> I long ago met a unique ULF sound generator of very high acoustic power
> level.
> I've described it on-list before.
> I only ever heard it and never saw it but this is what I think it was:
>=20
> Two diesel electric locomotives run up together in an engine shed on a co=
ld
> winter morning.
> (Temperature and season optional probably).
> Shed probably corrugated iron. Acoustically somewhat constrictive, but
> flexible at low frequencies.
>=20
> Set engine speed of both locomotives to the same speed with governors.
>=20
> Engines run ALMOST at correct speed and hunt gently around correct speed.
>=20
> Pressure waves from acoustic signals mix in confined space to produce
> difference frequency.
>=20
> SO:
> We lived near what was then probably NZ's largest shunting yard.
> Lie in bed on said cold winter morning at about 4am in house about 100
> metres from engine shed.
> Whump .... whump a whump a whump ..... whump a whump a whump a
> .........
> Loud softly explosiveish waves repeating at sub-Hz to a few Hz - maybe li=
ke
> somebody hitting house gently with a 10 ton bean bag.
> Lasts for maybe 10 minutes on and offish.
> Then stops suddenly. Until tomorrow.
>=20
> Fun, after the first few times.

I had a somewhat similar experience as a teenager.  My grandparents lived j=
ust a little bit north of Wellington (NZ), beside the main line north. One =
summers day while sitting reading at their place I became aware of a strang=
e sensation that felt like something pumping my chest, but couldn't hear an=
y sound. A short time later a diesel loco with a short goods train came pas=
t, at which time the chest pumping sensation was about its maximum. Just af=
ter passing me the loco driver started to push the loco as the train was ab=
out to go up an incline, at which point the chest pumping sensation ceased =
and the loco produced standard exhaust type noises.

I suspect that as the train was tootling along the level it was being fed w=
ith just enough fuel to keep the motor ticking over at idle, but by the tim=
e the cylinder was being exhausted the combustion had fully finished and so=
 there was no combustion noise out the exhaust, just the compressed cylinde=
r gasses being pumped out creating a pressure wave that I was sensing.


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