Dampen metal cone resonance: How much is enough?

[gary f]

Hi

When working with metal cone woofer, some people say it is
necessary to filter the cone resonance with a paralel LC trap, or use a steep filter (18dB/oct or more) or both.

But, how much is enough?

If the peak of the main resonance is filtered and is 12 dB below the average level, is it OK? Is 15dB better?

Should I use the full arsenal to eliminate the resonance like a 4th order electrical filter plus a LC trap?

[mcp]

Hi gary f

I'm afraid what you heard is quite true. It is a pretty sharp peak at break-up. And not a small one, quite a few dB. The further away you are from the peak, the better. It can be used for a 2-way, but I find little benefit considering the hassle and the extra cost in crossover. Should you decide to go 2-way, you may well need the "full arsenal". I would rather use them for subs, especially bandpass.

Cheers

[SY]

It depends on how far away it is from crossover and the crossover slopes. If it's a a point where the driver is 50 dB down anyway, I might not bother. If it's anywhere near the crossover point, I'd go at it with everthing I've got- including some experiments with viscoelastic damping.

[catapult]

If the peak of the main resonance is filtered and is 12 dB below the average level, is it OK? Is 15dB better?

I'd want it at least 40dB down which usually means deploying the whole arsenal to kill it. If you loosely define the breakup peak as a form of distortion,

12dB = 25% distortion
20dB = 10%
40dB = 1%
60dB = 0.1%

[gary f]

Well, I agree that the lower the better, but I have seen a few design with Seas woofer where the peak was only 15-20 dB down. Maybe those resonance were audible, I don't know.

Thanks for the replies
F

[jcarr]

Audiophysic is presently using metal-cone drivers which employ what it calls "active damping." The basic idea is to encircle the periphery of the metal cone with a tensioned band. The tensioning alters the resonant behaviour of the driver, and dramatically lowers the amplitude of the high-frequency break-up peak.

The active damping technique will likely allow the design of the crossover network to be simpler and more elegant than it would be otherwise.

hth, jonathan carr

[kelticwizard]

JCarr:

You mean basically glue a rubber band around the back side of the edge of the cone, just below the point where the surround is glued to the cone?

By any chance, would you have any tension recommendations for various size drivers-say 4", 6", etc.?

What I would consider doing is: cut a rubber band, glue it back together at the length I wanted, then stretch it to where it is the proper length to go around the outside edge of the cone, and use a kitchen scale to judge the tension.

Just a thought.

[kelticwizard]

Gary:

I haven't worked yet with metal cone woofers.

However, I can add that it is generally understood any output that is down 12 dB is considered "out of the running", sound-wise. That is why the crossover region of a 6 dB crossover is considered to be four octaves. It goes from -12 dB on the low frequency side to -12 dB on the high frequency side. Same thing for the crossover region for a 12 dB crossover-the region lasts two octaves-from -12 dB on the low frequency side to -12 dB on the high frequency side.

Having never worked with metal cones, I cannot say for sure. But that would be my assumption going in. Unless proven otherwise, minus 12 or 15 dB would make me feel safe.

[ThomasW]

For those with access to AudioXpress, the Sept issue has part one of a three part article detailing the design and construction of a 2-way using the Hi-Vi Research M8a metal cone woofer.

The author goes into great detail regarding the use of a high order Cauer/Elliptical XO's to deal with the issue that's being discussed here.

Jon Mark Hancock (the author) is a twice published member of the AES and seems to have a reasonal grip on loudspeaker design.

[kelticwizard]

Originally posted by catapult
I'd want it at least 40dB down which usually means deploying the whole arsenal to kill it. If you loosely define the breakup peak as a form of distortion,

12dB = 25% distortion
20dB = 10%
40dB = 1%
60dB = 0.1%

Very true.

I have an idea on how Gary can test this, though. Run either a steady tone or "white noise" through his setup. He could use either a tone generator for a single tone, or use the "white noise" from tuning his radio to a frequency between stations. Using a voltmeter, he should adjust the "balance" control until one side gets 2 volts, and the other side gets 0.6 volts. Then he should hook the speaker which gets the 0.6 volts to the B speaker if his amp has one.

The speaker that gets 2 volts will now play 11 dB louder than the speaker that gets 0.6 volts. Starting off with A+B on the speaker selector switch, he will get both channels playing. Then he can switch the speaker selector switch to A, thereby cutting off the speaker that is 11 dB down. See if he can hear the difference. 11 dB is awful close to 12 dB.

If he can hear a difference, then he should consider going to a filter that suppresses more than 12 dB. I have a hunch he won't have to, but he should try it.

Just a suggestion.