The items to be tested should be held during the test in the same position and configuration present on their standard installation. Vibration should be transmitted to the item in the same quantity from any fixation point of the item.
The structures that connect the item held in the proper test position and the shaker armature are called Fixtures.
The fixture is usually customized on the item dimension and shape, but a custom fixture can also be mounted on a standard fixture like and expander or a prism.
Standard fixtures are general purpose items that are part of the facilities of a vibration test laboratory. The most common one is the expander.
The fixture should be studied to be as light as possible to use less force as possible from the shaker to be vibrated, but must be as rigid as possible in the test frequency range.
Any resonance of the fixture present in the frequency range can alter the test results and cause non reproducible tests.