By the numbers

243-249 decibels

Loudness of seismic exploration blast directed downward

140 decibels

Loud enough to cause permanent hearing loss

100 decibels

Loudness of old-fashioned air horn

65 decibels

City of Charleston noise limit

Sources: Centre for Human Performance and Health, Geo ExPro magazine, Harbor Freight Tools

The blast from a seismic exploration air gun is loud enough to kill tissue in the human ear. The blare of whistles Tuesday in Marion Square could barely be heard above traffic.

That’s how it goes when you protest oil and gas drilling off South Carolina, a state where most elected officials and apparently much of the population support it.

Activists tried to rally public opposition to proposed exploration offshore by raising a din with air horns for a few minutes, as the rally did in Savannah a few months ago. The noise was intended to give a sense of just how loud the blasts are.

But city officials would not permit the demonstration to exceed 65 decibels, the city’s noise limit, about as loud as street traffic. The air horns would have reached about 100 decibels, as loud as a chain saw.

The blasts themselves are about 250 decibels, according to Geo ExPro, a petroleum science magazine. That’s far louder than what would deafen a person. Environmental groups say the blasts deafen and disorient sea creatures such as whales, dolphins and sea turtles.

About a dozen people blew the whistles, barely drawing attention at the noisy intersection of King and Calhoun streets. The whistle blowers largely were from environmental groups such as Oceana, which is holding the rallies, South Carolina Aquarium and Southern Alliance for Clean Energy.

“I think we got our message out,” said Katie Parrish, of Oceana. That the city considered air horns too loud for its environs, “I think adds credence to what we’re doing.”

Nine companies have applied to explore for oil and natural gas reserves offshore; federal regulators are reviewing those applications. Exploration companies argue the blasts are sounded well out to sea and directed downward; the noise emanating horizontally through the water isn’t nearly as loud. Parrish shook her head at that.

“These sound waves travel hundreds of miles,” she said.