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View Full Version : Re-foamed Fs exactly as the originals??!!



Flodstroem
03-11-2005, 12:34 PM
Hello again

Have just finished refoaming a pair of LE15A (grey model)
Everything went stright on and there was no problem.

Its amazing how easy the cone and voicecoil align in the "old center position"
when gluing the outer lip of the foam. There is no need for to cut out the center dome/dustcap.

Equipment that will make it easy to measure fs of speakers: audio signal generator, frequency counter, 2ch oscilloscope, 1k ohm resistor. :)

This is how it looks when measuring fs :blink:

Flodstroem
03-11-2005, 12:49 PM
When measure the fs at the LE15 refoamed I got the value 28 Hz for both speakers. JBL specs point at 20Hz.

Then I got the idea: how about my old LE15A (blue model). The speakers was re-coned 10 years ago at a JBL authorized factory in Sweden. What is fs at these?

This is how they looks when measure fs

Flodstroem
03-11-2005, 01:16 PM
When measure fs at the original LE15A (C16R2215) the results was this
#1=28.2 Hz
#2=26.4 Hz

New re-foamed grey LE15A
#1=28.0Hz
#2=27.8Hz

Interesting. and absolutely not what I had expected also taking to account the speakers are not new and have played for a long time.

When looking at the new foam surround roll ther is a different to. The old foam surround roll at the C16R2215 is wider than the narrower "Rick Cobb" model. The differance is about 3 mm and it can be easely seen with the bare eys.
Original JBL:

Here is the original JBL

Earl K
03-11-2005, 01:16 PM
Hi Flodstroem

I'm glad you are measuring Fs . :D

- You should suspend the woofer from something very strong while hanging it in the air ( say 4 to 6 ft / 1.2m to 1.83 m off the floor and away from walls & tables ). This gives a more accurate reading .

- If you don't have something strong to bear all the weight - then ignore this advice. It's better to be safe than sorry.

:)

Flodstroem
03-11-2005, 01:18 PM
and this is "Rick Cobb"´s: (comments will be appreciated). Thanks

Flodstroem
03-11-2005, 01:21 PM
and this is "Rick Cobb"´s: (comments will be appreciated). Thanks

Flodstroem
03-11-2005, 01:40 PM
Hello Earl K

Thanks for comments.
Reading eg. measure the fs at the speaker was done with a wery weak (small) signal (via the 1.0k Ohm resistor)

When moving the transducer out from the table I could registrate that fs was change but only marginaly and only do to the holding the speaker (in the frames) by my hand, not becouse of moving the speaker. The differance was only 0.35Hz with both hands on or off. This was the only changes when moving the speaker from table to free air.

regards

GordonW
03-11-2005, 01:59 PM
I've seen changes in Fs, because of the driver SITTING on the pole vent in the back of the speaker. I usually elevate the speaker (using something like three small identical pieces of wood, three electrical tape rolls, anything really, where all three are the same size), so that the vent has an un-obstructed path.

That said, I'm not surprised that the Fs on a speaker that hasn't been in service for a while, is higher than normal. I bet if you played the newly-refoamed and the original for a while, they BOTH would go down in Fs...

Regards,
Gordon.

Earl K
03-11-2005, 02:40 PM
and this is "Rick Cobb"´s: (comments will be appreciated). Thanks

- If it was me,,, I wouldn't be worrying much about the difference in foam widths. I think it's great that there are vendors like Rick Cobb who offer what he does and delivers a product that is really "quite" close ( & for peanuts I may add ) .
- Also, I think an Fs figure of mid 20's is better suited to the weight of that Le15 cone. :D

:)

Flodstroem
03-11-2005, 05:30 PM
Thank you for comments
It seems to be a very small if not any differance between the foam surround from R.Cobb and the original JBL foam surround at their speakers. This is very good.

What surpriced me most was the "big" differance between JBL T/S listed fs and what was found on the original speakers (eg the two LE15A Blue).

It should be interesting to hear if anybody knows where JBL got this 20 Hz from.

Yes Gordon, there is a differance measuring fs if the speaker is "sitting" on the magnets pole vent. This was the case with the LE10H-1 that was reacently re-foamed. I had to do as you described and also used pieces of wood. The differanse could be as much as 0.9 Hz at an LE10H-1

Its a differency with the LE15. This model does not have any pole vent and it doesn t matter if it sitts on the magnet when measuring.

When braking them in for a couple of hours in time I registrated that fs was lowered for about 1-1.2 Hz.

This rises a delicate question: what data should we use when building a speaker cabinet with a driver that does not have the same specs as in the JBL´s T/S list. If Fs is different. then is all T/S variables different from originals (Qts, Qms, Qes etc...). Hmmmm......... or should we not bother?

