Jbl Sub1500 15" Subwoofer 4 Ohm

[Mr. Widget]

Originally posted by boputnam
Hey, John...

You can put a bend in the duct, if need be. You may know that JBL often did this, for space reasons, and also perhaps to keep the duct-end a minimum of 3-in from any cabinet wall. You can use 90-degree elbows...

....Or, using your available 3x13.5 in rectagular opening, you could build a duct of 9.566 in length for the 373.064 cu.in.

NB: All duct lengths include the baffle thickness.

If you use an elbow, I would certainly make it longer than you think you need and follow Giskard's advice to trim and measure.

Bo,

Helmholtz resonators aren't based simply on volume. According to BBP6 the 3"X13.5" port would need a duct that was over 44" long. To over simplify it, as the mouth area increases the duct length must increase to maintain the same frequency.

Widget

[boputnam]

Hey, Widget...

Yea, I caught my error (thanks Claus! ) - that's why I deleted that portion of the post.

If John was to use the 5.25 opening, it looks like the duct would have to be increased to at least 21 in length - right?

I dunno - I need to check all this when I get home...

[John Y.]

"Helmholtz resonators aren't based simply on volume. According to BBP6 the 3"X13.5" port would need a duct that was over 44" long. To over simplify it, as the mouth area increases the duct length must increase to maintain the same frequency.- - Widget"

How, then, can the JBL 4641 or 4645C cabinets be tuned to 25 Hz (as they claim) with an (approx) 8" duct when the cabinet is less than 18" deep, unless they have a bend in the duct (which I doubt)? Yhese are 8 cu ft cabinets. Does cabinet size make that much difference?

Mr. Widget - I gather that measuring the resonant peaks is the best way of determining the system resonance, that is, resonance occurs half way between the peaks. That right? I like your idea of bottom duct placement being a good compromise, Now, if only I could find some JBL stilleto aluminum legs (circa 50's), I could have a real retro (looking) rig.

[jcdahl]

I wonder what two of these things would sound like replacing LE15s in a Paragon?

Wow I couldn't wait to try them so I just ordered two for myself. They must still have some left.

[Chas]

Maybe a stupid question, but I gotta ask: When you guys are talking about cabinet volume, do you mean net of bracing and driver displacement?

If so, what are you using for the driver displacement?

Charles

[Mr. Widget]

Originally posted by John Y.


How, then, can the JBL 4641 or 4645C cabinets be tuned to 25 Hz (as they claim) with an (approx) 8" duct when the cabinet is less than 18" deep, unless they have a bend in the duct (which I doubt)? Yhese are 8 cu ft cabinets. Does cabinet size make that much difference?

Mr. Widget - I gather that measuring the resonant peaks is the best way of determining the system resonance, that is, resonance occurs half way between the peaks. That right? I like your idea of bottom duct placement being a good compromise, Now, if only I could find some JBL stilleto aluminum legs (circa 50's), I could have a real retro (looking) rig.

For an 8 cu ft box with an 8" duct, BBPro recommends a 19" port length for a 25Hz tuning. I would guess JBL is using an elbow. I have no practical knowledge of either of these systems so I can only speculate. (An 8" duct 13" long would achieve a tuning around 28Hz with typical fill. They may be gaining virtual volume and lowering the tuning with heavy fill.)

Yes, measure the lowest impedance between the two peaks. The absolute minimum should be at the tuning frequency.

Widget

PS. Chas, yes you subtract bracing, port volume, woofer and other driver volume, and make allowance for amount of and type of fill. (The fill part is kind of guess work, but BBPro gives 4 levels of fill and their "Typical" seems to match my style of fill.) As for woofer volume, I have a chart somewhere but can't find it at the moment.

[Niklas Nord]

My K2 are using elbow-duct Så there can´t be anything
wrong with them...

[4313B]

Originally posted by Chas
Maybe a stupid question, but I gotta ask: When you guys are talking about cabinet volume, do you mean net of bracing and driver displacement?

If so, what are you using for the driver displacement?

