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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Walter W Wright on April 26, 2011, 12:42:17 AM
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you guys might appreciate this one;
i got into it about the old "underpowering/square wave" argument over on the gear page, with a couple of seemingly expert posters who insisted that square waves in and of themselves killed drivers.
i tried my best to source my arguments with pertinent quotes from the august masters on this very forum (who the hell am i, after all?), but it finally got brought home by one jay mitchell, a speaker designer who Knows What He's Talking About.
tons o' fun.
http://www.thegearpage.net/board/showthread.php?t=884601
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I read thru this thread, and the associated threads from this forum that were referenced there, and some of the documents from speaker manufacturers. As a side topic, I'm curious what are the failure modes of drivers? The gear page thread was about subwoofers. As far as I could tell there was only one failure mode here- too much current thru the voice coil resulting in the wires melting and then either shorting out or opening. Are there more failure modes, including different ones for higher frequency drivers? For example, could a short transient cause a failure from excessive excursion that would be too short to damage the coil? In other words in a post-mortem the coil would look fine but there would be mechanical damage? I would think that perhaps the design of the speaker could account for and prevent mechanical excursion damage?
Update- this info was easy to find elsewhere! No need to post responses.
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I read thru this thread, and the associated threads from this forum that were referenced there, and some of the documents from speaker manufacturers. As a side topic, I'm curious what are the failure modes of drivers? The gear page thread was about subwoofers. As far as I could tell there was only one failure mode here- too much current thru the voice coil resulting in the wires melting and then either shorting out or opening. Are there more failure modes, including different ones for higher frequency drivers? For example, could a short transient cause a failure from excessive excursion that would be too short to damage the coil? In other words in a post-mortem the coil would look fine but there would be mechanical damage? I would think that perhaps the design of the speaker could account for and prevent mechanical excursion damage?
Update- this info was easy to find elsewhere! No need to post responses.
Yes two dominant failure modes, over heat and over excursion.
I have heard LF drivers bottom out loudly and keep working, but there is a fundamental conflict in driver design between keeping the moving mass light so it can be accelerated easily and simultaneously robust to survive repeated operator abuse.
Smart application of DSP can reliably predict over-excursion (frequency "and" level "and" box tuning) and prevent it from happening. Likewise smart DSP can model VC temperature rise, or better yet measure directly the temperature or markers for that temperature rise and protect against that failure vector. This is not trivial for end users to apply generic solutions to specific loudspeakers but some power amps with built in powerful DSP are beginning to offer different flavors of protection.
In my judgement it is possible to design a powered loudspeaker with DSP built in, and set up by the loudspeaker design engineer(s) such that it would be very hard to kill by simple operator abuse. Of course the operators are constantly evolving.
JR
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JR, I'm not certain "evolving" is the right term.....
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JR, I'm not certain "evolving" is the right term.....
we are devo.... 8)
JR
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Another quick question. I'm about midway through the thread, so forgive me if my questions is already covered.
Most people in the thread blame the glue that binds the voice coil windings to the voice coil as the culprit when a VC fails. I was under the impression that when a VC overheated, the metal wire windings melted and opened the circuit. Is this the case, or is it as the TGP members suggest, the glue burns allowing the windings to break free and open.
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Another quick question. I'm about midway through the thread, so forgive me if my questions is already covered.
Most people in the thread blame the glue that binds the voice coil windings to the voice coil as the culprit when a VC fails. I was under the impression that when a VC overheated, the metal wire windings melted and opened the circuit. Is this the case, or is it as the TGP members suggest, the glue burns allowing the windings to break free and open.
Both can happen. Sometimes the glue will heat up and produce bubbles. If the copper survives the heat/incident and the circuit is still working these bubbles can harden into shape when cooling off and they might be touching the magnet assembly, creating a scratching noise when the cone moves which shows up as a horrible, very noticable noise whenever the speaker is playing.
In this case the driver needs replacement, even though the copper wire survived.
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the insulating coating on the vc wire can melt causing a short
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I am not a speaker guy, but one of the improvements made in recent decades is improved high temperature adhesives and insulation, so VC can get hotter before actually releasing smoke, but that alone doesn't buy a linear improvement in power output, because wire has a thermal coefficient that raises the resistance of hot wire to more ohms than when it was cold. This higher resistance in the voice cold means less output for a given drive voltage after it heats up (AKA power compression).
JR
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Ok, that all makes sense.
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you guys might appreciate this one;
tons o' fun.
http://www.thegearpage.net/board/showthread.php?t=884601
Having gone through the whole "square waves are DC" thought process myself, I can appreciate the discussion there. Edgewound is local to me; Maybe I should go knock some sense into him :)