Thank´s

GordonW
03-11-2005, 11:10 PM
Yes, the other T/S parameters will be different... but if the differences are all caused by ONLY the different stiffness (stiffer spider and/or surround), then they should pretty much "cancel out" as far as box volume and tuning is concerned... and give pretty much the SAME response.

What happens, is that compliance is really a "second order" affector of response... the MASS of the cone, the effective CONE AREA, and the MAGNETIC STRENGTH (Bl product), are the PRIMARY affectors of overall efficiency and bass extension. The compliance difference, if severe, can cause a different "shape" in the bass response (ripple in the response near rolloff), but we're talking BIG differences here- like the difference between, say, 20 Hz and 40 Hz, all else equal.

Especially in this case, where I expect, once the driver has been playing a while, the resonance will drop significantly... the sound of the driver in its current state, probably won't be perceptably different in a factory-spec-tuning box, than a driver that's dead-on in specs will. It's still the same cone, voice coil and magnet...

Regards,
Gordon.

Flodstroem
03-12-2005, 07:26 AM
Thank´s Gordon

Ok. I can understand that a slightly higher fs wouldnt make any big differance to T/S parameters. After all we are talking about "only" 4-8 Hz higher fs (LE10/LE15) than JBL´s fs specs. A decent fs, and as you say, probably will drop to fs specs after some time of use.

BTW, a reacently re-foamed 136A ("C8R2231A"): fs was measured to be 20.7 Hz. (with a zero braking in). Aiso a very decent fs to start at.

Thanks all for your answers :)

regards :D

Dougie
03-12-2005, 01:25 PM
Hi Flodstroem

I'm glad you are measuring Fs . :D

- You should suspend the woofer from something very strong while hanging it in the air ( say 4 to 6 ft / 1.2m to 1.83 m off the floor and away from walls & tables ). This gives a more accurate reading .

- If you don't have something strong to bear all the weight - then ignore this advice. It's better to be safe than sorry.

:)

I'm using the LMS system for my loudspeaker measurements and when I do an impedance sweep, I just clamp the driver down on a WorkMate made by Black and Decker. I find suspending the driver in mid air makes NO difference at all to the Z peak at resonance ( Fs ) and its difficult to get the driver to stay still :) Personally, I don't worry to much if Fs varies 2 or 3 Hz either way because to many factors affect your measurements, ie: temperature and the humid x, cable impedances, resistance when using aligator clips for test connections etc etc.

With regard to cable impedances, LMS gives you the option of using a 2 wire or a 4 wire impedance measurement. With the 2 wire connection, you first have to short out the measurment cables and do 1 sweep to measure the cable Z, then connect the driver, do another sweep of the actual driver Z, then the program will do a routine that will subtract the cable Z from the drivers Z. Makes somewhat of a difference in the Fs result. This tecnique is a bit of a pain ( takes longer ) so the 4 wire measurement is quicker. It allows you to use both the line input and line out ( oscillator out ) at the same time and only involves doing one sweep and LMS automatically deducts the cable impedance......needless to say, I use the 4 wire system :)

Doug

Earl K
03-12-2005, 01:41 PM
I just clamp the driver down on a WorkMate made by Black and Decker.

Hey, I have one of those around here..I'll have to start using it. :D

The suspension thing is definately a PITA .

:)

Dougie
03-12-2005, 02:06 PM
Hey, I have one of those around here..I'll have to start using it. :D

The suspension thing is definately a PITA .

:)

For those of you less informed than Earlk and think that a workmate is a co-worker;) here is a link:

http://www.amazon.com/exec/obidos/tg/detail/-/B0000302VO/103-0431064-7543006?v=glance

Doug

Flodstroem
03-13-2005, 06:18 AM
Hello
Thanks Dougie for your comment on measuring Fs

Personly, I measur the Fs at a speaker with the help of my oscilloscope (X-Y setting).

This is the most easy way to do it (I think if you have the equipments). You only have to look at the lissajous patterns from which phase can be measured (Fs when voice coil voltage and current is "in phase" = a stright line lissajous pattern).

This measuring methode is described in Joseph D´Appolito´s book: TESTING LOUDSPEAKERS pp16-26, from Audio Amateur Press (1998).

The drawback to this methode is that you need the equipments: audio generator (scaled in minimum 0.1 Hz), a counter and a 2 ch oscilloscope including a 1 k ohms 4W resistor.

This methode is very accurate and could be demonstrated to just move one of your hands over or at the side of the speaker: the lissajous at the scope changes the pattern immediatly. :D

Regards