Soft Dome vs. Metal Dome Tweeters - Audio Myths

In the first of our common audio myths series, and I said that I’d like to make this an on-going series. We talked about woofers and if woofer size matters. Does it actually mean bigger woofer equals bigger bass or more bass? And we dispelled that common myth.

Let's shift to the other end of the speaker spectrum and talk about tweeters. Now, this may be one of the most controversial and misunderstood topics about loudspeakers out there.

I suspect there’s going be lots of comments about people telling me I don’t know what I’m talking about, but I’ve been in this industry in for over 24 years now. I have lots of experience. I don’t know where this idea came from. It’s almost like somebody made a comment years and years ago about this topic and it just spreads. It’s deeply ingrained in people in audiophiles especially, that may otherwise look to science, look to some sort of support for arguing one side or the other. So, enough about that. What am I talking about?

I’m talking about metal dome tweeters versus soft dome tweeters. And I hear this, not only from customers or potential customers calling and asking questions about our products and the tweeter material, but I see this discussed all the time.

"I don’t like metal dome tweeters because they sound harsh and brittle and metallic."

or

"I prefer soft dome tweeters because they sound smoother."

As if how hard, how stiff or the type of material has anything to do with how necessarily that tweeter is going to sound or going to perform in a loudspeaker.

It’s complete, absolute bunk and nonsense.

Now, let me stop the people right away starting crazily writing going “Oh, my God! He’s nuts. I know what I hear.”

I’m not disagreeing with what people hear. Don’t get me wrong. I’m also not saying that different materials won’t sound different. All loudspeaker diaphragms are part of the speaker and they are the radiating surface. So, of course, differences in materials can lead to a difference in sound.

What I want to dispel is the notion of a blanket statement saying that a metal dome tweeter is always going to sound harsher and brighter than a soft dome tweeter because it is really complete nonsense. It’s like anything else: a well-designed tweeter along with a well-designed loudspeaker system. And remember, we have to filter out the base frequencies with a crossover network so we don’t burn the tweeter out because it is not designed to produce low frequencies or move very much.

So, when you’ve got a well-designed tweeter and a well-designed system, really, you can make a soft dome tweeter and a metal dome tweeter or a hard dome tweeter sound good if they’re good tweeters and well-designed and you deal with the crossover and the system correctly.

Now, I’ll give you an example.

I tried this experiment about 15 years ago because I started hearing this rumour as it crept up from some dealers, people saying “Oh well. You guys use metal dome tweeters.” (This was another company, not Axiom, but it doesn’t matter.) "Metal dome tweeters, they always sound bright, they sound harsh." Well, these dealers were going to be visiting and doing some listening, visiting our factory.

So, I decided to do an experiment. I took a good quality soft dome tweeter and one of our great metal dome tweeters that we were using at that time, and I made two identical pairs of little two-way 6 ½ inch woofer bookshelf speakers.

I purposely designed the crossovers so that the soft dome tweeter would have a little bit of lift and brightness of the top-end and I made the balance of the metal dome tweeter neutral, as you normally would.

And in the listening test (which was done totally blind, people didn’t know what they were listening to) the soft dome tweeter was the one that was said to be bright and speedy and harsh sounding.

In the wrong hands, with wrong designs or with not paying attention to the characteristics of a different tweeter, despite what material it might be, any tweeter can be made to sound bad.

So, what is – why do people have this hang up and say that the material is soft or metal dome? And I’ll throw in there that there are other types of materials other than metal and a soft dome. Soft domes can even be made of different materials. They can be made of synthetic fabrics, natural fabrics. And by coating a soft dome tweeter made of a fabric material, I can actually make it as stiff and as hard as a metal dome tweeter.

There are other things, there are different plastics and polycarbonates and things that are used for tweeters and really the sky is the limit.

So, what does it really come down to?

I think that where people are hang up is that they think that metal dome tweeters tend to have fundamental resonances that some frequencies that they will almost uncontrollably vibrate a natural resonance – like a tuning fork, for instance. And they tend to be on a good metal dome tweeter very high in frequencies. Some would argue above the audible frequency band or above 20,000 Hz, which is the limit of what people can hear.

