Speaker Failure article

Hi all,

this short article, by yours truly, sets out to debunk the many absurd myths surrounding sudden failures of loudspeakers - particularly woofers used in live music and disco sound systems.

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The article is rather tightly written, so you may need to read it a bit at a time and cogitate.

Take a careful look at the links provided at the end of the article too.

And before anyone asks, the 40mm voice coil and magnet gap are from a 10 inch woofer used in a very old AR2a.

The pics shown in the article were taken by me with a pocket size Cannon A430.

.... Phil

Reply to
Phil Allison
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Actually, it's terse and to the point. It needs a bit of editing here and there, but everything does. I'm currently editing articles for "Electronic Design". If every one were of this quality, I wouldn't have a job.

I'm reminded of Robert Stroud, the Birdman of Alcatraz.

Talk about Freudian slips...

Reply to
William Sommerwerck

"William Sommerwerck"

** That's the nicest thing Bill has ever said about me.

Tears .........

** Because I misspelled "Canon" ??

..... Phil

Reply to
Phil Allison

Well, you are rather a loud, loose cannon.

Reply to
William Sommerwerck

"William Sommerwerck"

** No way - I am a real straight shooter, every time.

As us Aussies would say, Sommerwanker is the one with:

" A few roos loose in the top paddock "

.... Phil

Reply to
Phil Allison

The following is meant seriously and sincerely...

You're not aware of your Jekyll-and-Hyde behavior? If you are, do you think it's normal or justifiable?

Reply to
William Sommerwerck

"William Sommerwanker"

** No such thing for me or anyone else to aware of.

But I have long ( something like 8 years long ) been very aware that YOU are merely another half witted, autistic, mental defective.

Usenet is flooded with hundreds of similar, defective individuals, attracted just like moths are attracted to lights at night.

So is the entire academic world, the public service and all forms of bureaucracy.

Human garbage and the root cause of all trouble and misery on this planet.

... Phil

Reply to
Phil Allison

Great summary, Phil, thanks for posting. And, thanks Rod, for the site!

Reply to
Wond

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Pretty good! The write up does raise a few issues I would like to address:

  1. Back in the day we found that the pole piece vent is useless for cooling. Without the vent, when the woofer compresses the air in the enclosure, the dustcap (assume solid) pushes air between the voice coil former and the pole piece. This causes air to flow out of the space between the magnet and the pole piece, over the wires of the voice coil and out the gap. On the expansion stroke, air flows back over the voice coil and then between the VC former and the pole piece.

With the vented pole piece, most of the air simply goes in and out the vent, and not past the voice coil. The forced air movement past the voice coil caused by the unvented pole piece should cool it better than simple conduction (plus some convection) to the pole and to the top plate when the pole piece is vented.

Now, all this air movement past the voice coil may affect the way the speaker responds to an audio input. Thus the vent may make for less distortion even though it's counterproductive for cooling. In that case, a dustcap that lets air flow through it might be simpler and cheaper than drilling a vent hole through the pole piece.

  1. While the point is well taken that attempting to eliminate clipping by using higher powered amps merely increases power supplied to the speaker, for a given amplifier, tweeters are significantly more likely to be destroyed when the amplifier clips, because amplifier clipping sharply increases the amount of high frequency signal, and tweeters' small excursions produce less cooling. Tweeter failure due to clipping may be the origin of the silly idea that higher power amplifiers cause less damage.

Showing what happens to the pink noise spectrum between input and output of a clipping amp would be instructive -- the article's graphics refer only to the time domain.

  1. Saying that the minimum impedance is purely resistive grates a bit, because the voice coil always has some inductive reactance, however small, and there is some intrawinding capacitance as well. The minimum impedance is just the spot where the high electrical impedance of the mechanical resonance rolls off and before the inductive reactance takes hold.
  2. Just for interest: At the speaker company, our tech built a working cutaway model. I forget how he wound the coil, but he could drive it at the mechanical resonance to produce visible woofer excursion.
Reply to
spamtrap1888

You're just the sort of person I enjoy fluttering around.

So this is all serious? It's not some out-of-control joke?

Why would anyone want you as a friend, let alone be around you?

Reply to
William Sommerwerck

Ahhh !! That's better lads, I was starting to worry about you. You know getting all sentimental and stuff :-)

Rheilly

Reply to
Rheilly Phoull

"spamtrap1888" "Phil Allison"

-------------------------------------------------------------------------- Pretty good! The write up does raise a few issues I would like to address:

  1. Back in the day we found that the pole piece vent is useless for cooling. Without the vent, when the woofer compresses the air in the enclosure, the dustcap (assume solid) pushes air between the voice coil former and the pole piece. This causes air to flow out of the space between the magnet and the pole piece, over the wires of the voice coil and out the gap. On the expansion stroke, air flows back over the voice coil and then between the VC former and the pole piece.

