JBL E-145 excursion capabilities ?

[Dr.db]

Hi,

I`d like to know the real Xmax values of the E-145.
So far I have found in some old datasheeds 7mm Xmax, but I think this cannot be correct.
In my eyes this woofer doesn´t look capable of such extensive cone excursion.... Maybe 3-4mm one way I´d guess.

Or is 7mm Xmax meant to say in- and out together? This would make sense...


Regards,
Olaf

[HCSGuy]

I'll try to photograph a VC and top plate measurement for you tonight as illustration, but I am sure the 7mm XMax is peak to peak.

[HCSGuy]

Here's some pictures of handheld measurements - approx 15.1mm for the gap height, and 7.6mm for the VC height. That makes the difference 7.6mm, which would be your P-Pmax, or 3.8mm excursion either way from center.

[Dr.db]

Thank you so much for the effort you have put into your answer!!
You´ve been a big help allready!

Just being curious; if the JBL 2235h is labeled with 8,5mm xmax is this peak to peak as well or has JBL used different measurement methods?

[HCSGuy]

The E145 is underhung; it is designed so that the entire voicecoil is always in the magnetic gap, so that the maximum amount of input power is transferred to the cone; hence its legendary status for transient response, but unsuitability as a high output subwoofer - short throw. The 2235H is overhung, which to say the voice coil is longer than the magnetic gap. In this case, the voicecoil winding is 19mm long and the magnetic gap is only 7mm. This lowers efficiency, as only 37% of the voice coil is ever directly in the magnetic gap. However, this lets the magnet control the woofer over a longer excursion - I would measure XMax as the 19mm winding height minus the 7mm gap height, giving you 12mm P-P or 6mm one way before the voicecoil starts to leave the gap and go non-linear.

However, I'm guessing JBL is using a calculated Xmax instead of a measured one, and is allowing the VC to start going non-linear and have part of the gap void of VC windings. If the cone did move 8.5mm, there would be 2.5mm of gap with no voicecoil in it.

Btw, my understanding is that if you look at Theile-small specs, Xmax is one way, but manufacturers can state it in marketing however they want, including peak to peak, or even be a little misleading by stating Xmech, which is how far things can go before the suspension stops the cone, or the former hits the back plate, regardless of where the VC is. This is more common in Car Audio than in the marketing of individual drivers for mature adults. JBL's data sheet for the 2235 shows "Maximum Excursion Before Damage: 22mm", which I would read as XMech.

[Dr.db]

Thanks a lot for your time!

I think this is the first time I really understood the difference between overhung and underhung voicecoils.

Your calculations seem logical to me. I´m just a little confused of the data-sheets JBL has puplished.
Have a look at my attachement; in the same list of thiele-small-parameters the E-145 has 7mm xmax and the 2235h 8,5mm xmax. One would assume that they used the same measurement e.g. calculation as this is puplished in the same document....

[1audiohack]

As has been said there are different "opinions" of what determines X max. One of which is the physical measurement method as was shown by HCSGuy. Another is setting a nominal percent distortion figure like 10%.

One of the variables here is the stray magnetic field or the leakage jumping across the top and bottom of the gap. In many motor assemblies this leakage is significant and often asymetric in shape or strength. This leakage extends the magnetic gap "depth or height" beyond the phslysical thickness of the top plate and pole peice. This can give the coil a couple of mm or more of apparent operation "in the gap".

In short, there is no absolute. I prefer a percent distortion at a given excursion and then of course the Xmechanical measurement limit so you know where it breaks.

My memories from when I studied all this are getting long in the tooth so please accept the lack of precise descriptors.

Barry.

[HCSGuy]

I thought about the stray magnetic field, but I assumed that using it would raise your distortion spec dramatically - using my measurements, if an E145 did go through 7mm of excursion, over 40% of its voice coil would be out of the gap. But as you said, if the allowed distortion is some number like 10%, this makes sense.

The elephant in the room remains - I should have been a hand model

[Dr.db]

Thanks for your help, both of you!

[gtimbers]

The The E-145 has the same magnet assembly and voice coil as the LE-15. The gap is 0.600" long and the coil is .280 long. We didn't pay much attention to x-max back then and there wasn't a standardized method of stating it. Usually transducer would mechanically bottom out (spider to top plate) before a realistic excursion had been reached. This was due to our having to reuse old frames rather than tool new ones. The frames were designed in the 30 watt tube days and were sufficient for the times. As amp power went through the roof, the frames didn't keep up. It was mid 80's or so when the frames got the necessary travel capability.

Back to the E-145. The fringe flux on that motor allowed the coil to come out of the gap by maybe 20% of its 0.280 length which is 1 mm plus a little. Including the coil gap numbers I get about 5.5mm peak. 7 mm is way high (or low if it was p-p). That particular surround was very stiff and was limiting by the time the coil tries to get out of the gap.

it was designed for bass guitar use, quite often in an open back enclosure. This is why it is stiff and able to protect itself. As I recall, it was quite efficient, running in the mid to high 90's for a 2.83 v input.

[Dr.db]

Thanks a lot for that very interesting backround!
It is great to have you participating in this conversation, really appreciate that.

Back to the E-145. The fringe flux on that motor allowed the coil to come out of the gap by maybe 20% of its 0.280 length which is 1 mm plus a little. Including the coil gap numbers I get about 5.5mm peak. 7 mm is way high (or low if it was p-p). That particular surround was very stiff and was limiting by the time the coil tries to get out of the gap.

5.5mm peak would be one way, so 11mm peak to peak?
Did I get you right ?

[gtimbers]

Yep.