The effects of a longer subwoofer port in a subwoofer can be disastrous. The length of a port in a speaker is critical to its audio performance. Fortunately, lots of online help exists for all aspects of subwoofer and loudspeaker design, making a complex combination of science and art something any do-it-yourselfer can master. A caveat is increasing port length always means reducing port diameter or volume.
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Ports, Vents and Bass Reflexes
They're all the same thing in speaker design: means to use the out of phase sound from the rear of the driver to enhance the sound produced from the front. Computer modelling has been used since the mid-1980s, and Thiele-Small parameters merge electrical filter theory with the acoustic behaviour of loudspeakers in enclosures. These developments allow speaker builders to reliably construct the optimum port length and area for any enclosure size.
If you've ever been next to a car whose exterior vibrations seem louder than the sound in your own vehicle, you know the effect of distortion resonances. Too long a port in a speaker can cause the same rattle and hum at the bottom of the speaker's range. Consider increasing box volume instead of increasing port length. For a given tuning frequency, as box volume increases, the port length decreases.
Lower Bottom-End Response
Longer port designs are often employed to extend bass response inordinately lower than a loudspeaker is designed to produce. Below a certain tuning frequency, however, lower response is not meaningful. Audio frequencies are supposed to be produced accurately. The lowest audio tones must move massive amounts of air, but doing so is a cooperative function of speaker size, magnet weight and correct porting.
Figuring the Correct Size Port
There's a reason why top speakers can command prices in excess of £6,500. Physics, engineering and acoustic science are involved in producing a top-flight speaker. A specific formula is employed for the port dimensions of any loudspeaker based on its loudspeaker driver size. Though technical, it's as reliable as the sun coming up in the morning:
Use this equation to tune a ported speaker box to a specific frequency:
Fb = the desired tuning frequency of the enclosure in Hertz.
Lv = the length of your port in inches.
R = the inside radius of your vent tube.
Vb = the internal volume of your enclosure in cubic inches. (Convert cubic feet to cubic inches by multiplying the volume number by 1728.)
LV = [(1.463 x 10^7 x R^2)/(Fb^2 x Vb)] - 1.463 x R
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