# At a distance r from the source point, the sound intensity...

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banaglorioso (kmb4543) – Homework 09 – yao – (88200) 1 This print-out should have 23 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A piano emits sound waves with frequencies that range from a low of about 29 Hz to a high of about 4300 Hz. What is the range of wavelengths spanned by this instrument for a speed of sound in air of approximately 359 m / s? r from the source point, the sound intensity is I = P 4 π r 2 P = 4 π r 2 I = 4 π r 2 I 0 10 β/ 10 r = radicalbigg P 4 π I 0 10 β/ 10 = radicalBigg 86 W 4 π (1 × 10 12 W / m 2 ) 10 11 = 8 . 27264 m . 002 10.0points A point source emits sound waves with a power output of 86 W. At what distance will the decibel reading be 110 dB, which is noise level of a loud indoor rock concert? At a distance r from the source point, the sound intensity is I = P 4 π r 2 P = 4 π r 2 I = 4 π r 2 I 0 10 β/ 10 r = radicalbigg P 4 π I 0 10 β/ 10 = radicalBigg 86 W 4 π (1 × 10 12 W / m 2 ) 10 11 = 8 . 27264 m . How many buzzing mosquitoes will produce a sound intensity equal to that of normal conversation? 004 10.0points Heard at a certain distance, a car horn emits a sound with a loudness of 50 dB. If instead there were 100 identical cars all sounding their horns at this certain distance, how loud would the sound be?
banaglorioso (kmb4543) – Homework 09 – yao – (88200) 2 1. 70 dB correct 2. 90 dB 3. 5000 dB 4. 150 dB 5. 5 dB 6. still 50 dB 7. 500 dB 8. 100 dB Explanation: The sound level in the case of one car is β = (10 dB) log parenleftbigg I I 0 parenrightbigg = 50 dB . Thus when 100 identical cars sound their horns, the loudness becomes β = (10 dB) log parenleftbigg 100 I I 0 parenrightbigg = (10 dB) bracketleftbigg log(100) + log parenleftbigg I I 0 parenrightbiggbracketrightbigg = (10 dB) log 10 2 + (10 dB) log parenleftbigg I I 0 parenrightbigg = (10 dB) 2 + 50 dB = 70 dB . 005 10.0points The figure below shows a complex wave pat- tern on a string moving towards a rigid hook at the wall on the right. After some time, the wave is reflected from the wall. v Select the wave pattern for the reflected wave. 1. v 2. v 3. v correct 4. v 5. Explanation: Consider the wave pattern image reflected about the rigid hook on the wall. v v After the time it takes for the wave to be reflected from the wall, this image is the wave pattern traveling to the left along the string. Note: Reflection about a point (hook) is the same as reflection about the y -axis (wall) fol- lowed by reflection about the x -axis (string). The leading part of the wave must remain in front and the wave is flipped over. 006(part1of2)10.0points Two speakers are wired to emit identical sounds in unison. The wavelength in air of the sounds is 6 m. Do the sounds interfere constructively or