Here's why:
* Gerund: A gerund is a verb form ending in "-ing" that functions as a noun. In this sentence, "ringing" is the gerund.
* Gerund Phrase: A gerund phrase includes the gerund and any modifiers or objects that go with it. Here, "of alarm" modifies the gerund "ringing".
The entire phrase "ringing of alarm" acts as the subject of the sentence, describing the action that jolted the speaker.