Subwoofer port noise is an annoying issue that tends to pop up in audio systems. The good news is that there are many ways you can go about fixing the issue.
Normally, you can determine with root cause with a quick troubleshooting process. Read on below for more details.
What Is Subwoofer Port Noise and What Causes It?
Subwoofer port noise is also known as chuffing. It happens when a sub is attempting to push more air out of the port than the port can handle. In other words, the port is too small for a large amount of bass production and so the air does not get transferred out smoothly, causing the noise.
There are a few ways to fix subwoofer port noise:
- Reduce subwoofer volume and change placement
- Increase the port diameter or port length
- Flare the port
- Tune the port
- Add polyfill
We get into these in more detail below. First, let’s understand more about chuffing.
When Does Subwoofer Port Noise Happen?
Many audiophiles will tell you something fancy like subwoofer port noise is ‘sound created by the port tube during periods of high excursion of a bass driver’. They may also tell you that the chuffing noise is to be heard when the subwoofer produces low-end bass around 30 HZ.
All they mean by this is that chuffing is created by large movements of air within a speaker port. As the air gushes against the port, noise occurs. This is why chauffing often occurs in SPL subwoofers.
This normally occurs at the lower end of the audible frequency spectrum, from say 20 to 30 Hz or so.
Is Subwoofer Port Noise A Major Issue?
Subwoofer port noise can occur on a wide variety of subwoofers. Moreover, it can happen in any speaker system, not just subwoofers.
In most cases, it just sounds like a weird farting noise. Most people will tell you it’s a common problem and that’s true. However, the bigger issue really is how easily subwoofer chauffing occurs in your system. If a subwoofer is chauffing easily, it may be hard to manage and fix.
How To Fix Subwoofer Chauffing Port Noise
Since chuffing is created by large movements of air within a speaker port, the obvious solution is to alter the speed at which the air is moving through the port as well as the volume of air moving through.
This can be done in a few ways. The option you choose should be depending on your circumstances.
1. Subwoofer Volume and Placement
Firstly, if you are using your subwoofer beyond its limits, this can easily cause port noise. Ensure that your subwoofer is operating within the prescribed limits of the device.
Another quick solution is to move the subwoofer close to the wall, place the subwoofer behind the couch or even place your subwoofer facing the wall so that you can lower the volume while achieving greater loudness by taking advantage of boundary interference for bass reinforcement.
2. Increase The Port Diameter
Increasing the port diameter is one of the easiest ways to fix subwoofer port noise. However, doing this requires a change in the length as well. The more port area you have, the longer the ports need to be to keep tuning the same.
Of course, this only makes sense for you if your subwoofer port is too small. How do you know if it’s too small?
Well, a good rule of thumb for your port diameter is that it must be at least 40% of the woofer diameter. This is the best way to guarantee that the port will work well at high volumes and very low frequencies without chuffing.
Let’s take an example. If your woofer diameter is 8 inches, your port diameter should be 3 inches. If your woofer diameter is 12 inches, your port diameter should be 4 inches. If your woofer diameter is 15 inches, your port diameter should be 6 inches. Here is the simple calculation:
Port diameter = Woofer diameter * 0.4
Once you have increased the port diameter, use our subwoofer port length calculator to determine the correct port length to keep tuning the same.
3. Increase The Length of the Port
Sometimes, the actual port itself can be too short. The result is that the air is not effectively forced out of the diameter, especially if your box design is a slot-vented box.
How do you know how long your pet should be? Well, you can use our subwoofer port length calculator to determine the correct length for your port.
Once you have determined the correct port length, you can use a piece of MDF to extend the current port length. On the other hand, if your box uses an aero PVC port design, you can simply replace the current port with a longer one.
4. Flare The Port
Flaring a port simply means smoothing the edges of the port, creating a more curved shape.
This changes the velocity of portions of the air mass going through the port enough to reduce chuffing to a reasonable level. In other words, it causes air to rush through the port more smoothly.
