relationship between 'b' and 'm' sounds


There are several words that have a common etymological origin but are
sometimes pronounced with a 'b' and sometimes with an 'm'. Here are some
examples:

おもえる、おぼえる （思える、覚える）

さむらい、さぶらい （侍）

ぶ、む （無）

ばく、まく （幕）

ば （馬）

There are many more examples. For 馬, I'm almost certain it is pronounced with
an 'm' initial in every dialect of Chinese but its onyomi has a 'b'.

Is there a reason why these two similar, yet definitely distinct, consonants
are related in this way?




There are two different reasons.

For native words - the historical pronunciation of Japanese voiced consonants
involved prenasalisation (so /d/ was more like [ⁿd]). While in most cases the
prenasalisation has been lost, in a few instances the voiced stop was the part
that was dropped.

This is also the reason for the modern language's [ɡ]~[ŋ] variation.

For Sino-Japanese words - Japanese has multiple sets of on'yomi that were
borrowed from Chinese at different times. The variety they were borrowed from
underwent an initial denasalisation ([n]>[d]) between times when Japanese
borrowed, so for example, one word which was borrowed as まく before the change
was later reborrowed as ばく after the change.

