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Sealed vs Ported Subwoofers: Which Is Right For You?

by April 01, 2014
A question for the ages: ported vs. sealed subwoofers, which reigns supreme?

A question for the ages: ported vs. sealed subwoofers, which reigns supreme?

One of the more common questions on the Audioholics forums is the matter of which is superior: sealed or ported subwoofers. Unfortunately, there are a lot of popular misconceptions out there: you might hear some folks say that ported subwoofers are no good for music, and are only useful for delivering big sound effects, or conversely that sealed subwoofers are “musical”, but lack the depth to deliver the bass called for in today’s blockbusters. While some subwoofers may certainly fit these stereotypes, the truth is considerably more complex. Ultimately, sound quality is far more a function of good engineering and choosing the right tool for the job rather than a question of sealed vs ported. Nonetheless, each alignment does come with specific strengths and weaknesses, the balance of which may make one type more suitable to your situation than the other.

Sealed vs Ported Subwoofers YouTube Discussion

Sealed Subwoofers

The simplest type of subwoofer to design and construct, sealed subwoofers consist of a driver, an enclosure, and an amplifier; the driver is responsible for 100% of the system’s output. 

SVS SB13-Ultra

The SVS SB13-Ultra, a compact yet powerful sealed subwoofer.

Overall system performance is a function of the driver’s Thiele/Small parameters and enclosure volume, which together will determine system Q and the system’s resonant frequency. Below the resonant frequency, sealed subwoofers typically feature a shallow roll-off of 12dB/octave, which also corresponds with relatively low levels of group delay and ringing in the deep bass.

Frequency Response

Frequency response of driver X modeled in boxes with a Qtc (system Q) of 0.5 (critically damped), 0.707 (optimally flat), and 1.0 (moderately underdamped).

As you can see in the graphs above, it’s possible to get a wide variety of response profiles from sealed subwoofers. The simulations above were achieved by simply varying box volume, with a Qtc of 1.0 being achieved in a small 54L box, 0.707 in a medium sized 136L box, and 0.5 in a very large 525L enclosure. Subjectively speaking, lower Q boxes (0.707 and lower) tend to be characterized as relatively tight, while high Q enclosures can be a bit boomy without equalization due to their response hump in the mid-bass range. On the other hand, one benefit of higher Q enclosures is that they offer a greater degree of protection for the driver against high-energy, low-frequency transients.

 Excursion

Excursion of driver X modeled in boxes with a Qtc of 0.5, 0.707, and 1.0, with 200W of power applied. Note that while a Qtc of 0.5 corresponds with a relatively extended response, there is a price to pay as this requires tremendous amounts of excursion at low frequencies.

Bottom Line

While not all sealed subwoofers are created equal, properly done the alignment has a lot to offer. Size is typically manageable; the earlier pictured SB13-Ultra is effectively a 17.5” cube, giving it a lot of flexibility in placement as well as a high SAF (spouse acceptance factor). While small size tends to come at the expense of extension, sealed subwoofers generally have a shallow low-end roll-off profile, which corresponds with good performance in the time domain (i.e. group delay / ringing). Last but not least, sealed subwoofers offer some degree of protection against bottoming out the driver, though it is still possible with sufficient power and the right content.

Ported Subwoofers

Where sealed subwoofers are relatively simple devices, ported subwoofers add a bit of complication to the mixture, i.e. the port. On the upside, porting augments system output at the vent’s resonant frequency, which extends the subwoofer’s response and allows for substantially more output capability at the tuning point relative to a comparable sealed subwoofer. 

SVS PB13-Ultra

The SVS PB13-Ultra is a deep-bass monster, but is substantially larger than its sealed brother.

However, below the tuning frequency, the driver is no longer loaded by the enclosure, and acts as if it is in free air. This results in a much steeper roll off rate of 24dB/octave relative to the 12dB/octave slope typical of sealed subwoofers; as a consequence, group delay is typically higher in ported models. In addition, below the tuning frequency, the woofer is in danger of over-excursion without appropriate filters for protection, which can further exacerbate problems related to group delay. Of course, like sealed subwoofers, many different response profiles are possible by varying enclosure size as well as port length vs diameter (larger enclosures and longer ports result in lower tuning points). It should also be noted that ported enclosures are typically much larger than their sealed counterparts.

Ported Subwoofer Frequency Response

Frequency response of driver X modelled in a maximally flat ported alignment and a BB4 ported alignment. Relative to the maximally flat alignment, the BB4 utilizes a smaller enclosure (221L vs 305L) but longer port, and achieves a broader “knee” at the low end.

Some folks may also be familiar with variable tune subwoofers, made popular by brands such as SVS and Hsu Research. In such cases, you have a subwoofer with multiple ports, one or more of which can be plugged to lower the system’s resonant frequency. This in turn changes the system’s frequency response profile, and adds a bit more work for the woofer. In addition, with one port open versus two, port air speed increases dramatically, increasing the possibility of chuffing.

