SOUND

POINTS TO REMEMBER

•Sound is produced due to vibration of different objects.

• Sound travels as a longitudinal wave through a material medium.

• Sound travels as successive compressions and rarefactions in the medium.

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

 • Sound cannot travel in vacuum.

• The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.

• The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ.

• The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.

• The number of complete oscillations per unit time is called the frequency (n), n = 1/ T .

 • The speed v, frequency n, and wavelength λ, of sound are related by the equation, v = nλ.

• The speed of sound depends primarily on the nature and the temperature of the transmitting medium.

• The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface and the three lie in the same plane.

• For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s.

 • The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.

 • Sound properties such as pitch, loudness and quality are determined by the corresponding wave properties. • Loudness is a physiological response of the ear to the intensity of sound.

• The amount of sound energy passing each second through unit area is called the intensity of sound.

• The audible range of hearing for average human beings is in the frequency range of 20 Hz – 20 kHz.

• Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”. • Ultrasound has many medical and industrial applications.

• The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines, icebergs, sunken ships etc.

1. Explain how sound is produced by your school bell.
Answer: When the bell continues to move forward and backward, it creates a series of compressions and rarefactions. In this  way a bell produce the sound .

2. Why sound waves are called mechanical waves?
Answer: Sound waves need the material medium to vibrate and propagate. Hence, these waves are known as mechanical waves. Sound waves propagate through a medium because of the interaction of the particles present in that medium. 

3.Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Answer: Sound needs a medium to propagate. Since there is no material medium on the moon due to absence of atmosphere, you cannot hear any sound on the moon.

4.Which wave property determines (a) loudness, (b) pitch?
Answer:  (a) Amplitude of sound wave determines the loudness of a sound. The amplitude of a sound directly proportional to amplitude of wave. If the amplitude of a sound is large, then the sound produced will also be loud. 
(b) Frequency of sound wave determines the pitch of a sound . The pitch of a sound is proportional to its frequency. High pitched sounds have high frequency

5.Guess which sound has a higher pitch: guitar or car horn?
Answer: The frequency of the vibration of a sound produced by a guitar is greater than that produced by a car horn. As we know that the pitch of a sound is directly proportional to its frequency, the guitar has a higher pitch than a car horn.

6. What are wavelength, frequency, time period and amplitude of a sound wave? 
Answer: Wavelength: The distance between two consecutive compressions or two consecutive rarefactions  is known as the wavelength. Its SI unit is metre (m).
Frequency: The number of oscillations produced by vibrating body in one second is known as the frequency of a sound wave. It is measured in hertz (Hz). 
Amplitude: The maximum height reached by the crest or trough of a sound wave is called its amplitude.

7.How are the wavelength and frequency of a sound wave related to its speed?
Answer: Speed of sound wave is product of wavelength and frequency of a sound wave 
Speed ( v) = Wavelength × Frequency = ν λ
Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.
Frequency of the sound wave, v = 220 Hz Speed of the sound wave, v = 440 m/ s 
Speed = Wavelength × Frequency 
Wavelength of a sound wave = Speed of the sound wave/ Frequency of the sound wave 
= 440 m/s ÷ 220 Hz = 2m
Hence, the wavelength of the sound wave is 2 m.

8.A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?
Answer: You know that the time taken to complete two successive compressions or two consecutive rarefactions is called the time period of the wave.
Time period of the wave = 1/Frequency Hence, time period is reciprocal of the frequency of the wave
Now, Time period of the wave=1/500=0.002 second

9. Distinguish between loudness and intensity of sound.
Answer: a) Intensity of sound waves is defined as the average energy transported per second per unit area perpendicular to the direction of propagation.
b) It is measured in Js-1m-2 or Wm-2.
c) The intensity of sound in air depends on the square of the frequency and the square of the amplitude.
a) The loudness of sound is defined as the degree of sensation of sound produced in the ear
b) The loudness of a sound depends on its amplitude.
c) The amplitude of a sound decides its intensity, which in turn is perceived by the ear as loudness.

10. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
Answer: The speed of sound depends on the nature of the medium. Sound travels the fastest in solids. Its speed decreases in liquids and it is the slowest in gases. Therefore, for a given temperature, sound travels fastest in iron.

11. An echo returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s ?
Answer: Speed of sound, v = 342 m/ s Echo returns in time, t = 3 s
Distance travelled by sound = v × t = 342 × 3 = 1026 m

In the given time interval, sound has to travel a distance that is twice the distance of the reflecting surface and the source.

Hence, The actual distance of the reflecting surface from the source =1026/2=513m

12. Why are the ceilings of concert halls curved?

Answer: Since, concert halls are big, so audience at the back rows of the hall may not hear clear sound of speaker. To overcome this problem, the ceiling of the concert halls is made concave. Concave ceiling helps the sound wave to reflect and send to farther distance which makes the concert hall enable to send clear sound to the audience even sitting in back rows of hall.

13. What is the audible range of the average human ear?

Answer: 20 Hz to 20000 Hz

14. What is the range of frequencies associated with

(a) Infrasound

(b) Ultrasound

Answer:  a) Infrasound: Less than 20 Hz (b) Ultrasound: More than 20000 Hz

15. A submarine emits a SONAR pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff ?

Answer:To return the SONAR pulse back, its wave has to travel two way.

Here, given,

Velocity (v) of sound wave = 1531m/s

Time (T) = 1.02 s

Thus, Distance = speed X time

Distance=1531 ms^(-1)×1.02 s=1561.62 m

So,the distance between the source and reflecting surface=1561.62÷2=780.81m