one decibel reduction at what reference pressure

The implication is that we're talking about sound pressure level in air, therefore the reference pressure would be 20 µPa.

and for which frequencies?

Again, the implication is annoyance and in that context I'm looking at overall SPL in A-weighted decibels (A-weighted decibels, while not perfect, is reliably correlated with annoyance)

I think you don't really know

For the record I'm an expert [0] in acoustics and noise control. It's how I've made my living for the past 30 years. So yes, I really know.

or you would have specified

I wasn't trying to get into a detailed discussion here, but I'm happy to oblige for anyone that wants to learn.

as with most things relating to acoustics, the truth appears to be extremely complicated

Absolutely. That said, if you look at the link, the author mentions 8-9 dB of excess attenuation with 50 meters of intervening foliage. That correlates to about 1 dB of attenuation per 18 feet of foliage. Again, that demonstrates that a strip of foliage would do almost nothing to reduce sound levels. And for what it's worth, the phonemea the author is describing is not "absorption" - it's a combination of partial cancellation of the reflected/direct wave interaction in porous soil (same reason why snow covered ground makes things quieter) and refraction from leaves/trunks (which is why the foliage needs to be _dense_, otherwise soon waves travel through the gaps and provide no reduction).

[0] By "expert" I mean a) studied the subject as an undergrad at MIT b) worked for 30 years in the field, producing or contributing to several hundred Environmental Impact Statements in the USA, authoring/co-authoring a couple dozen papers and presentations including one peer-reviewed study, c) authored or contributed to acoustics guidance manuals for the U.S. Federal Transit Administration, Federal Aviation Administration and National Academies, d) have been admitted as an expert witness in acoustics/noise control in criminal and civil trials in seven states, e) have certification demonstrating noise control expertise [1], f) been recognized by my peers as having contributed to the field, g) have had research referenced by international researchers

[1] https://www.inceusa.org/board-certification/about/ (sample test questions available at https://www.inceusa.org/pub/?id=6FBAEF10-B2FE-1D7D-AFCA-55D5... if you want to see the type of acoustics knowledge that is tested.

My stupid noise journey (2023)