The noise made by the submarines, which compromised their ability to stay hidden, was another major problem with the design. In the original requisition, the RAN guidelines for the noise signature of the new submarines were vague; for example, asking that they be "twice as quiet" as the "Oberon"s. Expectations and operational requirements also changed between the 1987 contract signing and when the submarines began operating in the late 1990s. The major element of the noise signature for the "Oberon" class was machinery noise transmitted through the hull; this was successfully avoided during construction of the "Collins" class by mounting machinery on platforms isolated from the hull.