The Benefits of Ported Enclosures and Calculating Port Size

***Ed Blown Vert*** · 10-20-2010, 01:50 AM

So, you’ve built your share of sealed “boxes”. The guy that works the MDF isle at Lowes knows you by name, all your jeans have glue or silicone RTV on them, and your friends buy you pizza when they need some bass for their ride. What you’ve built so far has been ok, but now’s the time to step up your game, get scientific and move on to the advanced world of ported enclosures.

Why Ported Enclosures
The first question would be “Why?” To understand the benefits of porting an enclosure, we have to understand the drawbacks of an acoustic suspension (sealed) enclosure. When a woofer is playing, the front of the cone and the back of the cone both displace air equally, but in opposite phase. Let’s say we mounted a woofer in a wall between two rooms. If we were in the room that the front of the woofer faced and we pulsed it so that its cone moved outward into the room, the air in that space would be slightly compressed. At the same time, the air in the room behind the woofer would have a slight vacuum. We hear this compression and decompression as sound because it causes our eardrums to move relative to the pressure. It wouldn’t matter which room we were in, the amount of compression or decompression would be the same, so the sound would be the same. But, if we opened a door between the two rooms, the pressure difference would equalize and we would lose output in both rooms.

A woofer in a sealed enclosure works the same. The front wave of the speaker is projected into the listening area. The back wave is captured by the enclosure where it bounces around colliding with itself until all of its energy is converted into heat. It never gets a chance to cancel the front wave. This can be demonstrated by taking your woofer out of its enclosure while it’s playing. Almost all the bass will disappear the instant the woofer clears the box.

A problem with all this is that out of the total power the amplifier has supplied to the woofer, half is heard; the other half is wasted as heat in the enclosure. Another problem is that to get lots of bass, we need lots of excursion of the cone. The pressure inside the enclosure is resisting the motion of the cone, trying to keep it centered at all times. The more the woofer moves, the higher the force trying to push it back to center. While this self-centering property is good for sound quality, it does lower efficiency and that means less BOOM. So, you say, “Let’s let out that pressure by cutting a hole in the box,” but that’s just like opening the door between the two rooms—the back wave of the woofer will mix with the front and voila, bass cancellation.

How Porting Works
The trick to porting is making a hole that slows down the back wave of the woofer enough so that when it finally exits the port, the woofer has changed direction and is now in phase with the sound exiting the port. The way we do this is by creating a mass of air in the port. We don’t normally think of air as having mass, but it does. In fact it has enough mass that the column of air over our heads weighs over 14 pounds per square inch. Do the math and you will see that our bodies have over ten tons of pressure pushing on us at all times. We can see the mass of air by filling a balloon and then tying a rubber band to it. Punch it, and you will see its momentum carry it away and then the rubber band will slow it and return it. Try the same thing with the balloon deflated and it doesn’t work. The air in a port acts much like that inflated balloon. It’s hard to accelerate, and once moving doesn’t want to stop. That restriction tends to delay the wave as it exits and that’s what allows it to be in phase with the woofer.

Unfortunately, there is no such thing as a free lunch. The mass of air will resonate at only one frequency. That means that it will work perfectly at one bass note, and to a lesser degree a range of notes above and below the one it is tuned to. If we play notes in the enclosure much higher than the tuned frequency, the restriction in the port will not allow the sound to exit, so the port has basically no effect. If we play notes much below the tuned frequency, the port will not delay the sound like it should and out of phase waves will exit, canceling the bass. When this happens, essentially, the woofer has no enclosure. We say that it unloads. This is not only bad for sound quality but can allow for excessive, woofer-damaging excursion. So if we tune the port too low, it helps output at frequencies that aren’t even in the music. If we tune too high, deep bass unloads the woofer and bad things happen.

Software for Port Design
This means that the port has to be carefully matched to the speaker, airspace and even the intended use of the enclosure. The diameter of the port, its length and the size of the enclosure all affect the tuned frequency. To figure out the proper size and length of the port requires formulas and math that would bring back nightmares of high school algebra. Thankfully, there is computer software available that can not only design an appropriate port, but can also predict the response of the finished speaker system. For this article we’ll use a downloadable application called WinISD.

Download, install and then open the application. Click the button to maximize so things will be easier to see. You will start with a window named “Plot window” This window can be dragged to a larger size for easier viewing. Just click on an edge and pull. The plot window is a graph with frequency on the bottom and relative volume on the side. It will show you the output of the enclosure at any given bass frequency.

