Eric Alexander behaves as if he’s a great speaker designer, but he proved the opposite with his own speakers, so the evidence is out there.
He makes a feature of using a tweeter array to perform mid-range duties.
The argument is that they’re lighter, so they can mechanically respond to the electrical input more quickly, but there are a few problems with this:
First, tweeters are small, so they have move further than a mid-driver, which undoes the supposed advantage.
Second, the ability to react quickly is reflected in the frequency response. If it's flat – it’s right.
Third, speaker drive units are dynamic transducers, not static, which is a crucial difference that few understand, including Eric.
Finally, if it really was a good idea, everyone would do it.
Here’s what dynamic means. At very low frequency, the time varying displacement of the drive unit diaphragm is roughly in phase and in proportion to the applied electrical signal. The resistance to motion comes from the mechanical and acoustic suspension. The displacement amplitude is roughly constant with frequency, and the acoustic output increases with frequency. In this state, a light drive unit would indeed respond to the electrical input more quickly than a heavy drive unit. This is static transducer behaviour, and this is what happens below the drive unit’s normal operating frequency range (the pass band).
Fortunately drive units don’t work like this in their pass band, otherwise you’d never get a flat frequency response. Within their pass band , drive units work as dynamic transducers, and the resistance to motion is predominantly the mass, not the suspension. The force generated from the electrical signal into the motor goes into accelerating the heavy drive unit back and forth, and it’s that acceleration of the diaphragm and hence the air that creates the loudness – not the position, nor the displacement, nor the velocity. When a signal is applied the drive unit starts accelerating and when the signal is removed the drive unit carries on what it’s doing – moving, stationary, whatever.
So – reducing the mass must improve the “speed”, right? Wrong – it increases the efficiency, not the speed or the bandwidth. The drive unit accelerates the instant the force is applied, not after it’s had time to move, it’s immediate. Reducing the mass will increase the acceleration, so you get a higher output for a given input, which is higher efficiency. The upper frequency limit is generally constrained by the driver’s ability to maintain pistonic motion, and not it’s mass.
So reducing the driver mass doesn’t make it faster. Proper speaker designers know this, and unless it’s all a cynical con, Eric clearly isn't one of them.
