I'm going to start reading this GAO Helicopter Noise study and put my comments below
https://www.gao.gov/assets/720/711651.pdf

Page 1 nice chart though this footnote raises some concerns: "666 flights for which FAA could not identify an operator or mission based on available
historical records."

I like visual aids

Would be interested in how practical the first two points are from a pilot's perspective @Chopper4Brad

I think I've only identified 2/3 of these heliports! Thinking about adding relevant heliports to map based on helicopter's known pad so people get a sense when Park Police hovers over "eagles nest" or sits on pad with transponder on for an hour...

MINIMUM altitudes over residents not mentioned here. And we have on occasion seen helicopters hovering at ~4000 feet surveying (surveiling?) PEACEFUL protests...
There's that mysterious Navy H60 with the orange door! @gifwiththetimes @SharkParsons

Made it to page 16 without a drink

This is a good "helicopter route map for dummies" tho @1crazyarchitect taught me about the layers button in http://globe.adsbexchange.com  admittedly I still look at street map view most of the time.

p.17 operators
conducted some portion of about 19,900 flights (23 percent) during evening hours. Specifically, certain missions—including state and
local law enforcement, air medical, and news—required a higher percentage of flights during evening hours.

p.17 Prince George’s County Police Department officials told us
90 percent of their helicopter flights took place during evening hours,
including observation, law enforcement support, and surveillance
flights. @alanhenney

Approximately 15% of military helicopter flights occurred during some portion of evening
hours... Coast Guard officials told us that their mission to help secure the D.C. area airspace by intercepting unidentified flights requires training
flights during evening hours

to simulate real world conditions,
including the use of night vision goggles.

Army National
Guard officials told us that their pilots are required by regulation to
conduct a certain number of training flights during evening hours in
order to be prepared to conduct rescue flights and security services. {Tho not during insurrection on Federal property}

p.18 "Limited noise data on helicopter flights in the D.C. area are available
because most [FAA and] stakeholders do not collect it, partly because studies to
measure and map noise from helicopters over a certain area are
technically challenging and resource-intensive."

#Copterspotter's: if I put together a weatherproof computer with microphone for such a noise study would you connect to your WiFi and put outside your house? #DataScience @fergindc @JoannaDBare @alanhenney @1crazyarchitect

"Army National Guard officials told us they do not need to
study noise from their helicopter flights because they fly in compliance
with all FAA guidance and regulations and their mission responsibilities
do not include further efforts to study helicopter noise"

"Almost all operators (16 of 18) told us they fly at the maximum posted altitude when possible" ...and the other two?

Picking this up from page 21 tomorrow!

In Jan. '17, the Army issued a policy that restricted the transportation of senior officials from the Pentagon helipad. Helicopter missions transporting personnel to and from the Pentagon helipad decreased from 226 in 2016 to 124 in 2019.

Register your official helicopter noise complaints with @FAANews they may get back to you in under 2 months!
https://noise.faa.gov/noise/pages/noise.html

Literally 100% of confusion about who was operating helicopter overhead would be solved by MILITARY USING TRANSPONDERS. In the meantime play #CopterSpotter and we will all learn to identify offending aircraft. @DonBeyerVA @EleanorNorton @FAASafetyBrief @usairforce @USArmy

Here are the #copterspotter details again https://twitter.com/HelicoptersofDC/status/1343989388636807170?s=19