a woofer module is used for outputting alow - frequency audio signal , and can be inserted into and removed from anexpansion slot of a portable computer . the woofer module has a housing , a speaker unit positioned inside the housing for generating the audio signal , a predetermined space positioned inside the housing for resonating the audio signal , and a bass reflex duct positioned inside the housing which is connected to an output vent of the predetermined space and an output vent of the housing for transmitting the audio signal from the output vent of the housing to an ambientenvironment .