Now, I’m not talking about perception and I’m not talking about how those frequencies can interfere with audible frequencies. I’m not going to go in that sort of textbook level detail right now, but the fundamental issue is because that resonance tends to be very high, it’s a very sharp resonance.

It’s very concentrated usually in a small band, but it’s outside of the audible range. People will happily look at raw tweeter measurements with no compensating wave guide or phase plug or anything like that on it. And they’ll see a spike on the response at 24,000 Hz, or something for example, and declare “Well, that resonance is why metal dome tweeters sound metallic and sound harsh.” And then, they’ll point to the soft dome tweeter, which will probably not have that sort of spike resonance.

We deal with that. We deal with that through the acoustic parts of the tweeter. We have wave guides, we have phase plugs, we have compensations that can be done in the crossover network. There’s all kinds of things that we can use to deal with that peak.

Now, I’m going to turn the tables for a second and say “Well, the soft dome has got just as bad problems is most cases."

One thing is that it’s not as pistonic because it’s a soft material. And by pistonic, I mean, at all frequencies, ideally, we’d like the entire tweeter dome to move in a uniform fashion and the entire dome to move at all frequencies. The reality is that that’s not the case. That’s not what happens. If you looked at actual laser imaging of a moving tweeter, you would see at different frequencies that it starts to bend. There are many, many nodes and bending modes that happen in the dome itself at different frequencies.

That’s for any material, it doesn’t matter. You can make the stiffest, hardest material on the planet and it will still have these resonant or bending modes. The difference, though, is that on a metal dome tweeter, they tend to be less extreme and they tend to be at higher frequencies because of stiffer material. And they tend to – a lot of them – be outside the audible listening range.

If we go now to our soft dome tweeter, it will tend to actually have much more severe modes and there’ll be more bending going on at different frequencies. Now, these don’t cause really sharp peaks and valleys like they may in a harder material like the metal dome. But remember, I said that those typically are above 20,000 Hz.

All of those bending modes are going to impact the amplitude response and they will impact it differently depending on how loud the system is playing.

That’s one big thing to consider here is that even though I can show you a static, on-axis measurement of the tweeter, and maybe – maybe – the soft dome measures more linearly.

As I drive it harder and harder, that may change.

And if I actually look at things like distortion of two different dome types or I start looking at time decay or waterfall graphs showing how different frequency bands decay, or the dome actually stops reproducing those frequencies after the signal is cut off, they can tell you a lot about the characteristics of the tweeter.

Like everything else, in any sort of loudspeaker or acoustic design, there are trade-offs. And those trade-offs have to be dealt with. And a good engineer will make the proper trade-off, depending on what goal they are trying to achieve.

One thing that I will say is a huge benefit of a metal dome tweeter over soft dome tweeter, which often doesn’t get talked about in this debate of smooth versus metallic sounding is power handling because the voice coil, is connected directly to an aluminum former that it’s wound on, which is now directly bonded to the metal dome.

That entire thing is acting as a heat sync and is able to pull heat away from the voice coil and dissipate it by using the dome surface itself.

That’s not going be the case, at least not to the extent of a metal dome, with a soft fabric dome tweeter. So, it’s a benefit because it means less distortion at high levels and better power handling. And it’s one of the trade-offs that we may, as loudspeaker designers say, that’s exactly why I want to pick a metal dome because I need this speaker to handle a lot of power without burning tweeters out.

So, I hope that gave you a little bit of insight. And I hope I wasn’t too technical in all the discussion of modes and distortions and things like that. But, just understand. It’s like anything else. Don’t judge simply by looking at a speaker and saying “Oh, it has a soft dome tweeter so it’s gonna sound soft and I like a brighter sound.” Or “I don’t want to even bother listening to that speaker because it has a metal dome tweeter and I hate how bright and fuzzy and metallic they sound."

That’s not it at all. A well-designed loudspeaker could use either or some other material. A well-designed loudspeaker is, at the end of the day, simply a well-designed speaker. And a good engineer will know how to deal with balancing their different problems and different issues that they may find with different tweeters.

So, thank you a lot for watching. And again, in the comments below, if there’s another audio myth you’d like me to cover, please mention it and we’ll try and take care of it in a future review.