Late last year, residents of Fishtown and Port Richmond began complaining about music flooding their homes at night. “Where is this awful music coming from?” one woman asked her neighborhood Facebook group. Another said it was shaking her windows.

Some insist it’s coming from across the Delaware River in Camden, where they say people load up cars with speakers for parking-lot parties. But across the water in the sleepy South Jersey suburbs, residents of riverside communities say they’ve been tortured by blaring music for years. And they blame Philadelphia.

It’s all part of a long history of cross-river finger-pointing over noise.

“This has always been a blame game,” said Jeff Stefan, a Westville resident who has spent years fighting the noise. After months of music disrupting nights in his riverfront home, Stefan traced the sound himself — eventually discovering parties with people blasting speakers in their cars in a lot near Lincoln Financial Field in South Philadelphia. The city, police, and the property owner worked together, the gates were locked, and the crowds stopped coming.

But the music, the boom cars, and the blame game continued.

» READ MORE: From 2019: ‘Loud booming’ from Philadelphia keeping South Jersey residents awake at night

Stefan and his girlfriend moved to their Gloucester County home on the bank of the Delaware River in 2013 assuming it would be peaceful. They soon began hearing noise from the Philadelphia sports stadiums and traffic on I-95, and a few years in, thumping bass from outdoor parties started rattling windows. It got worse during the pandemic, with bars closed and nowhere to safely gather indoors.

“If I can hear someone scream, ‘Go, Phillies,’ at a game, I can certainly hear subwoofers strapped to a roof,” Stefan said.

In 2019, he created “Inconsiderate Late Night Philly Music,” a Facebook group that has more than 3,500 members. People post reports of music heard anywhere from up and down the South Jersey riverside communities to landlocked towns like Cherry Hill. Philadelphians also joined the group, and in recent months more than a few have claimed the music they hear is coming from Camden’s Pyne Poynt Park.

Across the Delaware from a luxury townhouse development under construction on the edge of Fishtown, Pyne Poynt is home to baseball diamonds where a community little league plays, as well as a playground and boat launch.

Camden City Councilmember Felisha Reyes-Morton said one group of residents sometimes gathers in city parks to play music from custom speakers, but the parties normally end before dark. The men are hobbyists who take part in community events, she said, and after a party in Camden’s Von Neida park drew noise complaints, they have held most events in Pyne Poynt because it’s more remote.

Reyes-Morton, who lives close to Pyne Poynt, said she hasn’t heard neighbors complain about noise there.

Pyne Poynt is also outfitted with surveillance cameras, and Camden County police officers conduct nightly checks, county spokesperson Dan Keashen said. While officers break up occasional gatherings, Keashen said, the department hadn’t encountered large parties with blasting speakers.

But after an earlier version of this article posted online, a Philadelphia resident sent The Inquirer several videos shot in recent months that show cars parked near the back of Pyne Poynt Park at night, blasting music from speakers for a small group of people. In the videos, the sound can be heard at the entrance to the park but is fainter than what residents describe hearing on the other side of the river.

So what explains all the finger-pointing between Philadelphia and Camden?

Maybe it’s because water enhances sound waves in unexpected ways, said Peter Harnish, an undergraduate lab manager at the University of Pennsylvania’s department of physics and astronomy. Sound travels better over water than over land because it’s a flat surface with nothing to stop it.

“Just like you can get sound to echo off a building, you can get it to echo off a body of water,” he said.

The air above water tends to be slightly cooler, which creates a layer between the water and the hotter air. That boundary creates another surface from which a sound wave can bounce off and echo.

“Normally the sound waves would be trying to travel in straight lines in all directions, and instead they’re forced back down across the water,” Harnish said. “It’s almost like building a megaphone.”

That amplification also makes it harder to discern where a sound originated, Harnish said.

“You know what a car sounds like when it’s a block away, but on water something that sounds like a block away could be several blocks away,” he said. “You’re just not used to something being that loud from that far away. That ability to guess the range of the sound is completely different.”

» READ MORE: Judas Priest rocked Camden — and Queen Village — with help from the weather

The contrast between a warm summer night and a cooling rain can also affect sound. Residents of South Philadelphia’s Queen Village learned that in years past, including when a phenomenon called a “temperature inversion” made a 2018 Judas Priest concert on the Camden waterfront sound as though it was in their living rooms.

Stefan encourages members of his Facebook group to report loud music to police, but acknowledged it’s more complicated than complaining about a neighbor’s party.

“You call the police and they’ll say, ‘Where’s it coming from?’” he said. “I’m sitting on my couch in Westville and I can hear it — I don’t know!”

Police have taken the problem seriously, said Capt. John Walker of the Philadelphia Police Department’s 15th District, which covers much of Northeast Philadelphia. After Stefan began reporting the South Philly parties, Walker spent months working with local leaders on getting rid of them. In addition to locking the lot, rumble strips were installed nearby to discourage drag racing.

The parties moved north, where they’ve been spotted in parks and near boat ramps in Tacony and Frankford, as well as Graffiti Pier. Penn Treaty Park, in Fishtown, is a regular spot for parties, some of which also include music that carries into residential blocks.

Police have targeted areas known for parties, and officers can enforce noise ordinances. But Walker said educating partygoers also helps because they don’t always realize the sound carries so far.

“With these car parties, they need a place to play the music and they probably figure a park is a perfect place,” Walker said. “They’ll back up to the water, they turn the music on. Those sound waves bounce off the surface of the water and keep traveling until they hit something solid, but sometimes you can barely hear it when you drive up on the Philly side.”

In April, Pennsylvania State Rep. Joe Hohenstein met with police and leaders from Philadelphia and South Jersey towns to discuss solutions. Walker said that the city is working with police and neighborhood groups, and that development projects are underway to transform some empty lots into revitalized parks that will discourage parking-lot parties and muffle sound.

Stefan said the problem is less prevalent than it once was but remains an aggravation — whether it’s coming from Philadelphia or from his side of the water. Walker said the complaints have slowed in recent months, but said it’s likely that residents close to the water will always have to deal with unwanted noise.

“The way the sound is by the water,” he said, “it could be someone sitting in their car having a few beers and playing music and people hear it.”