Honbu / Hombu is Japanese for “Headquarters” or “Head Office” for a system of martial arts. There seem to be 2 different ways of pronouncing 「本部」and some people fiercely insist it is one or the other and their bias is not based on any facts.
A search of the internet will display a number of English language web pages with one or the other so that is not a valid source. Even English language martial arts text books flip /flop on the spelling. There are a few internet forums that have intelligently examined this issue, and I have included some extracts from those forums:
It should be ‘honbu’ , but many people pronounce it hombu.
To be a little more exact…the reason for the n/m differences are due to the use of “roman” letters (as opposed to arabic, chinese characters, etc) to express or write the Japanese language. There are several versions of “romaji” (Japanese for “roman letters”). And there are sounds in Japanese that can vary (like the tomato pronunciation example) depending on a number of factors (dialect, region, etc). You will see similar situations with words like Senpai/Sempai, Enpi/Empi, and Enbusen/Embusen, Kempo/Kenpo, Shimbun/Shinbun and so on.
We are trying to put another language into our own. And so we have these discrepencies. The linguistic terminology for this would be “reverse assimilation in the place of articulation.” The usual “n” sound is produced at the alveolar ridge (that “shelf” right behind your teeth) However, as Peter alluded to above, since there’s a bilabial (eg “b”, “p”) sound coming after the “n” sound, the lazy human mouth decides that it’ll save it some trouble and uses nasal bilabial consonant (ie “m”) rather than a nasal alveolar consonant (ie “n”). You’ll see such things even in English in words such as “impossible” (which most likely came from adding the prefix “in-” to the word “possible).
As far as actually pronouncing that part of the words goes, I think you’d be hard pressed to distinguish someone saying “honbu” (with a real “n”) and “hombu”, in regular everyday speech with their back turned towards you. (Similarly, the “th” and “f’ sounds are also hard to distinguish without visual and contextual cues.) Seriously, I kind of doubt there are many Japanese folks out there who would say “ho n bu” rather than “ho m bu” when speaking naturally.
Transliteration or the art/science of writing Japanese words in the Roman alphabet is not consistent. Some people write “kenpo” while others write “kempo.” Some people even write “si” rather than “shi” — probably since, technically, there’s no “si” sound (as in the Spanish “yes” or the German “if”) in Japanese. There are even other issues such as nigori (the voicing of certain consonants in a compound word) where some people write “katatedori” while others write “katatetori.” Also tricky is the case of long vowels — if we were really picky about it, we’d probably be writing the name of the art as “aikidou” rather than “aikido” since the last vowel is a long one…
The answer deals with the natural flow of speech.
Say honbu 5 times fast, then say hombu 5 times. The ‘n’ has a ‘hard’ or elongated sound to it. In natural speech, it is easier to roll that sound to an ‘m’ as opposed to an ‘n’. Prevents a slow down in speech.
The “n” or “m” distinction has to do with how characters are romanized when they are written – not how it is pronounced when spoken. Different romanized systems use one or the other.
Honbu is how it is written, Hombu is how it is pronounced. It is confusing as they look / sound so similar but consider say the Chinese written ‘x’ that is pronounced ‘shi’ and you might see what I mean.
Also try searching a japanese dictionary for hombu, you won’t find it. The Budo Jiten (martial arts dictionary) second edition, by Fredrick Lovret list only Honbu.