Is a Slot or Round Subwoofer Port Better? (Slot Port vs Round Port)




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When building a subwoofer enclosure, one of the most important decisions is whether to use a slot or a round port.

Both types of ports have their own pros and cons, and the choice will depend on your specific requirements and preferences.

Slot ports and round ports both have their benefits and drawbacks, so it comes down to personal preferences and the goals of your car audio. Slot ports tend to have a flatter frequency response but require more space in the vehicle. Round ports, on the other hand, are more compact but tend to produce turbulence that can affect the sound quality.

Tip: When you want to add a round port to the subwoofer box, look closely at the speaker’s specification regarding the required size for your sub.

With the dimensions in hand, check different designs in the audio store or online. For example, there is a large selection of round ports available on Amazon.

In this article, I will take a detailed look at the differences between slot and round ports, the factors to consider when choosing a subwoofer port, and common questions and concerns.

Types of Subwoofer Ports

To give some advice on the better option between slot ports and round ports for subwoofers, let me explain some pros and cons of each.

Firstly, round ports take up less volume than slot ports, but they can cause more port noise.

Round ports also have the least internal surface area per unit volume, meaning they have less air friction and support higher air speeds, which means better output. However, this also increases the risk of turbulent airflow and compressed output.

On the other hand, slot ports can be built with a higher cross-sectional area and longer lengths for the same tunings.

The downside is that increased air friction due to their higher surface area can lead to less port efficiency.

But this also helps to dampen unwanted cone excursions below the tuning frequency. The higher the port velocity, the better the driver is protected.

If port air velocity is relatively low, there’s no difference between using round or slot ports.

In this case, you can use round ports as they are easier to make and come in many ready-made sizes.

But if port air velocity is high, a slot port is preferable as port noise will be distributed over a large number of frequencies instead of one frequency as with a round port.

For most new sub boxes, slot ports will always be preferable to round ports.

However, if port noise is a problem, you can use larger or additional ports if you can make them long enough to maintain tuning.

Slot Port

A slot port, also known as a slotted port, is a rectangular port cut into the enclosure.

The width of the slot port is typically around the same as the diameter of the speaker, and the length of the port can vary depending on the desired tuning frequency.

Skar Audio Dual 12" 2400W Loaded SDR Series Vented Subwoofer Enclosure | SDR-2X12D4

Slot ports are known for their improved low-frequency response and reduced port noise compared to round ports.


#1. Improved Low-Frequency Response

One of the main advantages of slotted ports is that they provide a fairly significant difference in low-frequency response compared to round ports.

This is because slot ports allow more air to move in and out of the box, which results in a stronger bass response.

#2. Reduced Port Noise

These ports also tend to produce less port noise than round ports because the air moving through a slot port creates less turbulence, reducing the amount of noise generated.


#1. More Difficult to Build

Building a box with a slot port can be more complex compared to a box with a round port.

It does include the cuts for the slot, which need to be precise, and the edges need to be smoothed to avoid port noise.

#2. Less Flexibility in Enclosure Design

Slot ports also take up more volume in the sub box compared to round ports, which can limit the design options for the boxes. As a result, they have to be larger.

Round Port

A round port is a circular port that is cut into the box.

The diameter of the round port is typically around the same as the diameter of the woofer, and the length of the port can vary depending on the desired tuning frequency.

1000W Dual Bandpass Speaker System - Car Audio Subwoofer w/ Neon Accent Lighting, Plexi-Glass Front Window, 4 Tuned Ports, Silver Polypropylene Cone & Rubber Edge Suspension - Pyramid BNPS102

Round ports are known for being easier to build and having more flexibility in subwoofers design compared to slot ports.


#1. Easier to Build

Building a box with a round port is generally easier compared to a slot. There is no need to create an “L” shape or make extra internal walls.

#2. More Flexibility in Enclosure Design

Round ports take up less volume in the box compared to slot ports, which gives you more design options for the box design.


#1. Reduced Low-Frequency Response

One of the main disadvantages of round ports is that they provide less low-frequency response compared to slot ports.

This is because round ports allow less air to move in and out of the enclosure, which results in weaker bass response.

#2. Increased Port Noise

Round ports can cause more port noise compared to slot ports because when the air moves through a round port, it creates more turbulence, which increases the amount of noise generated.

Factors to Consider When Choosing a Subwoofer Port

When choosing a subwoofer port, consider the size, shape, and design of the port as well as the type of subwoofer being used.

The size of the port should match the size of the subwoofer driver, and the shape and design of the port can affect the overall audio quality.

#1. Enclosure Size and Shape

When choosing a subwoofer port, one of the most important factors to consider is the size and shape of the enclosure. Slot ports are typically used in larger enclosures, requiring more space and volume.

On the other hand, round ports are typically used in smaller boxes, as they take up less volume and are more flexible in terms of enclosure design.

