Air Tables

	Putting a relatively heavy object on blown up tennis balls, an inner tube,
etc., is a common way to get rid of floor and table vibrations. When you place a
mass on top of the air bladders, you should achieve a natural resonant freq lower
than the vibration frequencies transmitted from the floor. Its desirable to achieve
a 1 to 3 Hz major table resonance for a stereo turntable. Simply using air bladders
provides little damping of this resonance. My trick is to use bicycle or smaller
diameter tubes and feed the tube with a permanately attached air line.
The line will feed externally to another tube or bladder. When the isolation table
starts bouncing, the air line absorbs part of the pressure changes. This flow through
the air line acts as a resistance, damping the resonance which is the key to its success.

	In its simplest form, there is one inner tube which the table top plate sits. This is
fed through an air tube to another tube nearby. The line size can be regulated with a clamp. The
amount of pressure in the system and the table and component weight determine the resonant
frequency. This system controls vertically induced vibrations fairly well. Unfortunately,
horizontal movement can cause rocking. To combat this a more complex system must be employed.
Use 3 or 4 smaller tubes at the corners with lines comming from each tube to a central tee,
then feeding into another line going to the external bladder or inner tube. The table combination
fights vertically induced vibration in the same way. When an unequal distribution causes rocking,
the air lines between the tables main tubes start to conduct air flow, providing the
resistance needed to damp this rocking. I have not supplied any diagrams, but the system
is fairly self explanitory.

	There are laboratory systems which use very large tables, air bladders, level sensing
valves, tubes, and air supplies, so my basic is very much the same only smaller and a bit simpler.
The anti rocking effect can also be seen applied to anti-wave water beds. An inner
material placed to help block the waters flow from side to side provides the extra damping
to help stop the wave effect.

	I started looking into all this turntable business when helping my DJ friends cope
with major floor movement when everyone is dancing, and your supposed to set up on
terribly constructed podiums, which are placed upon a vibrating floor! I never did any
real construction of my practical air tables. I did gain more insight of the basic problems
and was able to minimize them with various foams and other materials.

	There is a very good foam that can be used alone or in conjunction with
other materials. This is the kind of foam rubber usually associated with can coolers
for pop and beer. It is also seen as piping insulation. This is usually black in
color and very soft, not to be confused with the polyethelene type. The foam
is very soft and seems to have a very good self damping quality, probably because its
kind of gummy. By cutting up appropriate sizes, this material can be placed under
sub platforms or alone at equipment corners.