Need Japan and English spoken help !!

[Gun[Lengend]]

Need someone can speak or write Japanese help me. Or someone can know english sfx.
I have a picture ( manga ) :

That in a first picture. The sfx mean : boof <= i can't search in dictionary.
But look like : だ
I search it and get the mean : " I " is Vietnamese mean : Tôi
It's not logical.
Someone can tell me what is mean. i'm sorry for my bad english.

[XexisKhan]

BOOF!

english sfx (sound effects) are just words that represent a sound. when you speak the word "BOOF" then you have made the sound the author wanted you to hear.

[Bùi Hoàng Hải]

Need someone can speak or write Japanese help me. Or someone can know english sfx.
I have a picture ( manga )
That in a first picture. The sfx mean : boof <= i can't search in dictionary.
But look like : だ
I search it and get the mean : " I " is Vietnamese mean : Tôi
It's not logical.
Someone can tell me what is mean. i'm sorry for my bad english.

In Japanese, だ pronounce 'da', now with a trailing, it should be 'daaaaaaaaaah...', maybe it has anything relating ti boof?