Intensity of Energy Transmitted by Sound

question 1

(UEL-PR) (adapted) Noise pollution in big cities is a public health problem. The classification of sound as strong or weak is related to sound intensity level I, measured in watt/m². The lowest audible intensity, or audibility threshold, has intensity I₀ = 10^–12 watt/m², for the frequency of 1000 Hz. The relationship between the sound intensities allows to calculate the sound level, NS, of the environment, in decibels (dB). The table below shows the relationship between sound level and maximum noise exposure time.

Based on this table, on the text and assuming that the noise on an avenue with congested traffic has an intensity of 10^–3 watt/m², consider the following statements.

I. The sound level for a noise of this intensity is 90 dB.

II. The maximum time in hours of exposure to this noise, in order to avoid irreversible hearing damage, is 4 hours.

III. If the considered sound intensity is equal to the audibility threshold, then the sound level is 1 dB.

IV. 1 watt/m intensity sounds² correspond to the sound level of 100 dB.

Check the correct alternative.

a) Only statements I and II are correct.

b) Only statements I and IV are correct.

c) Only statements III and IV are correct.

d) Only statements I, II and III are correct.

e) Only statements II, III and IV are correct.

question 2

(UFT-TO) A sound is produced by an outdoor speaker, which sits on a flat surface. The sound waves produced by this speaker reach a maximum pressure of 84 [Pa] to 10 [m] from the speaker. Assuming that the intensity of the sound waves is equal in all directions over the surface area of ​​the hemisphere, what is the acoustic power of the sound emitted by the speaker?

Consider the intensity of sound waves given by:

On what:

P_MAX= Maximum sound wave pressure [Pa]

ρ = air density = 1.20 [kg/m³]

v = Speed ​​of sound in air ≅ 350 [m/s]

a) 1000W

b) 1.68π kW

c) 4.12π kW

d) 13 kW

e) 13 MW

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