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Old 4th January 2006, 09:18 AM   #1
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Default What does the foam do?

Fundamental question;
I see drivers with corrugated paper surrounds and others with foam. What's the reason?
Production cost?
Fashion?
Cone Excurtion?
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Old 4th January 2006, 09:38 AM   #2
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All of the above.

There is no hard and fast rule, there will always be great and rubbish examples of both types of surround.
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Old 4th January 2006, 12:38 PM   #3
filgor is offline filgor  Australia
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Default Re: What does the foam do?

Quote:
Originally posted by Pbassred
Fundamental question;
I see drivers with corrugated paper surrounds and others with foam. What's the reason?
Production cost?
Fashion?
Cone Excurtion?
Both are cheap and nasty

Corrugated paper is the cheapest to produce as it is moulded as part of the cone, often it is doped with some kind of flexible laquer. I haven't seen drivers like that for ages, except really small, cheap replacement ones. You should only see this kind on an old radio or something!

Foam "roll" surrounds are better. They are more complext to produce and mount on the cone and therefore more costly to produce. High quality drivers used these up until a few years ago but nowdays they are a sign of low quality. In decent quality HiFi drivers foam has been replaced mostly by various types of rubber which lasts a lot longer.

You might be mistaking corrugated rubberised canvas for Corrugated paper. Corrugated canvas was used in high quality speakers in the past and is still used for high power PA applications ie. stage speakers. It allows for heaps of excursion, is very tough and lasts for ages

Hope this helps without offense..

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Old 4th January 2006, 12:51 PM   #4
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And don`t forget the surrounds ability to "absorb" standing wawes in the cone...

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Old 4th January 2006, 01:12 PM   #5
filgor is offline filgor  Australia
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Quote:
Originally posted by thinkbad
And don`t forget the surrounds ability to "absorb" standing wawes in the cone...

JB

Good thinking Thinkbad!

I'm all for high loss rubber surrounds personally. Especially if the driver is used in the midrange.
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Old 4th January 2006, 01:17 PM   #6
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No offence taken Filgor. Yes I did mean canvas. I thought that the foam WAS rubber ( also "rubber" has many level of quality).

Thanks. I never owned rubber surrounds except in cheap car audio.
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Old 4th January 2006, 01:20 PM   #7
Bill F. is offline Bill F.  United States
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I think doped paper surrounds get a bum rap. No, they're not usually as good as foam or rubber for handling high excursion, but they can be significantly better than foam at terminating HF flextural waves on the cone.

It's a question of mechanical transmission line impedance. Where there is a sharp discontinuity in material properties (paper to foam, for example), HF mechanical waves will tend to reflect off the boundary of the impedance mismatch. However, when the surround is basically an extension of the cone coated with damping compound, it is less likely to reflect, and more likely to terminate waves--a very good thing in midrange and wideband drivers.
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Old 4th January 2006, 01:42 PM   #8
filgor is offline filgor  Australia
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Quote:
Originally posted by Pbassred
No offence taken Filgor. Yes I did mean canvas. I thought that the foam WAS rubber ( also "rubber" has many level of quality).

Thanks. I never owned rubber surrounds except in cheap car audio.

Your right,

I guess technically the foam is actually a foamed rubber.
Most i've seen were either plasticised PVC or Polyurethane.

When I say rubber i really mean things like the Butyl or Nitrile rubber you get on drivers from Vifa, Peerless, Scan, Seas etc.

These rubbers can be foamed as well and as such would make a great surround.
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Old 4th January 2006, 02:03 PM   #9
filgor is offline filgor  Australia
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Quote:
Originally posted by Bill F.
I think doped paper surrounds get a bum rap. No, they're not usually as good as foam or rubber for handling high excursion, but they can be significantly better than foam at terminating HF flextural waves on the cone.

It's a question of mechanical transmission line impedance. Where there is a sharp discontinuity in material properties (paper to foam, for example), HF mechanical waves will tend to reflect off the boundary of the impedance mismatch. However, when the surround is basically an extension of the cone coated with damping compound, it is less likely to reflect, and more likely to terminate waves--a very good thing in midrange and wideband drivers.
I agree

That makes sense

If you have a larger driver producing midrange this style of surround could help to control cone resonances.

I try to avoid using drivers that require standing wave control. ie smaller midrange drivers with stiff PP or thicker paper cones. If the modulus of elasticity is high enough and the cone dimentions are small there is very little room for waves to form in the first place. Bell modes turn up but they flex the surround where it meets the cone at right angles so are absorbed quite savagely.

Unfortunately such small drivers dont have the same efficiency as a larger driver especially one with a lightweight paper cone
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Old 4th January 2006, 04:05 PM   #10
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Default Re: Re: What does the foam do?

Quote:
Originally posted by filgor
You might be mistaking corrugated rubberised canvas for Corrugated paper.
Otherwise known as accordian cloth surround. Not heard the term corrugated but with so many words in our hobby changing, it wouldn't surprise me.

Quote:
[i] It allows for heaps of excursion, [/B]
I'm not sure about the excursion as most "subs" have far greater excursion than PA drivers.

Quote:
[i] is very tough and lasts for ages [/B]
It is tough, it lasts for ages, can be repaired while on the road and I think it does a better job of keeping the cone linear during high excursion. PA drivers are the workhorses of the industry. Half roll surrounds are for the less used systems like home audio.
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