class 9 / physics / sound / QUICK BITS

PHYSICS - Class 9

Sound

           ·         SOUND - Sound is a form of energy which produces a sensation of hearing in our ears.

           ·         PRODUCTION OF SOUND – sound is produced by vibrating body. e.g. tuning fork.

           ·         PROPAGATION OF SOUND - A sound wave travels in the form of mechanical longitudinal wave and it requires a medium for its propagation. The substance through which sound travels is called medium. It can be solid, liquid and gases.

When an object vibrates, around particles will also vibrate.

The particles do not travel all the way; they displace from their equilibrium position and then exert a force on the adjacent particles, because of this adjacent particles vibrate. After vibrating the adjacent particles the first particles come back to original position. This process continues in the medium till sound reaches our ear.

Thus disturbance (sound) created by source of sound travels through the medium, the particles do of the medium do not move but the disturbance is carried forward.

           ·         BELL JAR EXPERIMENT -  A bell jar is placed on a base which is vented to a hose fitting that can be connected via a hose to a vacuum pump. By pumping the air out of the bell jar, the air pressure inside the jar can be varied.

It is done by placing an electrical bell in the bell jar. As the air is pumped out of the sealed bell jar, the sound from the bell jar fades. At a particular vacuum, no more sound is heard from the bell, but we can see that the hammer continues hitting the gong and sound is produced. However, the sound is not audible to our ears because of the vacuum inside the jar. This demonstrates that the sound wave cannot travel through vacuum. That is, a sound wave needs a material medium for its propagation. 

             Thus two astronauts can not communicate in normal manner when they are in space as sound doesn't travel through vacuum.

·         TYPES OF WAVES

(i) Mechanical waves: A mechanical wave is a periodic disturbance which requires a material medium for its propagation. On the basis of motion of particles the mechanical waves are classified into two parts.

(a) Transverse wave (b) Longitudinal wave

(a) Transverse wave: When the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, the wave is known as the transverse wave. For example, waves produced in a stretched string.

(b) Longitudinal wave: When the particles of the medium vibrate along the direction of propagation of the wave then the wave is known as the longitudinal wave. For example sound wave in air.

(ii) Electromagnetic waves: The waves which do not require medium for propagation are called electromagnetic waves these waves can travel through vacuum also. For example, light waves, X-rays.

               ·         CHARACTERISTICS OF A SOUND WAVE -  sound wave is a mechanical                                                                                                               longitudanal wave.

When sound wave travels through a medium, say air, the particles of medium disturb in the same fashion, i.e. compression and rarefaction (depression). When air particles come closer it is called compression. On the other hand, when particles go farther than their normal position it is called rarefaction.

1.    Frequency: The number of vibrations per second is called frequency.

The unit of frequency is hertz

2.    Amplitude: The maximum displacement of each particle from its mean position   is called amplitude. Loud or soft sound is determined by amplitude.

High amplitude cause loud sound.

The S.I. unit of amplitude is meter (m).

3.    Time period: The time taken to complete one vibration is called time period.

Frequency, f= 1/(Time period) or f = 1/T

4.    Wavelength: The distance between two nearest (adjacent) crests or troughs of a wave is called its wavelength. Denoted as λ.

5.    Velocity of wave: The distance travelled by a wave in one second is called velocity of the wave (or speed of the wave). The S.I. unit for the velocity of a wave is meters per second (m/s or ms-1).

6.    Pitch: Pitch is the sensation (brain interpretation) of the frequency of an emitted sound and is the characteristic which distinguishes a shrill (or sharp) sound from a grave (or flat) sound. More frequency will cause high pitch.

7.    Loudness: It is a measure of the sound whether it is loud or feeble / soft.

8.    Quality / Timbre: it distinguishes the pleasant and non-pleasant sound. If quality is high sound is pleasant e.g music. And noise is unpleasant to listen.

9.    Intensity: the amount of energy per unit area is called intensity. It is not same as loudness.

10. Tone: a sound of single frequency.

11. Note: sound produced by mixture of several frequencies.

·         SPPED OF SOUND IN DIFFERENT MEDIUM

Sound speed varies as

Solid > liquid > gases.

As temperature and humidity increases then also speed of sound increases.

·         REFLECTION OF SOUND

When sound waves strike a surface, they return back into the same medium. This phenomenon is called reflection. They follow same laws of reflection as light does.

Laws of reflection:

1.    Angle of incidence is equal the angle of reflection.

2.    The incident wave, the reflected wave and the normal all lie in the same plane.

·         ECHO

Phenomenon of hearing back our own sound is called an echo.

If‘t’ is the time at which an echo is heard, d is the distance between the source of sound and the reflecting body and v is the speed of sound. The total distance travelled by the sound is 2d.

Speed of sound, v = 2d/t or d = v t/2

To hear an echo time interval between reflected and original wave must be 0.1 second.

If we take speed = 344 m/s, then the minimum distance between obstacle and source must be 17.2 m.

This distance varies with temperature.

·         REVERBERATION

The persistence of sound due to repeated reflection and its gradual fading away is called reverberation of sound.

It can be minimized by sound absorbent material or highly cushioned materials.

·         USE OF REFLECTION OF SOUND

1.    Megaphones / bulb horns

2.    Stethoscope

3.    Ceilings of concert hall

·         RANGE OF HEARING

The audible range of sound for human beings extends from about 20 Hz to 20,000 Hz.

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. Infrasounds are produced by pendulum, whales, elephants, and rhinoceros.

Frequencies higher than 20 kHz are called ultrasonic sound or ultra sound. Ultrasound is produced by dolphins, bat, and rat.

·         APPLICATION OF ULTRASOUND

1.    Used to clean parts located in hard-to-reach places.

2.    Used to detect cracks and flaws in metal blocks.

3.    Used in ECG ( electrocardiography)

4.    Used to treat stones in kidney

5.    Used in various medical treatments.

·         SONAR

SONAR means Sound Navigation and Ranging. In this sound waves (ultrasonic) are used [microwaves are absorbed by water)]. Sound waves are emitted by a source. These waves travel in water with velocity v. The waves reflected by targets (like submarine bottom sea) are detected.  

2d = v x t

Where, d = distance of obstacle and source

t = time to send and receive signal

v = speed of ultrasound waves in water.

Uses

(i) The SONAR system is used for detecting the presence of unseen underwater objects, such as a submerged submarine, a sunken ship, sea rock or a hidden iceberg, and locating them accurately.

(ii) The principle of SONAR is also used in industry of detection of flaws in metal blocks or sheets without damaging them.

·         STRUCTURE OF HUMAN EAR

It is a highly sensitive part of the human body which enables us to hear a sound. It converts the pressure variations in air with audible frequencies into electric signals which travel to the brain via the auditory nerve.

The human ear has three main parts. Their auditory functions are as follows:

(i) Outer ear: The outer ear is called `pinna’. It collects the sound from the surrounding. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane. When compression of the medium produced due to vibration of the object reaches the ear drum, the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches. In this way the ear drum vibrates.

(ii) Middle ear: The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear which act as levers. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear

(iii) Inner ear: In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.