SOUND, CLASS 8

 

SHARMA CLASSES BHARATPUR

 

Sound: it is a kind of energy which gives us sensation of hearing.

• The sound is produced by vibrating objects.
• They travel from one place to another in the form of waves. Hence, the name sound waves.

 

Wave and particle motion of waves

• Mechanical waves are waves that travel through a material medium.
• It is of two types: depending on the direction of motion of the particle of the medium and the wave propagation:

 

Transverse waves

• Particle motion is to perpendicular the direction of wave motion.
• This type of wave is a mechanical wave called a transverse wave. E.g.: Light, or even  Mexican wave in a stadium.

Longitudinal waves

• When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction.
• It is also a mechanical wave.
• Example: a slinky

A longitudinal wave consists of successive compression and rarefaction that is formed due to continuous to and fro motion of a vibrating object.
A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium. It is a region of high pressure and high density.
A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium. It is a region of low pressure and low density.

 

·         Sound is produced by vibrating the objects and it is carried in all directions with the help of a medium.

·         Sound travels through air when air molecules vibrate. If air molecules are considered as small balls, a sound wave travels through air by pushing these balls close to each other and then pulling them away from each other. The air molecules come together in areas called compressions and they lie away from each other in areas called rarefactions. The air molecules do not move from their place but only vibrate about their original position. This vibrating effect propagates and allows sound to travel through air.

Speed of sound: speed of sound increase as the density of material increase. As more near the particle faster the energy transmits.

·         Speed of sound in air: 343 m/s

·         Speed of sound in water: 1480 m/s

·         Speed of sound in steel: 5100 m/s

 

Sound Properties

·         Sound needs a medium to travel. It cannot travel in a vacuum.

·         Sound needs a medium to propagate. The matter or material through which sounds propagates is called a medium.

How do humans produce sound?

• The sound produced in the voice box called larynx located at the upper end of the windpipe.
• 2 vocal cords get stretched across in the voice box. Has a slit, through which air is forced out by the lungs.
• Muscles attached to vocal cords make it tight or loose.

Hearing: We hear sound through our ears.

The eardrums of our ears sense the vibrations produced by a vibrating object and send them to the brain as the stimulus. This process is called a hearing.

 

Human ear : Human ear is made of three parts:

·         Outer ear – This is the visible part called pinna. It collects sound waves and directs them to the ear tube, at the end of which lies the ear drum. When sound strikes, the ear drum vibrates and passes sound to the middle ear.

·         Middle ear – It is a cavity containing three important bones, placed in a manner that they can move with the vibrating ear drum and transmit sound to the inner ear.

·         Inner ear – It contains fluid, which vibrates with transmission of sound and excites tiny hair in inner ear. The hair transform these vibrations into electrical impulses and these are transmitted to the brain through auditory nerve.

 

 

Amplitude, frequency and time period of vibrations

• The magnitude of disturbance in the medium on either side of the mean value is called as Amplitude(A). Larger the amplitude, louder the sound.
• The number of oscillations per second is called frequency. Expressed in Hertz (Hz).
• Time taken for one complete oscillation to travel across a point. T = 1/f. (Seconds)

 

 

Audible and inaudible sounds

• Audible range = 20Hz to 20kHz known as the Sonic range.
• Below 20 Hz (inaudible) → infrasonic range
• Above 20 kHz (inaudible) → Ultrasonic range

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The loudness of Sound: Larger the amplitude of vibration, louder is the sound.

The loudness is expressed in a unit called decibel (dB). It also depends on the amplitude of the sound.

Noise: Unpleasant sounds are called noise.

Oscillation motion: The to and fro motion of an object is called oscillation motion.

Pitch of the Sound: Higher the frequency of vibration, the higher is the pitch, or shrill of the sound.

Shrillness: The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we can say that sound is shrill.

Loudness and Pitch

• Volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.
• The number of oscillations per unit time. Directly proportional to frequency.

 

Noise Pollution: The presence of excessive or unwanted sound in the atmosphere is called noise pollution.

Major causes of noise pollution are sounds of vehicles, explosions including the bursting of crackers, machines, loudspeakers etc.

Presence of excessive noise in the surroundings may cause many health-related problems e.g., lack of sleep, hypertension and anxiety, Can cause hearing impairment, sleeplessness and also hypertension.

Plantation on the roadside and elsewhere is the best source to reduce the noise pollution.

The three categories of musical instruments are:

Stringed instruments – These instruments have an air chamber to enable increasing the loudness of the sound produced by plucking strings. The pitch is altered by altering the length of the vibrating portion of string. E.g. guitar, sitar, violin

Wind instruments – In this sound is produced by vibrating air column within the instrument. The pitch is altered by altering the length of vibrating air column. E.g. flute, trumpet, harmonica.​

Percussion instruments – Sound is produced by vibrating skin or membrane. Pitch can be altered to some extent by increasing or reducing the tension of membrane or by changing the force with which the membrane is struck. E.g. tabla, drum and dholak.