A Question About Reflex Ports

When calculating a reflex box response, a small, short (thickness of the wall) vent is mathematically equivalent to a long, large vent. But I can't help but think that there's more to the story - otherwise why would anyone go through the effort of using a port tube; you could just size the port (within limits, I understand) to require a length equivalent to the baffle thickness. This is rarely done in practice. So my question is this: what are the governing relationships? Is there an empirical relationship that has been derived over the years (for instance a target length:width ratio)?
TIA
Charlie


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