Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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RE: You local dudes have completely run out of ammo. So sad. Maybe take up macrame.

Posted by tomservo (M) on May 10, 2025 at 18:01:12

In Reply to: RE: You local dudes have completely run out of ammo. So sad. Maybe take up macrame. posted by Geoffkait on May 10, 2025 at 14:57:55:

How a woofer works would be the simplest thing to describe but even at that parts of it are not that obvious, but is appropriate for propeller talk.

Radiation
The acoustic radiation load on the radiator is actually quite small and it changes with frequency because the wavelength changes with frequency and so the radiator is effectively getting larger (compared to the wavelength) as the frequency climbs.

A woofer cone is tiny compared with bass wavelength BUT higher up, at a point they refer to as K=1, the radiator is about 1 wavelength in circumference and that point and above increasing radiation load with F stops.
I use horns at work often because one can look at it sorta like coupling the tiny driver to the radiation load of the big end .

How to make the velocity roll off to make a flat response woofer?
In order for the woofer to have flat response above the low end, the motion Velocity of the radiator has to fall off to offset that increasing efficiency below K=1

The motor does this;
The "wire in a magnetic gap" VC motor produces a force proportional to current and this is called BL and one way to look at this is Newtons of force per amp of current. The greater the force per AMP, the greater the Voltage it produces proportional to it's velocity if you move it and these two go hand in hand. The Voltage the motor produces if moved is also called Back EMF which some think is bad but in reality is only the inseparable counterpart of the force per amp property.

Our input signal is a Voltage that represents the acoustic pressure in the signal.

The motor has another part to it, the resistance of the wire its wound with and electrically equivalent it's in series with the motor / between the source and motor.

IF you blocked the woofers motion, you would see this series resistance and this resistance limits how much current flows when blocked with a given voltage input.
It turns it into a constant current = constant force vs frequency device with no motion.

Constant force means the motor is in the constant acceleration mode instead of velocity. Un blocked it is moving the mass of the motor and cone still constant acceleration and flat response.
One hard to picture fact is that one can glue a heavy lead ring to the woofer voice coil former and while it changes / lowers the sensitivity and low corner IT DOES NOT effect the high frequency response shape or corner.

The cones motion is constrained by the suspension and cabinet air stiffness at the low end, while the moving mass constrains the Velocity at the top end.
If you examine the impedance curve you see at box resonance that the two reactive things (mass as a parallel capacitor and a parallel inductor as the compliance) are equal but opposite and cancel out and the peak in the impedance curve represents the very slight radiation load and mechanical losses. On either side of the peak, the falling impedance eventually reach the wire resistance (except at the high end where the smaller series inductance of the wire comes into play)

IF you make the motor have the right strength for the box and driver stuff, you get flat response down to the low knee (Fb) which then rolls off at -12dB/oct. A woofer motor that is not strong enough produces a bump at the low end, a motor that is too strong has a rolled off (over damped) low corner.

This applies to the simple end of a sealed box woofers response where it's easier to describe.

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