Verdinut
Member
Well, it gets pretty hard to abuse at home pro Altec drivers when they have a sensitivity at 4 feet ranging from 99 dB to 108 dB for a 2.83 volt input. A pair of A7 VOTT speaker drivers in an appropriate cabinet can take a peak power of 700 watts! Just use an overall sensitivity of 100 dB (103 dB for a pair) and you would get peaks of over 125 dB at an 8 foot distance which are ear damaging levels.Actually, this is not correct. I even described how it happens (twice), perhaps you didn't read it. I saw other posters mention here too. Let me explain it again but maybe you won't read it anyway.
If a voice coil is overheated, two or more adjacent windings on the voice coil can short. These inter-winding shorts, the shorted windings no longer plays a role and the rest of the VC remains conductive. The reduction in resistance depends on the quantity of coils that got fused to each other. If 7% of the VC windings are shorted, the DCR reduces by ~7%, and the T/S parameters for the driver dramatically change (all of those shorted windings no longer play a role in the motor, inductance drops, and the defect introduces a non-linearity into the motor because the magnetic field in the vicinity of the short is quite non-uniform. Lots of stuff moves in this case, and the driver ends up testing much different than a good driver for the electrical parameters associated with the motor. Different drivers have different probability of this type of fail, some always blew to an open circuit, some have a large probability of the VC developing a partial short due to abuse. I have seen a few drivers with partial shorts due to misaligned VC rubbing the magnet.
And in case you don't want to read my text, here is a tutorial on voice coils that might help you visualize what is going on, and it touches on the problem of shorted turns, although more from the perspective of the type of bobbin the coil is wound on.
Voice Coils: A Tutorial
Mike Klasco (Menlo Scientific) provides an overview on how to specify or manufacture voice coils. In this reference article, Mike Klasco recaps on voice coil basics, bobbin materials and sizes, collars, winding techniques, configurations, tips and tricks and wire types and considerations - a...audioxpress.com
Yes I have. Perhaps you noticed I tested 22 drivers lying around my shop in this thread! It's not an accident! Back in the day, I installed thousands of car stereos, most built from separates. I worked at home hifi stores too, just slightly less blown tweeters! I can typically tell just from the sound if a driver has a VC rub or is partially shorted, so could all my co-workers. One thing for sure, if I got handed a pair of tweeter from the parts counter that were mismatched by 5%, no way I would begin the install... We tested every driver on an Ohm meter before install, no way you want to find out at the end of the build that you got a bad driver. So, yeah, experience. Literally thousands of drivers. YOu just don't see large DCR on a regular basis unless you measured wrong.
Perhaps you didn't abuse your tweeters the way some of my customers did! Interesting about your Altec Lansing drivers, I really don't know what to say about such a large mismatch without the data. Voice Coils are a length of wire wrapped into a precise number of turns. It is extremely unusual to see variation much larger than 1%, wire resistivity between samples, and hard to imagine the factory accidentally wound 25% more turns on the bobbin. It is very challenging to get the number of turns or the resistivity of the wire that different. In reality, VC matching is really quite precise, and not subject to large variation on a speaker that is working properly. That's why voice coil DCR variation is typically closer to 1% as I showed, not 7% as the OP measured, and certainly not 25% like you saw.
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