Cox, Johnson, and Alexander (2012) report cochlear dead regions (DRs) occur when inner hair cells at a given frequency are lost or malfunctioning. Previous publications have suggested hearing-aid high-frequency gain should be attenuated in the presence of high frequency DRs.
Precondition for a VdS approval procedure, among others, is proof of a certified quality management system in accordance with DIN EN ISO 9001 for the manufacturing facility of the product.
WDS *is* repeating. The generic technology is repeating, of which there are two common forms/implementations; WDS or Universal. WDS is a NOT a wifi-certified protocol, so incompatibilities are very common. So common, in fact, that many ppl have abandoned it except in those few cases where they have 100% assurance the devices are compatible.