#2. Subwoofer Characteristics

Another important factor to consider when choosing a subwoofer port is the characteristics of the subwoofer itself.

Different drivers will have different power handling and frequency response capabilities, and it’s important to choose a port that will complement these characteristics and provide the desired quality of your audio system.

#3. Desired Tuning Frequency

The tuning frequency, or the frequency at which the box is tuned, is another important factor to consider when choosing a port.

A lower tuning frequency typically requires a longer port, therefore, a larger enclosure with higher volume, while a higher tuning frequency will need a shorter port.

#4. Personal Preference

Ultimately, the choice between a slot port and a round port will come from personal preference.

Some people may prefer the improved low-frequency response and reduced port noise of a slot port, while others may prefer the ease of building and flexibility in the enclosure design of a round port.

Does Velocity Affect the Port Size?

Port velocity affects the sound quality and performance of the subwoofer. 

The velocity can be affected by various factors, such as the size of the port, the tuning frequency, and the wattage.

A higher port velocity can result in a more efficient subwoofer but also may cause port noise. The velocity is usually checked at 75% of the RMS rating or at the woofer’s maximum rating.

The velocity can differ for different subwoofer sizes. Larger subwoofers generally have a larger port diameter and can create higher air speeds before port noise becomes an issue.

However, the optimal velocity will depend on the specific subwoofer, the size of the enclosure, and the desired sound quality.

Also, impedance can affect the velocity to some extent. In many cases, a lower impedance will result in higher wattage and may increase the velocity as the subwoofer becomes stronger.

When you are thinking about modifying your subwoofer, I recommend checking the specifications of the subwoofer to determine the recommended impedance and to ensure that the port velocity is within acceptable limits.


In conclusion, when building a box, it is important to consider the size and shape of the enclosure, the characteristics of the subwoofer, the desired tuning frequency, and your personal preferences when choosing between a slot port or round port.

  • A slot port is typically used in larger subwoofers and provides improved low-frequency response and reduced port noise.
  • A round port is typically used in smaller subwoofers and is more flexible in terms of enclosure design but may create more port noise.

It is also important to have the right tools, such as a router, measuring tape or ruler, and sandpaper, to make the process of building or modifying your enclosure smoother and more efficient.


Can I Use Both Slot Ports and Round Ports in One Subwoofer Enclosure?

It is possible to use both slot and round ports in one box, but it is not recommended.

This is because the different types of ports will have different tuning frequencies, and although they may look “cool,” they may need to work well together.

Can I Avoid Port Noise by Using a Larger Port?

A larger port will not necessarily eliminate port noise. In fact, a larger port can actually increase port noise by creating more turbulence. But, again, it all depends on the subwoofer type and the tuning frequency.

The key to reducing port noise is to choose the right port size and length for the enclosure and subwoofer and to smooth the edges of the port to minimize turbulence.

Can I Increase the Sound Quality by Using a Larger Port Area?

A larger port area can decrease audio quality by creating more turbulence and reducing the low-frequency response.

It is good for the strongest subs, but when you use a too large port, it may generate an effect of free-air subwoofer significantly reducing the SPL.

The key to increasing audio quality is to choose the correct port size and length for the enclosure and subwoofer and to match the port to the subwoofer’s characteristics.

Can I Convert a Round Port Subwoofer to a Slot Port?

It is possible to convert a round port subwoofer to a slot port, but it requires some modification to the enclosure.

You can do this by cutting a slot into the side of the enclosure and smoothing the edges to minimize turbulence.

However, remember that this will change the tuning frequency of the subwoofer, which may result in less optimal performance.

Additionally, the dimensions of the new slot port will need to be carefully calculated to match the subwoofer’s characteristics and the desired tuning frequency.

When doing such modification, your project may result in a less aesthetic look of the subwoofer.

How Do I Know if My Subwoofer’s Port Is Properly Sized?

A properly sized port will be based on the subwoofer’s characteristics, the desired tuning frequency, and the enclosure size.

A good starting point is to use the manufacturer’s recommended port size, which can be adjusted based on personal and listening preferences.

Additionally, you can use software or online calculators to determine the optimal port size for your specific subwoofer and enclosure.

What Are the Best Enclosure Designs for Slot Port Subwoofers?

The best enclosure designs for slot port subwoofers will depend on the specific subwoofer and the desired tuning frequency.

However, a standard design is a vented enclosure, which is a sealed enclosure with a slot port.

This design provides improved low-frequency response and reduced port noise while still being relatively simple to build. Another option is a horn-loaded enclosure, which uses a slot port to increase output and efficiency.

How Does the Length of the Port Affect Sound Quality?

The length of the port can affect the audio quality in several ways. First, a longer port will result in a lower tuning frequency, which can provide a deeper bass response.

However, a longer port can also create more port noise and result in a less tight bass response.

On the other hand, a shorter port will have a higher tuning frequency, which can result in a tighter bass response but may not be as deep.