Metal drive units. What's all the fuss about?

[Lt Cdr Data]

the tweeters in the harbeths and mission 752 original are seas, out of interest.

I have had a few including the above with metal domes, and I wouldn't have a speaker generally with them, those were the best I have heard, and were fine, with a hint of sibilance on the missions, the harbeths still had a signature, I woudln't say bright, just a certain quality.

They do have a distinctive sound, can't put it into words, but I dislike them.

with woofers, had jamo concert 8s, one of my fave speakers, and they give really solid cracking bass, which I think is a feature.

the idea I think is that they can be lighter (alloy) and IIRC stiffer than paper?

I don't think you can beat paper woofers, its an odd thing, again, perhaps its my imagination but the older fashioned things seem to give more natural sound, technology does seem to make things sound less musical to my ears in general.

I like kevlar, carbon less so, they are all different, yet similar.

 

[bottleneck]

the idea I think is that they can be lighter (alloy) and IIRC stiffer than paper?

.

metal cannot be lighter than paper surely?

nb

Is the prevailance of metal tweeters not more about the fact that they can be mass produced cheaply? (is that the case? I dont know)

 

[PeteH]

Take a look at the "cumilative spectral decay" plot. Notice the four distinct back-front ridges on the plot (from below 10khz up to 20khz) ? This is ringing. What is significant is that they are there when the other frequencies around them have ceased to show on the plot. Hence they are "standing out" for our ears to hear.

That's interesting - thanks a lot GTM. As you noted though, despite the appealing bell analogy this still doesn't explain why metal domes should have universally generalisable properties as the ringing of a particular unit will also depend on its construction, but it does shed light on the issue of how different materials may act differently.

So just to make sure I've got this clear, the cumulative decay plot represents the drive unit continuing to move back and forwards - ie. continuing to operate as a piston - after the impulse from the magnet is removed?

 

[7_V]

metal cannot be lighter than paper surely?

Chris, aluminium cones are generally stiffer than paper for the same weight. Paper cones are generally far cheaper to produce.

PeteH, just to get to your basic question. If there wasn't significant flexure in drivers, ie if they acted as pistons, we wouldn't be able to hear the characteristics of the material used. But there is.

If they acted like pistons, the dispersion at high frequencies would be very beamy indeed, so the 'perfectly' rigid, lightweight cone may not be ideal anyway. Some 'good' drivers, like the Manger, don't even try to act like a rigid piston.

The material will influence the characteristic break-up sound (flexure) but the shape will also have an influence and will affect the cone's stiffness for a given mass and thickness.

GTM is right on the money with his descriptions of ringing, breakup and internal resonances. As far as cumulative decay plots are concerned, they measure the whole sound from the speaker and don't differentiate between the sound produced by the cone acting as a piston and that produced by break-up.

The general issue is more complex because there are basic differences between cones and domes, particularly when you examine the diffractions and reflections of the ripples as they reach the (usually rubber) surround of the cone or dome.

In addition, the spiders (suspensions) are not completely linear and exhibit a significant amount of friction, as can the surrounds themselves.

Various damping solutions, such as C37, can change the characteristic sound during flexure or break-up.

 

[Markus S]

IIRC (one of?) the original metal domed tweeters was fitted to the Celestion SL6/600 and that had a reputation for being a bit dull.

Paul

As well it ought to have. It was shelved down 2 dB against the bass/mid driver. And its oil can resonance was tuned out by a notch filter which featured hand-matched components for each speaker.

 

[PeteH]

GTM is right on the money with his descriptions of ringing, breakup and internal resonances. As far as cumulative decay plots are concerned, they measure the whole sound from the speaker and don't differentiate between the sound produced by the cone acting as a piston and that produced by break-up.

Thanks Steve - I realise that the plots must necessarily measure the whole sound, but I wasn't clear on how significant was the contribution of non-piston behaviour, or flexure if that's the correct term. So in fact tweeters are flapping about all over the place in general usage, and the behaviour isn't remotely "ideal" in the sense in which I described it earlier?

 

[Graham C]

'So in fact tweeters are flapping about all over the place in general usage, and the behaviour isn't remotely "ideal" in the sense in which I described it earlier?'

