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Calculate bass reflex port for a speaker

Pulkass

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I have got them : 2- 10 cm wide ports 7 cm long. Thanks
 

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andreasmaaan

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40 cm X 120 X 30 cm.

So that's an internal volume of 144 litres? Or 120 litres as you mentioned above?

And that's for just one woofer? If so, that seems far too large for this woofer IMO.

In the 4425 speaker, JBL places a single 2214H in a 54 litre enclosure with a port tuning of 34Hz.

When I model the driver in such an enclosure (using more or less standard/reasonable assumptions), this is the response I get:

1611440566905.png


That is with two 7cm-diameter ports of length 31.4cm each.

You could make the enclosure slightly larger than that, particularly if you're planning to use a passive crossover and there will therefore be resistance in series with the woofer.

Keeping the same tuning frequency (34Hz), increasing the enclosure volume to 70 litres, and adding 1 Ohm series resistance, this is the modelled response (with two 7cm-diameter ports of length 23cm):

1611440953710.png


Whether you're better of using longer/larger or shorter/smaller ports will depend on the frequency at which you plan to cross the woofer over, among other things. Two 7cm ports allows the woofer to get up over 100dB before the likely onset of turbulence, and up to 111dB before the likely onset of significant compression (this is just shy of Xmax above the tuning frequency).

When I increase the enclosure volume to 120 litres as you are planning to build, I get a very underdamped response, even with 1 Ohm series resistance. Would not advise going anywhere near this large:

1611441154312.png
 

Pulkass

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As l stated earlier, it will be the same volume used in the Original l 100 t. Calculations courtesy of JBL technology lab, was Greg Timbers involved ?, mine will have 5 internal braces a la Matrix B&W horizontal , plus two vertical. Braces 2cm in width. All holed. Glued to every cabinet s panel plus metallic holed piece of steel pressed in silicon to deaden the panels, the problem with l 100 t was it was a boom box. No internal bracing. !!!! A good woofer but horrible cabinet.
 

111db

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For a 140 liter box, Winisd recommends a quasiButterworth box tuning frequency of 32hz, resulting in a broad 1.5db dip centered at 65hz rising back to 0db at about 35hz, and a -3db point (f3) of 30hz. That requires larger vent diameters than the 25mm you propose, which in any case would have a high level of turbulence noise. If appropriate round port tubes are difficult to obtain, maybe you should consider a slot port that is integral to the cabinet. Once you have settled on port quantity and cross sectional geometry, Winisd can calculate port length quickly. Larger port cross sections require longer duct lengths to achieve the box tuning goal. The trick is to size the port cross sections large enough to avoid port noise and small enough to permit duct lengths that are practical.
 

Pulkass

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Thanks, I am settling on two 10 cm wide -7 cm long ports, not definitive, WINISD does not let you chose 2 ports, dont they ???
 

111db

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Thanks, I am settling on two 10 cm wide -7 cm long ports, not definitive, WINISD does not let you chose 2 ports, dont they ???
Actually it does allow for multiple ports. For a 140 liter box with 2x 100mm diameter ports, Winisd shows 255mm duct length, which will poach some cabinet volume, requiring additional compensation. But I suggest you consider the warnings about a too-large underdamped system per Andreasmaaan above, which I overlooked as I didn't realize a second page had emerged since I opened the thread this morning (USA). Or if you otherwise liked the L100 performance, just duplicate the volume and port parameters of the L100, brace the cabinet well, and hope the booming bass goes away. I'll also take the liberty to recommend that you reconsider actively multiamping your DIY system, despite your hesitations. Control is a beautiful thing, and optimizing passive crossovers is---for me at least---an art that takes a lot of testing and informed analysis to rise above the level of optimistic flailing. I'm very pleased with the performance I'm getting from an older 7.1 AVR with discrete analog inputs that I bought used for $50usd, used as a 7-bandpass power amp downstream from a DSP. But then, my drivers (sub excluded) are >100db sensitive.
 

