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Can you blow a driver with air?
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fieldman View Drop Down
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    Posted: 01 August 2016 at 4:40pm
We had two 18inch bins and a 15inch / 10inch / 1inch top. We were playing some heavy bass music in a small room when I heard a crackling from the top speaker. I had a look and it seemed the 10inch was blown - rubbing noise came from it when lightly pushed in and out.

So we got the other identical top out, tested it and it was fine. Then we unplugged the top and got the bass going again when I heard a crack. When I looked at the top both the 15inch and the 10inch were moving quite a bit in sympathy with the bass. And now the other 10inch is blown.

I think I have made a really stupid mistake - is it possible to blow a driver in this way?
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Points: 2186 		Post Options Post Options   Thanks (1) Thanks(1)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2016 at 4:51pm
So was the mid top connected to an amp but not playing?

Air pressure waves caused by the bass will move the other cones as you saw so you could theoreticaly have mechanically damaged the coil or the former that it is wound onto by smashing it into the magnet. This would explain the rubbing. I don't see how you could burn out a coil without  a signal going into it.
Keep pushing on, things are gonna get better.
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fieldman View Drop Down
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Points: 10 		Post Options Post Options   Thanks (0) Thanks(0)   Quote fieldman Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2016 at 5:08pm
The mid top was not connected. There was no burning smell or anything, but the driver is gone for sure - when you tap it in one place it makes a nasty sound. And both drivers have the same problem. What you say about the mechanical damage is what I think happened.

I will add that the amp driving the bins was in bridge mode for the first time, just as a test, and I am beginning to think that this introduced some distortion, cause we have used the same 2 bins / 1 top combo many times before without problem. And maybe this distortion in the smallish room was just too wrong...
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Meat View Drop Down
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Speakers should have their own enclosures within the box. If they don't then they will interact with the other drivers sharing an enclosure.

So, in a word, yes, you can blow a driver with air. I think what's happened here is that the (I guess large) movement of the 15" has pushed the 10" past its mechanical limits. It sounds like its smashed its voicecoil into the gap.
Don't test the champignon sound
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Teunos View Drop Down
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Mechanically you can, yes, but i have never seen it happen before. When it would have been connected, the damping of the amplifier will arrest any motion of the cone thats not present on the inputs.
Since it was unconnected, there was no such damping.
Best regards,
Teun.
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fieldman View Drop Down
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Points: 10 		Post Options Post Options   Thanks (0) Thanks(0)   Quote fieldman Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2016 at 5:24pm
Originally posted by Meat Meat wrote:

Speakers should have their own enclosures within the box.

I just had a look behid the wadding and the 15inch does have its own enclosure - there is a full panel between it and the top drivers.

Originally posted by Teunos Teunos wrote:

When it would have been connected, the damping of the amplifier will arrest any motion of the cone thats not present on the inputs.
Since it was unconnected, there was no such damping.

Looks like I have learned an expensive lesson. Can this type of damage be fixed with a recone? The 10inch are RCF - an old model but kits are available but 80euro each.
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jbl_man View Drop Down
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Points: 11155 		Post Options Post Options   Thanks (1) Thanks(1)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2016 at 5:38pm
That sure is an odd one....if the 10 inch drivers are in their own sealed part of the cabinet, (a cabinet within a cabinet) then i cant see anyway the bass motion of the 15 could interfere with it?

For the 15" to mechanically damage it, the entire cabinet would have to be infinate baffle,(not ported or reflex ) like the old hifi speakers....so the 10 inch acting as a passive radiator.

Very strange.
Be seeing you.
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fieldman View Drop Down
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Points: 10 		Post Options Post Options   Thanks (0) Thanks(0)   Quote fieldman Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2016 at 5:49pm
Originally posted by jbl_man jbl_man wrote:

That sure is an odd one....if the 10 inch drivers are in their own sealed part of the cabinet, (a cabinet within a cabinet) then i cant see anyway the bass motion of the 15 could interfere with it?


The 15inch wasn't connected, it was the two 18inch bass bins causing the air pressure. When I heard the crack I did notice both 15 and 10 moving quite a lot. There were a few other speakers in the room but they are all fine - as you say very "strange".
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even if drivers are in separate enclosures they are still in acoustic contact through the air in the room. connecting drivers to an amp or shorting the inputs reduces sympathetic cone motion but dissipates significant power as heat in the voice coils. If a driver is capable of reproducing a frequency it is also capable of acting as a microphone and picking up those frequencies.
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jbl_man View Drop Down
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Points: 11155 		Post Options Post Options   Thanks (1) Thanks(1)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 01 August 2016 at 6:40pm
Very doubtful if the acoustic contact of being in the same room could actually move the cone passively to mechanically damage a driver? Certainly a new one on me that.  If the pressure being created was that intense,how were you able to breathe? LOL
Be seeing you.
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the 10" will hit xlim at something like 125dB @ 100Hz. If the suspension losses are small then a level of 125dB is going to induce the same excursion. In practice losses, directivity, and distance between speakers reduces the induced movement but a few thousand watts going into a pair of double 18s is going to get you near 135dB.

try setting up a couple of bass cabs next to each other. put a signal through one and then short the contacts of the other with a piece of wire. the change in volume is ~2dB and should prove to you that this effect is significant.
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Points: 2654 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Muckerbarnes1 Quote  Post ReplyReply Direct Link To This Post Posted: 02 August 2016 at 8:43am
Many years ago in our test acoustic chamber we built, we tested drivers to destruction on many occasions with some monster amps I built. This give us lots of info during such tests. The sound pressure in the chamber was unbearable. In ear plugs and Peltor ear defenders were worn. My eyes were blurred whilst looking for issues under such tests. Many people wanted the toilet after the experience. Usually quite quickly too. You could feel the compression on yer ball bag. 2x 18's with 5" VC, and 2, 21" drivers with 6" VC were my favourite test. The results ended up with a modification in manufacture, and we tried again some weeks later. Always looking for the ultimate combination. 

In this test room we often had another enclosure with a driver in ready for such a test, or just to run for a period. Never once did we have a driver fail from acoustic coupling within this sealed small acoustic room. There was of course far more chance of a quick under garment change. 


Edited by Muckerbarnes1 - 02 August 2016 at 8:45am
Billy Dawg.
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