A or the?

[englishhobby]

My students and I are reading and discussing an adapted version of the novel "The Phantom of the Opera" in class. There was such a phrase:
1) He had the head of a dead man. (about the ghost's appearance)
And another one:
2) She had the voice of an angel. (describing a singer's voice)

Could you please tell me what difference it would make if I used an indefinite article in both sentences. What is the best way to explain the use of the definite article to my students in these situations?

Thank you in advance.)

[5jj]

1) He had the head of a dead man. (about the ghost's appearance)
Most people, dead or alive, have only one head each. You can have 'a dead man's head' but not 'a head of a dead man'.

2) She had the voice of an angel. (describing a singer's voice)
Angels, if you believe in them, have only one voice each. He could have 'a voice like an angel's (voice)', but not 'a voice of an angel'.

5

[englishhobby]

Thank you very much!