G11-6-3 PDF: Sound Intensity

Beats Beats are the periodic Juctuation heard in the intensity of sound when two sound waves of slightly different frequencies interfere with one another as shown in Figure 6.10. The number of beats per second (or) beat frequency is the difference in frequency between the two sources. fb=f2~fi where...

Beats Beats are the periodic Juctuation heard in the intensity of sound when two sound waves of slightly different frequencies interfere with one another as shown in Figure 6.10. The number of beats per second (or) beat frequency is the difference in frequency between the two sources. fb=f2~fi where = beat frequency, fi, f2= frequencies of the two sources speaker 1 high intensity sound sound wave 1 variation of amplitude sound wave 2 speaker 2 low intensity sound Figure 6.10 Beat frequency 6.4 INTENSITY OF WAVES Physics Textbook The intensity of a wave is the power (or) energy per unit time transported per unit cross-sectional area. where I = intensity of a wave P = power I= PA A = cross-sectional area In SI units, the unit of intensity of a wave is watt per metre squared (W m2). (6.10) If the air around the source is perfectly uniform, the sound power propagated in all directions is the Figure 6.11. same. In this situation, the propagated sound wave is represented by a spherical wave as shown in imaginary sphere area A=4 πr2 source power 1/4 intensity at surface of sphere I = Р/4л2 1/9 P WI P Figure 6.11 Variation of wave intensity with distance from the source By considering source of sound as a point source, sound wave can be distributed uniformly over spherical wave front of area 4π 2. Hence, the wave intensity at the distance r from the source is P P I= = A 4π 2 For a particular source, intensity varies inversely with the square of the distance from the source as follow. 1 I oc Intensity of a wave obeys inverse square law. The intensity of a wave is directly proportional to the square of the amplitude of the wave. 77 Noise Exposure Limit Noise is a sound especially that is loud (or) unpleasant that causes disturbance in hearing. Loudness of sound can be measured by decibel meter. The unit of loudness (sound level) is decibel (dB). Sound with very high intensities can be dangerous. Above the threshold of pain (120 dB), sound is painfully loud to ear. Brief exposure to levels of 140 to 150 dB can rupture eardrums and cause permanent hearing loss. Longer exposure to lower sound (noise) levels can also damage hearing. For example there may be a hearing loss for a certain frequency range. Table 6.1 is expressed the permissible noise exposure limits for maximum duration per day. Table 6.1 Permissible Noise Exposure Limits SUMMARY A stationary equal amplitu Progressive the medium. The intensity EXERCISE 1. There a (ii) How 2. The dis Maximum duration per day Sound level 0.4 m. E (hours) (dB) of the w 8 90 3. Draw a of the s 6 92 4. If the m 4 95 find the 3 97 5. What i length 2 100 the stri 12 1 12 102 6. Find th 105 (veloc 1214 110 7. What 8. A viol 115 a beat only c 9. How increa Example 6.8 Find the sound intensity for a person sitting 2 metre from the 25 watt sound box. P=25 W, r=2 m Intensity of a sound wave, 10. A stu notice that th of the I= P 4πr2 25 =0.50 W m -2 4×3.142×(2)2 Reviewed Exercise If the distance from a point source of sound is increased by three times, by what factor does sound intensity decrease? Key Words: intensity, amplitude, power, noise 78 arrivin