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?Screenshot 2016-12-10 09.46.22

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.

One thought on “Audiophile Wisdom – Ported vs Non-Ported Subs

  1. The first loudspeaker I ever built was a Theil aligned ported sub from a design published in the August ’76 issue of Audio Magazine. That was back in the days when we thought we only needed one sub because at that wavelength, your hearing can’t tell direction. Nobody was thinking about standing waves in rooms, let along acoustic treatments of rooms.

    It had 21 cubic feet of internal volume (think your fridge…), was 3 db down at 20hz and was ungodly efficient. I drove it with a 60 watt Southwest Technical Products tiger mono amp which was never pushed no matter the material or level. With the howitzer shot on the M&K Subwoofer test record, it could blow out match flames.

    The port was 6″ x 6″ and 11 1/2″ deep.

    Today we talk about subwoofer swarms. Given the size of this thing, can you imagine how big of a room you’d have to have in which to put 3 or 4 of these things?! In a later issue, one reader wrote into the magazine and said, “When I explained to my wife what 21 cubic feet meant, she explained to me what divorce meant.”

    Photos, details and a copy of the original article are at the link below.

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San Francisco Audiophile Society

San Francisco Audiophile Society