## 8.3 PRACTICE QUESTIONS (from MEC 702)

#1 a) A worst case analysis is being done on a four bladed propeller. Given the properties below, estimate the maximum force that will be applied to the engine mount if one of the blades falls off. (5%)

b) Describe how we would practically find the relationship between vibration at the engine mount, and a point in the cockpit. (5%)

c) Assuming the Bode Plots below was obtained in the test in part b), what would the transfer function be? (5%)

d) Given that the engine generates a vibration force of 100N at 6000 rad/sec, use the bode plots to estimate the vibration force in the cabin, with the missing propeller blade. (10%)

e) Using the conditions described in d), use a Fourier transform to find the vibration force in the cabin, with the missing propeller blade. (15%)

f) Given that the main source of sound in the cabin is a panel that shakes, and experimentally we determine the force to sound power to be related by the equation below. Find the sound power levels in the cabin. (5%)

g) Given that the cabin (2m by 2m by 2m) is finished with wood walls (alpha = .1), and 4 glass windows (40cm by 40cm each and alpha = .12). Find the reverberant sound pressure level in the cabin caused by the motor, and the missing propeller blade. (10%)

h) What is the total loudness in sones, based on the values found in g)? (5%)

i) What is the voice level that may be used from the curves below if the pilot and copilot sit 3 feet apart? (5%)

j) Outside, the aircraft engine is running, and noise POWER levels are as listed below. If the nearest neighborhood is 50m away, what are the sound pressure levels? What is the total sound pressure in each time period (add the A-weighted pressure values)? (10%)

k) What is the noise pollution level if a 10dB penalty is in place from 10pm to 7am? (5%)

l) A hangar is to be built to house the aircraft, and reduce the noise levels. (In effect this is a large hood.) If the hangar is to be built 100m by 100m by 20m, with a 1/4” plywood wall with insulation, what will the 1000Hz sound pressure level be near the hangar during the day? (10%)

m) As another alternative, a resonant muffler (an expansion chamber) is to be designed to reduce the 1000Hz sound. If the exhaust pipe has a 30cm radius, select the width and length for a Transmission Loss of 20dB. Assume c=300m/s. (10%)