Page 22 - January 2006

LOWERING THE BOOM
Victor W. Sparrow
Graduate Program in Acoustics, The Pennsylvania State University University Park, Pennsylvania 16802
Recently there has been substantial renewed interest by ASA members and others regarding the transient sounds of supersonic airplanes, called sonic booms. In fact, during July 21-22, 2005 an International Sonic Boom Forum was held in State College, PA, organized by the present author and Francois Coulouvrat of the French National Center for Scientific Research (CNRS) at Pierre and Marie Curie University (Paris VI). This Forum was a set of special sessions of the 17th International Symposium on Nonlinear Acoustics, co-sponsored by the ASA. There were 30 technical papers as well as panel discussions from indus- try and government. Participants were from seven coun- tries, and included presentations by the National Aeronautics and Space Administration (NASA), the Federal Aviation Administration (FAA), industry, and university researchers. Clearly there has been recent resurgence of interest in sonic boom, including new funded research. Where is all this new interest coming from? This article will try to answer this question and provide some highlights of recent sonic boom research.
The need for speed
In our busy, daily lives we try to find ways to save time in many ways. Technology can help us save time. A good exam- ple, pointed out by Sam Bruner of Raytheon Aircraft Co., is the cell phone. The cell phone links us to others with imme- diacy and convenience in a way unheard of in previous gen- erations. Twenty years ago mobile telephones were bulky, and only used by a few. They were expensive, but they did lay the groundwork for the personal communication system avail- able today.
Another way we can save time involves personal trans- portation. People who have jobs involving substantial travel have a strong desire to minimize the actual travel time. Our global airline network is at capacity and is limited by the fleet of existing aircraft. Current aircraft fly at speeds less than the
speed of sound in air, nominally 340 m/s (767 mph). It seems a reasonable way to save travel time is to simply fly faster, i.e., supersonically.
Commercial supersonic travel was possible on a handful
of airline routes during the 1980s and 1990s on the
French/British airplane, Concorde. Concorde was a technical
success, but economically and environmentally it was a fail- 1
ure . Concorde created substantial sonic boom noise while in supersonic flight, limiting its utility. In fact, until recently, all supersonic flight created objectionable sonic boom noise. But aircraft designers now have tools they believe will reduce or eliminate objectionable sonic boom noise.
A snapshot of a sonic boom
What is a sonic boom? It is a pressure disturbance creat- ed by the passage of an aircraft, or any other object, traveling faster than the speed of sound. A typical non-minimized sonic boom time trace measured at the ground is given in Fig. 1. The vertical axis is pressure in pascals and the hori- zontal axis is time in seconds. This particular graph is an example of a rounded sonic boom waveform produced by an
2
F-15 aircraft taken from the BoomFile database . Notice that
the pressure versus time waveform roughly looks like the let- ter “N.” Most conventional sonic boom waveforms look sim- ilar to this, although the atmosphere and aircraft maneuvers can alter the shape. Notice there is a distinct beginning and ending to a sonic boom waveform. This is because the air- craft has a finite length.
The peak pressure of this sonic boom is about 85 pascals. One poundforce per square foot (psf) is about 48 pascals, giving this boom a peak pressure of 1.77 psf. This is a very small fraction of 1 atmosphere (about 101,325 Pa), but most people would consider this a loud boom. In the U.S., units of psf are still used to describe sonic boom amplitudes instead of pascals as used by everyone else.
The most important point to gain from this article is that the sonic boom is made continuously during the entire time that the aircraft is moving supersonically. This means that the sonic boom is present everywhere along and adjacent to the aircraft supersonic flight path. This acoustically impacted area along the aircraft flight path on the ground is called the primary sonic boom carpet, and it may be on the order of 75 km wide (40 miles wide) or larger depending on the aircraft altitude and the meteorological conditions. The sonic boom wave sweeps out a conical shape as it propagates. The ground is impacted by the boom in the shape of a truncated hyper- bola. An overall picture, suggested by the work of Carlson3, given in Fig. 2 shows this cone being swept over the ground behind the aircraft.
The reader can also see the analogy between a superson- ic airplane and a fast moving speedboat in the water. The speedboat creates a V-shaped wake that spreads out behind
Fig. 1. Example measured waveform of F-15 aircraft. This particular waveform is somewhat rounded in shape.
20 Acoustics Today, January 2006