why is the second syllable a rising tone in สนามบิน ?

น beeing a low class consonant followed by a long vowel า and a stop final , shoudn't it be a mid tone ?

why is the second syllable a rising tone in สนามบิน ?

น beeing a low class consonant followed by a long vowel า and a stop final , shoudn't it be a mid tone ?

I mean a FALLING tone

why is the second syllable a rising tone in สนามบิน ?

น beeing a low class consonant followed by a long vowel า and a stop final , shoudn't it be a mid tone ?

I mean a FALLING tone

You were right first time. The term 'stop' in the tone rules means an oral stop, not a nasal stop.

What you see here is an example of the 'transferred tone rule', which is very similar to 'vowel governance' in Khmer. You will know that when a syllable starts with two consonants, it is the tone class of the first syllable that determines the tone. Well, the tone class of a consonant in a previous syllable of the same word may affect the tone if:

(1) this previous consonant is a stop or fricative

(2) no vowel symbol other than sara am comes between the previous consonant and the syllable (except that the ป of ประ may affect the tone, as in ประโยชน์ and ประวัติ)

(3) the syllable does not start with a stop or fricative consonant (counting อ and ห) of its own.

I am not sure where this rule comes from. The simplest explanation would be that the intervening vowel was optional, and thus one can imagine an alternative monosyllabic pronunciation สนาม *[R]snaam. However, that does explain the rule about sara am, in words such as ตำรวจ. It is conceivable that the rule applies here because the -am(n)- is actually an infix, borrowed from Khmer, and so you get pairs such as เกิด and กำเนิด.

An alternative explanation is that the effect we label tone class was actually once a distinction between 'clear' vowels (after high and mid consonants) and 'breathy' vowels (after low consonants), and that when a stop caused caused one vowel to be clear, the effect extended into the next syllable. In Khmer, the rule is expressed as

In any syllable which is preceded in the same word by consonants of different series, the series of the vowel will be determined by the last preceding stop or spirant.

(Khmer consonants are divided into two series, first and second, corresponding to the high/mid v. low distinction of Thai. With rare (borrowed?) exceptions, only stops and spirants can be first series consonants in Khmer.) The same rule is found in Cham, a language mostly spoken in southern Vietnam.