Start-Up screen of WinISD from LinearTeam

From the top options list Choose “New”. A new window called “New Project” will appear that will allow you to choose from a library of pre-loaded drivers. Chances are the driver you want to use won’t be on the list, but you can select “New” from this window and another new window called “Driver Editor” will pop up.

New Project Window

Driver Editor Window

You can enter the name of the driver you will be using and then fill in its Thiele Small Parameters. These parameters can usually be found in the manual or documentation that comes with the woofer, or can be downloaded from the manufacturer’s website. You will need a minimum of three Parameters (FS, QTS, VAS) for the program to be able to show you the predicted response of the enclosure. It’s best to enter all the parameters to make full use of the program. For example, if the manufacturer supplies the parameters PE and QES, enter them and the program will allow you to see whether the port opening is large enough for quiet operation. Some parameters may be given in either English or Metric units (for Example, Vas can be listed in cubic feet or Liters). By clicking on the units (in., mm etc.) next to the parameter box, you can scroll through all the possibilities until you find the one that matches the units that your speaker’s manufacturer supplied. For a list of major Thiele Small Parameters and their meanings, you can go here. When the parameters are entered, you must click “ok” at the bottom of the window to save the driver into memory (maximize the window to see “ok”). You can now close the window.

We’re now ready to load your new driver and work on an enclosure. Select “New” from the top menu bar, and in the “New Project” window, click “Own Drivers” and your woofer should be listed.

Click “Next” and the program will prompt you to choose between standard and Isobaric (cone to cone) mounting and allow you to select how many drivers will be in the enclosure. Click “Next” and the program will prompt you to enter the type of enclosure you wish to build. You can choose from sealed, ported or bandpass designs. We’ll focus on ported for now. Bandpass enclosures have some advantages in SPL, but are more difficult to design.

Also in this window is a bar graph, a value called EBP and a box type recommendation. EBP (Efficiency Bandwidth Product) is used to determine the best type of enclosure for specific subwoofer. It is obtained by Dividing FS by QES (EBP = FS / QES). Generally, woofers with low EBPs (Less than 50) prefer sealed enclosures. Those with high EBP’s (over 90) perform best in ported enclosures. In between, either enclosure can be chosen. Most woofers will fall into this middle range because manufacturers want the drivers to work in as many different enclosures as possible.

We can click “Finish” and the program will design an enclosure. It automatically tries to create the flattest most extended response possible. If the sub were being used in a home theater, those might be an appropriate goals but it usually makes for a sub that is not especially loud and either a very long port or a very large enclosure neither of which works well in a car. Fortunately, cars reinforce bass response of an enclosure through something called transfer function. Anytime a speaker is played in a small area (like the interior of a car), frequencies below a certain point are boosted at 12Db per octave. If our enclosure happens to start rolling off its response at the same frequency, the car will even out the response of the enclosure. This effect can be heard in headphones. Listen to a headphone off of your ear and it sounds tinny with little or no bass response. Put the phones on your ears and the small space (your ear canal) will cause a transfer function to restore the bass response. The smaller the space a speaker is placed in, the sooner and louder the bass is reinforced. This means that the same system installed in a small hatchback and a large SUV will have deeper, louder bass in the smaller vehicle.

Transfer function can be “mapped” by measuring the SPL of a range of frequencies of an enclosure out of a vehicle and then again in vehicle and comparing the result. This will tell us what notes the car will boost. One way to go about this is to use a laptop or smart phone as a tone generator. SigGen is a good one for the laptop and can be downloaded.

It can generate any tone from 20-20K through the soundcard of your computer. A simple stereo mini to RCA adapter will allow you to send the tones right into your car system’s bass amp, or you can save the tones and burn a CD to play in your deck. We usually measure from 20-100Hz in 2Hz steps. An inexpensive SPL meter to measure output will also be needed. Radio Shack has them inexpensively or you might be able to find a smart phone SPL meter application that would suffice.

SigGen Signal Generator for Windows

Once you have a tone generator and a way to measure SPL, the next step is to measure the response of a test enclosure. This enclosure can be anything as we are only interested in the change in response when it gets installed into the car, not the response of the test box. Set up the box in an open area away from walls and buildings with your measuring device in a fixed position about a meter away from the speaker. Play, measure and record the SPL of each tone on a piece of graph paper with the frequencies listed on the horizontal axis and the SPLs on the vertical axis. If you have Windows Excel, you can graph using that as well. When done you should have something that looks like this:

Notice that the test enclosure’s response stays relatively flat for all the upper bass frequencies, but rolls off starting around 42Hz. Now let’s take the same enclosure and place it in the trunk of a Civic sedan.

