For harmonic waves in layered elastic composites the results obtained by means of the Rayleigh quotient based on displacement and the Rayleigh quotient based on stress, are compared with those obtained by a new quotient recently proposed by one of the authors, in an effort to examine reasons for the astonishing accuracy of the new quotient for this class of problems. This comparison leads to a scheme for obtaining improved test functions which then give very accurate lower and upper bounds for the wave frequencies. Results are illustrated by numerical examples.

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