### Theory:

Sounds are of different frequencies, but not all the sounds are being heard by human beings. So, we classify sounds into two different categories.
• Audible sound
• Inaudible sound
Audible sound:
• Sounds, which can be heard by us, are called audible sound.
• Human beings can hear sound having a frequency from $$20$$ $$Hz$$ to $$20,000$$ $$Hz$$.
Inaudible sound:
• Sounds, which cannot be heard by us, are called inaudible sound.
• Human beings cannot hear sound having a frequency less than $$20$$ $$Hz$$ or more than    $$20,000$$ $$Hz$$.
Infrasonic:
• The sounds that have a frequency range below $$20$$ $$Hz$$ is called infrasound.
• Infrasound is employed in the earth monitoring system.
• It is also used in the study of the mechanism of the human heart.
Example:
Whales and elephants produce sound in the infrasound range.

Whale

Ultrasound:
• The sound waves that have a frequency range above $$20,000$$ $$Hz$$ are called ultrasounds.
• Children under the age of five, as well as some animals like dogs, can hear frequencies up to $$25$$ $$kHz$$.
Example:
Ultrasound is produced by animals like dolphins, bats and dogs.

Dog

Applications of Ultrasound:

Ultrasounds are high-frequency waves. They can travel along well-defined paths even when obstacles are present. Ultrasounds are widely used in industry as well as in medicine.
• It is extensively used in medical applications like 'sonogram' and SONAR system to detect the sea's depth and enemy submarines. It is also employed in dishwashers.  Another important application of ultrasound is the Galton's whistle. This whistle is inaudible to the human ear, but the dogs can hear it.
• Ultrasound is commonly used to clean parts that are difficult to reach, such as spiral tubes, oddly shaped parts, and electronic components. Objects to be cleaned are submerged in a cleaning solution, which is then bombarded with ultrasonic waves. Dust, grease, and dirt particles become detached and ejected as a result of the high frequency. As a result, the objects are thoroughly cleaned.
• Ultrasounds can be used to identify cracks and flaws in metal blocks. The cracks or flaws inside the metal blocks, which are invisible to our eyes, reduces the strength of the structure.
• Ultrasonography is a technique for obtaining images of internal organs like the kidney, liver, and uterus. It aids the doctor in diagnosing and treating problems in the patient's body. In this method, ultrasound waves are sent through the body to the organ in question, and the reflected waves are converted to electrical signals that can be viewed on a screen.
• Ultrasound waves may be employed to break small stones formed in the kidneys into small fragments or grains. These grains get flushed out with urine.
Reference:
https://commons.wikimedia.org/wiki/File:Dog_Breeds.jpg
https://commons.wikimedia.org/wiki/File:Humpback_whale_with_her_calf.jpg