What causes cone shake? Problems with porting?

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Gregory Roig

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What causes cone shake? Problems with porting?
« on: 7 Feb 2018, 09:01 pm »
I recently built a speaker and noticed part of the woofer cone shaking at certain frequencies.  Wondering if it's a problem with my porting and what might be a solution.

FullRangeMan

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Re: What causes cone shake? Problems with porting?
« Reply #1 on: 7 Feb 2018, 10:01 pm »
I recently built a speaker and noticed part of the woofer cone shaking at certain frequencies.  Wondering if it's a problem with my porting and what might be a solution.
What woofer is this?
There is other woofer in the same enclosure?

If part of the woofer cone is vibrating and part is not vibrating smt the cone is not correctly glued in the suspension or frame.

jules

Re: What causes cone shake? Problems with porting?
« Reply #2 on: 7 Feb 2018, 10:31 pm »
What's your source? This can happen with turntables in some circumstances.

richidoo

Re: What causes cone shake? Problems with porting?
« Reply #3 on: 8 Feb 2018, 03:46 am »
Two things happening:
As frequency falls below the tuning frequency of the reflex port, the port/cone resonance decouples, and the driver loses its internal acoustic load, allowing the cone excursion to increase quickly with falling frequency.

BUT, at such a low frequency, a small diameter cone has very little acoustic impedance and cannot grip the external air to create a sonic impulse because of the very slow velocity of the small cone at low frequency. It is moving but it can't grip the air that low, so you can see it move, but it is not making any sound. If the cone were much larger it could grip the air and make sound at that low frequency. Likewise, if the small cone were playing much louder (larger excursion,) the velocity would increase enough to catch some air and be audible, though it would sound like mud anyway.

As you suggest, you could increase the length of the port to lower the port tuning frequency, so the driver does not decouple from the load of the resonating port, but lengthening the port will have negative effects on sound quality at higher frequencies.
If it is a commercial speaker the port tuning is probably set correctly for the intended purpose of the speaker. A small speaker is not intended to be played loudly far below the port tuning frequency. You can break the driver if you allow it to bottom out due to too much power and no acoustic load below the tuning freq.

Never heard it called cone shake, so maybe you're trying to describe something else. Hope this helps

Folsom

Re: What causes cone shake? Problems with porting?
« Reply #4 on: 8 Feb 2018, 04:20 am »
What's your source? This can happen with turntables in some circumstances.


This can be a source. It can be infrasonics or jiggle somewhere in the stylus or arm.

jules

Re: What causes cone shake? Problems with porting?
« Reply #5 on: 8 Feb 2018, 05:25 am »
I guess the other key questions, given your design was DIY, would be ... how did you figure out the box volume and port dimensions for your driver if you weren't following a [proven] plan directly?

FullRangeMan

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Re: What causes cone shake? Problems with porting?
« Reply #6 on: 8 Feb 2018, 07:25 pm »
I guess the other key questions, given your design was DIY, would be ... how did you figure out the box volume and port dimensions for your driver if you weren't following a [proven] plan directly?
There is many duct calc online software.
http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL%20Port%20lenth%201.htm

Gregory Roig

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Re: What causes cone shake? Problems with porting?
« Reply #7 on: 13 Feb 2018, 07:09 pm »
 i think Richidoo probable has it right.  It's an 8-inch driver in a rather small box (.35 ft3) that I tuned to about 80 Hz. Playing below that at any volume causes the bottom half of the driver to shake, visibly and audibly (with louder volume). I'm limited to a rather small box but was hoping to get reasonable bass out of it.  Clearly I've gone beyond the limitations of the box/driver combination.
Perhaps I should stick with the box/port designs optimized for the driver. Comments and/or suggestions?

Also, what are the problems for higher frequencies you mentioned as a result of a longer port?

Folsom

Re: What causes cone shake? Problems with porting?
« Reply #8 on: 13 Feb 2018, 07:23 pm »
In that case, do you have any internal stuffing?

Gregory Roig

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Re: What causes cone shake? Problems with porting?
« Reply #9 on: 13 Feb 2018, 07:48 pm »
Unable to stuff because of internal amplifier--might start a fire!  Can multiple ports cause an unbalanced airflow that could cause similar problems?

FullRangeMan

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Re: What causes cone shake? Problems with porting?
« Reply #10 on: 13 Feb 2018, 08:29 pm »
Multiple ports are a solution used in pro-audio no much a hi-fi approach due additional resonances.

glynnw

Re: What causes cone shake? Problems with porting?
« Reply #11 on: 13 Feb 2018, 09:17 pm »
Have you considered that different surrounds can affect the issue you have?  Stiff cones, generally for ported or OB tend to wobble less than soft suspensions used in some sealed enclosures (or maybe I have this backwards?).  Anyway, I remember seeing some large speakers maybe 20 years ago with rubber surrounds that "danced constantly" - sometimes slowly pulsing when there should have been no noise and at other times moving all over the place with a loud busy signal.  I am just guessing here.

jules

Re: What causes cone shake? Problems with porting?
« Reply #12 on: 13 Feb 2018, 09:29 pm »
Quote
Perhaps I should stick with the box/port designs optimized for the driver.

Yes, probably. Have you fed the Thiele Small figures into a relevant number cruncher [WinISD is handy]

If you don't mind me asking, why are you limited to a really small box. At .35 cubic feet that doesn't leave much space for the amp. Have you managed to leave a reasonable distance between the speaker magnet and the amp and would it give you more space in the box if you took the amp out?


Tyson

Re: What causes cone shake? Problems with porting?
« Reply #13 on: 13 Feb 2018, 09:41 pm »
Put a small cap inline before the amp to roll off the low frequencies before they hit your speaker.