Think of flaring a port line bundling several lengths of a water hose. Water coming through the ends of a hose will come about at a different velocity depending on the length of the house and how the end is shaped.
Here’s how you can go about flaring your port:
How To Flare Aeroports: Place it on a metal bowl and use a heat gun while twisting it until the plastic at the bottom transfers into a flared shape.
How To Flare Ported Boxes: Just add some 90-degree angles in all of four corners of the boxes to smooth out the airflow. You can use pliers so something similar to do that.
5. Tune The Port
In general, port noise gets louder when you go below the tuning of the actual port. For example, the box might sound great and thumps until you get to lower frequencies around 30hz.
Around this point, you may get a lot of noise if you are going below the tuning frequency.
However, port tuning is more than just frequency. Port tuning is based on the relationship between the port area (diameter of the port), port length, and box volume. However, all of these things help to determine the particular tuning frequency.
For example, if you make the port longer, decrease the port area, or increase the box volume it will tune the port lower. The tuning frequency is important because, around the tuning frequency, all the output comes out through the port. Of course, you want that air to flow as easily as possible to reduce noise.
How do you know what your current tuning frequency should be? There are many subwoofer tuning calculators that you can use to tune your subwoofer box. Use one to determine what the tuning frequency should be and tune your sub to that frequency.
If you have done the following and you still notice loud port noise, you will have to make a new box that has a larger internal volume and a better-structured port design.
6. Add Polyfill
Stuffing your subwoofer box with polyfill is a trick that we use to make the subwoofer operate as if it is inside a bigger box. In other words, it will cause the system to act like a sealed enclosure.
This is because a small internal volume can sometimes be the main cause of subwoofer port noise. Stuffing the box with polyfill adds extra area inside the box and slows down the movement of air. You can get polyfill from DIY shops or extract it from an old pillow.
Now keep in mind that because the stuffing will slow down the air, while it will also help with cabinet resonance, it will also reduce the output. You shouldn’t add too much or too little poly-fill. If you need to learn more, please read our article on how much poly-fill do you need for your sealed subwoofer. As such, having the box work like a sealed enclosure can cause issues as well, so use this as a last resort.
Understanding Box Volume and Port area
There is a direct relationship between subwoofer port size (diameter and length) as well as your subwoofer box volume and the overall tuning frequency or resonance of your subwoofer box.
It is important that you understand that if you adjust any of these factors, you need to adjust others to compensate.
For example, if you double the box volume, you need to double the port as well. You could add a second port to the box, for example. To get the same frequency, the port needs to be wider and longer.
Does Adding A Low Pass Filer Fix Subwoofer Port Noise?
For some reason, many people seem to believe that adding a low pass filter will somehow reduce port noise.
Maybe it’s because adding a low pass filter may eliminate port noise, but it will eliminate your bass production as well.
This is because it’s the low frequencies that cause port noise in the first place, not the high frequencies. as such, filtering out the low frequencies will simply filter out all your bass production, effectively killing the subwoofer.
Is A Round Port Better Than A Square Port?
How Much Polyfill Do I Need For the Subwoofer Box?
It is dependent on the measurement of the enclosure. If the enclosure falls below 2.5 – 3.0 cubic feet in size, no more than one and a half-pound of polyfill per cubic foot should be used.
Verily, stuffing a box with polyfill makes it appear more prominent, which relates to thermodynamics. Adding polyfill to an enclosure changes the behavior of the air spring in the enclosure from “adiabatic” to “isothermal.” An isothermal process occurs once the polyfill has been added.
Subwoofer port noise is simply due to turbulent air exiting the subwoofer port. Port noise can be caused by a wide variety of factors including too much volume or poor sub-box design.
In fact, if your sub is running way too hot and pushing its limits, that could definitely be the cause as well.
Norvan Martin is the founder of BoomSpeaker.com. He is a professional Electronics Engineer and is passionate about home theater systems and AV electronics. BoomSpeaker was created as an online hub to share his knowledge and experiences as it relates to home theaters and home audio electronics.