Science of Hearing- I

We hear a variety of sounds ranging from children making noise, people shouting at the market place, cries of animals, melodious music, chirping of birds, sounds of thunder on a stormy day, the rhythm of rains on a rainy day, the sound of wind and the movement of falling water etc.

Let us see how different types of sound are created, how we are hearing them and interpreting them. We are hearing with our sense organ ears and interpreting and distinguishing the different types of sound with the help of nerves of the brain.

Sound is a form of energy. Therefore, sound can do work. Sound is created by 

vibrating objects. Sound is propagated in the form of waves. Sound waves 

require a medium for their propagation. The medium is either air or water. 

Speed of sound in air is 343.2 metres per second.  The speed of sound varies 

from substance to substance. Sound travels faster in liquids and non-

porous solids than it does in air. It travels about 4.3 times faster in water.

Sound is a type of energy made by vibrations. When any object vibrates, it 

causes movement in the air particles. These particles strike the particles close 

to them, make them vibrate and causing them to strike into more air particles. 

This movement is called sound waves.

Sound will be heard if your ear is within the range of the vibrations.

When a stone thrown into a still body of water. The rings of waves expand 

indefinitely. The stone disturbs the water particles to produce waves.

Sound is called Noise when the waves are repeated irregularly.

When sound waves are repeated at regular intervals, it produces Musical Note.

When the vibrations are fast, we hear a high pitched note. When the vibrations 

are slow, we hear a low pitched note.

Echo is produced when sound is reflected back after some time. Sound travels 

approximately 343 metres/s (1100 ft/s). If a sound produces an echo in 2

seconds, the object producing the echo would be precisely the same distance 

away. Animals uses echo to catch their prey. Eg: Dolphins, Whales and Bats.

Echo-location is biological sonar used by several kinds of animals. Echo-locating 

animals emit calls out to the environment and listen to the echoes of those calls 

that return from various objects near them. They use these echoes to locate 

and  identify the objects. 

What creates sound?

Vibration of the air particles in the medium creates sound. 

Whenever an object in air vibrates, it causes compression waves in the air. 

These waves move away from the object as sound.

Examples:

• The back and forth movement of a loudspeaker cone, guitar string or drum head result in compression waves of sound. When we speak, our vocal cords also vibrate, creating sound.
• Blowing across a bottle top can also create sound. In this case, the air inside the bottle goes in a circular motion, resulting in sound waves being formed. Wind blowing through trees can also create sound.
• Sound can also be created by vibrating an object in a liquid such as water or in a solid such as iron. A train rolling on a steel railroad track will create a sound wave that travels through the tracks. The tracks will vibrate, creating sound in air that we can hear when the train may be a great distance away.

Hearing range describes the range of frequencies that can be heard by humans 

or other animals. The hearing range of human beings is 20Hertz to 20000 Hertz 

or 20 KHz.

Anything above 20 kHz is called "ultrasound," because those sounds are higher 

than we can hear. Anything below 20 Hz is called infrasound.  Human beings 

cannot hear ultra sound or Infra sound.

Some dolphins and bats, for example, can hear frequencies up to 100 KHz. 

Elephants can hear sounds at 14-16 Hz, while some whales can hear subsonic 

sounds as low as 7 Hz (in water).

Ultrasounds are the sound waves with frequencies higher than the upper audible limit of human hearing. Human beings cannot hear. This limit varies from person to person and is approximately 20 Kilo Hertz. (20,000 hertz) in healthy, young adults. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.

Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultra Sound or Sonography is often used in Medical Technology. Industrially, ultrasound is used for cleaning, mixing, and to accelerate chemical processes. 

Medical Sonography is an ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, to capture their size, structure and any pathological lesions with real time tomographic images. Ultrasound has been used by radiologists and sonographers to image the human body and has become a widely used diagnostic tool.

Sonar is a device used underwater for locating submerged objects such as submarines and mines. It is used for submarine communication by means of sound waves. Sonar is simply making use of an echo. When an animal or machine makes a noise, it sends sound waves into the environment around it. Those waves bounce off nearby objects, and some of them reflect back to the object that made the noise. The reflected waves help to locate distant objects and can sense the shape and movement.

Ultrasound has been used since the 1940s by physical and occupational therapists for treating connective tissue: ligaments, tendons, and fascia.

High power ultrasound can break up stony deposits or tissue, accelerate the effect of drugs in a targeted area, assist in the measurement of the elastic properties of tissue, and can be used to sort cells or small particles for research.

Infra sound is a low frequency sound. Sound that is lower in frequency than 20 Hz per second is called Infra sound. This is the "normal" limit of human hearing. The study of these sound waves is referred as infrasonics covering sounds below 20 Hz down to 0.001 Hz. This frequency range is utilized for monitoring earthquakes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the heart.

Infra sound can result from Earth Quakes, Volcanoes, waterfalls, lightening, calving of ice bergs etc.

Whales, giraffes, elephants, alligators communicate using infrasound over distances.

Infrasound can be generated by processes such as sonic booms and explosions (both chemical and nuclear), or by machinery such as diesel engines, wind turbines and specially designed mechanical transducers.

 ( To be continued in the next post)