It's not just about being a straight through design. You have to take into consideration the sound pulses as part of it. Sound is simply energy that you can hear. You can pass it along, you can try to cancel it, you can absorb it.....anything other that just letting it pass freely will cause a disturbance in the system.
If you try to absorb it, you generally slow the flow. (Most glass packed mufflers)
If you try to cancel it, you have use something that has an off setting frequency against it and this disturbs the pulse. (Flowmaster type mufflers)
The magnitude of the disturbance will cause some pressure against the exhaust flow. Also, pipe diameter, length of bends, radius of bends, etc., all play a part.