If the human brain is exposed to one frequency in the left ear which is 10 Hertz below another audible frequency played in the right ear, rather than hearing either of the two audible frequencies, the brain chooses to “hear” the difference between them, the “beat” frequency.
Mount Pilchuck (Wikipedia)Mount Pilchuck (Lushootseed: bəlalgʷəʔ) is a mountain located in Snohomish County, Washington. It is 37 miles (60 km) northeast of Seattle. It is part of the Cascade Range.