Victor W. Sparrow (vws1@psu.edu), and
Steven L. Garrett (sxg185@psu.edu)
Graduate Program in Acoustics,
The Pennsylvania State University,
201 Applied Science Bldg.,
University Park, PA 16802, USA
Lay Language version of 3aSAa4
"Create an audio system to sonic boom an entire house?"
Presented at 9:30 a.m. on Wednesday, November 12, 2008 in Room Legends I
156th ASA Meeting, Miami, FL
How do you evaluate the annoyance produced by a sonic boom that would be created by an aircraft that does not yet exist? If the aircraft does not exist, who cares about the annoyance of a non-existent sonic boom? It turns out that lots of people care because billions of dollars ride on the answer to what sounds more like a Zen koan than a scientific research challenge.
The answer is both simple and complex. The simple answer is that conventional sonic booms are so startling that current Federal regulations prohibit the flight of supersonic aircraft over land. For that reason, it is hard to make a convincing business case for the development of a new class of supersonic jets if they cannot fly over land. But in the past several years, new aircraft design strategies have been developed that reduce the magnitude of the sonic boom. The complex part of the answer is the subject of the research we are doing at Penn State. Our research is directed toward determining the answer to the central question: “How quiet is quiet enough?” To provide that answer, we first need to understand what creates the shock wave that supersonic aircraft “drag” behind them and produces the sonic boom on the ground.When an aircraft travels faster than the speed of sound, the air directly in front of the flight path cannot get out of the way fast enough, so it “bunches up” creating large pressure fronts known as shock waves. When that bunched-up air reaches ground level, the pressure front is called a sonic boom. The effect of that shock wave is usually so startling that supersonic flight by commercial aircraft over land is prohibited. Because of these legal restrictions, only people who live near military bases or along the descent path of the Space Shuttle are exposed to sonic booms.
Although most people have not experienced the sonic boom created by supersonic flight, everyone has observed an analogous effect when they watch a duck or a speed boat on a lake creating a V-shaped wake behind it. The wake is a result of the duck or the boat traveling faster than the speed of surface waves on the lake – the water bunches up in front and these bunches travel all the way to the shore. The breaking of the waves at the shoreline is similar to the sonic boom created when the aircraft’s shock wave reflects from the ground and the sonic boom is heard.
Ducks haven’t changed in millions of years. For the most part, boats have not changed in thousands of years. But manned controllable flight by machines that are heavier-than-air is just slightly over one hundred years old – compared to ducks and boats, aircraft are evolving very rapidly.
In just the last few years, new supersonic aircraft designs have been demonstrated that can cause a significant reduction in the “bunch up” that causes the sonic boom on the ground by giving the air a hint of what is coming. One way to do this is by putting a long spike on the aircraft’s nose as shown below, or making other modifications to the shape of the aircraft.
Supersonic jousting: A retractable, 24-foot-long retractable lance-like Quiet Spike, produced by Gulfstream Aerospace Corp., is mounted on the nose of a NASA Dryden's F-15B research aircraft. Combined with other technologies, that spike can create a significant reduction in the amplitude and annoyance of the sonic boom on the ground. [NASA photo by Tom Tschida].
The development of a new class of quieter supersonic business jets or commercial supersonic jet transports will cost at least a billion dollars. Before making an investment of that magnitude, aircraft manufacturers need to know how loud the sonic boom can be and not be a source of annoyance to people on the ground. It is very important to determine this threshold-of-annoyance because the amount of boom reduction must be traded-off against the additional weight and drag caused by the boom-reduction technologies that reduce their efficiency. Aircraft fuel efficiency has never been more important!
Thus, the conundrum that motivated our research: How do you evaluate the annoyance of a supersonic aircraft that does not yet exist? One way to break the impasse is to build audio simulation devices to play computer-synthesized sonic booms of varying strengths and have people rate their acceptability. This has been done, and is ongoing, in laboratory environments. The difficulty is that people might react differently in a science lab setting compared to where they spend most of their time: at work or at home, where the boom can cause the dishes to rattle.
Earlier this year, the FAA began to recognize that they needed a capability for reproducing sonic booms outside people’s homes or offices to find out how they would perceive them. Would they always be able to hear the new low booms inside their home? Do they still cause pictures to bounce against the walls? Does the china cabinet rattle? Will it wake you if you are sleeping? Are they annoying?
The Pennsylvania State University is now engaged in a research study to design a sonic boom reproduction device that can present sonic booms outside an entire house, or a portion of a house. This has tremendous technical challenges, so much so we call it an Audio Grand Challenge. The aircraft manufacturers are providing the synthesized sonic boom waveforms for playback, thus we have those. The work is funded by the FAA though the PARTNER Center of Excellence (www.partner.aero). So what are the challenges?
Firstly, one needs to produce frequency content that is very low. The frequency content of sonic booms usually peaks somewhere between 2 and 8 hertz, or cycles per second. This is below the lowest frequency that most people with normal hearing usually perceive, which is 20 hertz and an octave below the lowest note on a bass. But the low frequencies between 2 and 8 hertz match up very well with the frequencies of structural vibration of typical American home construction. Thus, the walls, ceilings, and floors of your home react very strongly to pressure waves at these frequencies, and you might react strongly to your home vibrating, so the low frequency content can be very important indoors. Some of our preliminary calculations have shown that we may need to move as much of 15 cubic yards of air per second for adequate reproduction, and that is a lot of air! The calculations show that it will be difficult to produce adequate low-frequency sound with multiple, off-the-shelf 15 inch diameter subwoofer loudspeakers.
Another challenge is spatial coverage over your home. Sonic booms are very long spatially - as long as 60 yards - as their sound travels by your house. Thus your home is completely engulfed by the sonic boom wave as it passes by, and this is very difficult to recreate with an audio system.
Boom versus house: A still picture, to scale, taken from a computer simulation of a conventional sonic boom about to impinge on a portion of a house. [S. Cho and V. Sparrow].
A further challenge is that we need this reproduction system to be portable. It would be a nice academic project to set up a system that could only boom one individual home. But for us to get a good cross-sampling of different homes, with different building construction, in different parts of the country, and with differing community sensitivities to noise, we need to be able to take this reproduction system on the road. A reasonable turnaround time for setup, test, and removal might be one or two days at most.
One purpose of this paper is to solicit input from members of the Acoustical Society of America and other interested parties on how such an audio reproduction system would be built and operated. In early 2009, a down-selection activity will occur to decide upon the system with the best potential of meeting the goals for booming an entire house. This system will then be investigated more thoroughly to come up with an estimated cost of construction and operation.
With the results of these sort of subjective tests, and in consultation with other international regulatory bodies, the FAA may decide to reevaluate the regulations that restrict overland supersonic flights, which could then allow for the construction for a new fleet of supersonic jets.
[Work supported by the FAA/NASA/Transport Canada PARTNER Center of Excellence, www.partner.aero . Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the FAA, NASA, or Transport Canada.]