One port vs two

Frequency response for driver X when modeled with two ports open resulting in a maximally flat alignment, and one port open offering an extended response.

1 vs 2 port excursion

Excursion of driver X in maximally flat two-port mode vs one-port mode with 200W of input power. In one-port mode, the woofer must dig deeper before the port begins contributing, increasing its excursion requirements. Note in both cases below tuning, excursion goes off the charts quickly.

Bottom Line

Relative to their sealed cousins, ported subwoofers typically offer better low-end extension as well as greater output around their tuning point. However, there is no free lunch; deeply-tuned ported subwoofers tend to be quite large, making them less décor friendly as well as reducing placement options. Further, while ported subwoofers have a big output advantage down to their tuning point, below tune, frequency response drops off steeply while driver excursion goes off the charts. While most commercial subwoofers employ filters to protect the driver from over-excursion, this usually results in an even steeper low end roll off, and consequently problems with group delay and ringing.  

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About the author:

Steve Munz is a “different” addition to Audioholics’ stable of contributors in that he is neither an engineer like Gene, nor has he worked in the industry like Cliff. In fact, Steve’s day job is network administration and accounting.

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Recent Forum Posts:

Verdinut posts on January 25, 2017 17:23
Back spring pressure affects ported configurations too, but to a much lower extent than is the case with a sealed box.
shadyJ posts on January 25, 2017 17:10
Verdinut, post: 1168948, member: 80194
However, I have never heard of anyone questioning the linear response in a sealed cabinet. This phenomenon definitely must exist due to the air compression and decompression effect caused by the cone displacement. Probably, our hearing is not sensitive enough to notice this phenomenon which nevertheless cannot escape the laws of physics. This is one of the reasons why I always built bass reflex enclosures.

I wonder if anyone ever attempted to measure such non-linear displacement, and if so, what were the results.
Any comments anyone?
The effects of back spring pressure would be manifest as even-order harmonic distortion products. Backspring pressure also effects ported alignments as well. Even-order harmonics have to be severe in low frequencies to be audible, especially second-order harmonics.
Verdinut posts on January 25, 2017 16:19
Bear123, post: 1026954, member: 66726
There are two advantages to sealed subs: they are smaller than ported, and have a shallower rolloff for lower extension. I don't think the servo has any impact on those two things.

As far as the sealed versus ported question goes, as long as one can acommodate the size of the ported sub, I think it is generally the better design. Most people use their subs for both movies and music to some extent or another, and ported offer vastly superior output for movies, while being equally suited for music. The only reason to go sealed imo is if you have to have the small size.

Having said that, I have a sealed sub. But, it took two 15“ drivers to match the output of a 12” ported sub in the same size box. But, since that was the biggest size box I could fit, the two larger drivers kept my output for movies the same, but double my mid bass output.
Greetings!
You installed the two 15 inch woofers in the sealed box. Did you ever explore the feasibility of rather using them in an isobaric configuration in the same cabinet for better low frequency performance?
Verdinut posts on January 25, 2017 15:42
Sealed vs Ported:
There are several reasons why many audiophiles prefer the sealed enclosure. Of course, sealed boxes are usually more compact and there is also the spouse acceptance factor, as everyone knows.
However, the ported enclosure allows a more efficient rendition and, provided IMHO that you use a subwoofer with a Qts of between 0.38 and 0.5 and a very low Fs, you should be able to obtain a smooth and strong response to a lower frequency than with a sealed box, provided it is of adequate size and properly tuned. Also, transient response should not be a problem under these conditions.

I have heard of several comments with regard to ported enclosures with poor transient response as opposed to that with sealed cabinets. Driver and box have to be properly, not married, but matched to obtain successful results.

However, I have never heard of anyone questioning the linear response in a sealed cabinet. This phenomenon definitely must exist due to the air compression and decompression effect caused by the cone displacement. Probably, our hearing is not sensitive enough to notice this phenomenon which nevertheless cannot escape the laws of physics. This is one of the reasons why I always built bass reflex enclosures.

I wonder if anyone ever attempted to measure such non-linear displacement, and if so, what were the results.
Any comments anyone?
crossedover posts on April 14, 2014 16:49
mychaelp, post: 1028032
Thanks for the article.
Subs play such an important role that most people don't realize and it's important to understand the differences between options.
My SVS has a “phase” dial of which when I turn it, there are changes in the bass sound, but I always thought Phase would be either one way or another and not a dial adjustment. I just turned it until I had a good mix of loudness and depth bass which is tight but not boomy as it was on the other end of the dial. I may never know how to really adjust that dial.
I've always seemed to notice that to get the best sound and deepest bass from a sealed enclosure, you have to spend more than a ported version. SVS is a good example of how sealed cost more to get the same “on paper” response. I had found a site that listed some layman's information but can't find it now, might post later if I find it then.

Engineering a small enclosure to play loud,low and good involves a an exceptional driver, higher power and EQ. If you want a 15" cube to compete with a large enclosure it takes more Rd and money for the most part.
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