Super High Velocity* & Wide Coverage. The cross-flow fan delivers high air velocity and wide coverage, creating an invisible air curtain that reduces unwanted environmental air flow, such as hot or cold air from an open door. * FY-4009U118, FY-4012U118 & FY-4015U118 only The wavelength of sound in air is given by; Eq. 1 λ = co /f Where λ is the wavelength in feet (m). co is the speed of sound, which is 1120 feet per second (341 m/s) at sea level. f is the frequency in Hz. Decibels The very large range in sound pressure makes a logarithmic scale more convenient. Decibels (dB) are always referenced to base signal.