Subwoofers
- Thread starter mkillio
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Helmholtz resonator...
http://scienceworld.wolfram.com/physics/HelmholtzResonator.html
http://www.phys.unsw.edu.au/~jw/Helmholtz.html
Basically the "hole" is where a lot of the "bass" is expelled. In a sealed enclosure the bass is expelled in front of the speaker cone. The size of the hole and the length of the tunnel connected to said hole determines the frequency that's outputted from that hole.
Try looking up acoustic suspension, bass reflex, and band pass enclosures.
http://scienceworld.wolfram.com/physics/HelmholtzResonator.html
http://www.phys.unsw.edu.au/~jw/Helmholtz.html
Basically the "hole" is where a lot of the "bass" is expelled. In a sealed enclosure the bass is expelled in front of the speaker cone. The size of the hole and the length of the tunnel connected to said hole determines the frequency that's outputted from that hole.
Try looking up acoustic suspension, bass reflex, and band pass enclosures.
Assuming you mean a tuned port then Helmholtz resonator is correct, but the port is not where the bass is spilled out. The idea of the infinite baffle is to seperate the front/back energy waves, & hence stop their cancellation in esp the LF domain where 360 degree radiation occurs. However, this wastes the rear propagated energy. A tuned port allows the sympathetic rear wave of the dynamic driver to be output in correct phase thereby gaining 3db efficiency in the passband. Around the box tuning frequency (fb), the compliance of air/tuned-enclosure works to control cone excursion & allow greater power handling, thereby giving the notion of bass eminating only/mainly from the port. Below fb, however, the cone behaves as though it's operating in free air & can certainly become damaged if driven above it's mechanical excursion limit (X-Max). A rumble filter or subsonic filter can help, at the cost of your HT experience...
There are many alignments possible & are largely characterized by the seminal work of Thiele & Small. A "bass reflex" type alignment may give greater power handling & lower distortion over its design passband, but the downside is higher rolloff & higher distortion (> X-Max) below fb. A similar sealed box design may achieve a lower f3/f10 with more linear response aided by roomlift (12db) at the cost of lower efficiency. Each design must employ carefully chosen drivers which tend to be mutually exclusive unless box volume is not of concern. Other dynamic driver designs include transmission lines & Voight pipes, horns, semi closed & open baffles.
The advocates of sealed vs ported are almost as zealous as our two favoured IHVs... Perhaps the latest craze of equalized open baffles equates to PVR or Blair's "third way"... It depends on the intended use, integration, room & audio tastes.
There are many alignments possible & are largely characterized by the seminal work of Thiele & Small. A "bass reflex" type alignment may give greater power handling & lower distortion over its design passband, but the downside is higher rolloff & higher distortion (> X-Max) below fb. A similar sealed box design may achieve a lower f3/f10 with more linear response aided by roomlift (12db) at the cost of lower efficiency. Each design must employ carefully chosen drivers which tend to be mutually exclusive unless box volume is not of concern. Other dynamic driver designs include transmission lines & Voight pipes, horns, semi closed & open baffles.
The advocates of sealed vs ported are almost as zealous as our two favoured IHVs... Perhaps the latest craze of equalized open baffles equates to PVR or Blair's "third way"... It depends on the intended use, integration, room & audio tastes.
Yes, LF extension/gain is mainly what we're hoping to achieve with a ported alignment. If I wasn't clear, LF is definately emitted & most designs take care to avoid higher frequency dispersion/distortion from the port (less of an issue with subs). I dunno what I was getting at with "the port is not where the bass is spilled out", except it grates...
BTW, in my previous house with brick/plasterboard walls & timber floors I had my stereo subs crossed @ 40Hz. My new house with solid brick/masonry walls, timber floors on concrete slab, & 22" ceilings results in extreme bass lift. Plugging the ports for an overdamped response doesn't help much. I'm no longer using the subs until I mod the x-overs & slopes & am rejigging the x-overs for my mains to remove 3dB BSC. Shows how important room size, construction & furnishings are.
BTW, in my previous house with brick/plasterboard walls & timber floors I had my stereo subs crossed @ 40Hz. My new house with solid brick/masonry walls, timber floors on concrete slab, & 22" ceilings results in extreme bass lift. Plugging the ports for an overdamped response doesn't help much. I'm no longer using the subs until I mod the x-overs & slopes & am rejigging the x-overs for my mains to remove 3dB BSC. Shows how important room size, construction & furnishings are.
MfA said:
Yeah. Isn't it freaky how well signal theory applies?
When most of the "badass" deep ported subs have delays of >22ms, you have to wonder what you're actually listening to... Anything with >16ms is an "overleveraged" alignment. Compare this to <4ms for a Qtc 0.8 equal ripple sealed design & you can see why the inherent clarity, speed & accuracy descriptions are made. Combined with a gentler <12dB/oct rolloff & crossed at <50Hz, a sealed sub can be truly useful instead of a boom effects channel. This is not to say an excellent ported design is not possible. It's just that Vas & Q needs to be manipulated in order to fit the bill for today's point estimate hungry consumers who demand small enclosures & low f3 specs. The benefits of greater efficiency, max SPL & generally lower f3 values are tough to pass, esp when the strength of the sealed design is at extreme LF <30Hz, where there is very little content except for pipe organ & some .1 effects channel material. Given most consumer equipment is unable to produce enough output <40Hz, there's little point going down the sealed box route.
Wavelengths at this end of the spectrum also require rather large room dimensions & node excitation & interference patterns kill uniformity of response. My previous house layout & construction caused standing waves & LF deadspots, whereas the new house is a revelation. I've experimented with a 4" LF driver in a transmission line tuned to 27Hz, & output is astonishing. So much for 12" drivers a minimum requirement...
BTW I read in another thread something about sofas vibrating, etc. In fact there are specific transducers, for just such an HT effect.
A port or vent is a way to increase output form a speaker system, the vent is tuned to a certain frequnecy point, this frequency point will effect the response curve of the speaker system.
Above that tuned frequnecy there is extra load against the driver to reduce excursion, an disadvantage (as highlighted by others in this thread) in an ever increasing slope below this point driver excursion is not controlled and can lead to the driver unloading past its safe mechanical travel limits.
With regards to group delay above 35hz it can be a issue but in the 30-20hz region GR is close to unnoticeable.
What has to remembered is that sealed and vented systems even within there alignment types differ quite a lot.
You can make sealed systems range from High Q peaky and boomy to Low Q smooth and deep.
The same with vented systems, you can get them sounding great or make them utter boom boxes.
Above that tuned frequnecy there is extra load against the driver to reduce excursion, an disadvantage (as highlighted by others in this thread) in an ever increasing slope below this point driver excursion is not controlled and can lead to the driver unloading past its safe mechanical travel limits.
With regards to group delay above 35hz it can be a issue but in the 30-20hz region GR is close to unnoticeable.
What has to remembered is that sealed and vented systems even within there alignment types differ quite a lot.
You can make sealed systems range from High Q peaky and boomy to Low Q smooth and deep.
The same with vented systems, you can get them sounding great or make them utter boom boxes.
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