Liquid-sealing type variation isolating apparatus

A liquid-sealing type vibration isolating apparatus comprising:

• a coupler attached to a vibrating body;
• a holder attached to a vehicle body side;
• an insulator provided between said coupler and said holder that absorbs and isolates vibration from said vibrating body;
• a vibration isolating mechanism which directly follows said insulator and is formed with a liquid chamber containing a liquid in the form of an incompressible fluid, wherein said vibration isolating mechanism includes a main chamber having a wall formed by a part of the insulator and seals the liquid;
• an auxiliary chamber connected to said main chamber via a first orifice, wherein said liquid flows through said first orifice, and said auxiliary chamber is formed by a second diaphragm;
• an equilibrium chamber divided from said main chamber via a first diaphragm which allows the volume in the equilibrium chamber to change;
• a switching mechanism which switches a pressure in said equilibrium chamber alternatively to a negative pressure or atmospheric pressure in synchronization with engine vibration, wherein the switching of the pressure in said equilibrium chamber causes said second diaphragm to vibrate in synchronization with engine vibration;
• and
• a control mechanism which controls the switching operation of said switching mechanism;
• said vibration isolating apparatus further comprising
• a length (L1) of a duct line extending from an atmospheric pressure inlet to said equilibrium chamber which is set at a value determined by the following formula:
• 0.85 cT/4≦L1≦1.15 cT/4
• where c is the sound velocity and T is a period of time, in seconds, during which the open air is introduced into said equilibrium chamber by said switching means.