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In page Lucent Technologies:

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The Murray Hill anechoic chamber, built in 1940, is the world's oldest wedge-based anechoic chamber. The interior room measures approximately 30 feet (9.1 m) high by 28 feet (8.5 m) wide by 32 feet (9.8 m) deep. The exterior concrete and brick walls are about 3 feet (0.91 m) thick to keep outside noise from entering the chamber. The chamber absorbs over 99.995% of the incident acoustic energy above 200 Hz. At one time the Murray Hill chamber was cited in the Guinness Book of World Records as the world's quietest room.[3] It is possible to hear the sounds of skeletal joints and heart beats very prominently.[citation needed]