We can see that the output of the enclosure has been boosted at almost every frequency, with more boost at some frequencies than others. The differing amount of boost (or cut) is the transfer function or frequency response of the car. This varies from car to car and even by changing the location of the enclosure in the car.

To see the actual graph of response of the car, we can subtract the SPL of each frequency measured in the parking lot from the SPL measured in car. This will give us a new graph of the car.

Looking at this graph, we can see that the deeper the bass, the more boost the car applied. This is because the car boosts at approximately 12Db per octave. To see this, look at the boost at 90Hz. It is just below 5dB. Look again an octave lower (an octave is half or double a frequency) at 45Hz where it boosts 15Db for a total boost of 10Db. Theoretically this curve would be a straight line sloped at exactly 12Db per octave, but the shape, size and materials that the car is made of will add resonances that can alter this boost. We can use this curve to our advantage depending on what kind of system we are trying to build. You will notice that the curve has a peak at around 55Hz. If we built a box that was especially loud at 55Hz, the peak of the box and the peak of the car would match and SPL would be even higher. This would be perfect for an SPL type system. You will also notice that the curve has a dip at around 38Hz. If we built an enclosure that had a peak at that frequency, we could flatten the response of the sub bass and make the car sound smoother. This would be beneficial in an SQ type system. The ability of a port to tune an enclosure to any frequency makes it easy to custom match the response to a type of system or even a type of music.

Going back to WinISD, and the default enclosure it designed, we see that the program chose a .662 cubic foot box size with a 28.40Hz tuning. Although the response curve looks flat and the box size seems nice and small, there will be a problem.

If we click on “Vents” a window pops up telling us how to build a port for the enclosure. In this case it would be a 4” diameter round port almost 60” long. Obviously a 5 foot long port is not going to fit inside a box that is a little larger than ½ a cubic foot. Although we could leave the port extending outside the enclosure, it would make for a strange looking contraption. Theoretically we could construct a port that wound around using plumbing elbows. As long as the length through its centerline equaled 60”, it would tune the same. If we then wanted to install the wound up port inside the enclosure, we would have to compensate for the airspace it took up by adjusting the outside dimensions of the box so that the internal volume included both the airspace the port took up and the airspace the woofer needed to play in.

If we wanted a shorter port, we could change its diameter.

Rule #1) Large diameter (or area) ports need to be longer, smaller ports can be shorter

If we change the Vent (port) diameter to 2 inches, we can see that its length is reduced to only 14.08”. This port is much more practical and might well fit inside the enclosure, but we’ve created another set of problems. Looking at the box labeled “Vent mach” we see that the velocity of the air in the port has increased by four times. .16 mach is the upper limit for port velocity. When it gets into this range, the port becomes noisy as the air races in and out of its opening. Also, as the port gets smaller, it begins to become too restrictive and cannot flow well enough to increase the SPL as we would expect. Think of a car with an exhaust system that was too small. Horsepower would suffer.

Ok, so what to do? Another thing that affects the port length is the tuning frequency. The box that Win ISD designed had a very low tuning. Low frequencies equal long ports. In our Civic, we had a dip at around 36Hz. If we wanted a smoother response in that particular car, we could tune the enclosure in such a way that its response would peak where the car was weak helping to flatten the response. Let’s see what happens to the port when we tune higher.

The tuned frequency has been increased to 36Hz while the box volume was left unchanged. Looking at the curve we can now see that the output of the box has been increased by about 3Db. This may not look like much on the curve, but to achieve the same increase in output with an equalization added to the original box, we would need twice as much power at that frequency. Scroll the frequency up and down and watch the response curve of the box. You will see that the higher the tuning frequency, the higher the output of the enclosure. This is why SPL competitors tend to use higher tuning frequencies. Also note that as we tune higher, the curve rolls off faster at the lower end of the frequency. If you like deep, rolling bass that can be heard for blocks, those missing low frequencies are what you want. A higher tuning frequency that would read well on an SPL meter at a competition would disappoint you on the street. Let’s leave the frequency at 36Hz for now. Tunings between 30Hz and 40Hz are common because they give some boost without sacrificing too much low frequency output. Now click on the “Vents” tab and let’s see what happened to the port size.

We can see that with the larger, more efficient 4” port, the length has dropped from 5 feet to three feet long.

Rule #2) High tuning frequencies will allow for shorter ports, low tunings require longer ports.

