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!

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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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Building your first Subwoofer: Before you start cutting

If you want to build a subwoofer, here is what you need to know before you start cutting wood.

You have the bug!  A friend with a great subwoofer (home or car) has shown you their setup.  You have been browsing various online retailers and you think you can build it better, or cheaper.  Perhaps you have been into audio for years and you are ready to up your game and DIY.  This is a multipart guide on how to get started!  Here is what you need to do before you start ordering parts and cutting wood.

1.  BEGIN WITH THE END IN MIND

This is an old quote from Steven Covey.  Focus on the result that you want, define the purpose of this project, and clearly understand why you are doing it yourself.  Here are some things to think about.

• Is this for music or home theater?  For home theater you need to reproduce bass frequencies that are below the normal human hearing range (20 HZ).  For most types of music you don't need to play as low.
  
• Is this for nullor home?  Car's have less space to work with, but benefit from cabin gain.  The design principles are the same, but the end goal is different.
• Do you want sound quality or do you just want to be loud?
• Are you going to build one subwoofer, or is this a lifelong hobby?  This will make a difference later when start to build your tool collection.
• How are you going to "finish" the enclosure?  Paint?  Carpet? Veneer?  Vinyl?  Make sure you plan ahead so that the end project will look good, or at least tolerable.  

 

2.  IDENTIFY YOUR CONSTRAINTS

Like any other DIY project you need to understand the factors that are going to prevent you from creating the perfect subwoofer.  The three most important constraints are:

• SPACE:  Big bass requires large drivers in large enclosures and it is very important to put the driver in an appropriate sized enclosure.  Putting a large driver in an undersized enclosure will give you horrible results.  So start by understanding your available space.  How much space do you have to work with?  Will your wife let you put a big wooden box in the living room?  How much room do you have under the seat in your four door pickup truck?  How much of your car trunk are you willing to sacrifice?   
  

• BUDGET: Most audio websites never talk about budgets, but they should.  This hobby ain't cheap.  If you are looking at entry level gear, you can buy it cheaper than you can build it.  You are only going to save money if you are trying to recreate a high end subwoofer, and only if you have access to the tools you need to do the job.   

• POWER:  If you are building a subwoofer for your car or truck there is one more thing to consider:  POWER.  Your alternator provides the juice while the car is running and your aftermarket subwoofer amplifier is going to compete with important things like headlights, spark plugs, and cooling fans.  Every car is different, so every car will have a different maximum amount of power available for your subwoofer.  Just remember that your car's electrical system is not an infinite source of instant power and modern car electrical systems are more complex than you can imagine.

 

3.  INVENTORY YOUR TOOLS

If you build things for a living (carpenter, electrician, etc) or if you are already an avid DIY'er you probably have the tools that you need.  Otherwise you will need to start collecting tools.  Don't be afraid to start small, and start with cheap tools.  Don't get stuck into the "buy once cry once" mentality, this is just a marketing slogan designed to get you to spend more money than you need to.  You don't need a dedicated workshop loaded with industrial grade woodworking tools to build a 2 cubic foot box with a single hole in it.  Here is a short list of basic tools that you will need to get started, along with links* to amazon so you can order the ones you don't have.

• A workbench, a pair of decent sawhorses and some scrap wood is a good place to start.  My first workbench was an old desk that somebody left on the side of the road.
  
• A jigsaw.  This is the cheapest tool for cutting speaker holes.  Don't buy an expensive corded jigsaw.  You will eventually want to upgraded to a cordless model. 
  
• A circular saw.  Again, don't spend a lot on a corded model.  You will eventually upgrade to a table saw and you will eventually want a cordless circular saw.
• A cordless drill.  You can get buy with just one, and it does not have to be expensive.  But life is easier if you have two.  That way you don't have to waste time swapping out a drill bit for a Phillips head bit while you are assembling the enclosure.  Some speaker builders will glue and screw the box together.  But there are plenty of other ways.  You will, however, need to drill holes and drive screws to mount the driver in the enclosure. 
  
• Clamps. Some speaker builders clamp and glue their entire enclosure, this works great.  I use clamps as extra hands. 
  
• A sanding block or a handheld electric sander.  Again, you will upgrade to a cordless one eventually so don't spend a lot of money on this item.
• An edge guide, so you can make straight cuts.  Even if you just use a straight piece of wood you need an edge guide.  I like the kreg rip cut.
• Wire crimpers, strippers, and cutters.
  

If you want to kick it up a notch here are some more advanced tools that will make life a lot easier:

• A brad nailer, either battery or pneumatic.  Your enclosure will be held together with wood glue.  Clamps, nails, and screws are just used to hold things together while the glue sets. My preferred method is to apply glue, clamp the pieces in place temporarily, drive in a few 18 gauge brad nails, remove the clamps and move on to the next piece.   
• A router with a circle jig.  Cutting perfect circles with a jigsaw is hard.  A circle jig on a router is almost idiot proof.  You can also use the router to flush trim and round-over the edges.  This will really improve your end product.
• A table saw.
• A torch for solder and shrink wrap

The DIY Audio Guy has a video showing how to build a box with basic tools:

 

4.  INVENTORY YOUR SKILLS

What do you know how to do?  What are you good at and what is beyond your ability? Are you good at drawing?  How about basic arithmetic?  Do you know how to read a tape measure?  Are you a professional cabinet maker?  If you don't have some basic DIY skills, don't let that stop you.  Instead, practice a bit.  If you have never used a circular saw, then go get one, get some wood and start cutting.  Don't know how to solder?  Go get a soldering iron and spend a few hours a night practicing.  You will get better.  Your first box will look like crap.  So what?  Go get some more wood and try again.  Learn from your mistakes.  You will get better.  Jump on YouTube and watch some woodworking videos.  I recommend Steve Ramsey's Wood Working for Mere Mortals.  He teaches basic woodworking techniques and he uses affordable tools.  Here is his video on how to make a simple box:

Another great resources is Toid's DIY Audio, Toid covers topics and builds from beginner level to advanced.  Here is one of my favorite videos where he helps us understand and fix a very common mistake that is often made when building a ported subwoofer enclosure.

 5.  START SIMPLE, NOT SMALL

Large speakers are no harder to build than small speakers.  In my experience small projects (boomboxes, small bookshelf speakers) are much harder to work with because of the tight space inside the enclosure.  So don't start with a 6.5" woofer, start with a 12.  But, keep it simple.  Start with a sealed enclosure.  If you start with a ported enclosure then build an existing design, or a kit, or stick to the manufacturer's recommended specifications.  Don't try something complex until you have mastered something simple. Avoid exotic speaker coverings at first, then expand as your skill increases.  Don't start with plexiglass windows, exotic shapes, or advanced techniques. Sure, that stack-fab, fiberglass, passive radiator, eighth order bandpass you saw online looks nice.  But you need start with a simple rectangular box with a single hole in it.  After you can make a simple box look and sound nice then you can advance to the next level.

*Most of the links on this page are affiliate/associate links, if you click on the links and make a purchase I may earn a small commission.  As an Amazon associate I may earn a small commission from qualified sales. 

Welcome to the DIY Subwoofer Network!

Welcome to the DIY SUBWOOFER NETWORK, brought to you by The DIY Audio Guy!  Here we learn how to turn a pile of wood and raw speaker drivers into complete subwoofer systems for home, car, and anything in between.

We use this blog to post detailed instructions, plans and build logs.  We also post build videos for a wide variety of audio projects over on the DIY Audio Guy YouTube channel, but this website is focused specifically on SUBWOOFERS.