Is the question clear? Idiomatic scheme is not a term of art, I guess, but it's idiomatic and it follows a schema.
It's a weird one, for sure. Some thoughts:
The Adjective can't be removed
* The explosion was [easy] to hear
In idiomatic English, this would probably be "could be heard". But In German, the direct translations work: "Die Explosion war zu hören" (was audible), "Die Explosion ist gut zu hören" (was easily heared). In English, "I am to", "I am not not", have developed different meanings (whereas Ger. "Ich bin nicht zu haben" means I'm taken, not to be had).
This syntax doesn't work with all adjectives, at least not idiomatically. Compare
The castle is easy to see from here
* The castle is possible to see from here
This is good to know
* This is terrible to know
I'm using it rather freely though, and want to find out to what extend that's wrong. I was informed that "It is X to Y" might be the more usual form.
In my mind the adjective modifies the infinite verb. As such, it should be an infinitive.
The syntax of the phrase is similar to "good to him", as if to was not the infinitival marker, but a preposition; Which works in German, where nominalized verbs may take the form of the Infinitive (where English has the gerund), and nouns may take prepositions. In effect I'm not sure how to parse this.
Cases of to-infinitives abound, obviously. There's probably an interesting summary of it's origin somewhere. The phrase in question is probably a tangent in that story.
The passive voice usually takes is to be, "I am not sure how that is to be explained". German sees the passive expressed with the finite verb alone "Ich weiß nicht, wie das zu erklären ist", although "... wie das zu erklären sein soll" exists.
I don't know a proper term, so I don't know what to search for. Therefore, the title is makeshift. I don't even know what the problem is, so I'm trying to keep it general. What's the grammatical analysis of this phrase in those contexts? Posting here not ell.se for the historic, comparative aspect.