Quick demo of simple car sound proofing
The nice thing about sound proofing a vehicle this way, is that you can basically just keep adding more sound proofing indefinitely, and it'll always get a little better every time you do it. The van in the video has these pads stuck onto pretty much every large-ish surface, including the insides of all doors and covering something along the lines of 80 % of the ceiling, and a considerable portion of the "engine hood", i.e, underneath the seats.
This has brought the vehicle from "raise-your-voice-I-can't-hear-you rattlebox" to "wow-this-is-almost-like-a-normal-car".
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