The carrier must be capable of storing as many active oscillations as possible, retaining them over a very long period of time and also emitting them again constantly. A great many of different carrier materials were analyzed and tried out in long test series.
Not all materials have the ability to meet the desired requirements. High-silica aluminum ultimately proved to be the material with the best cost-value ratio and is used predominantly today. For practical reasons two half shells are assembled into the ready to use device. That way the installation can be completed with minimal expenditure of time and without much effort.