Compression Drivers - Why so heavy?

[BigT]

I am rather new at this horn thing. Today the compression driver I bought - a JBL 2445J - arrived. I have never seen one of these before. It's supposed to weigh 32lbs. but, my God, if feels as if it weighs 100! Why does it need to be heavier than a typical 18" woofer? There are lots of tweeters that can do the job that weigh so much less; why does a compresssion driver need to be so heavy? In other words, why does it need such a massive magnet? And how should this be supported? I certainly am not going to let it just hang there supported only by the bolts holding the horn.

Finally, how do you test a compression driver? Just attach an amplifier and let 'er rip? I asked in another post how hard was it to recone a woofer; how hard is it to replace a diaphram? It looks ok but there seems to be a few things around it that probably ought not to be there.

[Maron Horonzakz]

Why did you buy the driver??? if you didnt know what it was?

[BigT]

Why did you buy the driver??? if you didnt know what it was?

I didn't say I didn't know what they were - I only said that this was the first time I had seen one.

I love your solution, Scott! I hadn't thought of that. JBL has a mounting brackett but I didn't want to spend as much for the brackett as I did for the 2445J.

I wished I could afford one of those lightweight 2450H/J drivers but they always go for more (on Ebay) than I'm willing to spend, whenever they come up which is not often enough.

How good are those "baby butts" (I like the nickname)? Everyone seems to want the 2405 (077).

[SMKSoundPro]

Dear Big T,

In my 4733/4870 style of cabinet, Jbl used either a single 2404 baby butt cheek tweeter or two of the in the concert 4800 series.

I have a quite a few 2402 bullets and will cross them over at 7k using a 3105 crossover.

I just don't have enough 2404's and everybody seems to outbid me on them.

I built four of these cabinets out of baltic birch plywood, paying special attention to the joinery details. They are to go to work downstairs i my larger ballroom here at the nightclub for disc and live shows.

I am using these components because it what I have, and have found that JBL used them too at one point in time. Powered with Crown Macro-tech amps.

[SMKSoundPro]

Dear Big T,

I have many 2445 drivers and the question of support is right on the mark! It is heavy, no doubt about it, and there are newer lighter weight alternatives using n-dyn magnets. The 2445 is very old-school and I think a very worthy driver from 800hz - 16k. Then go into a couple of 2404 baby butt tweeters for the sizzle.

"And of course, never mount a large diaphramed compression driver without protection!"

I have a couple of pictures of the cabinets I am working on that show the plywood support of the driver.

Others will chime in and help to discuss your drivers.
Good Luck!
Scott.

[John]

Nice!!!

[Andyoz]

Why does it need to be heavier than a typical 18" woofer?

I think the question is why does the magnet driving a very light tweeter diaphragm have to be nearly as heavy as one used to drive a larger (and heavier) woofer cone.

Good question and one I've been afraid to ask myself.

[Mr. Widget]

I think the question is why does the magnet driving a very light tweeter diaphragm have to be nearly as heavy as one used to drive a larger (and heavier) woofer cone.

Good question and one I've been afraid to ask myself.

The short answer is that it takes gobs of magnetic energy for these things to perform as designed... read over this tech note, it explains it far better than I could.

http://www.audioheritage.org/vbullet...ead.php?t=4410


Widget

[Thom]

Not being a carpenter or cabinet maker I'm fond of unistrut and/or plumbers tape.

[1audiohack]

So, after reading the "vbullet" tech note shown by Widget, and doing the math on something changing direction close to 20,000 times a second, the acceleration forces are approaching 100 G ?!?

Scratch head,,, if that is even close to right,,,

No wonder they need so large a motor to play so loudly at very low distortion levels, and small wonder they fill a small room at less than one watt!

Even with way heavier moving mass, the acceleration forces for even subs could be less,,, scratch head,,,

[Hoerninger]

... and doing the math on something changing direction close to 20,000 times a second, the acceleration forces are approaching 100 G ?!?

Scratch head,,, if that is even close to right,,,

Ya, a car which reaches 80 mph in 6 seconds instead of 12 seconds needs a much stronger engine. Time has been halfed in this example, but when you consider 20 kHz against 200 Hz the time for one period is only one-hundredth.
May be this comparison gives a picture.
___________
Peter

[clubman]

Finally, how do you test a compression driver? Just attach an amplifier and let 'er rip?

Well I wouldn't recomend that! First I would do a test with a OHM meter to see if you get a open, short, or value of the DC resistance (ohms)

If you get a OHM reading then there is a chance everything is fine. Allthough just cause it ohms out doesn't mean that it won't act up at a given freq. A speaker repair shop could do a freq. sweep to test it more indepth than just a ohm meter.

You can just hook it up to a amplifier as long as the program material you are runing through the amp is within the range of the driver, 800Hz and higher. Youl would not want to run full range into that driver unless you want to get new diaphrams anytime soon.

These drivers sound amazing, and will last for years. I am runing 8 of them matched up with jbl 2226G's in SR cabs in a night club powered with macrotechs.....never had to replace a diaphram in 3 years

Best luck to you with your drivers!

[Steve Schell]

This is a great thread, BigT. First of all, I'm not surprised that that a 2445J prompted your initial question, as they seem almost absurdly huge and heavy. Fortunately they have been superceded by the modern neodymium magnet drivers which have only a fraction of the size and bulk of the ceramic magnet monsters.

In general, a compression driver needs a really big motor to cope with its task. The diaphragm is feather light, but the air load on it is great as the exit path on the surface of the phasing plug is only about 10% of the size of the diaphragm. Imagine pushing and pulling the diaphragm at super high speed against such a constriction!

Even the strongest compression driver motors cry uncle by 3 or 3.5 kHz., and the power response falls by 6dB per octave above that point. Up to that point they have an efficiency (ratio of electrical input to acoustical output) of 30% or so, really terrific compared to the dismal 1 or 2% efficiency of most direct radiator speakers. This is the reason for their existence, as they produce as much sound as a number of direct radiators, and at less distortion, cost and bulk.

To test a compression driver you need to use a crossover network to keep lower-than-designed frequencies out of the driver. You also need to install the driver on an appropriate horn, as the loading provided prevents overexcursion of the diaphragm.