MV Chetzemoka Ferry
Air Vent Pipes
I recently rode on the MV Chetzemoka Ferry which was running between Tacoma’s Port Defiance and Vashon Island’s Tahlequah terminal. The ship is powered by a pair of EMD 12-710 diesel engines rated at 3000 hp each at 900 rpm, or 15 Hz.
I took a sound measurement with my Android Smartphone near the air vents, which are excellent conductors of engine room sound. The fundamental acoustic frequency is 10.4 Hz which is about 69% of full power. There is no need to run the engines at maximum power given that the distance between the two terminals is only 1.9 miles.
Integer harmonics of 10.4 Hz are also present. The highest individual peak is at 93.4 Hz which is nine times 10.4 Hz. The reasons for this harmonic having the highest amplitude are unclear but may be due to the directivity of the sound source. Or perhaps the vent duct had a coinciding natural frequency that was excited into resonance.
– Tom Irvine