Please help me understand speaker fundamentals

[jnb]

Imagine a speaker were suspended in space with the roll surround and spider removed but with the cone still in place. If a dry cell were then applied to the terminals I imagine the cone would travel outward and keep on travelling.

If I then guess that if the cone were held still, a few mm forward of the central position like it might normally be with 1.5V on the coil then it would still travel outward when let go. Is this true?

If it is then it seems to suggest that the suspension doesn't just damp movement but also pulls back on the cone like a spring. It follows then that one would normally prefer a speaker in an acoustic suspension box or other box where the air damping is more significant than the speakers mechanical damping. Do you agree?

Or that the electrical damping should be significant where one is building a dipole (as I'm planning to). Would this mean a valve amplifier is a bad choice for a dipole woofer unless it has a really large travel or significant mechanical damping?

My first thought/question (just so you know) was going to be whether an applied voltage either positions or just accelerates a cone. I was thinking that a closed box with a very small air leak would cause rectification either on the way out, or on the way back in depending on the answer to this.

[Andy G]

These are answers to some of your thoughts, but I'm pretty rusty on this stuff, so other may want to correct me. I have tried to answer paragraph for paragraph.


It would still be pressing against the air, hence would stop fairly quickly, depending on its mass x velocity = momentum vs air friction and weight etc

If given enough momentum, it would keep going significantly after power is removed

Yes, the spider is the main centralising device, with the surround also playing a part, particularly in PA style drivers

Air damping works more in compression than in rarefraction inside the box. (try it sometime with a bike pump). It is possible to overdamp a cone using a sealed box. IMO, using a large box or open baffle will allow the cone to move more as the signal dictates.. ie more detail for small excursions.

Dipole bass requires large amounts of power, because big hi-movement cones are required to get decent bass. this alone makes tubes rather pointless because of the size of the amp required.

An applied voltage applies a force which leads to acceleration (force = mass x acceleration) but applying a small constant force to a driver with spider and surround will lead to a balance of forces which in turn leads to an equilibrium position (just like a spring balance). This applied force also liberates heat within the voice coil, so it is NOT good to hold a constant electrical force on a speaker

[footstony]

Hi jnb,
you said:
"My first thought/question (just so you know) was going to be whether an applied voltage either positions or just accelerates a cone"

An applied D.C. voltage will cause the cone to move one way and stay there (and heat up the voice coil!). The cone will return to the center when the voltage is removed.

A cone with no suspension will not return to the center.

An open baffle speaker will still have the damping of the suspension, but no additional damping from a closed box.

A lower power valve amp may not have enough grunt to drive a dipole woofer as dipole is generally less efficient in the bass region unless the baffle is large (say the size of a door). EQ and high power amps are often used with the woofer to correct the bass response for smaller baffles.
see here for heaps of info:
http://www.linkwitzlab.com/

A closed box with an air leak would be an aperiodic enclosure.
http://www.t-linespeakers.org/classics/dynaco.html

I hope this helps.

Regards Philip

[jnb]

Originally posted by Andy Graddon
Air damping works more in compression than in rarefraction inside the box.

This sounds as if it would produce 2nd HD. Another reason to try a dipole

Originally posted by Andy Graddon
Dipole bass requires large amounts of power, because big hi-movement cones are required to get decent bass. this alone makes tubes rather pointless because of the size of the amp required.

Originally posted by footstony
A lower power valve amp may not have enough grunt to drive a dipole woofer as dipole is generally less efficient in the bass region unless the baffle is large (say the size of a door). EQ and high power amps are often used with the woofer to correct the bass response for smaller baffles.

Did I read correctly from Linkwitz that the woofer's excursion is the limiting factor and regardless of EQ I just need enough "stroke" from the amp to match the stroke of the woofer? I'm also thinking that a typical class A SS amp has power similar to a class AB valve amp.

Originally posted by footstony
An applied D.C. voltage will cause the cone to move one way and stay there (and heat up the voice coil!). The cone will return to the center when the voltage is removed.

A cone with no suspension will not return to the center.

So the suspension is a centring spring, and as such would probably produce symmetrical distortion. I guess that makes it a good match for the closed box and its assymetrical distortion

Originally posted by footstony
An open baffle speaker will still have the damping of the suspension, but no additional damping from a closed box.

Would it have electrical damping, though?
If fed through a low Zout amp, as the signal drops through zero volts wouldn't the voice coil simply be shorted? Would this on it's own centre the coil?

[Andy G]

Originally posted by jnb

Did I read correctly from Linkwitz that the woofer's excursion is the limiting factor and regardless of EQ I just need enough "stroke" from the amp to match the stroke of the woofer? I'm also thinking that a typical class A SS amp has power similar to a class AB valve amp.

You would basically be crazy to use a class A amp to drive the dipole bass. Dipole bass systems almost always require active EQ and x-o, so save the Class A for the mids and treble.
I don't know what power the Orion (for eg) uses, but whatever it is it is driving a pair of 250w Peerless drivers, iirc, so is going to be well out of class A range !!!

[CeramicMan]

Originally posted by jnb
Imagine a speaker were suspended in space with the roll surround and spider removed but with the cone still in place. If a dry cell were then applied to the terminals I imagine the cone would travel outward and keep on travelling.

Correct. Ignoring transient effects and practical limitations because of the voice coil length, a DC voltage applies a force to the cone. Therefore it will accelerate at a rate that depends on its mass, and will keep accelerating until any opposing forces (spring tension + resistive losses) are equal to it. When that happens the cone will move at a steady rate, or it will slow down if the opposing forces start to outweigh the applied force.

It follows then that one would normally prefer a speaker in an acoustic suspension box or other box where the air damping is more significant than the speakers mechanical damping. Do you agree?

That depends on how big the box is compared to the speaker's VAS. If they're the same volume then it will be like two springs working in parallel, providing twice the stiffness compared to either one on its own. A relatively large box or open baffle will place more reliance on the linearity of the speaker's own suspension, whereas a small box will place more reliance on the linearity of the air (it's still a simplification... but close enough).