Charles

Actually it is a very good question. It's a good idea to determine what is being discussed, gross physical volume or net effective volume. The system is "looking" for a net effective volume and it is the builder's responsibility to take the gross physical volume, internal volume displacements (ports/ducts, subenclosures, bracing, drivers, etc.) and system losses (leaks, fiberfill, flexure, etc.) and create the required net effective volume. If you use fiberglass it will tend to negate the volume displacement of bracing, small subenclosures such as the LE5x/104x subenclosure, smaller port/ducts, smaller drivers, networks, etc. Volume displacements and system losses tend to cancel each other. In other words system losses increase effective volume while volume displacements decrease it. The better JBL enclosures generally measure total system loss at "10" or above. Software packages usually default to "7" which has always seemed to be considered typical. "3" is consider junk, worthless as a vented system.

The displacement rating for the 1500 SUB is 0.25 cubic feet. It has extremely low loss due to it's surround, cone, and dust cap construction.

"Yes, measure the lowest impedance between the two peaks. The absolute minimum should be at the tuning frequency."

The most accurate method is to measure the vented system and note the frequencies at which the two impedance peaks occur. For instance - a specific JBL L-96 has two peaks at 15.0 Hz and 54.0 Hz. Then seal the port and measure the single impedance peak and note it's frequency. In this instance it was 47.0 Hz. Fb will be the square root of (15.0^2 + 54.0^2 - 47.0^2) = 30.5 Hz.

In this instance the trough between the two impedance peaks occured at 30.0 Hz, which can definitely be considered "close enough". I believe D.B. Keele determined that Fb can vary by up to 10% without becoming a "problem".

[Chas]

Thanks guys. I notice mention of BBpro, is this software that allows plugging in T/S parameters and trying different ideas to achieve lowest F3? Sounds like fun, about twenty years ago I had to do it the hard way from Thiele and Small AES reprints. I shudder at the thought of doing that again with my sub 1500's.

If it is software, where can I source it? Is it reasonably priced?
TIA,
Charles.

[4313B]

"about twenty years ago I had to do it the hard way from Thiele and Small AES reprints. I shudder at the thought of doing that again with my sub 1500's."

I simply refuse to do so


You can get BassBox 6 Pro or BassBox Lite from Parts Express.

You can try Unibox from here:

http://home20.inet.tele.dk/kou/ubmodel.html

and WinISD from here:

http://www.linearteam.dk/

[johnaec]

How do these newer enclosure programs compare with JBL's SpeakerShop 1.0 from the mid 90s? Harris Technologies produced JBL's version, but it appears to no longer be available. I picked up a version on the web some time ago, and it makes dealing with the T/S parameters really easy, asuming the math in the program is correct. I'm just wondering if anything new involving T/S parameters has been incorporated into the newer versions that might be missing from the JBL version?

John

[4313B]

BassBox 6 Pro is from Harris Technologies. You can look on their site for revision changes. The errors in some of the published TS parameters of various JBL drivers becomes readily apparent in BB6P. (Errors originating at JBL, not Harris Technologies)

[James Nobel]

Hello JBLers,

This is my first post to the forum.

I just ordered two Sub 1500s from Parts Express a few days ago. The girl who took my order was named Amy. She was very nice. I hope she didn't notice how nervous I was. You see, I'll have to confess straightaway that I'm very frightened about confronting the raw power of those beasts. Will I even have the courage to open the shipping boxes when they arrive? My God....thoses things weigh as much as a child. I am not so sure I'm man enough to control them....Lord.....why did I have to order two of the devilish things? Last night I had a nightmare that when I opened the Parts Express boxes.....out leapt two raging pitbulls that devoured my wife as they barked and woofed and woofed at something like 115 db. I woke up screaming.

Ok, Yes, I'll admit it.....here and now in this public place....I do.....yes, I do crave the brutal and awesome savagery of massive, thrusting, Kevlar impregnated pulp laying waste to any and all who would enter my domain without protection.

So, I guess that's the truth of it, I am both afraid AND hungry for the spoils of sonic violence.

Please...please....please......don't judge me. My judgment will come later.

[4313B]

"This is my first post to the forum."

Neato

[mikebake]

Originally posted by James Nobel

Ok, Yes, I'll admit it.....here and now in this public place....I do.....yes, I do crave the brutal and awesome savagery of massive, thrusting, Kevlar impregnated pulp laying waste to any and all who would enter my domain without protection.

Oh no, not another classical music lover...............................