A slotted port is an acoustical device designed for high efficiency and low distortion within a specific bandwidth of frequencies. This type of port uses two walls with holes or slots cut out between them so that air can flow in and out through the opening.
The air flowing through these openings makes a noise similar to the sound produced by blowing across the open top of a bottle, hence its name ‘Blown Port’.
An example of how it works would be you cupping your hands over your mouth and breathing deeply into them; then, if you lean forward slightly you’ll hear a louder noise (the air rushing through your hands as you breathe).
Many people use slotted ports because it is cheap and easy to make. A port that goes straight through the box wall provides airflow for better SPL (higher volume) at a given bass frequency.
Slot Port Design
The port typically consists of several parts, including the barrel, an obturator, and a guide sleeve. These all work together to produce sound in the loudspeaker system when air passes through it.
There are many types of Slot Port systems, each with its resonant frequency and bandwidth characteristics, but all have the same basic characteristic of an enclosed volume of air connected through a relatively long tube or channel to the outside world.
The length of the channel determines the cutoff frequency at which it will start to resonate, very much like blowing across the top opening of a bottle produces sound only at specific frequencies based on the length of your lips.
A short slot (area between openings) will produce lower bass than if you had made it longer.
A simple inertance tube cut to the appropriate length can be used as a slot port. Inertance tubes are usually made of rigid PVC plastic.
Slot ports have one or more slots, which creates an opening that allows air into the enclosure. Slot ports are typically found on small subwoofers and amplifier cabinets, either on old equipment or by design.
Why Use a Slotted Port?
The purpose of these holes is to allow air from inside the speaker enclosure to escape in phase with the audio signal from the amplifier driving the speaker cone – which produces sound waves in front of and behind it.
For this reason, they are called “slotted ports” or sometimes simply ‘ported enclosures’. This technique is known as “vented box” construction because of the port’s resemblance to a ventilation shaft.
Slotted ports are normally compared to the popular round ports that you will see on many speaker designs. Check out our guide on slot vs round ports to learn more.
Advantages of Using Slotted Ports
- The first advantage of Slotted Ports is that they provide for Low-Frequency Extension beyond what would be possible with a closed box loudspeaker enclosure design. In other words, they enable deeper bass from a given sized enclosure and/or from a smaller sized enclosure than otherwise possible.
- The second advantage of Slotted Ports is that low-frequency extension remains fairly constant with increasing volume levels since port air mass resonance stays more or less constant with increased sound pressure levels.
- In addition, the third advantage of Slotted Ports is also related to their ability to maintain accurate bass response at high volume levels. This is because they are relatively linear within a given set of parameters for a wide range of input signal frequencies and amplitudes.
- The last advantage of Slotted Ports is that they use the Helmholtz resonance principle to boost low frequencies instead of the usual bass-reflex method used by sealed enclosures which typically emphasize midbass frequencies and cause a resonance null at low frequencies.
Disadvantages of Slotted Ports
- It makes the speaker enclosure “one-note” sounding, meaning that one single frequency can be boosted or weakened depending on where you put your microphone in front of the box. This is not very desirable as a well-rounded sound system would have even response over a wide range of frequencies.
- A slotted port results in decreased low-frequency extension, output at low frequencies, and efficiency. This is why it is common for a speaker to have a flared port instead of a straight one when going low in frequency – this allows for unrestricted airflow, no turbulence, and no decrease in efficiency or output at the lower end.
Speaker Port Tuning
The port tuning frequency is a term used in the audio community to express the amount of low-frequency extension a speaker enclosure provides.
It is a number that represents the speed at which sound waves travel through the length of the port, and therefore an indication of how low-pitched notes can be played before significant levels of distortion starts to become present.
In short, it defines how “big” a bass-reflex cabinet can go before noticeable problems arise with its performance.
Slot Port Vs Aero Port
A slot port is an aerodynamic enclosure that uses a round hole with the diameter of the tube (usually 1-2 inches), which acts as a vent for air to move in and out. Aero ports are usually shorter than slot ports, but they both affect sound reproduction.
Aerodynamic speakers are designed to reduce air turbulence caused by the rapid movement of air moving through the speaker’s port.
Slot ports will not be affected by airflow turbulence because all airflow exits through the front of the speaker, but they do require more surface area on your speaker enclosure since there must be enough space to accommodate for more length or width of tubing.
