Mountings - Neoprene Mountings
It is possible to reach frequencies as low as 84 cpm(1.4 hz) or even lower using steel springs and provide excellent isolation at RPM or the fundamental frequency, but noise transmission is another issue. The high pitched whines and hums developed by very high speed centrifugal, screw and sometimes reciprocating compressors, as well as transformers, travels axially through the spring wire and comes through the bottom spring coil with little reduction. While a rubber pad will reduce this noise, it has been our experience on many occasions that no matter how many layers of rubber pads were used (we have built up to 6”[150mm]), the noise will still find its way through. We never use pads harder than 60 durometer, but harder pads (65 duro and higher) are even more sound transparent.
Air springs completely eliminate this problem. Support is provided by pressurized air within a thin walled rubber container similar to but lighter than the side walls of an automobile tire. We have never experienced high frequency noise transmission when using air springs, as the sound is not transmitted by the air or the tire cord reinforced neoprene body.
There is always some air leakage no matter how slow. After systems are installed, they are seldom inspected and maintained. Our experience has proven it is mandatory that the air springs are installed with an air supply and height sensitive (not pressure sensitive) control valves. The valves provide additional air if elevation is lost or vent if the elevation increases due to rotational forces or temperature increase. Temperature increases occur on hot roofs or within transformer vaults.
Air usage is minimal. Most of the time the system is supplied by control air. If not available, the smallest available tank type compressor is more than adequate if it will operate at the air spring rated pressure. This compressor is often mounted on the primary equipment so it is self isolated.
We strongly recommend air springs in critical areas where noise transmission is a major worry.
Multiple bellow air springs shall be manufactured with powder coated upper and lower steel sections connected by a replaceable, flexible Nylon reinforced Neoprene element to achieve a maximum natural frequency of 3 Hz. (We have found 3 Hz adequate when using air springs. Should the specifying engineer require a lower frequency, change the 3 Hz to the lower number). Burst pressure must be a minimum of 3 times the published maximum operating pressure. All air spring systems shall be equipped with 3 leveling valves connected to the building control air or a supplementary air supply to maintain elevation plus or minus 1/8”(3mm). An air filter and water separator shall be installed before the air distribution system to the leveling valves.
Submittals shall include natural frequency, as well as load and damping tests, all as performed by an independent lab or acoustician. Air springs shall be type MT and leveling valves type LV as manufactured by Mason Industries, Inc.