Chirp chirp! 1 point possible (ungraded) Consider an idealized bird (treated as a point source) that emits constant power. If you double your distance from the bird, by how much does the sound level change? O2dB O6dB O6dB OZdB Suppose we have a speaker with power P0 placed a distance it: from a microphone, which has area A. We denote the intensity and sound level registered by the microphone by I and ,8 respectively, and the power received by the microphone by P. Now suppose that we increase P0 by a factor of 4, increase :13 by a factor of 4, and increase A by a factor of 2; Le, we take (P0 > 4P0) and (:1: > 451:) and (A 5' On A A 712,], 5 Which of the following is correct? 33 O (I>4I) and (>B+(10d Q (Is 0.51) and (tiw (1c 0 (I>(L25Tl and (B>B(] E learning.edge.edx.org 1:43 .Ill - 45 Student O (I - 0.251) and (B -+ 8 - (1 O (I -+ 21) and (B - B- (10 d None of the above Submit Designing a siren 4 points possible (ungraded) You are attempting to design an ambulance siren. You would like the siren to be distinct enough that it will be clearly audible in case of emergency, but not unnecessarily disruptive. (a) Suppose that you would like to have a siren which is loud enough that its intensity will exceed that of typical street traffic (which has a sound level of 85 dB) by at least a factor of three for cars that are 300 m away from the siren source. However, you also do not want it to that it1:43 45 Student sound level Of 85 UD) by at least a factor of three for cars that are 300 m away from the siren source. However, you also do not want it to be so loud that it can cause hearing damage (caused by a sound level above 125 dB) for a bystander at a distance of 10 m from the siren source. What is the range that the power of the siren can take so that it is consistent with these requirements? (You may neglect damping/attenuateon). Maximum power: W Minimum power: W (b) Suppose that you perceive a sound level equal to x dB when you are situated 110 m from an ambulance siren. Annoyed by the sound of the siren, you start to walk in the opposite direction. How much further do you need to walk in order for the sound Inual to dran to m learning.edge.edx.org1'44 .ul "5' E d Student direction. How much further do you need to walk in order for the sound level to drop to a: 5 dB? (c) Suppose an ambulance produces a sound level of 90 dB at a distance of 100 In. How many ambulances would need to be at the same position in order for the sound level at this distance (100 In) to exceed the threshold for hearing damage (125 dB ) Speed trap 5 points possible (ungraded) Police RADAR speed detectors bounce microwave light radiation off of moving vehicles and detect the reflected waves. These waves are nhjgdwmnnnnlnr E learning.edge.edx.org 1:44 45 NOUIICE IIICTUWave IIyHIL LaulauVII VII of moving vehicles and detect the reflected waves. These waves are shifted in frequency by the Doppler effect, and the frequency shift Af = \\fr-fs between the outgoing wave and the received waves provides a measure of the vehicle's speed. Though the waves here are light instead of sound, we will assume that the Doppler shift works exactly the same way, except that v = 343 m/s (the speed of sound) becomes c = 3.0 x 10 m /s (the speed of light). Transmitted RADAR wave Reflected Wave RADAR Transmitter Moving 2' car Police (a) An officer sitting in a stationary police car observes what appears to be a speeding school bus and uses her Doppler radar gun to send out a microwave of frequency, fs . Write an expression for the frequency fbus observed by the bus as it moves towards the police car at a speed Vbus (fbus should be a function of Ubus, c, and fs). To input your solution, use v_b for Vbus and f_s for fs fbus = learning.edge.edx.org1:44 .ull 'v? E fbus = (b) The radar waves are now reflected off of the moving bus, so the bus is still moving at speed vbus becomes an emitter of the radar wave (a moving source of the wave). Write an expression for the frequency detected by the stationary police car, fr, in terms of 115113, c, and f3, which is the frequency of the reflected wave. fr: (c) The Doppler shift is what is measured by the radar gun. Find an expression for the Doppler shift Af = lfTfs , in terms of vbus, c, and f3. Your expression should be simplified to have a common denominator. Afz E learning.edge.edx.org 1:44 45 Af = (d) Note that the vehicle speed v