The Science Behind KEF's Vented Tweeter

 

You've heard us proudly extol the virtues of our vented tweeter, but here's an in-depth discussion of what exactly a vented tweeter is and why your ears deserve them.

 

The Problem the Vented-Tweeter Solves

Air build-up behind a tweeter dome results in excessive force pushing back on the tweeter dome which in turn develops unwanted harmonics.

 

The Details

In order to be fast enough to properly replicate high-frequency vibrations, tweeter domes are far smaller than mid-range drivers (and certainly low-frequency drivers), which also means the volume of space behind the tweeter dome is also very small. Since soundwaves vibrate off the front and back of a driver, this makes tweeters especially vulnerable to the non-linear vagaries caused by pressure build-up behind the tweeter dome itself.

 

In typical tweeter designs, the energy that radiates out from the rear of the tweeter dome has no where to go, so since it exists and has to do something (every action has an equal and opposite reaction) it pushes back on the tweeter dome. When the dome moves backward the volume of air behind the dome is decreased causing the pressure behind the dome to increase which in turn causes the return pressure on the dome to become non-linear. That non-linearity produces unwanted harmonics in the signal.

 

The Solution

By increasing the cavity volume (without changing the essential shape and size of the tweeter itself) we have reduced to the point of elimination this pressure build-up and subsequent pushback, resulting in better high-frequency response, clarity and articulation.

 

The Science

  • • When the tweeter is resting (not creating sound) the air pressure behind the dome is equal to the air pressure in front of the dome, so there is no force on the dome from air pressure
  • • When the dome moves backward as it vibrates, the volume of the space behind the dome shrinks (the cavity volume decreases)
  • • This reduced cavity volume increases the pressure on the dome (from the rear cavity)
  • • The change in pressure causes a restoring force that pushes the dome back to its original, resting, position
  • • This force becomes a non-linear "spring" that because of its non-linearity, causes harmonic distortion

 

The science behind the problem of cavity volume behind the tweeter dome can be figured out through the Ideal Gas Law.

 

In the following video, think of the plunger as the tweeter dome and the barrel as the assembly the tweeter dome sits inside of. 

 

The smoke from the incense helps us visualize what happens to the air behind the tweeter dome as it moves in and out. With little or no pressure behind the dome, the restoring force is proportional. If we inject a steady signal into the tweeter, say at 3kHz, with a proportional restoring force, no harmonics are created and all we get out of the tweeter is the 3kHz signal. As the pressure builds, the restoring force on the dome is increased and harmonics are created. Those harmonics are not part of the music we are trying to replicate and are processed by our ears as harmonic distortion. 

 

All Of This Science Is Messing With My Dua Lipa, Or Is It?

In theory, the solution is to make the cavity behind the dome bigger but that's not as easy as it sounds: We still have to create a tweeter that is responsive and actually sounds good in it's intended frequency range.

 

With all of this in mind, and with a lot of notes on scraps of paper and calculations that would make the heads of mere mortals explode, our engineers have placed a sealed duct (vent) behind the tweeter dome assembly which serves to increase the volume of the cavity. Acoustic damping material that neutrally absorbs the rear-firing sound energy was added to the vent, eliminating a build-up of pressure and standing waves in the expanded cavity space.

 

There's more to it of course, but we'll let all those speaker companies that don't have vented tweeters figure out how to do it on their own. 

 

 

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