Sound Notes for SSC CGL, CHSL, IB & Railways Exams

Sound

Sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid.

```   1 kHz = 1000 Hz
```
• Sound can not travel through vacuum.
• Sound waves are longitudinal waves.
• Frequency Range of Audible Sound for Humans: 20 Hz to 20 kHz
• Children under the age of five and some animals, such as dogs can hear up to 25 kHz.

Infrasound / Infrasonic Sound

Sound below 20 Hz

• Whales, elephants, giraffes, etc. communicate using infrasound over long distances.
• Avalanches, volcanoes, earthquakes, ocean waves, water falls and meteors generate infrasonic waves.

Ultrasound / Ultrasonic Sound

Sound above 20 kHz, produced by dolphins, bats, etc

Applications of Ultrasound

• Ultrasounds can be used to detect cracks and flaws in metal blocks.
• Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called echocardiography.
• Ultrasound Medical Test for human internal body organ imaging.
• Ultrasonography is used for examination of the foetus.
• Ultrasound treatment sometime used for breaking small stones formed in kidneys into fine grains.

Supersonic Speed

If the speed of any substance is more than the speed of sound in air, then the speed is called supersonic speed.

Mach Number

The ratio of the speed of a body and that of sound in air is called the Mach number of the body.

• If the Mach number of a body is more than 1 , it is clear that the body has supersonic speed.

In sound propagation, it is the energy of the sound that travels and not the particles of the medium.

Characteristics of Sound Wave

1. Wavelength (λ)

The distance between two consecutive compression (C) or two consecutive rarefaction (R) is known as wavelength.

• SI Unit: m

2. Frequncy (ν)

The number of complete oscillations per unit time is called the frequency.

• SI Unit: Hz
• A Sound of single frequency is called as Tone.
• The Sound which is produced due to a mixture of several frequencies is called a note and is pleasant to listen.
• Noise is the sound of undesirable frequency and is unpleasant to ear.

Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound, and infrasound.

3. Time Period

The time taken by two consecutive compressions or rarefactions to cross a fixed point is called as the time period of the wave.

• SI Unit: s

Relation between Time Period and Frequency

```   F = 1/T
```

4. Amplitude

The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate.

• SI Unit: m

5. Pitch

The quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone.

• The faster the vibration of source, higher is the frequency and the higher is the pitch.

6. Intensity of Sound

Amount of sound energy passing each second through unit area is called the intensity of Sound.

7. Loudness

Loudness is perceived as how “loud” or “soft” a sound is and relates to the total number of auditory nerve stimulations over short cyclic time periods, most likely over the duration of theta wave cycles.

• Larger the amplitude of vibration, louder is the sound.

8. Timbre

Timbre is perceived as the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice) and represents the pre-conscious allocation of a sonic identity to a sound.

9. Echo

It is a reflection of sound that arrives at the listener with a delay after the direct sound.

10. Reverberation

The phenomenon of prolongation of sound due to successive reflections of sound from surrounding objects is called reverberation.

• In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.
• Sound absorbent roofs and walls are used in auditorium or movie halls are used in auditorium or movie halls to reduce reverberation.
• Megaphones, horns, shehanais are all designed to send sound in a particular direction without spreading it in all direction.

SONAR (SOund Navigation And Ranging)

SONAR is a technique that uses sound propagation to measure the distance, direction and speed of underwater objects.

Let the time interval between transmission and reception of ultrasound signal be t and the speed of sound through seawater be v. The total distance, 2d travelled by the ultrasound is then,

```   2d = v × t
```
• SONAR is used to determine the depth of the sea and to locate underwater valleys, hills, submarine, icebergs and sunken ships, etc.

Speed of Sound in Different Media

• Speed of the sound decreases from solid to gaseous state.
• In any medium, as we increase the temperature, the speed of sound increases.
• The velocity of sound through a gas is inversely proportional to the square root of the density of the gas.
• At 22° C and 0° C, Speed of Sound in the air is about 344 m/s and 331 m/srespectively.

A sonic boom is the sound associated with the shock waves created whenever an object traveling through the air travels faster than the speed of sound.

MCQs

Q1. The distance between two consecutive compression (C) or two consecutive rarefaction (R)?

• A. Frequency
• B. Time Period
• C. Wavelength
• D. Amplitude

Q2. Which of the following is incorrect about Sound?

• A. Sound Wave is a transverse wave
• B. Audible range for humans is 20 Hz to 20 kHz
• C. Sound cant travel through vacuum
• D. All of the above are correct

Q3. Ultrasonic?

• A. < 20 Hz
• B. 20 Hz to 20 kHz
• C. > 20 Hz
• D. Faster than sound

Q4. Speed of Sound at 22° C?

• A. 244° C
• B. 231° C
• C. 344° C
• D. 331° C