Hello Sattawat Wayu. My name is Christoph Lampl and I am the Head of Powertrain this year. The noise problem is a big problem especially with dbc. I see you are going to use an absorption silencer and cant pass noise with it. Have you tried thingds like this http://www.bodis-exhaust.com/ckeditor/plugins/doksoft_uploader/userfiles/20.jpg, or http://www.stein-dinse.biz/images/product_images/popup_images/300038192.jpg so db killers? Try different ones and try to measure noise at most far away point in this measure triangle of the rules and if it is possible near to the tire. Read through this articel http://tobiasmic.blogspot.co.at/2013/09/rules-corner-possible-rules-changes-for.html, maybe you have to change to an reflection silencer and you will lose power but luckily you pass noise.You also need to know which frequences your engine and your exhaust produce and after that you can construct an silencer.