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Old 24th July 2012, 06:10 AM   #1
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Default How do mfgs' lower free air resonance of a speaker?

Google fu reveals nothing. Okay I know adding mass to the cone will lower the Fs.
Does making the ridges or ripples in the spider bigger lower the Fs? I know the spider compliance and cone weight is the main thing affecting resonance. All things being equal does a smaller voice coil have a lower Fs than a larger voice coil due to the greater area of fabric the spider has in that case.
I've also heard that widening the space between inner and outer magnet pole will increase Fs at the expense of reduced bandwidth. I heard that from some Sub builder on youtube. Thanks in advance.

Last edited by asberrys; 24th July 2012 at 06:18 AM.
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Old 24th July 2012, 06:53 AM   #2
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Free air resonance Fs is entirely dictated by cone moving mass and suspension compliance, nothing else.

Magnet pole piece design has no effect and the only contribution from the voice coil is the small amount of mass it adds. Not all of the moving mass is the moving parts of the speaker, some is what is called the air mass load - the air immediately on either side of the cone. For a 12" woofer the air mass load is about 5 grams.
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Old 24th July 2012, 07:38 AM   #3
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Lowering Fs by adding weights has it's limits too because the suspension should be able to handle what you add. Light cones with floppy suspensions most likely will not like added weights. Heavier cones with more robust suspensions could handle additional weights easier.
But for a particular system, you will loose efficiency with added weights.
You cannot change the compliance of the spider or surround without possible damage to them . Unless you change them, which is practically a rebuild of the driver with a magnetic circuit that you cannot change. You still loose efficiency !

Manufacturers design the speakers for a target resonant frequency for which they have all parameters under their control, moving mass, surround compliance , spider compliance and the magnetic circuit to handle all that to get the target efficiency. Once they design all the components , it isn't easy to simply change the Fs after that without loosing out on the drivers designed parameters.
Spiders and surrounds look very simple but they aren't that simple when you design them !
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Last edited by ashok; 24th July 2012 at 07:47 AM.
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Old 24th July 2012, 08:19 AM   #4
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Okay thanks guys however what if I want to design a new driver. How is it one establishes the compliance of the surround? The depth of the ripples/ridges? Thicknes of fabric? etc,etc. Will a surround of a particular overall diameter have a lower Fs with a smaller hole in the middle, i.e,smaller voice coil or with a larger hole,i.e, larger voice coil? If you gave me the parameters of 18" woofer with a particular bandwith how would I achieve the Fs for that given that I can determine and know the voice coil size by analyzing upper cutoff frequency and inductive rise. Lets say I had to go with a particular weight of a cone already established by what the mfg had in stock. Thanks

Last edited by asberrys; 24th July 2012 at 08:22 AM.
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Old 24th July 2012, 10:53 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

Lowering Fs has no practical value unless you understand the total
design of a driver and the enclosure it is intended to be used in.

e.g. take a driver that is say Vas 100L, Fs = 30Hz and Qts 0.35.
In a 33L sealed box Fb = 60Hz, Qb=0.7. Lowering the driver Fs
to near 21Hz by doubling Vas to 200L, halving the suspension
compliance will have next to no effect on the sealed box.

For vented boxes you want Fs to be as high and Vas as low as
you can get away with, to limit the driver excursion below the
port frequency, as Vas is the only restraining force.

rgds, sreten.
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Old 24th July 2012, 10:58 AM   #6
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Spiders (the central cloth suspension) can be formed from a variety of cloths. They are heat formed with a resin binder that gives them the stiffness. Both the amount of resin and the heaviness of the cloth contribute to high stiffness. Corrugation depth has much to do with travel and the overall linearity but not stiffness. In fact a flat spider would be more compliant (but with little or no travel).

The goal is seldom extreme compliance as long as the woofer Q is placed in the target region. High linearity isn't always the goal either, since a progressive tightening can give more controlled excursion and compensate for Bl roll off.

Long term durability is always an issue. Achieving high stiffness with lots of resin can backfire if the resin cracks out as the driver breaks in.

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Old 24th July 2012, 11:49 AM   #7
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Peter Snell's early Model A series had an example of seemingly neat woofer/cabinet design without the forthought of consequences. I know - after having replaced a few failed spiders and damaged VC formers.

What PS did was add cone mass by pouring resin around the dust cap. That's okay, but then he mounted the woofer in the base of the cabinet facing up into the cabinet. Over years (and probably long after Peter's demise), the force of gravity gradually caused the spider to sag to the point where long, rearward excursions caused voice coil to slam against the back plate, resulting in former damage.

So, if the OP has designs on adding mass to the cone, consideration should be given to the woofer's orientation in the cabinet.
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Old 24th July 2012, 11:50 AM   #8
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Slightly off topic, but had to mention it. The air load in a dipole H-baffle can lower Fs with at least 25%.
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Old 24th July 2012, 01:21 PM   #9
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Quote:
Originally Posted by sreten View Post

For vented boxes you want Fs to be as high and Vas as low as
you can get away with, to limit the driver excursion below the
port frequency, as Vas is the only restraining force.

rgds, sreten.
I would rather say that for a vented box Qt is of primary concern. For an optimum flat arangement, you need a Qt of about .38.

Below the port frequency, Vas as such is not a restraining force; the ported enclosure below the port frequency does no longer provide an accoustical load to the driver. High Fs implies a stiff suspension, so that helps minimize excursion, but it also limits bass extension. Therefore, for vented it is really better to have a linear Xmax that is smaller than the mechanical Xmax, in combination with a non-linear suspension. Plus, in order to maximize output, you need to filter out frequencies below the box tuning.

vac
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Old 24th July 2012, 02:04 PM   #10
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Quote:
Originally Posted by speaker dave View Post
Spiders (the central cloth suspension) can be formed from a variety of cloths. They are heat formed with a resin binder that gives them the stiffness. Both the amount of resin and the heaviness of the cloth contribute to high stiffness. Corrugation depth has much to do with travel and the overall linearity but not stiffness. In fact a flat spider would be more compliant (but with little or no travel).

The goal is seldom extreme compliance as long as the woofer Q is placed in the target region. High linearity isn't always the goal either, since a progressive tightening can give more controlled excursion and compensate for Bl roll off.

Long term durability is always an issue. Achieving high stiffness with lots of resin can backfire if the resin cracks out as the driver breaks in.

David S.
Thanks everyone.
What about aluminum spiders? They do make them don't they. Don't they have the added advantage of helping as a heat sink for the VC also?
I guess one could add additional spiders in this case to stiffen things up.
Also anyone know if a small voice coil vis a vis a large VC will give a lower Fs for a given overall spider diameter? In fact isn't that one problem associated with large VC's, not enough spider area to properly control things?
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