Consonants in contact rarely assimilate in Adûnaic (SD/420). The main exceptions are nasals (m, n) followed by a voiceless stop (p, t, k) or spirant (ph, th, kh) that has the same place of articulation. These develop as follows:

mp, mph	>	pp, pph
nt, nth	>	tt, tth
nk, nkh	>	kk, kkh

These assimilations are an echo of the older phonetic rule that nasals became voiceless stops before voiceless stops, aspirates and [s]. Tolkien only explicitly listed the combinations mp, nt, nk as subject to assimilation (SD/420), but the example he gave of Amân + thâni = Amatthâni combines a nasal and a spirant, so it logical to assume that this assimilation rule applies to spirants as well. Perhaps Tolkien described the assimilations as an orthographic rule instead of a phonetic rule, since spirants were represented by the digraphs ph, th, kh.

Tolkien also mentioned some assimilations that appeared in spoken Adûnaic but were not represented in their writing. In particular, voiceless stops and spirants tend to be voiced before voiced stops (SD/421): “sd” pronounced [zd], “pd” pronounced [bd], though the voicing was less pronounced than similar assimilations in English. Other spoken assimilations are mentioned in the phonetics section of Adûnaic. Since these assimilations only affect pronunciation and serve no grammatic function, they are not discussed further here.