Dictionaries suggest 透 (tou) is pictophonetic where 秀 (xiu) provides the sound. Is this through a classical chinese pronunciation?

Question

I have several resources and dictionaries that suggest the word 透 (tou) is pictophonetic where 辶 (chuo) suggests the meaning and 秀（xiu) provides the sound. This confuses me because 透 and 秀 do not seem to be related in sound. I assumed that this may be due to a classical chinese pronunciation but was unable to verify that or find more information.

What is the correct etymology here? thanks!

Answer 1

Pronoucing them in Cantonese will make sense to you.

透，tau3
秀，sau3