Aero port reduces sound turbulence caused by the rapid movement of air moving through a port tube.
A speaker with an airport has a shorter tube, which removes the need for the extra surface area on the speaker enclosure. Aero ports reduce turbulence, but since they have a smaller diameter, you will lose some efficiency as less sound is produced compared to a slot port.
Slot ports are also more efficient than aero ports because anything that disturbs airflow from going in or coming out of them will increase turbulence and dampen the bass response.
Aero ports disturb airflow by creating a blockage effect, so they negate this problem and may actually increase bass reproduction due to their design, although it does mean that adding additional speakers with aero ports reduces the total “free” airflow.
DIY Speaker Port Tube
These are tubes that help guide sound waves from your speaker to the listening position. They’re normally round but you can make them in any shape you want.
How to Build DIY Speaker Port Tube?
Materials: scissors, ruler, pencil, paper towel roll (or toilet paper roll), knife (to cut the paper towel roll).
Step 1: Cut out a square 12 inches by 12 inches from your piece of paper; you will use this as your template.
Step 2: Cut the edge of the square to give it a shape similar to that in image 2 (you can choose any shape that will fit into your speaker box easily); then cut 8 inches into this shape along one side; now fold this long triangle in half so each side is 6 inches.
Step 3: To attach the BRT to your speaker box, measure from the back of the speaker porthole up 12 inches then make a sharp 90-degree angle cut at that point.
Step 4: Now place the BRT inside of your sharp corner and secure it by taping or hot gluing it into that corner; finally use scissors or an object with a sharp blade to poke 8 holes evenly around the tube for air to escape through.
How to Choose Port Tuning Frequency?
Choosing the proper port tuning frequency is a critical consideration when designing a system. The main factors that affect the choice of a particular one will be:
1). Size and output level of Woofer – Generally speaking, the larger its’ cone area, the lower the nominal tuning frequency – for an equivalent size enclosure. This is because more air needs to be moved in order to create a certain amount of SPL at any given listening distance from the speakers.
2). Acoustical Environment – Generally speaking, if your room is clean-sounding (lots of absorptive surfaces), you will probably want a higher port tuning frequency to optimize low-frequency extension. If your room is ‘live’ i.e., lots of reflective surfaces or has standing waves, then consider choosing a lower port tuning frequency for more output at low frequencies.
3) Free Air Resonance vs. Suspension Limited Excursion – A speaker’s ‘free air resonance’ means what it sounds like: at a particular frequency (Fo), the speaker will behave as if it were in free space without an enclosure around it, i.e., no air loading or other factors involved other than the simple driver itself.
How to Tune Aero Ports?
It is best to have two people, one on each side of the car at a time. It’s not as hard as it looks and gives you much better results than just doing one side at a time.
Step 1: Start with both vents pointing straight ahead towards the ground about 10-20 degrees off parallel with the ground if possible (nice trick: put them both in their lowest position)
Step 2: Have person #1 stand on the driver’s side and hold down or clamp down the center air nozzle so it points directly back at where person #2 is standing about 10 – 20 degrees off parallel with the ground if possible (he’s the one holding down the center vent). You can make a little holder for it or use some other method to keep it from moving around.
Person #2 should be standing up high enough so that he is looking directly at one of the aero holes while he holds down/clamps down (again very securely) the outside air nozzle pointing straight back towards where person #1 is standing. Again you can make a little holder for this if necessary or just hold it in place better than I did.
Step 3: Person #1 adjusts the vent angle until he sees two glowing yellowish spots on each side of both aero ports. Make sure they are exactly even and not skewed left or right! This makes your audio hit perfectly level with your windshield.
Step 4: Person #2 adjusts the center air nozzle so that you are hitting about 6″ – 1 ft. in front of the center of each aero port. You may have to tug back and forth on it a couple of times for each side until you get it just right, but again they must be identical!
Step 5: Now go do the other side. After both sides have been tuned move on to step 6 if you’d like.
Step 6: To polish off this mod, fold down your vent wings that point out into the sky as high as they will go then raise them all the way so they are pointing at where your vents used to be pointing off into space.
The slotted port is a modification of the traditional ported speaker enclosure, which has been in use for many years. Although there isn’t much difference in SPL efficiency between either type, this design seems to offer an advantage in the lower frequency range (i.e., more bass).