn]

So to make the numbers make sense, you have to know that SPL reduction is a logarithmic scale.  Every 3dB is a factor of 2 in actual noise level.  The formula is dB = 10 * log (reduction factor).  So if you have a reduction in noise of 0.5 (half) then the SPL reduction is -3dB = 10 * log (0.5).  Likewise a reduction of 6dB = 1/4 as loud, and 9dB = 1/8th as loud, 12dB = 1/16th as loud.

Clearly the best stuff is a solid fence with no gaps.  It's thin and has the best sight and sound reduction of anything in the list.  That's the reason they build the giant concrete walls along highways here...they work.  A generic lattice fence was 6.9dB worse than the solid plywood, so even 50% open area means a lot of noise gets through.  Other interesting observations:

• The foliage on palms was all around the same 4.5-5 dBA range.  
• The trunks on the Caryota Mitis and dense trunks on Bambusa Gracilis Textilis were remarkably good at 9.4-10.6 dBA.
• Thick waxy non-flexible leaves on the Zamia Furfuracea didn't do much at all.  I think the sound waves just bounced around and went right through with no absorption.  It was only 3.7dB = 60% reduction in sound for a whopping 8 feet thick dense thorny mess.
• Dense leaves and dense culms on the bamboo were very effective at around 7-10 dBA.
• Dense Viburnum was only a little better than a clustering palm (5.4 vs 4.7 dBA) and much worse in sparse areas (3.2 vs 4.7 dBA)
• Podocarpus is a good visual block, but generally much worse than other options
• Dense bamboo trunks were really good at absorbing mid-band noise like voices, lawnmowers, car exhaust and tire roar.  They were not very good at absorbing high frequency noise like the "whoosh" sound of cars going past.  The bamboo foliage was VERY effective at the high frequency stuff.

[Harry’s Palms]

I have a green wall of Ficus trees down at the bottom of the hill . I have it topped every other year so it is like a tall hedge, very thick . When cars go by on the other side of the hill I can tell the difference from before they are behind the hedge as they approach . It does work . Harry 

[Jdash]

There are a lot of factors when it comes to acoustics for sure.  are the road ways at higher elevation than your property? is all the new development positioned in a way where reflected sound has increased to the property? All the additional hard concrete around you has so much impact.  single point sound sources(reflection isn't considered a source since there's no energy at reflected points but can lead to resonance) should perhaps differ from multipoint(noise sources originating along the path of roadway). Your levels going up to conversation level is pretty wild considering highway is 1/2 mile out and the new toll road 1/3 mi doesn't help either, providing another range of incoming source direction. 

According to that FHWA source, they claim noise attenuation of 3-5dBA per 100ft of thick foliage coverage. 

solid walls and proper berms  seem like the way to go, unfortunately costly or not possible in many circumstances. Another solution I've come to has been water features or your own sources of sound that are more pleasant than the highway. i'm sure bamboo swaying would be one too. at whisper levels, a water feature would completely drown out the noise. working toward conversation levels, more of  challenge.