 Most sounds are frequency-modulated (FM) down-swept infrasonic pulses from 16 to 40 hertz frequency (the range of sounds that most humans can hear falls between 20 hertz and 20 kilohertz).Each sound lasts between one and two seconds, and various combinations of sounds occur in patterned sequences lasting 7 to 15 minutes each.These sequences are then repeated in bouts lasting up to many days.