Resistive Acoustic Vent
The Scanspeak 290001 Variovent or 'Flo-Vent' is essentially a resistive reflex port that offers controlled cabinet venting at low frequencies. Installing this vent in a sealed cabinet effectively lowers the woofer Q factor & increases the theoretical cabinet internal volume by a fifth, as seen by the woofer.
The aperiodic vent bridges the gap between infinite baffle & reflex designs, it allows deeper bass from sealed cabinets whilst offering a 3rd order final rolloff, resulting in an improved transient response over the sharp 4th order of traditional bass reflex designs. It is tempting when designing smaller systems with high compliance woofers to use small diameter, longer reflex ports to obtain a low tuned frequency - this can give rise to chuffing noises from the port - the Variovent does not suffer from this.