The number of shakers is often a difficult question to answer. Frequently we are limited by the total number of output sources in the data acquisition system or shakers available in the test lab for modal testing. Usually two to four shakers are sufficient for most tests, particularly when testing larger structures like automobiles or aircraft; tests with more than five shakers are rare. Ultimately, there need to be enough shakers acting as reference locations positioned so that all of the modes of the structure are adequately excited and observed and good frequency response measurements are obtained. This includes having multiple shaker/reference locations to resolve repeated roots and/or closely spaced modes.