ทร, as in the heading of this section, เสียดแทรก (pronounced เสียดแซก siat saek), when accompanied by a vowel (implicit in ทรง (ซง song an element in forming words used with royalty); a semivowel in ทรวง (ซวง suang chest, heart); or explicit in ทราย (ซาย sai sand).
All consonants have an inherent 'a' sound, and therefore there is no need to use the ะ symbol when writing Sanskrit.