Thank you!!!! That formula is exactly what I've been looking for!!
Please let me know if you happen to have a thermal coefficient spec for Cu wound on Ti formers for the 42 and 54mm VCs used in Scanspeak and Peerless Neo models ie. NE149W, NE180W and NE315W. I'd like to know how hard these drivers can be pushed thermally in smaller sealed compartments.
A very reasonable worry.
For practical reasons, I use both calculated values and real world destructive testing, each way has its merit.
In theory Epoxy I use,CIBA type 2 part component, "chemical" not "UV" curing, stands up to 220-240C and I trust their datasheet, what actually means it does not melt, bubble or crack below that temperature.
BUT that´s not the main or only point: fact is this is an electrical motor use, voice coil or more precisely VC wire is pushing-pulling that cone both ways, under high acceleration, meaning under HIGH force/stress.
"Micromanaging" or going the OCD route (which in this Pofession is an asset, not a handicap), the actual force transmission chain is: copper wire > its own enamel > adhesive > former coating > former material (paper - kapton - nomex - aluminum - titanium) > again former coating (if any) > neck adhesive > cone material
Any link failure will mean chain failure.
I have actually seen on my bench:
* "peeling" enamel, leaving shiny copper behind (and a rats nest of loose copper wire)
* cracked/toasted/bubbling/burning/molten adhesive.
Old school Nitro adhesive bubbles at 90C, toasts above 100C and catches fire above 140-160C ... not surprising since it´s actually smokeless gunpowder (not kidding) dissolved in ethyl ether + acetone. 😱
* of more relevance here, "normal" Epoxy (never tested UV cured type, I should one of these days) loses A LOT of mechanical strength above 140C, again leading to forced VC disassembly.
* paper in formers, like the cellulose it is, dries up and becomes brittle above 90C, starts toasting above 100C, cracks above 120C and easily cathes fire above that.
Not common today, but I remember 60´s early 70´s Rock bands (think Jimi Hendrix or Johnny Winter) having cabinets actually catch fire on stage 😱 , attribute that to old Celestion and Jensen speakers still using Nitro coated and glued paper formers.
Many factors involved, so my final test is to subject my own speakers (testing is cheap, since I have tons of cones, voice coils, etc.) to "worse than EIA" conditions, meaning I apply 10W RMS for 1 hour, then rise power in 10W steps, 1 hour each, until failure.
1 hour is ample time for magnet structure reach a steady temperature.
EDIT:
Guess you´ll have to determine that by destructive testing, sorry.
Or trust factory datasheet.
In the case of smell, I will never forget the 'incense' left in my room when I smoked a very expensive set of drivers in an OB arrangement.
Quite informative. Nitrocellulose is no joke. It goes up like a flash.
I can't imagine using a paper former is that advantageous given all the newer materials available these days. I know the purists like paper formers for fullrange and guitar amp applications, but a good nomex former can be light and very durable while still having the same acoustic advantages.
Celestion had some great guutar cone drivers, but the power ratings were very low on the older models with paper formers, usually 25W on the orignal 12" Greenback models. Not much to work with but very loud at even miniscule input levels.
A small 25mm VC on aluminum former can deal with almost 50W in contrast and its usually the wire enamel that gives up first. You can push VCs on Alu formers pretty hard if the conditions are favorable, but power compression becomes an issue.
I can't imagine using a paper former is that advantageous given all the newer materials available these days. I know the purists like paper formers for fullrange and guitar amp applications, but a good nomex former can be light and very durable while still having the same acoustic advantages.
Celestion had some great guutar cone drivers, but the power ratings were very low on the older models with paper formers, usually 25W on the orignal 12" Greenback models. Not much to work with but very loud at even miniscule input levels.
A small 25mm VC on aluminum former can deal with almost 50W in contrast and its usually the wire enamel that gives up first. You can push VCs on Alu formers pretty hard if the conditions are favorable, but power compression becomes an issue.
I was testing an E-waste Polk 12" and did several hours of 25-45hz sweeps with just vent warm air and 12mm Xmax with a 200W AB amp. I dropped the
frequency to 10hz (youtube test tone) , Started smelling the varnish after 20 min. I did not feel safe to use this driver again , even as it still measured 3.2Re.
I just Bought a new KRK HO yellow kevlar woofer with 65mm voice coil. , thought it was good but heard strange noises at @ 30hz. 10hz , 12mm xmax , 40 hz good ... but
The amp is clipping at just 30hz. Woofer has a 34hz Fs , does that impedance "bump" make the amp clip ? Does the load the amp "sees' vary that wildly
to make it clip over at narrow band. I noticed that band became lower 25-30 hz as I broke this woofer in.
I'm going to make a much larger amp for it (Slewmaster EF3). I don't have a higher powered one at the moment.
OS
The impedance of a driver rises at Fs (free air resonance), an amplifier delivers less current into a higher impedance- if anything should be less prone to clipping diving at the impedance maxima.
The "youtube test tones" you are using may not be the same voltage at each frequency, and may not be pure sine waves.
The "youtube test tones" you are using may not be the same voltage at each frequency, and may not be pure sine waves.
Theres a strong argument in favor of Alu VC formers as they're likely the strongest and most thermally conductive material, but the transmission of heat into the spider and cone require a very strong and temp stable adhesive, which makes the moving mass heavy and the dampening properties of Alu create lower Qes and take away some low level resolution. Ti is a great material, but its expensive and not commonly used on larger VCs. Fiberglass and nomex are the best compromise for dampening, stiffness, reliability and cost. Nomex has the edge and it needs to be reinforced for stiffness. There is no perfect scenario, but al around Alu is by far the most therrnally durable former material.
Ti shim is magnitudes cheaper than aluminum and kapton from what I've found. I am winding my voice coils with .1mm grade 2 titanium and I am getting 200mm x500mm rolls of it for like $15 delivered to my door. One roll is exactly enough for me to cut down and wind 4 coils with. Morel uses it for their 5" voice coil subs. Stainless steel is another option too.