We started Mason Industries in 1958. Our revolutionary designs of high deflection free standing spring isolators, as opposed to housed mountings, and our literature with down to earth information made its mark. This generated phone calls from acoustical consultants asking us to get into the floating floor business. We asked why. In addition to wanting more than one source, the implication was that some competitive information was unreliable and they would rather work with us.
In those wonderful days we were doubling our volume every year and keeping up with demand and continued development of our mechanical systems, led us to answer, we simply were not ready to enter the floating floor market.
In 1965 one of our representatives ordered Bridge Bearing Neoprene Pads. While we had been molding rubber for years, we were not familiar with this specification.
DuPont manufactures Neoprene, and they were a great help. In addition to the Bridge Bearing formulations, they provided publications and back up information on Neoprene’s excellent aging characteristics.
After this exposure to Neoprene, we realized we had a proper floating floor material. If Neoprene could survive in outdoor applications, exposed to sunlight, temperature extremes, snow and rain, it would certainly last for the life of the structure when located in the dark, cozy, moderate temperature environment, under a floating floor. We immediately phoned the acoustical consultants, and asked what frequency they needed
We were told they wanted an isolation frequency of 8 Hz in a 2” air gap. Since the lowest audible frequency is 25 Hz. 25/8 provided an acoustical ratio of 3/1, similar to minimum vibration isolation, and at the higher frequencies, sound loss would improve dramatically.
We learned that rubber materials are often deflected 10% of the rubber thickness, and many publications refer to 15% deformation as a good conservative compression limit. That is why our 2” thick isolators have published deflections of 0.2” and a maximum of 0.3”.
Dynamic Stiffness is simply defined as the ratio between the spring rate in vibratory motion and the static spring rate.
When working with steel springs, the ratio is 1, as spring steel is a completely resilient material. Rubber materials are quite different. Dynamic stiffness increases with hardness and in broad terms, the filler ratio of the materials to the rubber content as well as the type of carbon black reinforcement, plasticizers, etc. It is also very sensitive to the polymer.
We ran our Kodaris Neoprene Dynamic Stiffness test in 1972. The corrected data showed that at 0.2” deflection, the poorest situation using 60 duro with a dynamic stiffness of 1.63 increased the frequency to 9 Hz at 0.2” and 7.3 Hz at 0.3” as compared to a steel spring where 0.2” deflection would be 7 Hz and 0.3” 5.7 Hz.
In negotiating a recent building support project, we convinced the client that Neoprene should be used in place of Natural Rubber. We were not concerned that the specification required a new dynamic stiffness test, because we believed the Kodaris test data showing our 50 durometer Neoprene compound had a dynamic stiffness of 1.50 and 60 durometer 1.63. However, the dynamic stiffness tests run today are very different, and much more sophisticated. It is a forced frequency test for resonance at specific frequencies of 5, 10 and 15 Hz. We were dismayed to find that rather than 1.5 to 1.63, the new results ranged from an average of 1.8 for 54 durometer to 2.4 for 64. Using the same test techniques, our new LDS rubber
compounds are below 1.3 in 50 durometer and 1.35 in 60. This meant the continued use of Neoprene represented too great a sacrifice in performance.
LDS stands for Low Dynamic Stiffness. In addition to exceeding all AASHO Bridge Bearing structural requirements, we had worked for years to develop compounds with extremely Low Dynamic Stiffness characteristics even in 60 and 70 durometer as published. Using these compounds lowers frequency response for a given deflection to improve both vibration isolation and reduce sound transmission. Other than oil resistance, Mason LDS compounds are far superior to Neoprene in physical characteristics as well. Building Support Pads can have a lower profile than Neoprene for the same frequency. This is true of floating floor mounts too, but mounting heights are often maintained to achieve a specified air gap.
In Europe, virtually all isolation work was and is done with Natural Rubber. In this country, specifications for bridge bearing rubber supports allow the use of Neoprene or Natural Rubber. The very great majority of bearings, if not all, are Natural Rubber. However, there is no requirement for a
low dynamic stiffness, so the compounds are made less expensive by using more fillers and are considerably less efficient than our designs acoustically. In supporting bridges, this is unimportant as bearings are used in shear to accommodate expansion and contraction and not for vibration isolation.
There is a mechanical aspect too. Most engineers and architects are in the habit of pouring concrete on forms with the bearings directly underneath or erecting steel directly on the bearings. In this first stage, the loadings are very low so the bearings hardly deflect. As the building progresses, the bearings deflect in response to the added weight, which is not always uniform. The more deflection required to achieve a frequency, the greater the complication of uneven deflections that may distort the structure or induce cracks. LDS compounds minimize that problem, because deflections are minimal for the same frequency.
The use of Natural Rubber has been guided by the Malaysian Rubber Institute, and just as DuPont has been promoting Neoprene and other excellent special purpose polymers, the Natural Rubber industry has been working with the chemical people to perfect the antiozonants and antioxidants. Other additives reduce sunlight damage. The new Natural Rubber materials have become completely reliable in long term aging tests, so there is no longer any reason to continue with the Neoprene. We have always improved our offerings, and hopefully, our learning curve will continue.
Based on these conclusions, all acoustical isolation materials, including the mounts used in Jack-up systems, the EAFM series, or bearings to support and isolate structures will be made of LDS materials. (Low Dynamic Stiffness.) Hanger elements and hanger cups are included as well.
While the danger of oil contamination is minimal, all floor mounted pads under spring isolators, spring holders, etc., will continue as commercial grade Neoprene.