port length


2008-02-10 1:13 pm
Maybe a simple question. When you calculate a port length does this include the baffle thickness? Is this strictly speaking the actual "cut" length of a tube and then placed in the enclosure? What if the length of the port is shorter than the baffle thickness? Does this change the tuning frequency?
The calculated port length (whatever software you're using) is just the resulting effective port length. In real designs the measured port length is usually shorter than the calculated length, depending on the length, diameter, and "style" (i.e. straight, single/double sided trumpet, exponential opening, ...) of the port duct. As a rule of thumb for straight ports, take the calculated (and reasonable) length and subtract approx. the diameter (round port) from the length as a starting point. For any fine tuning (if necessary) use variable ports with the same diameter. As soon as you know the correct length you can swap to fixed ports and cut to the length as desired.

A length shorter as the baffle thickness would be a bad design :xeye:.
In this case increase the diameter of the port, (re)calculate your port length again, and you'll see that the length will increase too. :)

One design rule would therefore be the following:
A short port with a small diameter might have the same tuning frequency as a long port with a large diameter (see also the formula for the Helmholtz resonator - which I do not attach here).

If the port is to long the air will resonate within the bandwidth of interest of that bass (headword: comb filter). If the port is to short, the diameter is ofter very small though, the air speed inside the port is very high (larger or equal to i.e. 0.2mach) and will introduce unwanted vent/port noise at higher sound pressure levels. That's by the way the reason why ducts have very often a round-off shape at the/both port end(s) to lower that effect noticeably.