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** With most speakers, there is a sealed volume behind the magnetic gap so very little air flow goes on. Hot air adjacent to the voice coil circulates with cooler air and this serves to heat the magnet structure, along with conduction and radiation of heat from the voice coil.

The main reason for having a hollow pole piece is to relieve pressure on the cone during large excursions, but it also allows moving air to cool the pole piece too.

--------------------------------------------------------------------------- With the vented pole piece, most of the air simply goes in and out the vent, and not past the voice coil. The forced air movement past the voice coil caused by the unvented pole piece should cool it better than simple conduction (plus some convection) to the pole and to the top plate when the pole piece is vented.

----------------------------------------------------

** JBL designed a way to force cool air over at least part of the voice coil in operation - provided the cone is moving significantly at low frequencies of course. Their idea, called " Vented Gap Cooling " takes advantage of the AES 50Hz to 500Hz testing method to get around double the previous published power ratings.

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See the final two paras on the first page.

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  1. While the point is well taken that attempting to eliminate clipping by using higher powered amps merely increases power supplied to the speaker, for a given amplifier, tweeters are significantly more likely to be destroyed when the amplifier clips, because amplifier clipping sharply increases the amount of high frequency signal, and tweeters' small excursions produce less cooling.

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** Tweeters burn out for the exact same reason woofers do when the ampler clips - cos the *average power level* has gone up !!

With normal unclipped programme, a tweeter may receive 5% of the applied power or 5 watts out of 100. With 6 dB of clipping, that same tweeter will now receive 20 watts, a direct result of turning up the gain by 6dB.

Others have done tests to show that the increase in high frequency energy due to peak clipping music programme is small in comparison to the above.

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  1. Saying that the minimum impedance is purely resistive grates a bit, because the voice coil always has some inductive reactance, however small, and there is some intrawinding capacitance as well.

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** Real speakers (woofers) test purely resistive at some frequency in the 200Hz to 500Hz range and this condition corresponds with the impedance minimum.

If the impedance minimum for a particular driver is at 250Hz, then below that frequency the impedance is capacitive and above that frequency it is inductive. ( There is usually about an octave range where the impedance varies by only 10%. )

If you sweep test a driver using a dual trace scope, one channel for voltage and one showing current, you can observe the two traces coinciding in phase at any impedance minimum or maximum.

The minimum impedance is just the spot where the high electrical impedance of the mechanical resonance rolls off and before the inductive reactance takes hold.

** Correct.

If you were to jam the voice coil tight in the cap, then all you have is the R of the coil plus some ( lossy) inductance.

.... Phil

Reply to
Phil Allison

One quick observation, Phil.

In section 2 - Voice Coil Construction, your photo reference for a destroyed voice coil is wrongly stated as "Figure 5". The photos that show destroyed / damaged coils are actually figures 6 and 7.

Good article, though

Arfa

Reply to
Arfa Daily

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This is a very well written article Phil however I have a question. In "voice coil facts" you state:

"When a current is passed through a voice coil, heat is generated. The amount of heat in watts is given by the very simple formula:

P =3D I=B2 * R - where I is the RMS current and R is the actual resistance of the wire, at any temperature"

Admittedly my AC theory is a bit rusty, as am I but since a voice coil has an inductive component to it shouldn't the formula incorporate the J factor, (R+JX)? Or is this just a theoretical example of perhaps a DC current being passed through the voice coil? Lenny

With eith

Reply to
klem kedidelhopper

Mr Allison is quick to call me and others autistic idiots. Well, this autistic idiot will be the first to give the correct answer.

The reactive component of the voice-coil impedance -- or any impedance -- doesn't dissipate energy. It's as simple as that.

Reply to
William Sommerwerck

"klem kedidelhopper"

This is a very well written article Phil however I have a question. In "voice coil facts" you state:

"When a current is passed through a voice coil, heat is generated. The amount of heat in watts is given by the very simple formula:

P = I² * R - where I is the RMS current and R is the actual resistance of the wire, at any temperature"

Admittedly my AC theory is a bit rusty, as am I but since a voice coil has an inductive component to it shouldn't the formula incorporate the J factor, (R+JX)? Or is this just a theoretical example of perhaps a DC current being passed through the voice coil? Lenny

** Sometimes the facts are SOOOO simple folk refuse to believe them.

In any complex, reactive circuit ONLY the resistive components dissipate actual heat.

Inductive or capacitive components in series with resistive ones only act to REDUCE the current flow.

So, with a speaker voice coil, one only has to know the RMS current flow and the actual R value to calculate the HEAT in watts.

R varies with temp, so watch out.

... Phil

Reply to
Phil Allison

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Thanks William and Phil. It HAS been a while since AC theory. Lenny

Reply to
klem kedidelhopper

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