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Everyone who has taken a science class should know that sound is measured in decibels (dB). However, human hearing is measured in decibels A-weighted (dBa), namely because the human ear cannot hear lower audio frequencies. A-weighting is used particularly for environmental and industrial noise. A typical conversation is about 60 dB, and it is reported that prolonged exposure to sounds of 85 dB or higher can cause permanent hearing damage. On the opposite end of the spectrum, an audio lab at Microsoft’s headquarters was dubbed the quietest place on Earth at -20.6 dBa by the Guinness Book of World Records.

The audio lab is anechoic (free from echoes) so that Microsoft can test the latest in sound equipment. The room is covered in wedges of sound-proof material and built on springs to cut down on any vibrations that might occur during testing. Humans can’t hear anything below 0 dBa. “To give you a rough idea, the Brownian motion (movement of particles in a gas or liquid) is around -23 dBa. You can’t get any quieter because that’s just the air particles moving,” said an engineer at Microsoft.

They describe the room in Building 87 on the Microsoft campus as “on the edge of what are the limits of physics,” and are using it to test Windows 10’s Cortana app to handle inputs and outputs. This room breaks the previous record of -13 dBa, also achieved in an anechoic test chamber at Orfield Labratories in Minneapolis in 2012. Microsoft had achieved sound floors as low as -9.4 dBa in 2004. While -23 dBa is the theoretical low, it could be possible to go even lower in a vacuum in space, where there are no moving air particles.

On the opposite end of the spectrum, the loudest-ever recorded sound was the 1883 eruption of Krakatoa, a volcanic island located in Indonesia between Sumatra and Java. The sound was so loud that it was heard as far away as Mauiritus, around 3,000 miles away from the eruption, and it is reported to have reached as high as 180 dB. It was also reported that if anyone was within 10 miles of the volcano, they would have gone deaf immediately.

Decibels are measured on a logarithmic scale, meaning 40 dB is 10 times more powerful than 30 dB. While many may think 0 dB is absolute silence, it’s only absolute silence to the human ear. This means that anechoic chambers such as the one in Microsoft’s Building 87 are the perfect place to build and test audio products. “We designed this and other super-quiet acoustically-controlled chambers to engineer and build best-in-class audio products at Microsoft. We use these facilities for designing products like the Surface, HoloLens and Cortana, that we take great pride in,” said Hundraj Gopal, Ph.D., Microsoft principal human factors engineer.

“We are absolutely thrilled with the results of our chamber for Microsoft,” said Jeff Morse, vice president at Eckel Noise Control Technologies, the company responsible for designing and building the chamber. “Not only did we fulfill the requirements they needed for their testing, but we were able to deliver a facility that set a new world record. We couldn’t be more honored to have been part of this exciting project.”

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