How fast are your drivers?

Oct 23, 2018

What are we talking about when we ask about a drivers’ “speed”?

Are we discussing acceleration or velocity?

Question: I’m partial to 12″-15″ because they have a tighter low end. The smaller the speaker the quicker it moves with the air and that gives you a tight sound. The smaller the speaker, the faster the voice coil moves, right?
So… do a smaller driver’s voice coils move faster than a larger driver’s voice coils? Or is the speed of your speaker dependent on the frequency of the signal you’re sending?

Answer – David Lee: The smaller speaker has a lighter, less massive diaphragm and therefore can accelerate more quickly. A larger diaphragm/cone can potentially move at a higher velocity, because it has a greater distance to cover. It’s like drag racing vs. speed trials. A tweeter makes a better drag racer, a woofer may achieve a higher top speed.

However, the word “speed” in this context is somewhat misleading. The relevant factors are acceleration and velocity. Also, there is the assumption that a smaller driver’s voice coil and diaphragm are less massive, but this assumption can be misleading, too. There are relatively small drivers with very high-mass cones.

The driver diaphragm/cone’s peak velocity occurs as it passes through the midpoint of its travel and the peak velocity is influenced by the demand for amplitude as well as frequency. A diaphragm making a 1 mm peak to peak excursion 200 times per second will have a lower peak velocity than an identical diaphragm making a 5mm peak to peak excursion 200 times per second. They have to repeat the cyclical motion over the same time but the one that has to go farther must travel at a higher velocity to complete the journey in that time.

If two diaphragms making these excursions are the same mass but not the same size (the larger one might be carbon fiber, and yes, factoring in the air mass) then the difference would be the displaced volume of air. Thus the larger diaphragm will generate a higher SPL for the same excursion. There would be no difference in velocity or acceleration.

If the two diaphragms making these excursions are the same size but not the same mass, the heavier one will lag behind the lighter one in phase due to the inertia of the additional mass. In other words, it will complete the cycle 200 times per second but it will arrive at any given point in the excursion slightly later than the lighter diaphragm.

If the motors of these two same-size drivers are identical except for the mass of the cone, the heavier cone will require more energy (power) to achieve the same excursions. In other words, the heavier one will be less sensitive.

These factors are independent of the driver’s interaction with the enclosure.

A smaller and therefor lighter diaphragm can accelerate, decelerate, stop, reverse direction and repeat the process with less energy loss at higher frequencies than a larger, heavier diaphragm. At higher frequencies, the lighter diaphragm has an advantage.

A smaller diaphragm displaces less air than a larger diaphragm and so to produce the same SPL as a larger diaphragm it is necessary for the smaller driver to make larger excursions, which results in higher peak velocities. Under matched conditions, such as in a sealed box, the smaller driver must move farther and therefor faster to produce the same SPL as a larger driver operating at the same frequency. In such a situation the speed of the smaller diaphragm isn’t an advantage, it is a consequence and a necessity.

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