Both art and science come into play when figuring out the best enclosure for your speakers. One of the design options to weigh is whether or not a ported or sealed enclosure will work best for your application (and your budget).
A ported enclosure, also known as a bass reflex or vented box, is a design that incorporates a tube or vent that is open to the outside of the box to allow air to escape.
At a high level, a port helps a speaker produce lower frequencies better. Experts in speaker design can use a port to tune a speaker to achieve a specific resonant frequency. They do this by altering the length and diameter of the port to increase or decrease its surface area and volume.
All other things being equal, the longer the port is, the lower the enclosure will be tuned for. Decreasing the diameter of the port will also lower the tuning frequency of the speaker. However, it’s critical that the diameter is large enough that it allows the air to escape slowly enough so it doesn’t create unintended sounds like chuffing or whistling.
All this can get tricky in smaller enclosures and may require a labyrinth-style twisting tube that wraps throughout the available space. In general, a ported enclosure will cost more than a sealed enclosure because of the additional design required. However, you have to weigh if the benefits are worth it.
Another benefit of a ported enclosure is that it can help a speaker be more efficient, meaning it will require less power to transmit at the same volume as the same speaker would inside a sealed enclosure.
Sometimes the decision for the type of enclosure can simply be a matter of personal preference. Some people prefer the tight and punchy sound a sealed enclosure offers. Some people like the aesthetic appeal of a passive radiator speaker (which can offer many of the same benefits as a ported enclosure). However, sometimes a port is needed to achieve either a frequency response or bass volume level that can’t otherwise be achieved in the available space.
Let’s take a look at the Oaktron by MISCO 305-WF08-02 12” pro-audio woofer (part # 93061) and how it can be deployed in different applications.
This speaker is considered a “low Q” driver, which means that the magnetic motor is very strong and allows it to work in a variety of enclosures. Many customers looking for tight, strong bass punch, and a small enclosure, opt for a sealed enclosure that yields a Qtc (a measure of frequency response flatness) of 0.707 or lower. This 12” speaker needs only 1 cubic foot (roughly 29 liters) of enclosure volume (not including the volume of the speaker) to achieve that.
The f3 (-3 dB point in frequency response) is 97 Hz, which sounds a little high but the f6 (-6 dB point) is 74 Hz and the f10 (-10 dB point) is 56 Hz. In a 400 square foot room, the woofer in this alignment would be great for use in a 2-way speaker system.
For extended bass response in larger rooms, or outdoor venues, a larger ported enclosure is often used. Since we will not be getting much boundary gain, we would want to tune the woofer’s enclosure to achieve a lower f3. This woofer appears to work best in a 4.5 cubic foot (130 liter) enclosure tuned to 40 Hz, which yields an f3 of 45 Hz.
When calculating the port diameter, we test the model to see if it is large enough to prevent port chuffing or whistling. Too small of a port diameter increases the velocity of the air that moves within it. If air velocity exceeds 10% the speed of sound (34 meters per second), unpleasant noises will be heard.
Below is a graph that demonstrates the difference in using two 2” diameter ports (red) versus one 4” diameter port (blue) with 100 watts of input power.
As you can see, the surface area of two 2” diameter ports (6.28 square inches) is not large enough to prevent port noise, whereas the single 4” diameter port (12.57 square inches) does not exhibit the same issues at this power level.
Now that we know a good port diameter to work with, which can also be turned into a slot or vent with the same surface area, we determine the length we need to achieve our desired tuning with modeling software. A 4.5 cubic foot enclosure needs a 4” diameter port that is 1” long.
When you’re working with a speaker manufacturer and can present them with exact information on the amount of space you have to work with for your application they should be able to advise you on whether a ported enclosure is a good option.
The speaker spec to know about when determining if a port makes sense is the speaker driver’s Total Q or Qts. It will typically be a decimal number below one, however there are high Qts speakers (with a value above 1) that are meant to work best on an infinite baffle. The Qts tells you how several of the components of a speaker (spider, voice coil, and particularly the magnet) are operating when the speaker is at its resonant frequency. The power handling and dampening effects of the driver also figure in. In general, a Qts below .6 means a speaker should be put in a ported enclosure. A Qts above .7 means it’s better suited for a sealed box.
The formula to calculate Qts looks like this:
Qts = (Qms X Qes) / (Qms +Qes)
Qms is a measurement of the speaker's mechanical suspension system (the surround and spider) and Qes is a measurement of the speaker's electrical suspension system (the voice coil and magnet).
One of the best ways to help a ported speaker perform even better is by incorporating DSP (digital signal processing) into it. With DSP you can have what is called low frequency, high pass filters. This will prevent over-excursion by a woofer or subwoofer—which can occur when the speaker is getting too much power below its tuning frequency. DSP can also help flatten out the frequency response of a speaker overall.
If a ported speaker is the best option for your application, the best approach is to start with a CAD design. The next step is to 3D print a prototype to be tested and verified. Today’s design software programs are pretty accurate and you can expect your prototype to be within 10% of your goal. Transducer engineers who have experience solving all sorts of audio problems will be your best bet for making those final tweaks to arrive at the finished design.
There are pitfalls with poorly designed ported enclosures. If the design isn’t aligned you can get a sloppy or boomy frequency response. A properly designed subwoofer with a ported enclosure should sound very lean, very articulate, very dynamic, and without a dominating resonance or a dominating peak in its response.