The Subwoofer DIY Page
A ported enclosure system consists of a driver mounted on one side of a box that has an open tunnel or port which allows the passage of air in and out of the box. At low frequencies, the vent contributes substantially to the output of the system.
The ported enclosure system is characterised by lower distortion and higher power handling in the system's operating range, and lower cutoff frequency than a sealed enclosure system using the same driver. Distortion rapidly increases below the cutoff frequency however as the driver becomes unloaded, and the transient response of a ported enclosure system is usually inferior to that of a sealed enclosure system using the same driver. However, the lower cutoff frequency and better power handling within the system's passband often makes ported systems the alignment of choice for many speaker builders.
Ported enclosure systems are much more sensitive to misaligned parameters than sealed enclosure systems, which makes their construction more difficult for the beginning DIYer. I advise that you don't attempt to build these systems, unless you're certain that the T/S parameters for the driver that you want to use are correct.
Almost any driver can be used in a ported enclosure system, however, only drivers which have a Qts value between 0.2 to 0.5 will generally give satisfactory results. If the driver has a Qts above 0.4, try using it in a sealed enclosure or single reflex bandpass system instead.
Designing a ported subwoofer system
The following image is the Hornresp input screen for a ported subwoofer system, based on the Dayton Audio PA310-8. I've selected a box size equal to the driver's Vas, and a vent size of 90 sq.cm, which is about the cross-sectional area of a 4" vent. I've also set the box's depth, Lrc, to be 1cm (to start). I've also enabled Hornresp's "semi-inductance" feature, to simulate the impact that this has on the overall response of the system.
Now, let's use the Loudpeaker Wizard, and adjust the length of the vent, Lpt, until Hornresp tells me that the resonance frequency is 39 Hz, the same as the driver's Fs. On completion, the Loudspeaker Wizard screen should look like the image below. Looks like the vent will have to be 9.9 cm long to achieve a resonance frequency of 39 Hz in a box that size. However, the response is not really flat. There's a bit of a ripple to it, suggesting that the box is a little too big for the driver.
Let's see what happens if we bring the box size down a bit. This should reduce the "ripple"...
Well, the ripple is mostly gone, but now the resonance frequency has shifted up to 49 Hz. Why? Well, if we decrease the size of the box without changing the size of the vent, the resonance frequency will go up. So let's try changing the size of the vent to bring the resonance frequency back down again. The image below shows what the response looks like if I lengthen the vent a bit.
Now, I could stop the design work at that point. However, what I want to obtain is a slightly smaller box with a resonance frequency of 42 Hz, not 48 Hz, as the resonance frequency of a ported subwoofer defines its lower cutoff point and for this particular design I want that to be 42 Hz. So fiddling with the Loudspeaker Wizard some more results in the image below.
That looks pretty usable. The response is not as flat as the previous iteration of the design, but the target resonance frequency has been obtained and the passband is smooth. There's one more thing to adjust though - the depth of the box. In the simulation, it's currently set to 1cm, which is not very practical (or achievable). So let's adjust it until the box is about 40 cm deep...
Will that have any effect on the response? Let's have a look..
So, the simulation is showing some impact above 400 Hz, which is usually way out of a subwoofer's normally-used passband. The change in the response curve is caused by reflections within the box. These can be reduced in level by lining the box with acoustic fiberfill.
Ported System projects on the Internet