Sealed vs Ported Subwoofers: Which Is Right For You?

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. 

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 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 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. 

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.

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.

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.

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.  

Sealed vs Ported Subwoofers: Wrap Up

While the simulations above do a reasonable job of characterizing the differences between sealed and ported subwoofers, the real work begins when we make the leap from simulated / outdoor performance to performance in real rooms, as well as take human hearing into consideration. Approaching sealed subwoofers first, their shallow roll off profile can make them quite suitable for smaller spaces thanks to a phenomenon known as room gain. Below a certain frequency (nominally denoted as F=(speed of sound / (2 x Longest Room Dimension)), the room naturally boosts the low end at a theoretical rate of 12dB/octave (less in the real world, as real walls are lossy barriers). This means that in spite of the fact that they may roll off earlier in simulations or outdoors relative to their ported counterparts, sealed subwoofers have the potential to offer extension down to 10Hz and below in the right room. In addition, their small size (excluding huge overdamped enclosures) makes them much more at home in smaller spaces. 

Outdoor (purple) vs potential indoor (blue) frequency response of the SVS SB-2000 (image courtesy of SVS).

As you might guess, where sealed subwoofers are a natural fit for smaller rooms, big, deeply-tuned ported subwoofers are right at home in larger spaces where room gain is minimal (though they can work well in smaller spaces with a bit of EQ). But what about all that nasty group delay / ringing business we talked about; surely that must affect music performance? That’s where the realities of human hearing -come into play. .

Group delay for driver X modeled in a maximally flat ported alignment as well as a “deep tune” one-port mode.

Looking at the graphs above, you might be concerned by the spike in group delay relative to a sealed enclosure (the simulations of which don’t break 15ms regardless of Q). However, because of the relatively deep tuning points involved, the frequencies where group delay becomes a problem are areas where human hearing isn’t terribly sensitive to the issue; moreover, they’re frequencies that just aren’t particularly relevant to the vast majority of music. As such, when put in appropriate rooms and/or EQed to scrub off a rising low end resulting from room gain to achieve relatively flat in room response, it’s possible for ported subwoofers to be very capable performers, both for music and movies.

Bottom Line

Small sealed subwoofers are a natural fit for smaller rooms: their size combined with their shallow roll-off mean easy placement and deep extension in small spaces (say under 2,500 cubic feet). Conversely, large, deeply-tuned ported subwoofers offer a real competitive advantage against their sealed brethren in larger spaces where the room doesn’t boost low-end output. In addition, when designed properly, problems in the time domain are pushed low enough in frequency to be non-factors, meaning they can perform well for music as well as movies.

Conclusion

Subwoofers play an important role in today’s A/V systems, shouldering the burden of reproducing the low end in music and movies. The two most common styles utilize sealed enclosures and ported boxes; naturally, since their development people have been debating which one is better. When approaching the question from a balanced point of view, the answer that they both have their strengths and drawbacks. As such, the question of which is right for you depends on your situation, specifically, the room you’re putting it in. Blanket statements like "sealed sub is more musical" or "a ported sub is better for movies" just isn't always the case. Design execution and application must be considered. Happy hunting!