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Topic: What's the purpose of a subwoofer's bass port? (Read 6423 times) previous topic - next topic
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What's the purpose of a subwoofer's bass port?

I recently bought a pretty decent Logitech Z-2200 2.1 system--best speakers I've ever had, though that's not saying much--and noticed a pretty big hole in the side of the pretty massive subwoofer.  It seems to narrow a little and curve down into the box.  What exactly is the purpose of it, and what does it do for the sound?

What's the purpose of a subwoofer's bass port?

Reply #2 –
Short answer: Makes the bass sound better / makes more bass.  Cover it up and see!

What's the purpose of a subwoofer's bass port?

Reply #3 –
It provides an extended frequency response over a sealed enclosure of the same volume.  The tube frequency is tuned to a certain frequency to supplement the ass response (acheived by varyign the tube length and cross-sectional area) However its transient response is not a good as that of a sealed enclosure.  ie the bass it will often sound more 1 note, but this can be minimised with careful design - there are other issues as well.    (note this is all an oversimplification).

What's the purpose of a subwoofer's bass port?

Reply #4 –
Some cabs are designed so the out-of-phase wave coming from the back of the speaker combines with the in-phase wave coming from the front. Because the out-of-phase wave is delayed a little from having to travel through a port it creates a comb-filter effect. They combine in-phase at certain frequencies. These frequencies can be changed by tuning the cab, like changing the shape and size of the cab or the length of the port.

What's the purpose of a subwoofer's bass port?

Reply #5 –
A sealed box is can be rougly modeled as a weight (the moving parts of the speaker) on a spring (the elastic parts of the speaker, plus the effect of compressing/rarifying the air in the box).  The coil and magnet allow your stereo to push the cone in and out.  How far it moves  at a given frequency and power is dependant on how the mass and spring react to the force.

WHen you add a port, it is sort of like adding another weight to the back end of the spring.  The mass is that of the air in the port.   

Now, imagine that toy that is a paddle and ball connected by a rubber band.  If you work the paddle at just the right frequency, the paddle and ball go away from each other at the same time and towards each other at the same time.. so the ball goes bounce bounce bounce.  If you go too slow,  everything goes up and down together.. no bounce.  If you go too fast,  the paddle shakes and the ball just sits there.

Back to the speaker..  if the port is well designed, the air in the port and your speaker will be bouncing in and out together (reenforcing each other) in the frequency range where the speaker without the port would be losing response at the low end.  Like with the paddle and ball, the speaker may actually not move much at all.. all the action is in the port.

SO you get another half  octave or so of bass response.  At high frequencies the port doesn't  do much.

Edit: Some pitfalls of a bad port design..  The resonant range may be narrow, or not well aligned with the natural response of the speaker, so you get a boomy response where all bass sounds like the same note.  It may also take longer for a bass note to die off if things keep bouncing a while after the signal stops.

As with the paddle and ball, if you go below the resonant frequency, the rubber band (air spring) gives no resistance... you could move the paddle 10 feet.  This is NOT good for a speaker.  A cautious user  (or speaker designer) will have a high pass filter to cut off  frequencies that are dangerously low.

What's the purpose of a subwoofer's bass port?

Reply #6 –
Quote
to supplement the ass response


Hmmm, what does ass response have to do with subs? 
"You can fight without ever winning, but never win without a fight."  Neil Peart  'Resist'

What's the purpose of a subwoofer's bass port?

Reply #7 –
Quote
Quote
to supplement the ass response


Hmmm, what does ass response have to do with subs? 
[a href="index.php?act=findpost&pid=251728"][{POST_SNAPBACK}][/a]


That's where the low frequencies come up through your chair and vibrate your butt.

What's the purpose of a subwoofer's bass port?

Reply #8 –
As far as I remember from a old book on loudspeakers that I read, the hole (or tuned port as it is called) is essentially to prevent loudspeakers from resonating at their natural resonant frequency.

Everything has a resonant frequency which it will hum the most at (Well not everything, but you get the idea). Take your table and bang it, and it will hum at a certain frequency. If it's restrained in some way it will resonate at a different frequency. In a loudspeaker, the table is the loudspeaker case and your hand banging the table is the cone which moves in and out based on the music.

With a loudspeaker the problem of resonant frequency is extremely important as it will typically make the loudspeaker sound boomy i.e. the resonant frequency of the loudspeaker case will drown out other frequencies that come from the cone, and will also last for longer than it's supposed to thus drowning out sounds that come after it.

The hole is designed such that at the natural resonant frequency of your loudspeaker case, most of the energy will come out of the hole, thus smoothing out the overall frequency response of the loudspeaker.

Here is an excellent website explaining all this and more (probably much better than I did.)

rajas

What's the purpose of a subwoofer's bass port?

Reply #9 –
Ass respsonse    heh heh.  Saw that and decide to leave it in.  Nice typo i thought!

What's the purpose of a subwoofer's bass port?

Reply #10 –
Quote
Ass respsonse    heh heh.  Saw that and decide to leave it in.  Nice typo i thought!
[a href="index.php?act=findpost&pid=251810"][{POST_SNAPBACK}][/a]


Beautiful

What's the purpose of a subwoofer's bass port?

Reply #11 –
Quote
As far as I remember from a old book on loudspeakers that I read, the hole (or tuned port as it is called) is essentially to prevent loudspeakers from resonating at their natural resonant frequency.
    .....

Here is an excellent website explaining all this and more (probably much better than I did.)

[a href="index.php?act=findpost&pid=251809"][{POST_SNAPBACK}][/a]


The references on that site seem to be mostly applicable to musical instruments.. lots on organ pipes, horns, flutes, etc.  Those dealing with speakers are generally older than the papers of Thiele and Small  which pretty much defined modern speaker box design.  Note that the characteristics of a woofer used by deginers are called the Thiele-Small parameters.

The site itself doesn't  get any more technical than "put a hole in the box to change the resonance"  and leaves you with the impression that any speaker will be improved by just cutting a hole in the box (certainly not true).