The soft dome vs. metal dome tweeter debate

Many debate is been had about tweeters in general and soft dome vs. metal dome tweeters in particular.

There is no single thing that makes a good speaker. It’s a juggling act between so many things. Which is exactly why the design is done by highly qualified engineers. One of the contributing factors in the whole good speaker equation is tweeters. 

When we received the Yamaha NS-5000 in store a few weeks ago, the Rapallo team was speechless in all the right ways. And it was the tweeter design that stood out. So what makes a tweeter a hit or a miss? 

There are a lot of opinions out there when it comes to good tweeters. Some based on science, some based on an experience, some just based on silly forum nonsense. Fact is that it’s quite the topic for debate. 

Gene Dellasala from Audioholics puts it like this: ‘Everybody’s right and everybody’s wrong and everybody is confused’, which kind of sums it up. 

So, what makes a good tweeter? Shape? Material? What else?

The lowdown on Tweeters

Starting with the basics, tweeters take care of the high-frequency sounds. When listening to music high frequency sounds are typically represented by the vocals, guitars, horns and the likes.

When talking tweeters, it’s good to keep in mind that high frequency wavelengths are quite small and only a few watts can generate piercing levels of sound. Just to prove a point, compare a tweeter whose radiating surface seems almost rigid to a woofer where the cone may move up to 5 centimetres and you know what we are talking about. Because of that, tweeters usually do not require very much power to create relatively loud sound.

But we just mentioned the word ‘rigid’. When designing a tweeter, designers want a tweeter to be light, so it can move quickly and easily. but at the same time it is vital for a tweeter to stop moving quickly when the music ends (damping). Both these things matter for the reproduction of accurate sound. But while you want tweeters to be agile, they also need to be stiff enough to hold their shape and not distort when you decide to turn the volume up. So, really, we want a tweeter to have an optimum balance between stiffness, agility and self-damping which means it can accelerate without flexing and stop very accurately without unwanted resonances.Challenging…

To achieve that manufacturers try to find the best balance between tweeter shape, material used and manufacturing cost. The race is on for the best possible performance at any price point.

Dome Tweeters

Domes are by far the most common tweeter shape. Probably about 95 percent of all speaker systems use dome tweeters, including the Elac Debuts and SVS speakers. You would think all avenues in dome tweeter design have been explored by now, but there are still engineers like Andrew Jones that are working on new shapes, materials and waveguide designs.

Overall, dome tweeters are relatively easy to manufacture and they are mass-produced, mostly in China. Typical dome materials are made in a wide range of products going from plastic, paper, soft materials like silk, metals and combinations of these. And this is where a lot of the debate lies.

Which one is better? Metal domes or soft domes?

Opinions, opinions! Here’s the lowdown. People in the metal dome camp say that metal as a material is easier to control and these days, with research and science to help out, metal domes can be amazing. Critics on the other hand go on about how they sound metallic or just plain bad. Guess what? Both camps are right.

Breakup point

Critics probably are thinking about a cheap metallic dome tweeter. Very often it’s resonance that is the issue. We’re talking breakup point, incorrect dampening, no correct phase plug.

Breakup point? What are we talking about?

Breakup point is kind of the magic word when you are talking about dome tweeter design.

As we said, we’re venturing in engineering territory here, so we’ll try to keep it simple. In normal conditions, a tweeter dome moves as a whole. (This is the ‘pistonic’ area of operation.) At higher frequencies the tweeter dome starts to flex (which is exactly why you want a ‘stiff’ material), leading to resonances, unwanted non-harmonic distortions. This is what is referred to as ‘breakup’. All tweeter domes break-up. The question is: at what frequency and how good is the internal damping to hide the breakup. In other words: there is ‘good breakup’ and ‘bad breakup’.

Metal Dome Tweeters

So with that in mind, we go back to our metal dome tweeters.

Metals that are most used in tweeters are aluminium, titanium and beryllium, all of which are chosen for their high stiffness to mass ratio.

Titanium tends to be the one used in the cheapest speakers and tends to have a more prominent breakup mode. Aluminium is a lot easier to control than titanium, which explains why a lot of companies opt for the aluminium choice.

It’s important to note that it’s not just the material that is important though. Engineers can do many things to tackle breakup. Dome shape for instance matters. The rounder the dome is, the less prominent the breakup mode is. And then there is the rubber surround that is used these days with metal dome tweeters to dampen the response. Add some more speaker gizmos that we won’t go into (like a phase plug) that can be helpful, and you will understand that it’s quite a simplified statement to say that metal dome speakers sound good or bad. A lot lies in the way they are implemented.

But back to the metals used. While aluminium is the most common metal used, it’s beryllium that is the choice for high-end speakers. It’s a very expensive metal to work with and quite toxic too.  What makes beryllium so amazing is that the break-up point is way beyond the audio band. Some are around a 30kHz, which makes them as smooth as a silk dome, but with less distortion than silk domes.

So the thing to remember about a metal dome is, that it’s a tricky one to do well. But when the engineering and execution come together, it can simply tick all the boxes and blow your socks off.

Soft Dome Tweeters

When it comes to the argument for soft dome tweeters, there is a lot validity in the argument that soft dome tweeters sound smoother. Soft domes tend to have more of a convex shape, more of a natural dome. Their breakup modes are less prominent. Silk-dome tweeters for instance tend to break up more often, but more ‘gently’. Some audiophiles feel that makes them more pleasing to the ear even when the break up mode is lower in the audio band (and therefore more prominent) than modes commonly seen in metal dome diaphragms.

And then to complicate things even further, there are hybrid dome tweeters that use a metal dome overlay over a soft material underneath in an attempt to get the best of both worlds. Ah well…

So the whole debate of soft vs metal dome tweeters? It just depends on the design and engineering.

Variations on a theme

Cone tweeters and Horn loaded tweeters are really just variations on a theme.

Very popular in the 60’s and 70’s , the cone tweeters fell out of favour because they were said to not spread the sound as widely as other designs, and the cheap materials used in many factory speakers where thought to do a poor job of accurately reproducing sound. But again, all is in the design, because Q acoustics 3000i series  is using cones with much success.

Likewise, Klipsch are the masters of the horn loaded speaker. Klipsch prides itself in offering a speaker they claim is capable of extreme output levels with ultra-low distortion and astonishing dynamic range. But critics will tell you that colouration levels of horn loaded tweeters are often high.

Do we spot a pattern here?

So…

Obviously, there is a world beyond the cone, dome and horn tweeters. ELAC prides itself in its ribbon tweeters used in the newly released Vela speakers (Sorry, we had to throw this one inJ). The technology behind a ribbon tweeter is very different to a dome tweeter, so we’ll have to go into the detail some other time. All we want to say for now is that they have an ultra-fast response and produce an extremely airy and transparent sound. The downside is that they are rather complex and therefore not cheap. But if it’s audiophile you’re after….

And with that, we leave you.

Image by Audioholics