The inherent error involved in the measurement of dynamic mechanical properties of viscoelastic materials by a conventional forced-vibration method is theoretically evaluated by considering propagation of strain and stress waves in a sample. The frequency f and the sample length l must satisfy the criterion fl<0.12(|E
*|/ρ)
1/2, when the dynamic modulus and loss angle have to be determined within 10% error. Here, E
* is the complex modulus and ρ is the sample density. For an ordinary polymer sample of a few centimeter length, the criterion sets the limit of applicable frequency below a few hundred Hz. To avoid these difficulties, we propose a new method of viscoelastic spectroscopy hopefully applicable in the audio frequency range, employing a technique of the mechanical impedance measurement.
The new method involves the following procedures: Two equivalent vibraters are attatched to the both ends of a specimen. The vibraters are operated simultaneously with the phase difference θ of 0, π/2, π, and 3π/2, or only one vibrater is operated with the other end fixed (without operating the second vibrater). The impedance Z(θ) or Z
fix is measured by an impedance meter attached to the end of the specimen. Then, the complex modulus E
* can be estimated by one of several simple algebraic relations among the Z. One of such relations, for example, is E
*=Z(0)Z(π)/ρA
2, where ρ and A are the density and the cross-sectional area of the specimen, respectively.
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