Rather than testing stiffness, for foams a better option is to do "Indentation Force Deflection" test.

http://en.wikipedia.org/wiki/Indentation_force-deflection

There's an ASTM and ISO standard protocols for this test.

Dynamical mechanical analisis (DMA)

http://www.perkinelmer.co.uk/CMSResources/Images/44-74546GDE_IntroductionToDMA.pdf

Dynamical mechanical analysis

DMTA ( Dynamic-Mechanical Thermal Analysis)

Universal Testing Machine (UTM) also useful for the determination of tensile strength and young's modulus of the polymer. (In most of the cases, tensile strength is directly proportional to the stiffness of the polymer)

There is a distinction between static and dynamic processes to measure the hardness. All refer to the same principle: a body dipper is pressed continuously in the material with a force of specific test. It is necessary to measure the local deformation that originates in the plastic and elastic. The static measurements determine only the part of the plastic deformation. According to the magnitude of the force of evidence is difference: macro hardness (force F > 30 N), hardness of small load (2 - 30N) and micro hardness (< 0.5 N). The measurement of hardness with Brinell, Rockwell Vickers or belongs to the processes for measuring hardness static. In the dynamic processes (method of Shore hardness or hammer suspended) moves a body piercer in part a measure with a kinetic energy from a defined interval. Well checked lines, shafts of turbines or castings. It has at its disposal DIN 50150 tables to compare the results obtained by means of the various procedures. Brinell hardness Abbreviated HB. A hardened steel ball (HBS) or a metal ball hard (HBW) diameter cone- acid is pressed with a force of F-test vertically on the surface of a faired test. The test force is exercised within a defined time interval (two to eight seconds); the time of incident, you must be between ten and fifteen seconds. The Brinell hardness is calculated by the diameter of the pressure and the strength of evidence. The Brinell hardness procedure applies on all base materials with ferrous alloys or not with ferric salts. A heating mechanism in the test installation procedure gives meaning to the Brinell in determining the hardness of metals heat. Vickers hardness HV procedure of penetration for the control of hardness (process of measuring the hardness DIN 50133). A diamond with a pyramid shape is pressed in a test with a defined force. The hardness is calculated by the diagonal of the pressure - a measure to release the test - . Depending on the strength of test distinguishes the load range small and micro hardness. The macro hardness in metals is independent of the strength of evidence. The Vickers procedure is used with very tough material; for testing small, hard and thin to surfaces and temperate. After the HV- the test of strength in abbreviated form and the time of impact - if it differs from the usual time -. Hardness Rockwell hardness Rockwell determines the value of hardness by means of the depth of penetration of a body of evidence. For the variants C and (for hard materials or very hard) as well as N (for testing thin) a ball diamond, it penetrates the test in a way defined. For the view- B and F (for materials semi-hard and soft) or T (for testing thin) used a steel ball. The body of evidence may have a more secure hold during the measurement with the pre-load. The test load varies depending on the variant. Abbreviations for the procedures Rock- well are: HRC, HRA, BHR, HRF, HR- 15N, HR15T, HR30N, HR30T, HR45N, HR45T. The value of duration - za is placed in front of the abbreviation, for example: 45 HRC; 76 HR45N. DIN 50103. Sheet 1 for processes C, A, B, F, sheet 2 to N and T. Shore Hardness procedure of dynamic hardness measurement. It determines the height of rebound head of a pin that drops on the surface of the test from a height of 250mm. Height 177 mm bounce co- nonregular Shore to 100 units. Hardness Test Procedure by impact dynamic hardness measurement. The basis of calculation is the diameter of the pressure of a ball produced by the impact with a hand hammer (hammer Poldi) or with a spring under tension. With the hammer Poldi ball is pressed in a control bar of known hardness.

Dear Alberto;

I agree with researches above about there are several tools and instruments that you can utilize to measure stiffness for Polyurethanes (PU). However; it becomes overwhelming for any researcher to come up with the best proper method. Therefore, I will ask you this; why are you measuring the stiffness? I guess, you mentioned that you would like to  study the comfort from wearing hearing protectors. My other questions, after measuring stiffness, how are going to correlate stiffness to comfort?? this is another challenge? Typically, from measuring stiffness for PU, you can say that this PU is soft or stiff, etc. But it will be difficult to correlate it to comfort. I am suggesting you to establish a sensory research by distributing several   wearing hearing protectors on people and let them give you scores from 1-10, for examples 1 being the worst protector and 10 being the best. Then you can correlate stiffness to sensory results. I hope this is will shed some lights on your research, but feel free to continue discussing this topic and I will be more than happy to further discussing it with you as well.