Although a 3 foot long port is more reasonable, it’s still a bit difficult to deal with. So, let’s see what else we can do. So far we’ve been keeping the box volume fixed. The next thing we’ll try is to vary the enclosure’s size. Click on the “Box” tab once more. Scroll the volume up and down and watch what happens to the response curve. As the box size increases, the output of the enclosure increases and the frequency of its peak decreases. Or, in other words, larger boxes play louder and deeper. We’ll pick 1.0 cu’ for now.

The output of the box has been boosted almost 6dB now, the equivalent of increasing system power by four times. If I had 250Watts powering this enclosure, I would need to have 1000Watts to be as loud at 38Hz with the same driver in a sealed box. Click on “Vents” and see what has happened to the port.

The port length has dropped to a more reasonable 22.69 inches.

Rule #3) Large enclosures need shorter ports, small enclosures require longer ports

One concern with making the enclosure too large is something called group delay. When the port is operating, it has the tendency to delay the sound waves exiting it. This means that the sound coming directly from the woofer cone will reach your ears sooner than the sounds from the port. When group delay becomes excessive, the woofer may begin to sound “sloppy” or “slow” An example of this would be sound a bass guitar riff. The pluck of the string should happen at the beginning of the note and should sound tight and crisp. With a large group delay the note could begin to sound blurred and fluttery. Depending on the type of music, group delay may be more or less important. If you’re playing a bass test disc, with one extended bass tone after another, you might not notice a problem, but on a classic rock or jazz recording with a tight bass line, a critical ear might not like the sound. How much group delay is acceptable? It depends on your ability to hear the delay and the music you play. Some say that keeping group delay under 20ms is preferable. Click on the “Group Delay” tab to check it. Ultimately however, designing an enclosure with the frequency response and low frequency output that satisfies your needs is more important than group delay.

Clicking on the graph will create a crosshair that can trace the curve and read the group delay at a given frequency. This enclosure has a delay of 27.49 milliseconds, which is a little high, but acceptable.

Ok, let’s say we’re happy with the curves. We can now move on to designing the actual enclosure. Click on the “Box” tab and you will see a field named “Box Shape”. Click on that field and a new wind named “Box Shape Editor” will appear. This window will help you calculate the dimensions of your enclosure. The first thing you will need to do is to fill in the field asking for board thickness. This will be the thickness of the material you will be constructing the enclosure from.

The most common material for constructing enclosures is MDF (medium density fiberboard). It’s used for its lack of resonance, inexpensive price and availability. Other options would include Baltic Birch Plywood a very high quality finely laminated cabinet grade ply. Baltic birch is stronger and lighter than MDF, but harder to find and more expensive. The most widely used thickness of Ply or MDF is ¾” although thicker material might be required if building a very large or high SPL enclosure. By entering the thickness of the material, the program can automatically compensate for the wood thickness when calculating volume. The next thing we need to do is enter two dimensions that will remain constant. These would be measurements made in the car that must be maintained in order for the enclosure to fit. Be sure to allow for the thickness that carpet or upholstery will add to the finished enclosure and measure to the smallest clearance on panels that are curved.

In this case I entered 9” for the width and 12” for the height and then clicked on the “D” for depth and the program filled in the final measurement. Boxes that are cubic or rectangular have only three dimensions, but the software will also allow designing sloped boxes by clicking on the “Cubic (normal)” window. Now that we have a design, a new problem has cropped up. The depth of the enclosure is 23.5 inches, and our port length from an earlier step was 22.69”.

Although the port would fit in this enclosure, the end inside the box would be blocked by the rear panel. A port must have a clearance of at least its diameter inside the box. We could add an elbow, but the easiest solution would be to go back to the “Vents” tab and make the port diameter smaller so it will be shorter.

A port diameter of 3” will be sufficient to support this small woofer and “Vent mach” is still reasonable. The software also gives us the option of making a rectangular or slot port by clicking the “Shape” field.

Here I entered the internal height of my enclosure (10.5”) and a width of 1” and the software tells me I would need a single piece of wood 17.20 inches long to create a slot port. Depending on how that piece was installed, you might need to compensate for the thickness of the front baffle of the enclosure. For this design, I used a piece of 3” PVC that was handy, but typically slot porting is faster and easier since it uses the same material that is used to construct the enclosure and wont require a trip to the hardware store for plumbing. In the next part we'll cover the actual construction of our enclosure.

Related article: Constructing a Ported Enclosure

Contributor Joe Padula is an instructor at Installer Institute in Florida.

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