4. A politician is giving a speech outside in a park. At top strength, her voice

Question

4. A politician is giving a speech outside in a park. At top strength, her voice produces a sound intensity level of 70 dB at a distance of 1 meter a) Approximately, what is the sound intensity level of her voice for someone who is standing 30 meters away? (Recall the rule for changes in SPL & SIL: doubling distance reduces SPL and SIL by 6dB.) 1 Mark] b) A 4m wide by 3m high reflective wall is placed 2m behind her. Will reflections from the wall cause an annoying echo for someone who is standing 30m away from her, or will her clarity be improved? Explain. (Hint: Consider times of flight tch a "Hoor plan" if it is helpful.) 12 Marks]

Explanation / Answer

Intensity level B = 10 dB log(I/Io)

I is intensity of sound = Power/area

Io is 10^-12 W/m^2

log(I/Io) = 70/10 = 7

I/Io = 10^7

I = 10^7 *10^-14

I = 10^-4 W/m^2

I = P/4pir^2 = 10^-4

Power P at 1 m is = 10^-4 * 4*3.14 * 1^2

P = 12.56 *10^-4 Watts

Now if distance r = 30 m

Inew = 12.56 *10^-4/(4*3.14* 30^2)

Inew = 1.11*10^-7 Watt/m^2

intensity level B = 10 dB * log( 1.11*10^-7/10^-12)

B = 50.45 Decibals

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time taken by the sound propagating towards the listener is t1 = S/V

t1 = 30/340

t1 = 0.088 secs

frequency f1 = 1/0.088 = 11.36 Hz

time taken by the reflected sound is t2 = 32 /340 = 0.094 sec

since here path difference between the waves is 2 m , Causes as Contructive interferecne resulting in more clear speech of sound

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