**2. Experiments**

As in the case of heat conduction [23], the experimental results are considered as a benchmark problem in order to test the validity and feasibility of the corresponding generalized model. Here, one measurement performed by Rhodes [50] is discussed in detail. There are many other data in the literature [51–54], but this one is going to be sufficient to present all the necessary conclusions and difficulties arising in that field, i.e., how the scaling properties appear, the interpretation of the experiments and more importantly, the role of the material parameters.

A sonic interferometer [55] is used to measure the sound speed for various frequency-pressure ratios [50], see Figure 1 for typical data. The interferometer is placed in a dewar to maintain a constant temperature within.

**Figure 1.** Speed of sound measurement performed by Rhodes [50]. The vertical axis denotes the relative speed of sound, i.e., *v*/*v*0, where *v*<sup>0</sup> is the speed of sound related to the normal state. The original data can be found in [50]. The relevant points are emphasized by red squares.

It has to be emphasized that the frequency was constant as well in the experiments [50], i.e., the pressure is varied over the whole interval. More appropriately, it was the density that changed during the experiment when the constant temperature has maintained. In Figure 1, the results related to normal Hydrogen is presented. The measurement is also performed using pure para-Hydrogen and the 50–50 mixture of para-ortho Hydrogen [50]. Now choosing the curve from Figure 1 corresponding to 296.8 K. Before making any advancement with the extended models, two essential aspects must be discussed. The first one is to investigate the density dependence of material parameters. Then, one can calculate the dispersion relation for the relating model (4)–(6) or (5) and (6) to model the experiment and analyze the frequency-pressure dependency, too.
