Compression ratio cube root

We take the cube root of the average compression ratio, because the three jets represent three compression stages. 

From Eq. (3.1) we see that the Debye frequency iw varies proportionally to the velocity of elastic waves, divided by the cube root of the volume, and from Eq. (3.9) wc see that the velocity of either longitudinal or transverse waves varies inversely as the square root of the compressibility times the density, if we assume that Poisson s ratio is independent of the volume. As we shall see later, this assumption can hardly be... 

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