Audiophile Wisdom – Ported vs Non-Ported Subs
A question asked by SFAS Sr. VP Leslie Lundin and answered by member and audio engineer Manny LaCarrubba.
Subwoofers: To Port or Not To Port?
Ports are used in loudspeakers to extend bass response. Woofers and the boxes we put them in form a resonant system. A sealed box has natural roll off below its primary resonance of 12dB/oct. When you add a properly designed port to the box you have now added a second resonance below the first one. This extends the low frequency output of the system. At the port resonance, virtually all output is from the port and the woofer cone barely moves. So, the system stresses the woofer less. When maximum output is needed, you use a ported box. This is why they are used exclusively in sound reinforcement subs. (We used to see horn loaded subs but they are relatively rare these days.)
You rarely get something for nothing in any field of engineering and speakers are no exception. In theater applications, which is what we are considering here, bass extension to 25Hz or better is the benchmark for high end design. It is difficult to tune a practical sub with ports that low. Secondly, ports have a tendency to “chuff” when placed under high SPL conditions. Related to the extraneous noises are issues surrounding port compression. Ports are non-linear. They start to become non-linear at much lower levels that most designers are willing to admit. So when a really loud explosion or dinosaur foot stomp comes, just when you need the highest output at the lowest frequencies, the port can compress the dynamics by as much a 3-6dB.
In addition to these issues, don’t forget that the output of the port is 180 degrees out of phase with the output of the woofer giving the system a steeper 18dB/oct high pass filter characteristic. This phase rotation is audible though not necessarily always objectionable to all.
Sealed boxes on the other hand require a substantial increase in the excursion of the cone to produce the same low frequency output. It is usually easier to design a practical sub (a more elegant box without big holes in it) with lower distortion and a more linear output this way. Not to mention, many people, myself included, think they sound better. You may need to use more of them to get the SPL you want so the real trade off is price.
Leslie is building a high end cinema so we want to use the gold standard for subs which is multiple sealed boxes. While more expensive, the price penalty these days is not extreme.