What does a speaker amp output?

[andreasmaaan]

I haven't worked it out yet, but my gut feeling is that it should still be in the same place.

What I am getting at is that the suspension/surround is not actually responsible for the restoration of the cone to its centre position; the signal's AC-only coupling does that anyway. And hopefully it is not responsible for limiting the travel of the cone either. If so, then 'overshoot' doesn't exist because momentum/inertia is simply another side of the result of F=ma.

The thing is, for your thought experiment to work, the moving mass must be zero, the motor infinitely strong, and the voice coil and cone fused infinitely strongly, etc.

It's because motors aren't infinitely strong, voice coils and cones have mass, and because voice coils and cones are attached by glue etc., that there is overshoot that needs to controlled by the surround/suspension.

 

[Cosmik]

The thing is, for your thought experiment to work, the moving mass must be zero, the motor infinitely strong, and the voice coil and cone fused infinitely strongly, etc.

How so?

I have a mass, I apply a momentary positive force to it, and it starts to move and keeps moving. I apply a momentary negative force of the same duration and it stops. (Note that in this case, even though the overall signal has had zero mean level, the cone has moved... this needs working out! I sense a computer model coming on, using real music. Others may prefer straight maths ).

Why do I need zero mass, infinite power etc.?

Edit: maybe I spoke too soon about the suspension not being an essential element of keeping the cone in the centre... More thought needed....

 

[andreasmaaan]

How so?

I have a mass, I apply a momentary positive force to it, and it starts to move and keeps moving. I apply a momentary negative force of the same duration and it stops. (Note that in this case, even though the overall signal has had zero mean level, the cone has moved... this needs working out! I sense a computer model coming on, using real music. Others may prefer straight maths ).

Why do I need zero mass, infinite power etc.?

Because with mass you have momentum and inertia, was my thinking. Granted, however, that with zero mass there is no need for infinite motor stength

 

[Cosmik]

Because with mass you have momentum and inertia, was my thinking. Granted, however, that with zero mass there is no need for infinite motor stength

I still don't get it. A finite mass is accelerated by a finite force. If there is no 'stiction' what's the problem?

 

[sergeauckland]

A thought experiment: suppose the cone has no surround/suspension but is on a frictionless track. After a few seconds/minutes/hours of playing music will it still be in the same place it started, or will it have shifted?

My thoughts on this experiment goes like this:- With a frictionless track, and a free cone driven only by the coil, with no suspension, hence no restoring force, it will start moving either forward or backwards depending on the initial current direction. With no restoring force it will keep moving as long as the current is flowing in that direction, regardless of the amplitude of that current. Only air resistance will limit the acceleration. In a vacuum, it would just keep accelerating as long as the current and hence force was applied (assuming also an infinitely long magnetic field). When the current reverses direction, the cone will stop accelerating and eventually slow down, reach zero velocity and start accelerating in the opposite direction. Consequently, after any arbitrary time interval, it will be at a completely different location to where it started.

All this of course is theoretical, as the assumptions made, especially 'frictionless' and an infinitely long magnetic field aren't practical, but it shows that what makes a loudspeaker driver able to work is the restoring force of the suspension, together with any air suspension from a sealed cabinet.

S.

 

[andreasmaaan]

I still don't get it. A finite mass is accelerated by a finite force. If there is no 'stiction' what's the problem?

It's been a long time since I studied physics, and even back then I didn't get very far. Is inertia not an inherent property of any object with mass, even in the absence of friction?

If so, a cone with mass will not respond instantaneously to changes in force applied to it, unless those forces are infinitely strong.

 

[RayDunzl]

 

[Cosmik]

With no restoring force it will keep moving as long as the current is flowing in that direction

I disagree with that part, Serge. I think it will keep moving after the current has ceased (an object floating in space with no forces on it).

 

[sergeauckland]

I disagree with that part, Serge. I think it will keep moving after the current has ceased (an object floating in space with no forces on it).

You're right in a vacuum, but in air, air resistance is a force that will slow it down and eventually stop it. In a vacuum, it will keep accelerating as long as a force was applied, once the force stops, it will stop accelerating but continue at a constant speed. At that point, a magnetic field needn't be present, it will just continue at a constant speed evermore.

I do love these thought experiments!

S.

 

[Cosmik]

You're right in a vacuum, but in air, air resistance is a force that will slow it down and eventually stop it. In a vacuum, it will keep accelerating as long as a force was applied, once the force stops, it will stop accelerating but continue at a constant speed. At that point, a magnetic field needn't be present, it will just continue at a constant speed evermore.

I do love these thought experiments!

S.

So I can now conceive of a sequence that continues driving the cone in one direction only even though its mean level is zero. I suspect the suspension *is* necessary to keep pulling it back - as those fairground flight simulators on hydraulic suspensions are constantly being pulled back, hopefully so slowly that it isn't perceptible to the passengers.

The only thing I can do now is to model it with real music...

 

[JanRSmit]

I'm not sure I agree with #4. If the driver had no inertia at all, it would follow the exact shape of the signal delivered by the amp. In the example of the kick drum, it will reproduce the inertia of the drum itself perfectly. However, when considering woofers, the mass being different, there will be a small difference in the natural movement (hence acceleration) of the driver compared with a drum skin behavior. There is overshoot, and also ringing, depending on the damping. The question being : how small is the effect? Is it audible?
This is actually true of any physical system response :View attachment 25661

Whist true what you state, it states a step-response. A kick-drum is not a step response kind of sound/electrical signal.
SPL refers to the magnitude of the at some point observed pressure variation in air which by itself has a relatively static pressure level. It is in case of a blow on the kick drum a variation with a start and an end.