Sound Waves

Sound is a longitudinal wave that is produced by vibrating objects. Examples of vibrating objects are:-

v  Turning fork

v  A whistle

v  As stretched string

v  A  stretched membrane(e.g. Ngoma)

v  The vocal cards of a human being

Production of sound waves

As object (turning fork) vibrates, let the surrounding air molecules into vibration.

As molecules vibrate regions of high pressure (compressions) and regions of low pressure (rarefactions) are observed, hence energy is transferred from one place to another.

Compressions:-

Are regions of high pressure of vibrations

Rarefactions

Are regions of low pressure of vibrations

Note.

          Sometimes sound waves are called Pressure waves.

SOURCES OF SOUND WAVES

Sound waves are produced by almost everything ranging from:-

v  People

v  Animals

v  Plants

v  Machines

Musical instruments are designed to produce specific types of sound.

v  Guitars

v  Violin

v  Piano’s

v  Flutes

v  Drums

v  Marimbas

AUDIBILITY RANGE

Is the range of sound waves which can be heard by a human ear. The human ear can detect sounds in the frequency range of 20 Hz to 20,000 Hz (20 KHz).

INFRASONIC

Are sounds with frequencies below 20 Hz.  Elephants, birds, whales are able to detect infrasonic sounds.

ULTRASONIC

Are sounds with frequencies above 20,000 Hz (20 kHz) some animals such as dogs, bats and dolphins can detect ultrasonic sounds.

THE HUMAN EAR

The human ear convert sound energy to mechanical energy and then to a nerve impulse that is transmitted to the brain.

Consist of three parts; the outer ear, middle ear and inner ear.

 

THE OUTER EAR

Consist of pinna and ear canal. In the outer ear, the sound is still in the form of pressure wave, with alternating pattern of high (compressions) and low – pressure regions (Rarefactions).

THE MIDDLE EAR

Is an air – filled cavity that consists of an ear drum, and three small interconnected bones, the hammer, anvil and stirrup.

The Eustachian tube connects the middle ear to the through. Its function into regulate pressure.

SOUNDS IN THE MIDDLE EAR

Vibrations from the outer ear force ear drum to vibrate at the same frequency. Vibrations (movements) of the ear drum set three bones into motion. The three bones hammer, anvil and stirrup amplify vibrations. Vibrations are then transmitted to the fluid of the inner ear by stirrup.

THE INNER EAR

Consist of the cochlea, the semicircular canals and the auditory nerves.

The cochlea and semicircular canals are filled with water like fluid.

Note:-

The fluid and nerve cells of the semicircular canals help in maintaining the body balance.

SOUND IN THE INNER EARS

Amplified vibrations by 22 times from the stirrup strike the oval window.

The oval window sets up waves in the fluids of the cochlea, the cochlea transforms sound into nerve impulses that travel to the brain through auditory nerves. The semi-circular canals send information on balance and head position to the brain.

Note:
If some fibers (nerves) are removed, the person becomes deaf to the frequencies corresponding to the removed fibers.

ECHO

Is a reflected sound which is heard distinctly from the original sound.

Note:

Echo as occur when the reflected sound reaches the ear more than 0.1 seconds after the original sound was heard- 

                                                              V = distance

                                                                       Time

 At 25°C, the speed of sound in air is about 340 m/s.

                                               V = ^d/_t

                                           340 = d/0.05

                                                d = 340(0.05)

                                                   = 17 m.

 \For the Echo to be heard, the obstacle must be at least 17 m away.

REVERBERATIONS

Is the persistence of sound in a particular space after the original sound is removed.

OR

Are the multiple reflections of sound waves when they are placed in a closed room or cavity

It is caused when a large number of echoes build up and then slowly decay as the sound is absorbed by the wall or water.

  

Example

The speed of sound in water is 1500 m/s. If it takes 2 seconds for the sound to reach the sea bed and return to the ship, calculate the depth of the sea.

                  \ The depth of the sea is 1500 m.

  

APPLICATIONS OF SOUND WAVES

i)        Used to locate shoals of fish or wrecks on the sea beds.

ii)       The sound of the engine of a motor car can give a warning to pedestrians.

iii)     Sound is used for communication with each other.