Vader

Blown VC, sub bottoming questions....

Well, I just dodged a bullet (again) after doing something extraordinarily stupid with my subs: when determining my room-curve, I ran pure sine tones at 80Hz, 32Hz and 23Hz. Even though the volumes were low (70dB – 82dB at 3m using duals), I guess even a mere 45 sec tone can fry the VC (based on my research). Thankfully, my woofers are fine (I hope). Out of curiosity, I have since been reading up on woofer construction, and getting a better idea just what can happen. What I could not find is a good description of what a blown woofer would sound like, assuming it could still make sound at all – only that it would be "distorted". Distorted how? I know that the VC would be rubbing in the gap, but would this manifest itself as a buzzing, or scratching? Something worse?

That was my second act of naïve stupidity; the first was pushing one of the subs too hard and bottoming it (I think). Based on a conversation with Ron (SVS), I have learned that bottoming a sub can rub off the coating on the voice coil, also resulting in rubbing and possibly shorting it out all together. Is this the same coating that is at risk of overheating from sine tones?

24 hours ago, how a driver worked was nothing more than a black box to me, and largely still is. But now I am wanting to understand more and more (that's the inquisitive kid in me, I guess)…

Peace... Derek

One sub to rumble them all. One sub to shake them. One sub to humble them all. And in the darkness break them.

Louvre attendant: Sacre bleu! ze frame on ze Mona Lisa broke and ze only one left iz too small. Andre, bring me ze scissors!

JohnRice

Re: Blown VC, sub bottoming questions....

Derek, one way you can check is to, very carefully, push on the driver cone (not the dust cap, not the surround), making sure to put equal pressure around the cone, so it is not pushed out of alignment. Just push it a few millimeters, let it return to it normal position, and push it again. Repeat as needed. Listen closely. If you hear any scraping, it is blown. Also, if you don't push it back in a perfectly straight line, it will tend to scrape. This won't typically cause damage, it just won't indicate it is blown.

BTW, it can be blown, and still not scrape, but if it scrapes, it is blown.

They flutter behind you, your possible pasts.
Some bright-eyed and crazy, some frightened and lost.

Vader

Re: Blown VC, sub bottoming questions....

Thanks, John!

I'm pretty sure I got lucky, as the bass still sounds awesome (perfectly smooth and tight, with no "distortion" that I can hear as such). Ed Mullen has been great in helping me to ascertain that, although I failed to do adequate homework, my woofers are fine. One of the telltale signs of a woofer being blown is the smell of anything melting or burning, which was not present (that I could tell). Everything I have come across tells me that, if I were lucky enough to have blown the woofer such that it could still make sound, that sound would be pretty bad - I was just wondering what "bad" is. My understanding is that the damage, while not instantaneous, is irreversible in the sense that the VC is either warped, or the adhesive is melted. I would assume that either of these would happen a lot faster at high volumes (> 90dB), and/or for extended periods of time. In my case, 70-80dB (actually more like 64-74dB/each since I am running duals) for less than a minute did not appear to reach that threshhold. I am just reading all that I can about what can happen (I used to think that the only way to blow a speaker was to overdrive it with a powerful amp, and that a speaker rated at 150W could take anything an 80W amplifier could dish out, and just keep on smiling)... Looking at driver diagrams, and "putting names to faces" of terms such as "the gap" and the "voice coil former" is just wetting my appitite further...

John, you've got my curiosity up: could you describe a scenario whereas a driver is blown but does not scrape?

Peace... Derek

One sub to rumble them all. One sub to shake them. One sub to humble them all. And in the darkness break them.

Louvre attendant: Sacre bleu! ze frame on ze Mona Lisa broke and ze only one left iz too small. Andre, bring me ze scissors!

Robert_J

Re: Blown VC, sub bottoming questions....

Technically, it would be when the wire between the terminal and the voice coil melts. You won't get any sound. Since this is braided wire, it is very difficult to impossile to re-solder. It normally requires rebuilding (aka recone) the speaker. In the picture below, the wire is integrated into the spider. This keeps it from hitting the bottom of the cone during high excursion (aka tinsel slap).


In the voice coil picture below, if you had started to smell something, the windings would not be a nice orange color. The shellac coating would have turned a dark brown to black. In a blown woofer case, it will melt completely and lose its insulating properties. Now you have a short and your sub's impedance goes from about 4 ohms down to 1 ohm or less. Your amp goes into protection mode or blows up as well.


If you bottom it out, it will deform and scrape against the pole piece (the raised, center section in the pic below). To simulate this, get a beer can and just crush it a little. See the dimples and creases. Since a voice coil is made out of high temperature kapton, it can bounce back from small bottoming out. Hit it hard or enough times and the creases become permanent. If you deform it enough, the wiring of the voice coils can scrape the top plate (the outer, top part of the pic below). Scrape the insulation off of the wires and you short the sub out and kill your amp.


-Robert

Vader

Re: Blown VC, sub bottoming questions....

Thank you, Robert!

Peace... Derek

One sub to rumble them all. One sub to shake them. One sub to humble them all. And in the darkness break them.

Louvre attendant: Sacre bleu! ze frame on ze Mona Lisa broke and ze only one left iz too small. Andre, bring me ze scissors!