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Rear port pros and cons (not an unbiased opinion)

Posted by Duke (D) on November 18, 2007 at 00:05:36

In Reply to: RE: HELP! Best front ported 2-way mini-monitor under $1K posted by chocolate_lover9999@yahoo.com on November 17, 2007 at 22:15:06:

Arguments in favor of rear-porting:

1. Any midrange energy emerging from the port will be less audible with the port on the rear than if the port was on the front. Whether or not this is an issue depends on several factors, but in most two-way ported speakers some midrange energy escapes via the port. The longer the port, the more likely that it's a source of coloration (imaging talking through a cardboard tube - the longer the tube, the more coloration; it's actually a bit more complicated, but that's a reasonable approximation).

2. Boundary reinforcement of the port's output is enhanced by the rear port's proximity to the wall - giving you more deep bass energy. A down-firing port usually works even better. Note that not all ported speakers sound good with boundary reinforcement of the port's output - many will sound too boomy. The solution is a port that tunes the box lower than "normal" - in anticipation of such boundary reinforcement. This is the approach Audio Note uses. It can be done with conventionally-tuned rear-ported speakers if you re-tune them to a lower frequency, which usually requires some creative modification of the port (either increasing its length or reducing its diameter) unless the port length is readily user-adjustable.

3. Theoretically it's desirable to have multiple low-frequency sources spaced apart some distance. If you've ever noted a significant change in bass response from moving a speaker a few inches, that illustrates that the room-interaction-induced peaks and dips can change significantly with a small shift in source location. Several researchers advocate multiple low frequency sources in a room, such that they "average out" at the listening position (and pretty much throughout the room). If a rear-ported speaker is toed in a bit, and if the rear port is not at the same height as the woofer, then the locations of these two low frequency sources are displaced relative to one another in all three dimensions. I do not know how much physical displacement of port and woofer it takes to result in an audible smoothing - that probably depends on the room acoustics to a certain extent - but at least with a vertically offset rear port and a bit of toe-in you're stacking the deck in your favor.

Arguments against rear-ported speakers, and my counters:

1. Rear ported speakers won't have good impact because the port's energy is firing in the wrong direction and therefore has a longer path length to the listener. This makes intuitive sense, but the problem is that at typical port tuning frequencies the ear doesn't even register the notes until more than one wavelength have arrived. In other words, the ear is so slow to hear low frequencies that the minor additional path length doesn't make a significant difference. Up close there would be a measurable and audible bass SPL difference from the port being on the rear, but back at the listening position the SPL difference would be negligible (remember, the port's low frequency output is omnidirectional). Boundary reinforcement of a rear-facing port's output can actually increase the subjective sense of impact, assuming the port is tuned appropriately.

2. Rear ported speakers sound boomy if placed close to a wall. This is certainly true in most cases! But when the port starts out tuned with boundary reinforcement in mind, or if the port can be adjusted, if done right the net result is deeper bass extension instead of bass boom.

3. If you place a rear-ported speaker too close to the wall, the narrow space between the port opening and wall will change the tuning. Yes this is true, but we're talking about the wall having to be something like 2/3 of a port radius away for this to begin to happen (and we're assuming the rear of the speaker is parallel with the wall). Also, note that any change in tuning from an increase in effective port length due to wall proximity will be in the direction we'd want to go anyway - the tuning frequency would be lowered (though probably not by very much).

Disclaimer: In case you haven't guessed, I incorporate user-adjustable rear porting into my designs... and I stole the idea of rear porting from Audio Note. In my opinion they have essentially proven the concept, and I don't know why more manufacturers don't use it. There's a free bass lunch out there for low-tuned rear-ported speakers if you place 'em close to the wall.

Duke

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