Slot Port vs Round Port - Detailed Guide
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Ports are used by speaker enclosure manufacturers as a way of controlling the sound output frequencies of the speakers and set resonant frequency. In this article, we look at the two commonly used port types – slot ports and round ports.

In general, round ports have lower surface area than slot ports for the same size. With slot ports, the more equal the sides (a square being optimal) the lower the wall surface area. Note that when air velocity through the port is low, there isn’t any difference between sot and round ports. However, as air velocity increases, there are differences between the two. In addition, you get the same tuning effects when tuned to a specific frequency

Slot Port vs Round Port

FeatureSlot PortsRound Ports
Better ForMoviesMusic
Ease of UseHarder to build and flareEasier to make, ready-made sizes, fares easily
VolumeTake Up More VolumeTake Up Less Volume
NoiseCan cause noise as we but not as easily. Avoid port noise by giving more port areaMay cause more noise if not sized properly
Port AreaCan extend as neededCan be made into an "L" or labrynth shape,
With Low Port Air VelocityNo differenceNo difference
With Low Port Air VelocityBetter for noise reductionSusceptible to noise
Internal Surface AreaMore - the more equal the sides, the lower the wall surface areaLess
EfficiencyLess - Greater friction due to greater surface areaGreater - less surface area and friction
FrictionLess - helps dampen unwanted cone excursions below the tuning frequencyGreater - More unwanted cone excursions
With High Port Air VelocityBetter - Noise distributed across various frequenciesPoor choice - will cause noise

Before we get further into the analysis, here is a quick rue of thumb:

With Low Port Air Velocity: If port air velocity is fairly low, then there’s basically no difference. In this case, just go with a round port because they are easier to make and come in many ready-made sizes.

With High Port Air Velocity: If port air velocity is fairly high, then a slot port is the better choice. In this case, port noise will be distributed over a wider frequency range instead of one frequency as with a round port.

Check out the following guide to learn more about speaker port air velocity.

What Is Slot Port?

A slot port generally uses 3 sides of the enclosure as port walls. if the port exists at one end of the enclosure, for instance, you only need to add just one piece of wood to complete the port.

It is considered the easiest and most effective way to build a slot-shaped port. However, some of the limitations include; port depths cannot exceed 16-inch in some box dimensions, and the end of the port should not be so close to the back wall as its width.

Pros

  • Slot ports are quite easy to build
  • Slot ports add strength
  • Greater tendency for turbulence.

Cons

  • The narrow duct can cause friction with the air and the walls this will bring along a lower tuning box.
  • Chuffing becomes a problem when using lower tunings, and as such results in higher port velocity.

What Is a Round Port?

Rounded ports are made using cylindrical PVC pipes. As a result, they take up fewer volumes. The most commonly used PVC cross diameters are 3 inches to 6 inches.

They have the lowest surface area (perimeter) per given port area and should, therefore, be made big enough to reduce the unwanted port noise.

Pros

  • Making rounded ports is relatively easier as you only need to drill a round hole in a sealed box then fix the round PVC.
  • It is much easier to retune the enclosure without the need to build a completely different box.
  • Despite its small size, you still are capable of enjoying a better sound than that of a slot port. This makes it more efficient and less demanding in terms of space allowing for fewer spatial requirements in the vehicle.

Cons

  • Rounded ports are more costly to install. This comes about when choosing diameter ranges of up to 6 inches.
  • The bass from round ports is always not enjoyable by most people.
  • Loud sound is not a guarantee as your subs could still end up with horrible sound.

Slot Port vs Round and Port Area

The quality audio a port produces is highly based on its area. If you have two ports – one slot port and a round port, they will sound the same if they have the same cross-sectional area.

If you consider a certain port size or area, then a round port will have a lower surface area than a slot port of the same size. As an example, a 3″ x 8″ port has a perimeter of 22″. On the other hand, a 1″ x 24″ port has the same area, but a perimeter of 50″.

When looking at slot ports, there is a ratio that has to be adhered to. For example, a 5:1 slot port is fine. In addition, for a sot port, the more equal the sides (a square being optimal) the lower the wall surface area.

Finally, it is easier to get more port area with a slot port. In terms of volume and area, we use the rule of 12-16 square inches of port area per cub of volume. 

To learn more, check out our article on how to calculate the port size of a subwoofer. In fact, here is a subwoofer port calculator.

Slot Port vs Round and Sound Quality

You won’t be able to tell the difference between a well-designed slot port and round port that are tuned to the same frequency.

This is because you will get the same tuning effects with either slot or round ports when tuned to a specific frequency. As long as the port is big enough and or flared enough to eliminate chuffing, then you should be ok.

Slot Port vs Round and Volume

In general, round ports take up less volume since PVC isn’t very thick. However, with slotted ports, no matter how long the port needs to be, you can bend the port along the inside of the box until you achieve the desired length.

In terms of volume and area, we use the rule of 12-16 square inches of port area per cub of volume. 

Slot Port vs Round and Noise

People generally expect round ports to produce more noise than sot ports. This is not necessarily true, but round ports are more likely to produce noise if they are not properly sized than a slot port that is not properly sized.

In addition, if your round port is small, you are likely to have more port velocity than that of a larger diameter port. This means more noise.

With slotted ports, however, you can easily avoid port noise by giving more port area. In fact, you can also round over the port entrances/exits of the port. Another benefit is that no matter how long the port needs to be, you can bend the port along the inside of the box until you achieve its correct length.

Slot Port Vs. Round Port: Which Is Better?

Both slot and round ports will sound the same when tuned to a specific frequency. Although many listeners argue that round ports give more port noise, it all depends on the port size used and the tuning achieved.

Also, if you are using slotted ports, you can avoid port noise by just adding more port areas. It is therefore not easy to say which is better as it depends on one’s preferences and listening experiences desired.

Choosing The Best Port

Here are some of the factors to consider when choosing or building ports for your subwoofer enclosure.

Difference In Sound Produced

Your greatest worry should definitely be the sound each enclosure will produce. To have the best results, always ensure you tune both slotted port and rounded port to the same WinISD.

Most researchers suggest that slotted ports produce flat sound whereas the rounded port gives a more tubular sound.

Velocity

Another factor to consider is the velocity peak of the air. Remember when using a slotted port, it should never be narrow for that will increase air velocity leading to friction that will eventually cause the noise. When considering velocity, rounded ports always come in handy.

Diameter

Ports with larger diameters have higher usable velocity than smaller ports. This velocity especially below 10 m/sec has lower compression and turbulence. Always ensure that you do a good calculation of the surface area to the ratio.

What Are The Best Ported Enclosures To Use Today?

If you are planning to install rounded ports for your sub, you can save time by getting a universal fit. Some of the best choices to consider include:

  • Skar Audio SK2X12V Dual 12” Subwoofer Enclosure
  • Skar Audio SK2X8V Dual 8-Inch Ported Enclosure
  • Powerbases PSWB121 550W Single Vented Subwoofer Enclosure

If port air velocity is fairly low there’s basically no difference. So usually you’d make a round port because they’re easier to make and comes in many ready-made sizes.