What is meant by “fixturing” in regards to vibration testing?

What is meant by “fixturing” in regards to vibration testing?

As stated in ISTA testing protocols, "Restraining devices shall be used to... (1) Prevent the test specimen from moving off the platform and (2) maintain test orientation of the packaged-product, but (3) not restrict the vertical motion of the test specimen."

This means that the test package should not be affixed directly to the vibration table. For safety, the test package should be prevented from falling off by some "fixture", "fence", "restraining device" around the sides, but not attached to table in such a way that the vertical motion is restricted. This allows the package to respond to the vibration of the table in a natural way.