9.1.4 Critical Mass  Critical mass in the field of sound preservation is where the size of collection is sufficient to justify the expenditure to undertake all the tasks in house. It is difficult to quote concrete numbers when defining critical mass; the more professional institutions there are available in a country or region, the higher the critical mass will be. If, however, there are only few institutions engaged in professional audio archiving, or if there is none available at all, then the critical mass would be lower. Critical mass should always relate to specific media formats; coarse grooved discs, microgroove discs, open reel magnetic tape, etc. In fairly developed countries or regions critical mass would be at least several thousands of items, but often institutions with tens of thousands of carriers of one type make a rational decision to outsource signal extraction. Under less developed circumstances the autonomous transfer of few thousand items/hours only can be carried out successfully.  The critical mass will also depend on the homogeneity of the material within the respective format. Homogeneous collections can be transferred with some degree of automation. The cost associated with fully automated systems generally suggests outsourcing to institutions or service providers that offer computer controlled parallel transfers. Collections consisting of many different carriers or standard of recording – as often found in research collections – demands reliable manual transfer, which may be available at lower cost in house, provided the specialised expertise is available.  Even large, professional sound archives may consider sending parts of their collection to specialised institutions or service providers for the purposes of transfer. This may especially be so for some historical analogue and digital carriers.