(ii) Loudness or softness :
Loudness or softness of sound wave is the sensations that depends upon is amplitude. When we strike a table to with more force, it vibrates and produces loud sound waves which have more amplitude. When struck with smaller force, vibrating table top produces soft sound waves which have less amplitude. A loud sound wave carries more energy and can be heard at large distance. Reduction in amplitude at large distance, makes the sound soft.
(iii) Timbre or quality :
Quality or timbre is characteristic of a sound which enables us to distinguish between the sound of same loudness and pitch. This characteristic of sound helps up to recognise our friend from his voice without seeing him. The quality of two sounds of same loudness and pitch produced by two different sources are distinguishable because of different wave form produced by them.
Eg. : The violin and flute (Bansuri)
(iv) Intensity :
Intensity of a sound is defined at the sound energy transferred per unit area placed perpendicular to the direction of the propagation of sound.
That is, intensity of sound =
Area Time
energy Sound
×
Intensity of a sound is an objective physical quantity. It does not depend on the response of our ears.
The S.I. unit of intensity of sound is joule s-1 m-2 watt m-2 (∴ Js-1 = 1W)
Difference between loudness and intensity of sound :
S.No. Loudness Intensity of Sound
1 Loudness is a subjective quantity. If depends upon the sensitivity of the human ear. A sound may be loud for a person but the same sound may be feeble for another who is hard of hearing.
Intensity of sound is an objective physical quantity. It does not depend on the sensitivity of a human ear.
2 Loudness cannot be measured as a physical quantity because it is just sensation which can be felt only.
Intensity of a sound can be measured as a physical quantity.
RANGE OF HEARING
The human ear is able to h ear sound in a frequency range of about 20Hz to 20kHz. We can not hear sounds of frequencies less than 20Hz of more than 20kHz, these limits vary from persons to person and with age.
Children can her sounds of somewhat higher frequencies, say upto 30 kHz. With age, our ability to hear high frequency sound diminishes. For the elder, the upper limit often falls to 10-12 kHz. We take 20Hz-20 kHz as the audible range for a average person.
Even in the audible range the human ear is not equally sensitive for all frequency. it is mot sensitive to frequencies around 2000-3000 Hz.
Sound of frequencies less than 20 Hz is known as infrasonic sound or infrasound. Sound of frequency greater than 20 kHz is known as ultrasonic or ultrasound.
Different animals have different ranges of audible frequencies. A dog can hear sound of frequencies upto about 50 kHz and a bat upto about 100 kHz. Dolphins can hear sounds of even higher frequencies. Animals such as elephants and whales can hear sounds of frequencies less than 20 Hz. Some fishes can hear sounds of frequencies as low as 1-25 Hz.
SONIC BOOM
When a body moves with a speed which is greater than the speed of sound in air, it is said to be traveling at supersonic speed. Jet fighters, bullets, etc, often travel at supersonic speed. And when they so son, they produce a sharp, loud sound called a sonic boom.
The source moves at a speed greater then that of sound waves traveling at the speed of sound, are left behind. The high-pressure layers due to sound waves originating at different points bunch together as shown in figure. Actually, these layers fall on the surface of an imaginary cone of which OA, OB is a part.
The total pressure on the surface of this cone is very high.
The source is at the apex of this cone. As the source moves ahead, It drags the cone together with it. When the surface of the cone reaches a person, the ears experience a sudden increase in pressure. After the surface crosses him, the pressure is suddenly reduced. This causes the person to hear a sharp, loud sound-the sonic boom.
A region consisting of a very-high-pressure layer followed by a lower-pressure layer travels through the space together with the cone. This is called a shock wave. This shock wave give rise to the sonic boom when it reaches a person.
EXERCISE
OBJECTIVE DPP - 17.1
1. A sound wave travels from east to west, in which direction do the particles of air move ?
(A) East - west (B) North - south (C) Up and down (D) None of these 2. In which medium sound travels faster ?
(A) Solid (B) Liquid (C) Gas (D) None of these
3. What is the name of short duration wave ?
(A) Pulse (B) Frequency (C) Time period (D) Velocity
4. What is the velocity of sound in water at room temperature ?
(A) 1500 m/s (B) 330 m/s (C) 1500 km/s (D) 330 km/s
5. The unit of quantity on which pitch of the sound depends is :
(A) Hertz (B) metre (C) metre/second (D) second
6. The unit of quantity on which loudness of found depends is :
(A) metre (B) Hertz (C) metre/second (D) second
7. Nature of sound wave is :
(A) transverse (B) longitudinal (C) electromagnetic (D) seismve 8. Pitch of high frequency sound is :
(A) high (B) low (C) zero (D) infinite
9. Voice of a friend is recognised by its :
(A) pitch (B) quality (C) intensity (D) velocity
10. Sound waves in air are :
(A) Longitudinal waves (B) Radio waves
(C) Transverse waves (D) Electromagnetic waves
11. Sound waves can not pass through :
(A) A solid liquid mixture (B) A liquid gas mixture
(C) An ideal gas (D) A perfect vacuum
12. A periodic wave is characterized by :
(A) Phase only (B) Wavelength only
(C) Frequency only (D) All the above
13. The spend of sound is maximum in :
(A) Air (B) Hydrogen (C) Water (D) Iron
14. When wound waves traveling in air enter into the medium of water, the quantity which remains unchanged is :
(A) Wavelength (B) Velocity (C) Frequency (D) None
SUBJECTIVE DPP - 17.2
1. In what form of waves, sound travel in air ? 2. Can sound waves travel in vacuum ?
3. Have you every wondered why we hear sound of a hom of an approaching can before the car reaches us ? 4. Distinguish between loudness and intensity of sound ?
5. Which characteristic of sound helps us to identify our friend by his voice while sitting in a dark - room ? 6. What is the loudness of sound ? What factors does it depend upon ?
7. Define the following related to sound waves : (i) Pitch
(ii) Loudness and softness (iii) Intensity
(iv) Quality
8. A person has a hearing range from 20 Hz to 20 kHz What are the typical wavelengths of sound waves in air corresponding to these two frequencies ? Take the speed of sound in air as 340 ms-1.
9. The wavelength and frequency of a sound wave in a certain medium are 20 cm and 1650 Hz respectively.
Keeping the medium same, if the wavelength is changed to 16 cm, calculate : (i) the velocity of sound (ii) the new frequency of the sound wave.