how to design a cat back exhaust?

[scions whoop]

this is going on my 05 scion tc. i need to lower my exhaust noise by 16db and im going to install a custom system while im at it. for now im just putting in a cat back, but i will eventually install a catless header, so the cat back has to be able to keep the noise and rasp down caused by a sraight through header.
im not sure what components to put in it. high flow cat? ive heard a well designed cat will not really affect power, so i would rather have it to pass emissions. i also bought a thrush glasspack and i think i might add a resonator. ill probably stick to the stock pipe size, 2.25". theres no need to go bigger as the car is mostly stock.

i need the car to be under 74db and i want a throaty sound with no rasp. how can i accomplish this?

right now, im running stock pipes with no muffler, very nice sound, only problem is that its 90db and i have to pass a stupid noise test for court....

[pelly3s]

74dB wow thats low. i hope thats at idle haha. mine is louder than that with just an intake done to it.

When you go to court that day request to see that the meter meets ANSI S1.4 Type II standards and that it is to be calibrated that day and present the calibration paperwork to you. I have talked to people who have had issues where the courts meters were off by 6dB or more.

youre best option is if you still have the stock muffler throw that on for the court date, then build one after. i highly doubt you'll build anything thats decent that will meet the laws you are up against

[scions whoop]

im at 90db right now with no muffler, so i think even the cheap glasspack i bought might drop it down just enough to pass. i dont have the stock muffler and i have a custom bumper so i can install the stock one even if i wanted to

[pelly3s]

good luck but i dont think i glass pack will help on that one.