## Thursday, August 4, 2011

### Quietest Places on Earth?

Monday we saw the longest truck in America. Yesterday the tallest building in the world. Today in this series of blogs about superlatives, I'd like to investigate the quietest places on earth. I'll try to mention numbers and concepts you might need for quietness. Can elementary school math (as taught in our Excel Math curriculum) help us comprehend a lack of noise?

Noise is measured in decibels. As with the temperature 0 degrees, 0 decibels is not equal to NO NOISE at all. It is equal to the lowest level of sound that people can commonly hear. Instruments (and some animals) are capable of hearing sounds that we cannot.

Sound is a bit like water - it fills all the available space, it bounces around in waves, it can be deflected by a hard surface or absorbed by a spongy surface.

I took a little survey to see what people think is the quietest place on earth:
• My wife said the quietest place must be Carlsbad Caverns. One visitor said, "This is everyone's idea of what a cave SHOULD be: quiet, clean, dry, flat, full of fantastic formations, high ceilings, and good lighting." The contractors at the site say they ensure "Soundscape protection" by using low-noise equipment, turning all equipment off at night, and using decibel meters to monitor, measure and reduce noise impact. On the other hand, their "janitors" use special vacuum cleaners at night to remove lint from the formations next to the trail!

• Friend A suggested Mount Whitney, at 14,505 feet elevation. Maybe, but it depends when you go. Besides the wind and the occasional aircraft, the Forest Service reports 30,000 hikers and climbers a year go to the top, and 200,000 pass through the Whitney portal, mostly in the summer - so 1500-2500 cars a day are squeezing into 300 parking places.

• Friend B choose Antarctica. Although it is windy, and only 4000 scientists (with some of their kids) live there, it's still free of most noises. Underwater is another story. I learned about multiple sound tests near the Antarctic: blasting up to 205 decibels in Acoustic Thermometry experiments; low- and mid-frequency active sonar research with naval vessels; seismic surveys, dredging and construction; offshore wind farms, and science experiments. Go here and click on Explore to hear underwater sounds. [click on the map to enlarge it]

• Friend C chose the Grand Canyon in Arizona, but what about the tourists flightseeing? The latest proposed noise controls still permit 364 flights a day or 65,000 flights a year over (but not within the walls) of the Canyon. This is a complex issue - we want people to enjoy the natural resources but doing so in a plane or helicopter shouldn't spoil the day for those on foot. [click on the map to enlarge it]

• I like Anza-Borrego State Park, where 600,000 acres are protected. Although off-road vehicles are allowed in certain areas of the park, and a few planes fly overhead, it's very isolated. In the Badlands, even the terrain seems specially-constructed to absorb sound.

I turns out we were all wrong. Have you heard about anechoic chambers? Places specifically constructed to keep noise out?

The quietest place on earth is inside the Orfield Anechoic Chamber, up in Minnesota. (Only about 25 miles from the Road Train factory!)

Orfield Labs has a room inside a larger room, both contained within a third room. The inner-most anechoic space is a 6-sided steel box floating on springs, inside a larger 5-sided steel box, again spring-mounted on steel beams. These chambers are within a larger room with 1-foot-thick concrete walls and ceilings. The smallest room is filled with 3.3 feet thick fiberglass acoustic wedges as shown in the photo. This inner chamber was measured at negative 9.4 dB (the lower limit for human hearing is 0 dB).