Concept: Sound waves are longitudinal mechanical waves and frequency can be defined as the number of wave cycles happening in one sec . The types of sound waves whose frequencies are lower than the audible limit of human hearing are called Infrasonic wav es. Frequency can be divided into three categories based on their frequency range : Audible sound waves: The frequency range of this wave is 20Hz - 20000Hz . Humans can easily detect these types of waves. Example: Sound produced by Vocal cords. Infrasonic waves : The frequency range of these types of waves is below 20Hz . Humans cannot detect it. Example: Sound produced by Earthquake, Volcanic eruption and ocean waves, etc. Ultrasonic waves or Ultrasound waves: The sound frequency above 20,000Hz is known as ultrasonic waves . Humans cannot detect it too. Examples: dog whistle is an example of an Ultrasound wave. Explanation: The average hearing range for a human is between 20 Hz to 20 kHz and uses three muscles to aid the hearing process. There are, however, animals who have better hearing than that of humans. These animals which communicate with each other at infrasonic noises include: Elephants - Elephants can communicate using very low-frequency sounds, with pitches below the range of human hearing. These low- frequencysounds, termed "infrasounds," can travel several kilometres, and provide elephants with a "private" communication channel that plays animportant role in elephants' complex social life. Their frequencies are as low as the lowest notes of a pipe organ. Pigeons - Pigeons can hear infrasound, sounds that are much lower than a frequency the human can hear. With the average pigeon being able tohear sounds as low as 0.5 Hz , they can detect distant storms, earthquakes, and even volcanoes. Whales - Whales too, communicate at infrasonic levels . Bats, on the other hand, emit sounds at typically 2 or 3 times higher pitch than we can hear. Sounds above human hearing are usually called ultrasonic . Hence, Statement 2 is NOT correct.