Anechoic Chamber is in Building 87 at Microsoft's headquarters in Redmond, Washington, is currently recognized in Guinness World Records as the quietest room in the world, with a measured background noise of -20.35 dBA, 20 decibels below the threshold of human hearing.
If you stand in it for long enough, you start to hear your heartbeat. A ringing in your ears becomes deafening. When you move, your bones make a grinding noise. Eventually you lose your balance, because the absolute lack of reverberation sabotages your spatial awareness. In this room at Microsoft's headquarters in Redmond, Washington, all sound from the outside world is locked out and any sound produced inside is stopped cold. It's called an "anechoic" chamber, because it creates no echo at all.
The background noise in the room is so low that it approaches the lowest threshold theorized by mathematicians, the absolute zero of sound the next step down is a vacuum, or the absence of sound.
An anechoic chamber (an-echoic meaning "non-reflective, non-echoing, echo-free") is a room designed to completely absorb reflections of either sound or electromagnetic waves. They are also often isolated from waves entering from their surroundings. This combination means that a person or detector exclusively hears direct sounds, in effect simulating being inside an infinitely large room.
Anechoic chambers, a term coined by American acoustics expert Leo Beranek, were initially exclusively used to refer to acoustic anechoic chambers. Recently, the term has been extended to RF anechoic chambers, which eliminate reflection and external noise caused by electromagnetic waves.
Anechoic chambers range from small compartments the size of household microwave ovens to ones as large as aircraft hangars. The size of the chamber depends on the size of the objects and frequency ranges being tested.
The room offers a very rare sensorial experience. "As soon as one enters the room, one immediately feels a strange and unique sensation which is hard to describe," wrote Hundraj Gopal, a speech and hearing scientist and the principal designer of the anechoic chamber at Microsoft.
"Most people find the absence of sound deafening, feel a sense of fullness in the ears, or some ringing. Very faint sounds become clearly audible because the ambient noise is exceptionally low. When you turn your head, you can hear that motion. You can hear yourself breathing and it sounds somewhat loud," he said. In the real world, Gopal explained, our ears are constantly subject to some level of sound, so there is always some air pressure on the ear drums. But upon entering the anechoic room this constant air pressure is gone, since there are no sound reflections from the surrounding walls.
To achieve extreme silence, the room is designed with an onion-like structure that isolates it from the rest of the building and the outside world. It's made of six layers of concrete and steel and it is somewhat disconnected from the surrounding building, because it sits atop an array of vibration damping springs. Inside, fiberglass wedges are mounted on the floor, ceiling and walls to break up sound waves before they have a chance to bounce back into the room. The floor itself is simply a grid of sound-absorbing suspended cables.
"The design, planning and construction took a little over one and a half years," confirmed Gopal.
"I needed a good location on campus where the measured noise levels inside the building were adequately low. I needed to plan the inside of the building so that I could nest the chamber six levels deep. We had to build a special 12" concrete wall encasing the chamber to block out further ambient sound. Careful attention was paid to every detail that could conduct sound from outside to inside the chamber, such as isolating the sprinkler supply pipes and the fire-alarm sensor, or lining the air-supply and return ducts with additional sound absorbing material."
The final result? "The noise level measured inside is -20.3dBA. This means that the ambient noise in the chamber is 20.3dB below the threshold of human hearing." By comparison, one of the quietest sounds that can be heard in a quiet room, calm breathing, clocks in at 10dB.
Microsoft uses the room as an acoustic research lab for several of its products and components, including speakers and microphones. Currently, the company is using the acoustic lab to test Cortana, the company’s voice assistant adding background noise in the room while simultaneously asking questions, to measure and ultimately improve upon performance of Cortana.
According to Microsoft, the same company who built the previous record-breaking room for Orfield Lab, Eckel Noise Control Technologies, helped create the new room for the company. Microsoft’s Audio Lab itself actually consists of multiple anechoic chambers, with the largest sitting on top of springs in order to keep the amount of outside noise to an absolute minimum. Highly specialized instrumentation from Brüel & Kjær was used to analyze and measure the room. A two-microphone coherent power measurement technique was used to achieve the extremely low ambient noise level measurement.
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