LISTEN: PHIVOLCS detects low-frequency sound amid Mayon's effusive eruption

State volcanologists on Monday recorded a low-frequency sound as Mayon Volcano continued its effusive eruption. 

On Tuesday, PHIVOLCS posted on Facebook an audio file containing the infrasound signal detected by the Camp One Observation Station in Albay amid lava fountaining and minor Strombolian activity between 11:17 a.m. and 12:57 p.m. Monday.

According to the US Geological Survey, infrasound has a frequency that is below the threshold of human hearing (20 Hz).

There are microphones made specifically to pick up these atmospheric sound waves. 

"Infrasound has an important role in volcano monitoring because it can help locate and learn about volcanic eruptions, lahars, debris flows, and more, especially when combined with other data such as seismic," the USGS said. 

"Infrasound waves can propagate for hundreds to thousands of miles due to their low frequency but are 10 times slower than seismic waves. They travel at the speed of sound, which is approximately 760 mph (340 meters per second) at sea level."

Alert Level 3 prevails over Mayon, and entry into the 6-km Permanent Danger Zone is prohibited. —VBL, GMA News