My 2c after the last year or two of builds I have noticed the following general statements to be often true, based both on measurements and listening tests.

- Rear ports can be used when the port is near a rear boundary, and the number of speakers used in total per passband is low. i.e a pair of floorstanding hifi speakers, or bookshelf speakers.

- Rear ports in PA cabinets can potentially make it hard to phase align the low-mids with the subs. The port gain is also potentially sending the "out of phase" waves rearward. This outdoors (not in a club environment for example) is potentially leaking SPL towards the performers. (Wrong! see video in laster post)

- The port gain from a port at the rear can be difficult to measure from the front of the enclosure, especially in half or full space, rather than indoors

Below shows a back ported 3-way hifi speaker with a 15" woofer designed to be used up against a boundary such as a wall. The orange and blue lines show measurements taken 60cm from the front of the enclosure (in a semi-anechoic/impulse gated half space environment), and the black line shows the measured port response at the rear of the enclosure.

The response (with added port gain) tracks both simulations and listening tests.

Would love to hear about anyone else's testing, measurements, related articles, etc regarding back porting vs front porting :)

Phase alignment is more on how the two non-identical reflex boxes are tuned. In addition to how the loudspeaker is reacting in the box based on its TS Parameters. Many don't like rear ports for they cannot keep the boxes flush against the wall. If you have enough bass, you do not need walls and corners as a means of room gain. All you need is the ground it sits on.

Best Regards, 

Just to expand on what I wrote. Based on my measurements I've done 20 - 25 years ago, 1 foot is the bare minimum you would have rear ported reflex near a wall. Anything closer would alter the tuning frequency. 

If the bulk of the sound (based on the tuning frequency of the port) is coming from the port, it would make sense to offer a front firing port. If the port is used for extended frequency response, it does not matter where the port is located. 

I might add, rear firing ports offers minimal (if any) port noise reaching to the listeners ear unlike front firing ports. Rear firing ports are definitely more High End as, the resonance above the tuning frequency of port is not noticeable to the listeners ears like front firing ports. 

I've used subs offering front firing ports for a little over 32 years and, rear firing ports for around 30 years. Personally, I like the sound rear firing ports. However, if you like that rolling type of baseline (which stems from a lot of resonance), front firing ports is the ideal choice.

Best Regards,  

As we know it's very easy to steer low frequency with multiple subwoofer enclosures by playing with delay and phase - should this not be also seen in the low frequency coming from an enclosure vent which as we know comes out out-of-phase from the woofer?

I would love to see a Kipple NFS scan of the same enclosure with a front port, back port, and side port, to see if there is a measurable effect on (not the overall SPL) the distribution/directionality/coverage pattern of SPL by frequency (specifically around port tuning frequency)

If anyone has links to papers on phase steering of low freq in enclosures by port placement I'd love to have a read!

It depends on the tuning. Here in the states, the majority is not going to tune a reflex cabinet above 35 Hz. Using 35 Hz as a guideline, you would need a boundary of 32 feet to control it's directivity. Controllability will be greater in terms of feet if you are aiming below 35 Hz. 

Best Regards,