Chart 2 illustrates the results of a second test wherein the IPA #115 perforated sheet, the one with the least Open Area, was used in conjunction with four different sound absorbing materials:
4" 6 pcf Mineral Wool with a NRC of 1.05
4" 3 pcf Mineral Wool with a NRC of 1.10
4" Fiberglass Blanket with a NRC of 1.05
4" Fiberglass Board with a NRC of 1.10
NRC stands for Noise Reduction Coefficient, a standard measure for sound absorption which is reflected in the y Axis Scale. A material with a NRC of 1.10 is approximately 5% more efficient as a sound absorber than a material with an NRC of 1.05.
The test results demonstrate, again, a high degree of transparency for the IPA #115 material. Additionally, we can see a rather significantly better sound absorption by Fiberglass Board in the lower frequencies and noticeably weaker performance of the 6 pcf Mineral Wool material below 1000 Hz. But, the differences are small and clearly the presence of the perforated metal had no effect on the sound absorbing performance of any of the materials.
