Did some digging and found this.
Alberich
Alberich consisted of synthetic rubber sheets of about 4mm in thickness
which had sound absorbing properties. The material was Oppanol which was
secured to the outer hull with adhesives, much like an outer skin.
Although no conclusive tests were performed, but it was claimed that the
echo reflection of a U-boat with Alberich was reduced by about 15
percent. In addition, it also acted as a sound dampener, containing the
U-boat’s own engine noise.
Although the principle was a sound one, problems were encountered with
the adhesive coat which was not strong enough to adhere the rubber sheet
to the hull. This resulted in the sheets being partially washed off,
which flapped in the wake of the ocean current, causing hydrodynamic
resistance and noise. Further research into more reliable adhesives were
conducted, but up to the end of the war, only a few U-boats had
received this treatment. A further contribution was that treating the
hull was a time consuming and laborious task.
The first U-boat to receive Alberich was U-11, a Type II coastal boat
for trials on its sound absorbing properties. On April 1940, the first
operational U-boat was treated, U-67 a Type IXC which was just being
laid down. Thereafter, problems with the adhesive prevented further
treatments until late 1944, when U-480, a Type VII was tested again
using a new adhesive. The results were satisfactory enough and it was
decided that all new Type XXIIIs and XXVIs would receive this coating,
but ultimately only one Type XXIII, U-4709 had been completed with the
coating.
Tarnmatte
This was a radar absorbing material used to coat snorkel heads to shield
them from allied radar. It consisted of a compound of synthetic rubber
and iron oxide and was claimed to have 90 percent effectiveness in
reducing radar signature.
Found the link in this interesting thread on SubSim.