At least six times a man felt the ground shake in Kinnelon on Wednesday coincided with times Picatinny Arsenal in Rockaway Township was conducting tests with explosives, an arsenal spokesman said Thursday.

Certain weather conditions could have led sound waves from the tests to bounce off the atmosphere toward Kinnelon, Montville and Bloomingdale where vibrations were felt this week, the military installation said.

Residents called Montville and Kinnelon police on Wednesday to report the ground was shaking. Police said the rumblings were due to testing at the arsenal.

The same tests were conducted the previous week without reports of disturbances in those towns, Picatinny spokesman Peter Rowland said. The tests in question are not scheduled for Thursday or Friday.

"We were not aware that people in that particular area were being disturbed until a gentleman from Kinnelon emailed me," Rowland said.

When he heard about the rumblings, he had test engineers check their logs. They found the reported rumblings were close to times the arsenal conducted tests, which they suspect combined with certain weather conditions to cause sound vibrations to reach the affected towns.

"We're always sorry to disturb our neighbors," he said. "We often don't know when these particular conditions are present, so we learn about them from people calling us and letting us know."

Temperature inversion, in which temperatures higher in the atmosphere are warmer than temperatures near the earth's surface, can lead sound waves to bounce off the atmosphere--in this case, apparently, on an angle toward Kinnelon--and that might have been a factor, Rowland said.

Rowland said it's unpredictable and sporadic as to where the sound waves might be felt, and that Picatinny rarely receives such reports. He said there isn't any danger from the sound waves.

The arsenal conducts tests most days of the week, with Mondays and Fridays typically being slower testing days, he said. Picatinny, which has about 5,000 people working for it, provides armaments and munitions products and services to all branches of the U.S. military. Picatinny has more than 800 buildings, including 64 labs, on the installation's nearly 6,500 acres, according to its website.

Two tests were pointed to as likely sources of the sounds, with the tests aimed at developing weapons that are less likely to accidentally fire and that have a longer range, Rowland said.

One involves a laboratory gun that fires projectiles through a large metal tube that gathers data, with the projectile eventually reaching a water trough that slows it down so it can be recovered.

"At the gun end, there's a fairly good-sized charge of propellent that can make a lot of noise under certain circumstances," he said.

The other test involves testing a propellent by firing into a sand cave.

"It's all contained," Rowland said.

But while the projectiles are contained on the base, the sound waves are not.

"There's an accompanying sound that's emitted from the weapon at the same times as the energy is released. ... That sounds then has to travel. It can be very deafening at the source and can also be loud and create vibrations at various other places."

Rowland said Picatinny tries to work with people if the sound bothers them, such as by waiting longer between tests.