Re: The Tuned-Mass Damper

[brian whatcott <inet@intellisys.net>]

At 16:11 11/17/97 -0500, Gene Mosca wrote:
> Hi All,
>
> The tuned-mass damper is used to dampen oscillations in tall building.  A
> large concrete block (400 tons) slides "floating" on a layer of oil. 
....
> What I don't understand is the 180 degree phase angle.  It seems to me
> that at resonance the driving force and the displacement of the
> oscillating mass are 90 degrees out of phase, and if the driving force is
> provided by a helical steel spring (not a pneumatic spring--whatever that
> is) the displacements of the opposite ends of the spring are 90 degrees
> out of phase.  Am I correct?  If so, do pneumatic springs operate
> differently so that at resonance the opposite ends are 180 degrees out of
> phase?
>
> Gene

I was hoping for an illuminating answer to this interesting physical
 question of considerable practical importance.

Lacking that...I can easily recall that though we would expect the phase
angles you describe for the components of a single tuned circuit, here 
we have two "close-coupled" tuned circuits.
  This arrangement was formerly a feature of the tube heterodyne radio 
sets to which your parents listened.

 This tuned circuit arrangement has the feature that although the central 
'peak' of the resonance is reduced (it may in fact dip between two
displaced peaks) the bandpass is now broader and with steep shoulders.

 I suppose this reduction in central amplification ('Q') would serve a
useful purpose of the sort described.

I would have liked to demonstrate this effect in mechanical terms.
I found the elastic bands and the mass.... but a compound pendulum is not
the  needed set up.

 I suppose a tuning fork could be said to demonstrate this anti phase
behavior: if one tine is struck - both tines vibrate in antiphase...

Sincerely,

brian whatcott <inet@intellisys.net>
Altus OK