Office of Naval Research—Vandenberg Space Force Base and Los Angeles International Airport (LAX)
By Kaustubha Raghukumar, Ph.D., Consultant
Grace Chang, Ph.D., Technical Director
This endeavor sought answers to the following science questions related to underwater sound generated from an in-air source of sound such as a rocket or aircraft.
- What is the signal strength underwater as the sound refracts through the air-sea interface?
- What is the loss in intensity across the air-sea interface?
- Is the signal strength sufficient to discern the bottom reflected path at full ocean depth, thereby allowing for the inference of bottom depth
A preliminary measurement was made of sound from a SpaceX rocket launch. The signal duration of the rocket launch is observed to last approximately 20 seconds and consists of a steep ramp-up in energy as incident angles allow for refraction into the water column.
NoiseSpotter(R) testing in Monterey Bay for ONR.
Spectrogram of underwater sound from a SpaceX rocket.
Noisespotter® was deployed approximately 7 km offshore of LAX on January 15, 2021, for a period of approximately 3.5 hours between 0930 and 1300 Pacific Standard Time. During this period, over 30 flights were documented taking off from the LAX runways. Aircraft types that transited overhead during the deployment were the Airbus A320 (Delta Airlines flights DL1041), Airbus 321 (Delta Airlines DAL358), Boeing 757 (DAL1129), Boeing 737-800 (Delta Airlines DAL2227), Boeing 737-900 (Delta Airlines DAL382), Boeing 767 (Delta Airlines DAL319), Boeing 777-300 (Japan Airlines JAL61), Gulfstream Aerospace G650 twin-jet (FlexJet LXJ651), EMBRAER 175 long wing twin-jet (SkyWest SKW3713, SKW3717) and the Canadair Regional Jet CRJ-900 twin-jet (SkyWest SKW3723, SKW3729, SKW5325).
Spectrogram of underwater sound from an aircraft for SkyWest Canadair Regional Jet CRJ-900 twin-jet SKW3729 from LAX to Bozeman, MT
The results above show that underwater noise from an aircraft can indeed be audible when aircraft are transiting at elevations that range from 600 m to 900 above ground. Therefore, louder aircraft, such as rockets or military aircraft, can be expected to produce significantly louder sound that can be exploited as a source of opportunity. Directional processing of recorded underwater sound was able to track the aircraft as it entered a refraction cone that allows for underwater propagation of in-air sound.