All drivers over the upper part of their useful range. There is no big advantage in making a driver 'pistonic' over its full bandwidth and beyond. You would sacrifice lots of other parameters [like mass] in the process. The nearest to pistonic was probably the KEF B139 bass driver which is just solid polystyrene foam block, with a 'kitchenfoil' skin. It only works up to a few 100Hz. Any sort of non flat driver [ie cone/dome] is better off in controlled [damped] bell-mode resonance, as it will help smooth cavity and pipe response irregularity which occur due to its shape regardless of other factors. As 7v said, controlled narrowing of the moving area allows the driver dispersion to be wider

 

[Lt Cdr Data]

Is the prevailance of metal tweeters not more about the fact that they can be mass produced cheaply? (is that the case? I dont know)

to understand this a little, one has to look back around ls3/5a time, I think prior, most were paper, and paper, whilst being a cheap product, can be a bit fiddly in drive units,

its the consistency, the way its made, then bextrene came out, and its repeatable, and fairly cheap, not cheaper than carbon or hi tech stuff, polypropylene now rules the roost.
Cheapish, repeatable, durable, sounds good, less break up than paper.

So I think metal is similar, repeatable, not necessarily cheap, but the ones you see so often must be, remember, of course in mass production economics is everything.

Perhaps a rough analogy is a tennis racket or golf club,now its all carbon and titanium, at the high end, is that cheaper to make than wood (perhaps if the wood is premium, sawn, treated, varnished, turned by hand)

...or maybe a roller coaster...wooden vs. metal, brings me back to the old fashioned stuff argument, note in these cases, traditionalists would always point to the incomparable feel of a wooden wood or roller coaster over a 'metal' wood or big dipper.

 

[merlin]

There was a slightly (only slightly) less pricey AR system driving some ProAcs, Michael.

Streuth Michael, you must really be earning these days

For the record the amps used with the Flabbers cost £40K. Those in the Proac room just £9K (UK prices of course). So I am glad you think the difference is "slight" You'll be buying that yacht next.

I understand that the speakers were the D80 btw. Again rrp £7,995 as against the £23,000 of the Flabbers. So £17k plays £63K - glad you could hear a difference mate - what cables were they using BTW

 

[michaelab]

I understand that the speakers were the D80 btw. Again rrp £7,995 as against the £23,000 of the Flabbers. So £17k plays £63K - glad you could hear a difference mate - what cables were they using BTW

Thanks merlin, I didn't know the price of the cheaper system but I was trying to impress on people that it was also a megabucks system well beyond the means of most of us.

The "cheap" system was using Nordost (you can just see a bit of it in the pic), not sure which type. I think the SF system was using Transparent, again, not sure which type but probably the top of the range if the rest of the system is anything to go by.

Michael.

 

[PeteH]

Any sort of non flat driver [ie cone/dome] is better off in controlled [damped] bell-mode resonance, as it will help smooth cavity and pipe response irregularity which occur due to its shape regardless of other factors.

Well now that's very interesting. I realise I could have just gone and looked all this up BTW, but I was feeling a bit lazy.

I still suspect that pre-belief plays a large part in how many people approach metallised drivers, but at least I've got a slightly better idea of what's going on now.

 

[GTM]

So just to make sure I've got this clear, the cumulative decay plot represents the drive unit continuing to move back and forwards - ie. continuing to operate as a piston - after the impulse from the magnet is removed?

No. It is due to the actual cone/dome itself actually emitting those frequencies. This happens at any displacement of the cone/dome. It's mechanical and nothing to do with the electrical input to the drive unit.

You know how if you pass your finger around the rim of a good quality wine glass it "sings". Same principle. Now imagine your wine glass doesn't have a stem and is the dome of your tweeter. It would still be able to produce these frequencies even if the dome isn't moving back and forth. Of course this is an exagerated example, and the actual modes causing the ringing in drive units are more complex, but the principle is the same.


GTM

 

[GTM]

I still suspect that pre-belief plays a large part in how many people approach metallised drivers,.....

You'd be right to think so too. There is a general perception that metal dome drivers must sound "metalic" so people often hear what they want to.


GTM

 

[PeteH]

No. It is due to the actual cone/dome itself actually emitting those frequencies. This happens at any displacement of the cone/dome. It's mechanical and nothing to do with the electrical input to the drive unit.

With you - thanks.

 

[technobear]

I still suspect that pre-belief plays a large part in how many people approach metallised drivers, but at least I've got a slightly better idea of what's going on now.

I deliberately approached the Bristol Show this year with an open mind on cone materials. I listened to a few speakers with metal dome tweeters. Nearly all of them played very nicely, smooth and tuneful, until something came along (usually a piano, but not always) that caused me to wince and place my hands over my ears. I didn't experience this effect with any silk dome tweeters.