Compression ratio/cone failure

[weltersys]

HF drivers often use 10/1 compression ratios or even higher.

I recall low mid horns having no problems with cones ripping or kinking with ratios above 6/1.

Warnings of cone destruction when using compression ratios much above 2/1 on bass horns seem common, yet I have heard no actual reports of failure.

I have replaced some shredded cones in bass horns driven well beyond Xmax below Fc (horn cutoff), got to look through eight 15” speaker holes after a one song use of a poorly set up foot pedal controlled aux sub set up.
Another time an amp’s power supply shorted, resulting in some arc welding level DC spikes ripping some 10” cones clean around near the surround, the voice coils and spiders remained working.

What are the highest compression ratios any of you are using in low frequency horns driven at high level, over what length of time?

Other than driving hard below Fc, what types of ratios and power levels have resulted in mechanical failure?

[jbell]

In TH's it seems that compression ratio is not much of a concern (at least to me) as getting a good smooth response seems to naturally eliminate extremely high compression ratios. 2:1 to 4:1 seems to be 'normal' for me, and I've not run any to cone destruction because I always run a high pass filter at 48db/oct.

In FLH's I know there are some concerns as getting a smooth response that is not -13db from low to high freq in the passband usually requires a pretty high CR. (at least in my experience) I don't know that there is a hard and fast rule on CR as various drivers have varying strength's of cones. Obviously the smaller the cone, the higher the CR it can stand, the tougher the cone, the higher the CR it can stand.


Here's an example where I ignored the 'common wisdom' of not going over 4:1. This sub is an 8:1 CR and I've run it at driver rated PE for hours on end with no cone failure. I can say for certain that aspect ratio (L vs W) of the throat makes a difference at high CR's and that a more 'square' aspect ratio throat sounds better.

Dual MCM FLH

[weltersys]

Originally Posted by jbell

In TH's it seems that compression ratio is not much of a concern (at least to me) as getting a good smooth response seems to naturally eliminate extremely high compression ratios. 2:1 to 4:1 seems to be 'normal' for me, and I've not run any to cone destruction because I always run a high pass filter at 48db/oct.

In FLH's I know there are some concerns as getting a smooth response that is not -13db from low to high freq in the passband usually requires a pretty high CR. (at least in my experience) I don't know that there is a hard and fast rule on CR as various drivers have varying strength's of cones. Obviously the smaller the cone, the higher the CR it can stand, the tougher the cone, the higher the CR it can stand.


Here's an example where I ignored the 'common wisdom' of not going over 4:1. This sub is an 8:1 CR and I've run it at driver rated PE for hours on end with no cone failure. I can say for certain that aspect ratio (L vs W) of the throat makes a difference at high CR's and that a more 'square' aspect ratio throat sounds better.

Dual MCM FLH

Thanks for the response, that thread was an interesting read.

The tougher the cone, the more compression ratio it can stand does seem obvious, while relating it to size "the smaller the cone, the higher the CR it can stand" is not obvious to me, unless a larger cone was made of the same thickness of material as the small cone.

Have you tried high compression ratios with larger cones and had failures?

[Djim]

I don’t want to restart the whole thing but the Xlim-7dB method seems to work fine for most drivers in any kind of cab. It tells you how much power you can use within safe mechanical limits. Although it doesn’t say anything about Pwr Compr, in many cases I found this point was around the same as Xlim -7dB excursion.

Mechanical strengths under high compression ratios is more a matter of experience. Once we had some 21” Beyma’s probably ruined by the ‘wobbling’ (correct English word?) effect as forces were not equally distributed over the cone surface. You could blame it on the design but we changed them for the first production of 21” from 18Sound. Better surround and cone and a very strong connection were the ‘coil bobbin’(?) is attached to the cone. They did hold up.

Before anyone thinks I’m connected to 18Sound, we did kill some 15W1200’ s. In my opinion they were unbalanced for the task they were marketed: "High Output Low Freq Transducers",High BL, low Xmax (6mm), light Mms and so on. 18Sound doesn’t produce this driver any longer. Nobody is interested in the rest of our 15W1200 spare parts. But hey, they seem to be a good candidate for TH’s as long we don’t ‘feed’ them to high (sorry Bertje ik heb ze al thuis liggen!)

These days we try not to use high compression ratios. With high excursion 12”s for low-mid we don’t need high compression techniques any longer for high crossover-points.

[weltersys]

Originally Posted by Djim

I don’t want to restart the whole thing but the Xlim-7dB method seems to work fine for most drivers in any kind of cab. It tells you how much power you can use within safe mechanical limits. Although it doesn’t say anything about Pwr Compr, in many cases I found this point was around the same as Xlim -7dB excursion.

Mechanical strengths under high compression ratios is more a matter of experience. Once we had some 21” Beyma’s probably ruined by the ‘wobbling’ (correct English word?) effect as forces were not equally distributed over the cone surface. You could blame it on the design but we changed them for the first production of 21” from 18Sound. Better surround and cone and a very strong connection were the ‘coil bobbin’(?) is attached to the cone. They did hold up.

Are you then saying that Xlim-7dB would work for a 10/1 ratio 21" just as it would for that speaker in a BR cabinet?

What part of the 21” Beyma speaker failed ?
Was it driven above Xlim-7dB :^)?

[Djim]

I’m not sure yet cause I’m still working on this Xlim -7dB. But in a horn design the Xlim will be reached with less power than the same driver in a bas-reflex design. So the -7dB (electrical power) point will shift with it. However, it doesn’t tell you anything about the mechanical capabilities of a driver in a high comp situation. Xmax or Xlim are not related to this. But as most TH’s around here don’t use a high comp ratio this is not a problem. But it would not surprise me if you could make a similar link between compression ratio and the –dB number from Xlim you need.

With the beyma's you could actually see that the cones were tilted. They made this strange mechanical sounds or showed even physical marks like cracks in the attachment at the cone/bobbin point. Some even had cracks that went into the cones and some had these 'fold' marks (sign of local pressure). I don’t know till what point they were driven. In those time everything was still analog and accessible for the sound techs.

[Don Snyder]

Danley had some problems blowing cones with the dual 15" servodrive speakers in his BT-7, and only used a compression ratio of 2 in the dual 12" LabHorn. A popular mod to the LabHorn design used a ratio of 3 with no problems.

[weltersys]

Originally Posted by Don Snyder

Danley had some problems blowing cones with the dual 15" servodrive speakers in his BT-7, and only used a compression ratio of 2 in the dual 12" LabHorn. A popular mod to the LabHorn design used a ratio of 3 with no problems.

What was the nature of the BT-7 cone failure?
Was this before or after the cones were soaked in epoxy?

[Djim]

Thanks Don, in that case there is no relation between comp ratio's and excursion. So what is the problem then... less excursion trough Pwr comp so its power will be transferred into heat instead of cone movement?

Maybe I should explain myself better. If the driver gets to hot it can deform the bobbin slightly. The bobbin starts to push the cone unequal. Together with the unequal pressure from the mouth it can rip a cone to pieces.

[cecaa850]

Don, you have PM.