A vented, or bass reflex, box has the ability to extend the low frequency response by using a tuned port. The tuned port and the woofer work in harmony to boost the low frequency response. In essence the sound waves from the tuned port are in phase with those of the woofer at or below its resonance frequency, amplifying the low frequency output, as an added advantage the woofer excursion is reduced. Below that the sound waves quickly shift to 180 degrees out of phase, which make the woofer excursion excessive. Another disadvantage is a raise in impedance around the resonance frequency, this increases the voice coil temperature and could cause for harmonic distortion to be introduced. In our case the XLS10 woofer doesn’t suffer from this at all, the well ventilated four-layer all-aluminum voice coil prohibits this.
Mounting the XLS10 in a 30-liter enclosure means that there’s a limit to the maximum length of the vent, as it will have to fit inside the enclosure. Obviously a long vent can be bent to fit the enclosure, but that adds to the complexity of the design. Regardless of port length we’ll need to use a double flared vent to prevent port noise, which is a real danger with the sound pressure level we set as a requirement. To have a flat frequency response tuning the vent at 25Hz is recommended, which will result in the below noted frequency response.
Obviously this is better than we’ve seen from the closed box, the graph shows that the vented box has higher low frequency output, but drops off sharply at 20Hz. What is now important to consider is the spl and excursion at 20Hz and whether the vent diameter size we picked, 72-mm, is large enough to prevent vent noise. As a rule of thumb vent noise becomes audible when the air velocity in the vent is higher than 35-m/s. The graphs below give the spl, excursion and the air velocity in the vent.
As can be seen from the graphs we’ve accomplished our goals with a spl of over 100dB at 20Hz, cone excursion below 12.5-mm and air velocity with a 34.1-m/s maximum. What’s also important to note is that fact that we only needed 160-watts of amplifier power, the XLS10 woofer in a closed box needed almost twice the amount of power. There are drawbacks as well, firstly the length of the vent is no less than 607-mm, which means we’ll have to bend it to fit the enclosure and secondly we’re close to hitting the woofer’s excursion limit and the air velocity limit in the vent. So how about we explore whether a box featuring a passive radiator gives us a bit more leeway?