What is "synchronizing" propellers and why was it done?

Date:         17 Dec 97 03:43:07 
From:         dpbsmith@world.std.com (Daniel P. B. Smith)
Organization: The World Public Access UNIX, Brookline, MA
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I can barely remember flying on four-engined propeller-driven airliners,
and it seems to me that the sounds of the engines tended to "throb" a bit
but would settle into a very steady sound at cruising altitude.  I believe
I have read in books that four-engined propeller aircraft had special
mechanisms for synchronizing the propellers, i.e. making sure that they
all turned at precisely identical rates.  Assuming this is so, the
question I have is:

Was this just done for acoustic comfort and aesthetics?  That it, were
propellers synchronized just because the steady sound was pleasant and
reassuring?  Or were there aeronautical reasons, and, if so, what were
they?  Did a plane fly differently, consume less fuel, etc. if the
propellers were synchronized?  Why?

Daniel P. B. Smith