How Does a Subwoofer Port Work?
This speaker enclosure has a hole it, and that hole makes
better bass. How can putting a hole in
your speaker box make better bass? Keep reading to find out!
Ports look very simple. They are nothing more than tubes or slots built into your enclosure. But they are actually quite complex. There are also a lot of misconceptions about ports and how they work. To those completely unfamiliar with subwoofers they look like a terrible idea. When the driver pushes out would it not suck air in, and thus the sound back into the enclosure? While those who are slightly more familiar with subwoofers, in an excellent example of the Dunning Kruger Effect, will argue that when the speaker moves inward the air in the box, and thus sound, sound comes out of the port. Thus taking advantage of both the backward and forward movements of the subwoofer. Both of these ideas use the same logic to reach opposite conclusions. So we need to dig deeper.
The air inside the port has mass!
We don't think of air as something that has mass, but it does. Think back to high school science class. Remember the Periodic Table? Every one of those elements has a mass, and some of those elements are gasses, like Hydrogen and Oxygen, some of those elements combine with others to form molecules, like carbon dioxide, a gas. What is our atmosphere made up of? Gasses. We don't feel the weight and pressure exerted by these gasses because we are used to the feeling it, but it is there. When you are visualizing a subwoofer port don't envision an empty round tube that is open on both ends. Don't think of a subwoofer as a fan that blows air in and out of that tube (well, under the right circumstances it does, more on that later). Think of air in the port as a chunk of lightweight material (like paper), that chunk of material vibrates to make sound waves. As the woofer moves it does not pump air in and out of the port. Instead, it causes the air mass in the port to resonate as the air in the enclosure pressurizes and depressurizes. I will let Sponge Bob show you what that looks like:
A port acts like an extra speaker cone.
What else is a vibrating mass that creates sound waves? A speaker cone of course! The port is a BONUS CONE! The port's job is to resonate and make sound waves in order to complement the subwoofer and help it play louder and/or lower. Sometimes we just skip the port all together and throw in a speaker with no voice coil or magnet, this is called a passive radiator.
Speaker cones and subwoofer ports "resonate."
Now comes the cool part, the part where we start to understand the physics of creating sound. Sound is nothing more than a waves propagating through the atmosphere. We quantify waves by measuring the number of times the wave peaks and falls (a cycle) per second. We call that hertz (hz). Your subwoofer plays the slowest and largest of these sound waves. Typically 120 HZ and lower. It takes a lot of energy to produce these waves, that is why the best subwoofers are huge and use a lot of power.
Every subwoofer driver will have a specification called the free air resonance, or Fs. This is the frequency where the mass of the cone is perfectly offset by the subwoofer suspension (the surround and the spider) If we put that subwoofer driver into an enclosure the air in the enclosure now becomes a part of the suspension, and we get a new specification for the system. The resonate frequency of the subwoofer system, or the box frequency, Fb.
At Fb the subwoofer has a very hard time moving. Why? This is the frequency where the suspension is most effective at holding the cone steady. The spider, surround, and air in the enclosure becomes a very stiff spring. If the cone is not moving, then you get no sound. But, do you know what has virtually no suspension? A mass of air sitting in a tube. At Fb the air in the port takes over for the cone. The port is now producing almost all of the bass. You can verify this with test tones. If you know your box tuning just play a test tone at Fb, the cone should move very little, but the port should be moving a lot of air. How is it that the speaker can pump so much air in and out of the port when the speaker is not moving? Because the speaker cone is not a pump and a port is not just a hole in a box. The entire system (box, cone, port) can be thought of as a Helmholtz Resonator. The same concept is used in performance car exhaust systems and intake systems and can be tuned to create a cool engine sound.
If you play increasingly higher frequencies the port starts to move less.At a high enough frequency the air in the port will stop moving and the system will behave like a sealed enclosure. The pressure changes in the enclosure are not enough to overcome the weight of the air in the port.
What happens if you start at Fb and then play increasingly lower frequencies? BAD THINGS HAPPEN. You don't want to do that. As the frequency gets lower the pressure changes in the enclosure begins to overpower the weith of the mass in the port. You eventually reach a point where the port now acts like a literal hole in the side of the enclosure, at this point the cone DOES become a pump that just pushes and pulls air in and out of the port. The port and the sub will start canceling each ohter out, and the cancellation gets stronger as the frequency decreases. Most importantly you risk damaging the subwoofer. The subwoofer driver will no longer benefit from the air suspension inside of the enclosure and the cone will move to much, causing distortion and at high volumes you will damage the subwoofer. The DIY Audio Guy has a great video on this topic: You should check it out!
The important thing to remember is that port behavior, and the boost you get from the port, is dependent on frequency. The goal is to design the port so it resonates at the exact frequency you need to get the sound you want. In the next port post we talk about how to design the port in order to get the most out of it. We call this "tuning" the port. In the mean time here are some awesome subwoofers that you should check out:
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