Thielesmall help

[delphiplasma]

Hi,

Can someone please help? I'm building a small bass reflex subwoofer and require help regarding the driver thiele parameters.

Am I right that:

CMS: The higher the numerical value, the higher the suspension compliancy. And the higher the compliancy, the bigger the VAS value is

MMS: The higher the mass of the cone, the lower the VAS value is.

Just need those areas cleared up.

Thanks in advance.

 

[Iain McNeill]

CMS yes, bigger number means the driver surround is more compliant, needs less force to displace the cone.
MMS yes, bigger number means cone & voicecoil weighs more.

BUT, just because it has more mass doesn't affect the compliance. They are independent (well, OK a heavier cone probably needs a stiffer surround to keep it in place but the designer can change both parameters independently.)

VAS is just CMS in different units

 

[Ron E]

Hi,
CMS: The higher the numerical value, the higher the suspension compliancy. And the higher the compliancy, the bigger the VAS value is

Correct. Vas = Cms*k, where k is a constant

MMS: The higher the mass of the cone, the lower the VAS value is.

No, but low Vas (stiff) drivers need more mass for a given resonance frequency.

The mass and compliance give the resonant frequency Fs = Sqrt(1/(Mms*Cms))

 

[delphiplasma]

Thanks, Ian and Ron for clearing that up for me. I had always thought that a highly compliant cone meant higher low frequency output for a smaller enclosure.
In an Isobaric design the VAS of the paired drive units is half that of a single drive unit. What parameters are causing the VAS to be halved? I thought the increase in cone mass would play a part in this?

Once again, thanks for the responses.

 

[Rybaudio]

Two suspsensions means you have to apply 2x the force to the cone to get the same displacement, hence half the Cms hence half the Vas.

 

[delphiplasma]

Thanks. Makes sense, double the mass, double the compliance stiffness. Isobaric usually is best for small sealed boxes, so am I correct to say that a low compliant drive unit will provide the best low frequency extension and highly compliant drive unit provide the best LF extension in a large enclosure and does that apply to both sealed and bass reflex designs?

Thanks.

 

[Ron E]

Remember, enclosure size also has a lot to do with Qts and your chosen Qtc. A lower Vas driver will usually be less efficient - the tradeoff is efficiency for box size.

 

[Rybaudio]

delphi,

I think you've got the right idea but I just want to clear things up:

- the moving mass does double in the isobaric pair (two cones) but that has no connection to the doubling of the suspension stiffness (I can't tell if you're saying one implies the other or not in the above post).

- "compliance" is the inverse of stiffness. In the isobaric pair, stiffness is doubled and compliance is halved. A low compliance suspension is stiff while a high compliance suspension is loose. Think of the usual meaning of the word comply.


As for LF extension, well it depends on what you mean by LF extension. Regardless of box size, the loosest suspension will always have the most very low frequency output. This is because below resonance the dominant cone controlling force is the combined spring forces of the suspension and air in the box. A smaller spring force from the suspension means more cone movement hence more output. However, if by LF extension you mean the shape of the curve near F3, then a Qt of .7 is optimal and yes, all other things held constant, a looser suspension is needed for a smaller box and stiffer one for a larger. In the two cases the net stiffness (box+sus) needs to be the same, so when the air is less springy (big box) the suspension needs to be more so, and vice-versa.

 

[delphiplasma]

Hi Rybaudio,

Sorry, my e-mail did seem a bit unclear. I meant that the mass doubled and that the compliance halved with respect to the isobaric configuration.

What confuses me is the fact that a drive unit with a low compliance usually has a low vas value, yet is best suited to a large enclosure, whereas a drive unit with a high compliance, which usually has a high vas value, is suited to a small enclosure?

I must be missing something here?

Very grateful for the replies, though.

Thanks

 

[Iain McNeill]

I'm not sure that's true.

Vas is the volume of air that provides the same returning force as the driver suspension. A stiff suspension has low compliance, low Cms and equivalently low Vas. As you reduce the volume of the enclosure the "Vas air spring" becomes the dominant returning force. This helps linearize the speaker and reduce THD but it increases F0 and so sacrifices bass. Increasing the enclosure volume beyond this point doesn't hurt but there's no real advantage.

You can design stiff and soft drivers in both large and small enclosures and the end result will depend largely on the actual driver.

<edit> and as RonE says, reducing the enclosure increases Q.

 

[Rybaudio]

What confuses me is the fact that a drive unit with a low compliance usually has a low vas value, yet is best suited to a large enclosure, whereas a drive unit with a high compliance, which usually has a high vas value, is suited to a small enclosure?

I must be missing something here?

You're not missing anything. The idea is simple:

stiff suspension (low Cms) = small equivalent volume (Vas) = less spring needed from enclosure (large box)