NOTE: This story has been updated with comments from an acoustics expert.

When the Heavy Montreal music festival kicked off this past Saturday at Parc Jean-Drapeau, sensors installed by the main stage registered a sound level of 90 decibels — about as loud as a lawn mower or a motorcycle.

At the same moment, across the river in Saint-Lambert, another sound sensor measured 54 decibels, or nearly the same as a normal conversation.

These numbers marked the first entries in a collaborative project between the Société du parc Jean-Drapeau, the paramunicipal agency that manages Île Sainte‑Hélène and Île Notre‑Dame, and the cities of Montreal and Saint-Lambert.

The goal is to figure out just how much noise reaches the South Shore suburb during summer festivals.

Longstanding complaints

For years, Saint-Lambert residents have complained that the music at events held at Parc Jean-Drapeau — such as Osheaga and ÎleSoniq — is too loud and a nuisance for the community.

"We can't really enjoy being outside because very often there is noise. It's not really sound; it's noise," said François Girard, who lives in Saint-Lambert.

That's where this new project comes in.

Geneviève Boyer, a spokesperson for the Parc Jean-Drapeau agency, told CBC Montreal's Daybreak that experts will analyze the data at the end of the summer and try to offer solutions.

Here are the locations of the three sensors measuring sound during this summer's festivals. (Google Maps)

CBC News obtained the data recorded by two sensors at 15-minute intervals during the weekend's heavy metal festival.

One was positioned near the main stage on Parc Jean-Drapeau, while the other was set up on Merton Avenue in Saint-Lambert.

The chart below compares the two.

Use data with caution, expert warns

Many factors can affect sound levels, such as physical barriers, wind speed and direction, humidity, and temperature.

A flash thunderstorm, like the one on Saturday night, registered with the Saint-Lambert sound sensor as far louder than music from the festival.

Also, the number used to measure loudness has limitations, said Romain Dumoulin, an acoustician at McGill University and a former noise inspector for the City of Montreal.

The decibel A measurement simulates human hearing by filtering out low frequency sounds. But amplified noise coming from concert speakers is rich in low frequencies, and people can still be affected by them, he said.

"I'm a bit skeptical of putting this data out in the public," Dumoulin said. "The idea behind it is good, and it could be useful for acousticians, but — sorry for the bad joke — it's just adding more noise."

Another limitation he sees: the sensors can't know what is happening when it registers a spike.

"Was there a lot of background noise? Did a dog bark nearby? Was there a lot of traffic?"

Roughly 25 decibels lower on South Shore

The festival sensor was set up near the Scène Heavy Black Label, one of four stages that were used during the event.

Shows played on stages further from the sensor didn't register as loudly, which explains the steep oscillations of the red curve in the chart above.

The sensor on Île Notre-Dame measured sound levels near the main stage. (GKM Consultants)

During the festival hours, the noise detected by the sensor in Saint-Lambert was roughly 25 decibels lower than the one at the main stage, and it rarely went past 55 decibels.

For a comparison, the average ambient noise on the island of Montreal registers at 58 decibels, according to a 2015 study that compared noise levels to socioeconomic status.

Still, the noise levels registered in Saint-Lambert during Heavy Montreal surpassed the level typically recorded in the area on weekends. That typical sound level is visualized by the yellow band on the chart above.

Saint-Lambert resident skeptical

Girard said he's suspicious of these low readings.

He claims the noise at his home was "unbearable," and a sound-measuring app on his phone picked up 74 decibels, which is as loud as a home sound system playing music.

"I'm skeptical about the location of the sound meter here," he said. "It's placed in the backyard of a house that's not in the most affected area."

Dumoulin cautions, however, that smartphone apps are not always accurate in their measurements.