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Flxible Metro - Exhaust Drone?


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Does anyone know how the Flxible Metro buses acquired their low-frequency exhaust drone sound that comes into play with the revving engine (be it by Cummins or Detroit Diesel)? Lack of resonation, perhaps? Too many examples I can think of to list here... so I'll draw out some examples:

TriMet #1701-1808, 1901-1910, 1809-1816 and 1817-1843

WMATA #9701-9785, 9801-9818, 9819-9835

VTA (San Jose, CA) #9201-9291, 9301-9355

Maryland MTA #9401-9425 and 9431-9435 (ZF transmission), 9426-9430 (Voith transmission)

The best known case of any Flxible Metro with this exhaust drone at all speeds is with any 1992 to 1996 model equipped with the Cummins L10E, L10G or M11E engines. Often times the exhaust plumbing is not resonated to block that part of the sound out.


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