Wednesday, May 9, 2012

Maximum theoretical SPL exceeded by NASA

As described earlier, NASA used numerous drivers at the mouth of a horn and used the throat of the horn to simulate the effects of extreme sound pressures generated during a rocket launch on payload instrumentation, mechanical apparatus, and materials. Driving the horn "backwards" amplifies the sum of each driver's individual maximum SPL and using many, many drivers permits huge sound pressures to be generated at the throat of the horn with very little power (as compared to trying to achieve this over a large area). Along with answering the questions of "what happens to the gear?" two additional question they investigated were "1) how loud can it go, and 2) what happens when the theoretical maximum level is exceeded?"  One may think of this experiment as "boys and their toys" but it was in fact an opportunity to explore an unknown. Sort of like going to the moon and not taking pictures would be a missed opportunity, so would be not driving this test apparatus to its physical limits.

What they found is that the theoretical maximum SPL could be exceeded (not in free air but within the test apparatus).  While the actual sound pressure achieved is considered to be classified (and a mystery to me as to how it was actually measured), the effect at such levels was disclosed to me back in the 1970s by one of the team members (if you are reading this, hi Burt!). The nice sinusoidal wave we all have grown to hear and appreciate starts to change its actual shape being less and less sinusoidal and more and more triangular. To get an idea of what that may have "sounded like," go to and listen to the difference between a sinusoid and a triangle wave. There is a distinct addition in high frequency content for the same fundamental frequency.

So as the air is compressed and rarefied inside the horn under such extreme stresses, a transition occurs similar to that of an airplane exceeding the speed of sound, but more like phase changes in Doppler distortions.

And as it is with boys and their toys, I was told that a piece of steak was fastened to the test plate at the throat to observe what would happen (presumably to see what effects there would be on a living tissue).  What happened? Think microwave on steroids.

Yours for higher fidelity,
Philip Rastocny

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