How Does A Port Work?????

OK, I know how a port works....the dimensions of a port are calculated from the port's cross-sectional area, port length, volume of the enclosure and box tuning frequency. When the driver moves in and out, a volume of air is pushed and pulled in and out of the port. Ports are "tuned" to a certain freq. via the cross-sectional area and the length. This generates a harmonic, like a pipe organ, correct?

My question is, what if I have a port tuned to 40 Hz based on the box volume & box tuning frequency . If the driver plays a constant 100Hz note, is the port putting out an audible 40Hz tone? OR does a port only provide audio noise at box tuning frequency?

I guess my question is, what is the relationship between the sound from the port and the freq. the driver is playing at?
 

roddyama

diyAudio Moderator Emeritus
2002-01-19 9:25 am
Michigan
Assuming the tuned frequency is 40Hz, well above 40Hz the mass of the air in the port will not react at these higher frequencies so the output is only the reflected sound from inside the box. As the frequency becomes lower and begans to approach 40Hz the air in the port will start to react. The port output level will icrease as the frequency nears 40Hz and reaches its peak output at 40Hz. As the frequency drops below 40Hz, the limited volume velocity of air in the port causes the output level to drop off rapidly (~24db/oct).
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
No, a port is not an organ pipe at low frequencies.

Imagine a rubber band dangling from your hand with a mass on the end - the mass is the port, the spring is the air in the box interacting with the area of the port. If you move your hand up and down slowly, the rubberband hardly stretches - this is the way the port acts at low frequency - the volume of air displaced by the speaker goes right out the port, this is why a port is said to unload at low frequencies.

At the resonant frequency of the mass spring system, the port moves a lot with almost no input from your hand - and out of phase with your hand.

Above the resonant frequency of the system, you can move your hand a lot and the mass hardly moves. A port effectively seals up at about twice the port frequency, except for unwanted effects below.

Way above the resonant frequency, the port will act as an organ pipe, the frequency being related to f=13562/2*L, where L is the length in inches. This is unwanted radiation, some programs can model it.