A Sonic Boom Box for Research and Persuasion
Want to hear a sonic bang? Just visit Gulfstream's
supersonic boom box down in the static park. There you can stand in the
seven-foot-high open bell mouth of a 32-foot-long folded horn (did any of you
see Back to the Future?), brace yourself—and hear almost nothing. And that's
the entire point.
The Supersonic Acoustic Signature Simulator II Road Show is
hardly worth listening to.
Now, the acoustic drivers are driving sound through this
enormous speaker and the bass trap is stopping it reverberating around the
trailer—but you hardly hear anything because what it is generating is a
representation of the quiet boom Gulfstream must achieve for its supersonic
business jet to be able to fly over land. And if the airplane can't do that,
then Gulfstream won't build it.
The challenge, says Pres Henne, Gulfstream's svp for
programs, development and test, is to shape the airframe to smooth out the
N-shaped boom-boom shock wave and its abrupt pressure changes into as near as
possible a smooth sine wave. "A perfect wave over the length of a supersonic
business jet would produce about a 6 or 7 Hz pressure pulse that your ear
doesn't respond to because it's too low a frequency," he explained.
The boom box doesn't go that far, but it produces what
Gulfstream today believes it can achieve tomorrow.
"A lot of people think that a boom is a boom, and that it's
very loud and you can't do anything about it. That's just not the case," Henne
says. "When people hear the difference [in the simulator] between the Concorde
and what we think we can get, it's dramatic. Skeptics come in, and after they
listen they are transformed."
The boom box is actually used by Gulfstream as a development
tool, and has been labeled by NASA one of the best in the world. It spends much
of its time traveling to conventions and meetings, helping turn the tide of
public opinion—and eventually legislation—toward acceptance of a supersonic
business jet whose boom has been finessed to a whisper.
"We play the Concorde boom and what we call the Gulfstream
whisper, and so far no one out of 700 people has said 'That's unacceptable,'"
Henne says. "A small percentage, two or three percent, have said 'It may be OK;
I don't know that it's quite quiet enough yet.'"
—John
Morris
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