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Re: Footstep sounds - was RE: Novas, Age of Dinos in S.A., pg 203 figure D



2009/10/14 Martin Baeker <martin.baeker@tu-bs.de>:
>> =A0 But they do actually cause vibrations on the ground. Not only are these=
>>  distinct=2C they are noticeable for quite a distance.=A0 The concept of tr=
>> acking animals by placing your ear to the ground is not fiction=2C for exam=
>> ple=2C and despite not impacting the ground=2C a train is capable of produc=
>> ing enough force acting on the ground to cause it to vibrate for a very lon=
>> g distance so that it can also be "tracked."=A0 Unless you think this is al=
>> so fiction.
>
>
> I think this is due to air actually being a lousy
> sound-conductor. here is a nice experiment which was really an
> eye-opener for me: Take a very long piece (10 m or so) of wood (I did this in 
> a
> kid's park with a tree trunk where this was an experimental setup),
> preferredly one that is only supported at a few points.
> Put your ear very close to one end of the trunk. Have someone else
> scratch the other end with their fingernails slightly, so that they
> themselves can just barely hear their own sound. You will hear the
> noise clearly at your end.
>
> One other thing to consider here: The sound also depends on how strong
> the impact is: elephants, with very soft and big feet, make almost no
> sound walking on hard ground, whereas horses do, because the
> deceleration of the foot is spread over a longer time. Furthermore,
> the pressure below an elephant foot is smaller than that below a horse
> hoof (didn't do the calculation, but considering the difference in foot
> size this should be obvious.) So with a very big sauropod foot which
> may be padded from below there will be not much noise produced (or,
> more precisely, the sound produced will be spread over longer time so
> that the peak sound amplitude is smaller).

Jaime,

Force and energy are two separate things.  Whenever a 50 tonne
sauropod is standing on the ground, it is exerting a 9.8 * 50,000 =
490,000 Newton force (its weight) but that in itself does not, of
course, produce any sound.  Its the dissipation of energy that does
that.  In efficient locomotion (i.e. walking) the foot is lifted,
shifted forward (which we can ignore for our current purposes, imagine
the sauropod walking on the spot if you like) and placed down again
gently so as to avoid wasting energy by converting it to
sound/heat/etc.  Large animals tend to be good at this due to the
possession of compliant foot pads.  Obviously that changes when an
animal shifts into a run, when efficiency is sacrificed for raw speed.
 Then the feet hit the ground hard, and then you hear it.  It's the
difference between carefully putting a heavy box down on the ground
and just dropping it.  The box weighs the same either way, but
dropping is dramatically louder.