Quantity of air then speed of intake. Thicker air not more is key factor. Consonant T is unvoiced, vowel sound is what you control sound with. You don't want pressure to keep diaphragm down. The diaphragm should be allowed to return to its contracted position.
Pelvic pressure keeps pushing diaphragm down.
Wind as wind not pressure. There cannot be wind without pressure so you don't have to add any additional pressure to the equation. Your body works in similar fashion. There is a moment when you have finished breathing when the inner and outer air pressure is equalized. Prolong this moment.