Pulkass

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This is for a friend, a much less hi fi maniac than me, I m giving those marvellous JBL woofers to him , he helped build my behemoths way back in 1999-(multiampled) --bracing will be 5 matrix B&W like boarda--2 cm with 8 holes each, plus two vertical. he won t multi ampli-plus metallic plates holed--drowned in silicon-I m serious aboud deadening--mass--de copuling,
-he s just a normal guy. eheheheheheh
 

111db

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Friends help friends build speakers.
I love working with vintage pro drivers. Your friend will love them too.
Can you elaborate on your behemoths? I'm a behemoth lover.
 

Pulkass

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Friends help friends build speakers.
I love working with vintage pro drivers. Your friend will love them too.
Can you elaborate on your behemoths? I'm a behemoth lover.
 

111db

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Thank you, Paolo, I like your behemoths very much! I wish I could hear them, but there's a little ocean in the way. :confused: We share some common preferences: active multiamping and cabinet damping, and of course lots of piston area. My avatar depicts one future version of my own behemoths, using old Wharfedale W70 cabinets with sand-filled backs as a nucleus, and constructed of birch salvaged from discarded mid 20th century furniture. Drivers for my behemoths-in-process are:
>TOA HLS38J-8 38cm woofers (Altec clones by Fostex or Coral).
>University ID40 alnico midrange compression drivers on hand-laid fiberglass radial horns similar to Community SRH or JBL 2345.
>superb TOA (Fostex/Coral) HT-371-8 alnico ring/slot compression tweeters similar to the famous JBL 2405. These are especially sought after in Europe, as I understand it.
> a 46cm B&C pro cinema subwoofer in a separate cabinet.
I have "breadboarded" these drivers together with 3.1-way active multiamping, and I am blown away by the sound quality, far beyond my initial expectations.
 

Pulkass

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Thank you, Paolo, I like your behemoths very much! I wish I could hear them, but there's a little ocean in the way. :confused: We share some common preferences: active multiamping and cabinet damping, and of course lots of piston area. My avatar depicts one future version of my own behemoths, using old Wharfedale W70 cabinets with sand-filled backs as a nucleus, and constructed of birch salvaged from discarded mid 20th century furniture. Drivers for my behemoths-in-process are:
>TOA HLS38J-8 38cm woofers (Altec clones by Fostex or Coral).
>University ID40 alnico midrange compression drivers on hand-laid fiberglass radial horns similar to Community SRH or JBL 2345.
>superb TOA (Fostex/Coral) HT-371-8 alnico ring/slot compression tweeters similar to the famous JBL 2405. These are especially sought after in Europe, as I understand it.
> a 46cm B&C pro cinema subwoofer in a separate cabinet.
I have "breadboarded" these drivers together with 3.1-way active multiamping, and I am blown away by the sound quality, far beyond my initial expectations.


Thanks. !!! Want to tell you that l moved ever since. I am in a detached villa, big treated Salon, l can really krank it now!
 

111db

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I'm also blessed with a big detached listening space of about 100 sq meters. Unfortunately it is unheated, so in winter I only listen on warm sunny afternoons. I have a theory that every recording is best listened to at a similar spl to that at which it was mastered. So for each recording I try to find that sweet spot level, and sometimes it turns out to be really loud. A great example is the thrilling live recording of the Phil Collins Big Band: Hot Night in Paris... wow!

P.S. I should clarify that I do have other good listening options in heated spaces: modified/updated Klipsch Fortes in my parlor and a Klipsch Heresy 2-based 5.1 home cinema system. But they can't compare to my behemoths for richness, clarity, dynamics, and imaging.
 
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Pulkass

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I'm also blessed with a big detached listening space of about 100 sq meters. Unfortunately it is unheated, so in winter I only listen on warm sunny afternoons. I have a theory that every recording is best listened to at a similar spl to that at which it was mastered. So for each recording I try to find that sweet spot level, and sometimes it turns out to be really loud. A great example is the thrilling live recording of the Phil Collins Big Band: Hot Night in Paris... wow!

P.S. I should clarify that I do have other good listening options in heated spaces: modified/updated Klipsch Fortes in my parlor and a Klipsch Heresy 2-based 5.1 home cinema system. But they can't compare to my behemoths for richness, clarity, dynamics, and imaging.


I ve just put that on, on your suggestion, forgot that LP--I saw Genesis 4 times live, so the Phil Collins connections, here it is very loud at 8 am--hahahahahaha heaven bliss !!!!
 
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