Here's why:
* Gerund: A gerund is a verb form ending in "-ing" that functions as a noun.
* Gerund Phrase: A gerund phrase is a group of words that includes a gerund and any modifiers or objects that go with it.
In the sentence, "ringing of the alarm" is a gerund phrase because:
* "ringing" is the gerund (a verb form ending in "-ing" acting as a noun).
* "of the alarm" is a prepositional phrase modifying the gerund "ringing".
The entire phrase acts as the subject of the sentence: "The harsh ringing of the alarm jolted me...".