# Physical Acoustics Research

Physical acoustics is a broad area pertaining to the underlying physics of acoustics. Dr. Sparrow's research in physical acoustics primarily centers on propagation research and on sonic booms.

## Sonic Boom Research

What is a sonic boom? When any object travels faster than the speed of sound, a weak shock wave is formed. The fluid ahead of the object is suddenly compressed, and the fluid behind is suddenly decompressed. Supersonic airplanes are classic examples. They continuously create these weak shock waves while in supersonic flight. Both people and animals find the noise of a sonic boom annoying, so any progress one can make in understanding them is important.

## Sonic Boom Noise Penetrates the Ocean Surface

In a relatively recent paper in the Journal of the Acoustical Society of America (Vol. 97, No. 1, January 1995) Dr. Sparrow elaborated on the theory of Sawyers, who in 1968 showed that sonic boom noise penetrates from the air into sound wave that penetrates into the top of the ocean for many meters. Dr. Sparrow showed that the faster a supersonic aircraft flies, the deeper the penetration.

Here is a visualization from that paper:

The nine graphs each show acoustic pressure (scaled by the acoustic pressure at the air-ocean interface) on the vertical axis. The horizontal axes show time scaled by the duration of the sonic boom wave at the air-ocean interface. Thus at the air-ocean interface the sonic boom is an acoustic wave shaped like the letter "N" of duration 1 with amplitude 1.

The three graphs in the first column show the sonic boom for an airplane traveling at mach 1.4 (1.4 times the speed of sound in air). For the second column the airplane is traveling at mach 2.4, and for the third column the airplane travels at mach 3.4. In each column the acoustic pressure waveform is shown at depths of 1, 10, and 100 meters. While it is apparent that for the mach 1.4 aircraft speed, the waveform decays relatively quickly, for the 2.4 and 3.4 (particularly) speeds the waveform persists to the 100 meter depth. Hence, one can see that a faster flying airplane implies deeper penetration by the sound waves. Please see the journal article for additional details.

This research was funded, in part, by NASA Langley Research Center.