Question

 

(a) The power of sound from the speaker of a radio is 20 mW. By turning the knob of volume control the power of sound is increased to 400 mW. What is the power increase in dB as compared to original power?  

(b) How much more intense is an 80 dB sound than a 20 dB whisper?

Solution

Correct option is

 

(a) As intensity is power per unit area, for a given source P ∝ I, so

             

  

  

(b) By definition of sound level,

            

SIMILAR QUESTIONS

Q1

A copper wire is held at the two ends by rigid supports. At 30oC, the wire is just taut, with negligible tension. Find the speed of transverse waves in this wire at 10oC if  

Q2

A uniform rope of length 12 m and mass 6 kg hangs vertically from a rigid support. A block of mass 2 kg is attached to the free end of the rope. A transverse pulse of wavelength 0.06 m is produced at the lower end of the rope. What is the wavelength of the pulse when it reaches the top of the rope?  

Q3

A uniform rope of mass 0.1 kg and length 2.45 m hangs from a ceiling. (a) Find the speed of transverse wave in the rope at a point 0.5 m distant from the lower end, (b) Calculate the time taken by a transverse wave to travel the full length of the rope (g = 9.8 m/s2)  

Q4

A pieze-electric quartz plate of thickness 0.005 m is vibrating in resonant condition. Calculate its fundamental frequency if for quartz,  and 

Q5

Determine the change in volume of 6 litres of alcohol if the pressure is decreased from 200 cm of Hg to 75 cm. [Velocity of sound in alcohol is 1280 m/s, density of alcohol = 0.81 g/cc, density of Hg = 13.6 g/cc and g = 9.81 m/s2]

Q6

 

(a) Speed of sound in air is 332 m/s at NTP. What will be the speed of sound in hydrogen at NTP if the density of hydrogen at NTP is (1/16) that of air? [Assume ρairH â‰ƒ 1.]  

(b) Calculate the ratio of the speed of sound in neon to that in water vapours at any temperature. [Molecular weight of neon = 2.02 × 10–2kg/mol and for water vapours = 1.8 × 10–2 kg/mol]

Q7

(a) Find the speed of sound in a mixture of 1 mol of helium and 2 mol of oxygen at 27oC. (b) If the temperature is raised by 1 K to 300 K, find the percentage change in the speed of sound in the gaseous mixture. (R = 8.31 J/mol K). 

Q8

The faintest sound the human ear can detect at a frequency of 1 kHz (for which the ear is most sensitive) corresponds to an intensity of about 10–12W/m2 (the so called threshold of hearing). Determine the pressure amplitude and maximum displacement associated with this sound assuming the density of air = 1.3 kg/m3 and velocity of sound in air = 332m/s

Q9

What is the maximum possible sound level in dB of sound waves in air? Given that density of air = 1.3 kg/m3v = 332 m/s and atmospheric pressure  

Q10

A window whose area is 2 m2 opens on a street where the street noise result in an intensity level at the window of 60 dB. How much ‘acoustic power’ enters the window via sound waves. Now if an acoustic absorber is fitted at the window, how much energy from street will it collect in five hours?