The Types of Sound : 

We can't hear all kinds of sound. On the basis of hearing there are three types of sound: 

(1) Infrasound : The sound of frequency less than 20 Hz is called infrasound or infrasonic sound. Such type of sound can't be heared by human being. There are some animals that produce infrasound. 

(i) Rhinoceroses : Rhinoceroses communicate using infrasound of frequency as low as 5 Hz.
(ii) Whales and Elephant : Whales and elephants produce sound in the
infrasound range.

(iii) Rats and snake : It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.

(2) Audible sound : The sound of frequency between 20 Hz to 20000 Hz is called audible sound. 

(3) Ultrasound : Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound is produced by dolphins, bats and porpoises. Rats also play games by producing ultrasound.

Hearing Aid: People with hearing loss may need a hearing aid. A hearing aid is an electronic, battery operated device. The hearing aid receives sound through a microphone. The microphone converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. The speaker converts the amplified electrical signal to sound and sends to the ear for clear hearing.

Applications of Ultrasound : 

Ultrasounds are high frequency waves. Ultrasounds are able to travel along welldefined paths even in the presence of obstacles.

Ultrasounds are used extensively in industries and for medical purposes.

Applications of Ultrasound:

(i) Ultrasound is generally used to clean parts located in hard-to-reach places, for example, spiral tube, odd shaped parts, electronic components etc.

(ii) Ultrasounds can be used to detect cracks and flaws in metal blocks.

(iii) Ultrasound is used in the medical field to detect illnesses/diseases.

(iv) Ultrasound is used in a device named SONAR, which is used to measure the depth of ocean in the ships.