It is probably the stiffest cone material currently manufactured for domestic high fidelity loudspeaker use, but are metal coned hi-fi loudspeakers truly represent the future of loudspeaker design?
By: Ringo Bones
Contrary to popular belief, it does not favor playing back heavy metal rock music and while discussion still continues on the pros and cons of metal-coned drivers, the number of designers and manufacturers using them does seem to be on the increase since the late 1980s, and it is not just the bass guitar amplifier maker Hartke that’s been famed for using metal coned drivers. In the United States, Platinum, NEAR, Joseph Audio and Thiel are well-known examples while in the UK, Acoustic Energy, Monitor Audio, JPW, Studio Power, B&W (in their up-market Nautilus), Musical Technology and even Mordant Short whose up-market Performance 6 loudspeakers got very favorable reviews back in 2005. In Norway, the popular driver maker SEAS were producing a range of aluminum and magnesium alloy units since the start of the 1990s while the famed metal-dome tweeter, despite its disadvantage of an oil-can resonance between 25-KHz and 30-KHz, is still currently being used in many commercially produced hi-fi loudspeakers and is still produced by a number of manufacturers worldwide despite of the advent of high-resolution digital audio and the vinyl LP revival whose wider-than-REDBOOK-spec-CD frequency response can easily reach the oil-can resonance mode of most metal-dome tweeters. Unlike Red-Book 16-bit 44.1-KHz sampled CDs whose bandwidth stops dead at around 22-KHz.
Fundamentally, the argument for metal hi-fi loudspeaker driver cones rests on its very high stiffness with the potential avoidance of any unwanted resonance or “breakup” in these drivers’ intended working frequency range. To optimize metal cone stiffness, special alloys are used. These are physically hardened and then reinforced by electrolytic anodizing which results in a thick coating of very tough “ceramic” made from the very oxide – i.e. aluminum oxide - of the alloy itself. This anodized surface lends itself to dye coloring, as in the case of Monitor Audio’s well-known “Gold” dome back in the mid to late 1990s. Both main drivers – i.e. bass midrange cones and tweeter domes benefit from the chemical reinforcement process.
When commonly used softer cone materials give or bend in their breakup frequency, and they generally do in their operating frequency range, their resonances must be carefully apportioned and controlled – i.e. damped – to try to attain the highest sound quality. Resonances do color the sound, and their presence is often seen as irregularities in the loudspeaker’s frequency response. Paradoxically, how these errors appear in the measured response may not always be a good indication as to how they actually sound. Like playing Classical string quartets on a typical metal coned hi-fi loudspeaker doesn’t always make the reproduced sound of the Classical string quartet recording sound as if it is always accompanied by a xylophone or by Zildjian and Paiste cymbals.
With softer damped materials – like fibrous paper or pulp card, Bextrene and Polypropylene plastic and bonded matrix composites – bending may be imperfect because these materials don’t act as perfect springs to begin with. A true linear spring recovers immediately from deflection or deformation with near 100% restitution after being stressed. In contrast, many composites and plastics show some memory effect, a slower recovery after bending, and some nonlinear compression with higher forces. This may result in a change in sound quality as sound level is increased. The nonlinear response to high bending forces, and the slowed recovery after bending – technically speaking, it is a form of hysteresis –is a factor in the overall linearity of the driver. In return for the favorable internal damping from a resonance viewpoint, non-metal-cone technology can provide a smooth response, nicely extended to the required upper limit, and then may often deliver a smooth acoustic rolloff beyond this point.
In contrast, the metal diaphragm or cone may be essentially perfect from its bass resonance to beyond the required range, and have no resonance whatever in its lower operating region. Potentially, it has a singular freedom from compression and hysteresis distortion. Subjectively, that manifests itself, if the overall system design is of sufficient quality and a great tonal neutrality, a sound with expressive dynamics and a high dynamic range. Clarity can be very high and low-level detail excellently resolved, in short, fine transparency is a typical of the genre.
However, there’s a price to pay. Like for like, metal cones are generally heavier than the alternative cone materials and magnet for magnet, this often results in reduced sensitivity, often a loss of 2 to 3 decibels. Metal cones are substantially more expensive than paper pulp or plastic equivalents – not to mention the seldom stated fact that metal cones destined for metal coned loudspeakers have a higher reject rate in their production in comparison to paper and plastic.
Finally, while there are no resonances in their primary frequency range, when a metal cone does finally give up and resonate, it lets go with a greater exuberance than a paper or plastic coned driver. So severe is the first resonance that it can rise 10 to 15 decibels above the main response and with sufficient energy to suck power out of the adjoining frequency bands. Thus, a 6.5-inch metal coned driver might resonate at 6-KHz – desirably higher than the 1-KHz typical resonance of a good paper or plastic coned driver – but it does so with such amplitude that the cone output decays prematurely into a pre-resonant suckout, tailing of above 2-KHz. This makes the crossover design of a metal coned hi-fi loudspeaker much more awkward.
This means that if the crossover rolloff isn’t sufficiently fast – i.e. not enough high ordered crossover slope – some of that 6-KHz peak may pop up into the treble band, roughening both the tweeter’s sound and its measured response. Some hi-fi loudspeaker designers resort to anti-resonant traps: electrical filters that seek to notch out the resonance and remove it from the system’s sound. This may seem like a brilliant engineering solution, but it limits the loudspeakers’ compatibility with power amplifiers – i.e. single-ended triode amplifiers with a low damping factor doesn’t like these kind of speakers.
Ultimately, it is up to the loudspeaker designer to make the best choices in the overall system build, regardless of the cone technology employed. Excellent sounding high fidelity loudspeaker systems have been produced over the years that use every conceivable combination of driver technology. Just hope that if you finally found your ideal hi-fi loudspeaker, sound quality wise, it is within your range of affordability.