September 10, 1998

It was a crazy summer. I was very busy at work, I took a class at Stanford and I got married...all in one summer!

Two months ago we finished up our wind-off calibration of the new 40- by 80-Foot Wind Tunnel test section. (Wind-off means we didn't turn the wind tunnel on. We will do the wind-on calibration later this month.) For the wind-off calibration, we were interested in how much of an echo we were going to get from the walls of the wind tunnel.

During the first month we fired off pistols and cannons and other noise sources to measure the reflections as I discussed before. The remaining part of the test was an assessment of the decay properties of the test section.

Imagine you are out in the middle of a field somewhere. In front of you is a loudspeaker with your favorite music playing. As you move away from the speaker the music will get quieter and quieter until you can't hear it at all. In fact, you may notice that the loudness of the music will decay rapidly as you first start to move away. However, when you are further away, you will have to walk a much greater distance to notice anymore change. Try it sometime.

This phenomenon is well known in science and applies to other "point sources" such as light and radio waves. Sometimes acousticians call it the "6 dB per doubling law." It says that the sound pressure will decrease by 6 decibels each time you double your distance from the speaker. So if the speaker is screaming at 100 dB at 10 feet away, it will still be at 94 decibels from 20 feet away. (A decibel (dB) is a measure of sound pressure. A whisper is about 50 dB, someone talking is about 70 dB, a jet plane might be 100 dB, and a rock band might get to 120 dB near the stage.)

The above is not true in a room, however. Because of all the reflections from the walls the noise can still be loud even from far away. This is why a basketball game is so much louder inside an auditorium than it is on an outside court. For our wind tunnel, we want the noise environment to be the way it would on the outside. One way to test this is to place a noise source at one end of the tunnel and position a microphone at different distances from the source and determine if the sound does indeed follow the "6 dB per doubling law." Of course it won't because we're in a big room, but we can get close to it if we designed the room correctly.

We used three noise sources to test the decay:

1) An 18-inch woofer in a speaker cabinet. The woofer provides low frequency noise.

2) The dodecahedron. The dodecahedron is a 12-sided ball with twelve 8-inch speakers mounted in each face. The dodecahedron is supposed to radiate mid-range frequency sound evenly in all directions.

3) The "Screaming Ted." The Screaming Ted Nugent, or Screaming Ted (named after a loud heavy metal guitarist from another era), produces ultrasonic white noise. It sounds like a high pitched hiss. Most of the sound generated by the Screaming Ted can't even be heard by humans, but dogs can probably hear most of it!

With each of these noise sources we measured the decay of the noise with distance at different frequencies. We are still analyzing the data but it seems that we have a pretty good anechoic wind tunnel. This is a good thing because it cost you $25 million.