CBSE Class 12 Physics Sound Wave Solved Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.
Ex.1 The human ear can hear sounds from a low of 20Hz upto a maximum frequency of about 20,000 Hz. If the speed of sound in air at a temperature of 0ºC is 330 m/s, then find the wavelengths associated with these frequencies
Thus sonic sound waves have wavelengths from 16.5 mm to 16.5m.
Ex.2 If speed of sound at 0ºC is 330 m/s, then what is the speed of sound at 20ºC.
Sol. v = v0 + 0.6t
= 330 + 0.6 × 20
= 342 m/s
Ex.3 If the bulk of modulus of elasticity of water is B = 2.30 × 109 N/m2 and density ρ = 103 kg/m3, then determine the speed of sound in water.
Ex.4 Calculate the temperature at which sound travels in hydrogen with the same velocity as in helium at N.T.P. The density of helium is
twice that of hydrogen.
Sol. Let θºC be the required temperature at which velocity of sound in hydrogen is same as in helium at N.T.P. (i.e.0ºC) According to Laplace's formula for the speed of sound in gases
Ex.5 A tuning fork of frequency 220Hz produces sound waves of wavelength 1.5m in air at N.T.P. Calculate the increase in wavelength, when temperature of air is 27ºC.
Ex.6 Calculate the speed of sound wave in an atmosphere of helium at 0ºC and 1atm pressure. Note that 4g of helium under STP has a volume of 22.4 litre. For He γ = 1.67.
Ex.7 At what temperature will the speed of sound in air become double of its volume at 0ºC.
Ex.8 The ratio in the densities of oxygen and nitrogen is 16 : 14. At what temperature the speed of sound will be the same which is in nitrogen at 15ºC.
Sol. If M be the molecular weight of the gas and T be the absolute temperature, then speed of sound in a gas
Ex.9 A trumpet player plays a note of frequency 400Hz with an amplitude of 8 × 10–3 mm. If the density of air is taken as 1.3 kg/m3, and the speed of sound 330 m/s, the find the intensity of the sound wave.
Sol. The intensity of sound wave is given by,
I = 2π2ρvn2a2
= 2 × (3.14)2 × 1.3 × 330 × (400)2 × (8 ×10–6)2
= 2 × 9.86 × 1.3 × 330 × 16 × 64 × 10–8
= 8.66 × 10–2
= 0.087 W/m2.
Ex.10 What is the intensity level of sound in dB for (i) threshold of hearing and (ii) threshold of pain.
Ex.11 A 5 watt source sends out waves in air at frequency 1000s–1. Deduce the intensity at a 100 meter distance, assuming spherical distribution. If v = 350 ms–1 and ρ = 1.3 kg/m3, deduce the displacement amplitude.
Sol. We know that intensity is given by,
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