Infrasonic sound - The sound which has the frequency less than 20 Hz .
Audible range to humans- from 20Hz to 20,000Hz .
Denser medium - faster transmission of sound .As speed of sound is directly proportional to the square root of elasticity of medium , therefore in a denser medium , sound travels faster than a rarer medium .
Ultrasonic sound - sound which has frequency more than 20,000Hz .
Rarer medium -slower transmission of sound .