# Physics : Sound Wave

What is sound?

• Sound is a form of Energy, associated with vibrations of matter.
• It is a mechanical wave which requires medium to travel through.

Propagation of Sound

• Sound wave is a disturbance which moves through a medium.
• The disturbance travels through medium, but the particles do not move themselves.
• Sound Propagates as a series of Compressions and Rarefactions.

Compression:- The region of high pressure and high density.

Rarefaction:-     The region of Low Pressure and low density.

Medium and Speed of Sound

There are three mediums

1. Solid
2. Liquid
3. Gas

The speed of sound is more in solids, less in liquids and least in gasse.

SOLID > LIQUID > GAS

Important Terminology:  –

• Amplitude :           The maximum displacement of partials from mean position.
• Wavelength :           Distance between two successive compressions or Rarefactions.
• Frequency :           The number of vibration per second.
• Time Period :           The time taken to complete one vibration.
• Wave velocity :           Distance Covered per second.

Some Interesting sound Energy Facts

• The study of sound wave is called “Acoustic”.
• Sound is measured in decibels and Pascal.
• There is no sound in space. Because there is no medium.
• Dogs can hear sound at a higher frequency than humans.

Important Questions

Q.1                   In which of the following speed of sound is maximum?

(a) Air                          (b) Vacuum                  (c) Steel                        (d) Water

Ans. (c)            Sound wave travel faster in solids because the atoms are bounded tightly and elastic content is higher.

Since there is no medium, sound cannot propagate in vacuum.

Q.2                   Conversion of sound energy into electrical energy is done by:

(a) Loud speaker                                   (b) Gramophone

(c) Microphone                                     (d) Audio Records

Ans. (c)            Microphone converts sound energy into electrical energy.

Q.3                   Sound wave of wavelength, greater than that of audible sound is known as –

(a) Infrasonic                                        (b) Supersonic

(c) Ultrasonic                                        (d) Seismic wave

Ans. (a)            Wavelength of infrasonic wave is greater than that of the sound wave.

Q.4                   When temperature of air increases, the velocity of sound: –

(a) Increases                                         (b) Decreases

(c) Remains unchanged                         (d) Decreases

Ans. (a)            In dry air, at , the speed of sound is 332 m/s.

• At  the speed of sound is 343 m/s.

Q.5                   In Sonar, we use-

(a) Radio waves                                    (b) Ultra sonic waves

(c) Electromagnetic waves                     (d) Infra Sonic Waves

Ans. (b)            SONAR (Sound, navigation, and Ranging) is a technique of location, detecting and determining the speed of an object through the reflected sound

Q.6       Hertz is the unit of  ________

(a) Wavelength                                     (b) Intensity

(c) Frequency                                        (d) Time Period

Ans. (c)            Hertz is equal to one complete cycle per second.

Q.7       Which of the following animals can hear ultrasonic sound?

(a) Rat              (b) Squirrel                   (b) Cat                          (d) Bat

Ans. (d) Bat can hear ultrasonic sound.

Bats can hear higher frequencies of sound beyond the range of human hearing.

Q.8      To hear a echo, the minimum distance should be :-

(a) 17 meter      (b) 34 meter                  (c) 51 meter                  (d) 68 meter

Ans. (a)   Considering the speed of sound to be 343 m/s, The minimum distance to hear distinct echo is 17.2  meter

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