by Jonathan » Fri Feb 24, 2006 1:45 pm
Metal cones can be very good, but only if done correctly. There is a balance in speaker design when referring to "soft parts", this balance is rigidity vs. damping vs. efficiency. Naturally, they are at odds with one another. Polk Audio had a good analogy on one of their white papers. Comparing rigid materials with well damped materials is like comparing an iron rod with a garden hose. If you had the iron rod, it would be very strong and light for the needed strength, but if you hit it with a hammer, it would ring due to low damping. A garden hose would have excellent damping if you hit it, but it lacks strength. Rigidity is critical for low energy storage and low distortion, while damping is critical for suppressing resonances that can develop. If you were to insert the iron rod into the garden hose, you'd have a well damped, stiff device that would be very heavy, thus low efficiency of the driver.
What I'm getting at is that only a few rigid cone drivers do the job well. A bad rigid cone driver will have resonances throughout midrange that are hard to control, and the driver can sound harsh and fatiguing. The driver can also have resonances past the intended freq. range, but a poor motor design that would still excite these resonances due to harmonics. The SEAS Magnesium cone drivers (Excel, Lotus) are an example of a good rigid cone driver, they have no edge resonances throughout the intended frequency range and have extremely low distortion throughout midrange. Rigid materials like magnesium have an inherently high stiffness to weight ratio, allowing you to cast more material into the cone body for a given weight target. Done right, this can make the cone so stiff that you can "push" resonances outside of the intended frequency range and suppress that resonance with a notch filter, call it damping via electronics if you want to. Sadly, there are more bad rigid cone drivers than good ones, thus the bad reputation. Same goes for metal dome tweeters.
On the other hand, you have well damped materials. Polyprophylene and paper are examples, with paper being the better example. These damp themselves well enough that you have no large upper midrange peaks like you would with a rigid cone driver, which also makes the crossover design much easier and more flexible. The drawbacks are that it has higher energy storage within the intended frequency range and also higher distortion. Due to damped cones being much more forgiving, and there are more examples of good drivers in this category than rigid cones.
It is important to note that most companies fall somewhere in-between. Focal is an example, who makes sandwich cones combining rigid materials like kevlar and fiberglass with well damped materials such as foam and polymer. Dynaudio uses a Magnesium Silicate Polymer composite as their material of choice. Many others use treated paper, mineral filled polypropylene, and other types of cone composites. It is also important to note that most of these drivers tend to be a jack of all trades, master of none. If you have very specific goals for a system, these drivers probably aren't going to do the best job for your intentions.
My opinion of the subject is that most biases in audio are developed due to incompetence moreso than personal preference. Rigid cone drivers are more difficult to work with, same as ported enclosures, TL enclosures, AP membranes, ribbon tweeters, horns, and many other examples. For this reason, they get bashed more because most people are incapable of using them to their fullest potential. For that reason, there are more "forgiving" drivers created than ones focused upon superior performance. Take a look at the newer audio design goals and you'll see that most companies are focusing their efforts upon fullrange and "extended range" drivers. I feel that these drivers exist solely to cater to people that are afraid of crossovers. 6-7 octaves from a midbass driver may sound impressive, they ALWAYS sacrifice a ton of performance to get it. In car audio, this can see more use considering the variety of installation options that have to be considered, but the funny thing is that car audio manufacturers aren't doing this and home audio manufacturers are.
Keep in mind that this is rather generalized, not all rigid drivers are the same, nor are "soft coned" ones. Companies differ in composite technologies and manufacturing methods, because of this performance varies by a large amount.
Metal cones can be very good, but only if done correctly. There is a balance in speaker design when referring to "soft parts", this balance is rigidity vs. damping vs. efficiency. Naturally, they are at odds with one another. Polk Audio had a good analogy on one of their white papers. Comparing rigid materials with well damped materials is like comparing an iron rod with a garden hose. If you had the iron rod, it would be very strong and light for the needed strength, but if you hit it with a hammer, it would ring due to low damping. A garden hose would have excellent damping if you hit it, but it lacks strength. Rigidity is critical for low energy storage and low distortion, while damping is critical for suppressing resonances that can develop. If you were to insert the iron rod into the garden hose, you'd have a well damped, stiff device that would be very heavy, thus low efficiency of the driver.
What I'm getting at is that only a few rigid cone drivers do the job well. A bad rigid cone driver will have resonances throughout midrange that are hard to control, and the driver can sound harsh and fatiguing. The driver can also have resonances past the intended freq. range, but a poor motor design that would still excite these resonances due to harmonics. The SEAS Magnesium cone drivers (Excel, Lotus) are an example of a good rigid cone driver, they have no edge resonances throughout the intended frequency range and have extremely low distortion throughout midrange. Rigid materials like magnesium have an inherently high stiffness to weight ratio, allowing you to cast more material into the cone body for a given weight target. Done right, this can make the cone so stiff that you can "push" resonances outside of the intended frequency range and suppress that resonance with a notch filter, call it damping via electronics if you want to. Sadly, there are more bad rigid cone drivers than good ones, thus the bad reputation. Same goes for metal dome tweeters.
On the other hand, you have well damped materials. Polyprophylene and paper are examples, with paper being the better example. These damp themselves well enough that you have no large upper midrange peaks like you would with a rigid cone driver, which also makes the crossover design much easier and more flexible. The drawbacks are that it has higher energy storage within the intended frequency range and also higher distortion. Due to damped cones being much more forgiving, and there are more examples of good drivers in this category than rigid cones.
It is important to note that most companies fall somewhere in-between. Focal is an example, who makes sandwich cones combining rigid materials like kevlar and fiberglass with well damped materials such as foam and polymer. Dynaudio uses a Magnesium Silicate Polymer composite as their material of choice. Many others use treated paper, mineral filled polypropylene, and other types of cone composites. It is also important to note that most of these drivers tend to be a jack of all trades, master of none. If you have very specific goals for a system, these drivers probably aren't going to do the best job for your intentions.
My opinion of the subject is that most biases in audio are developed due to incompetence moreso than personal preference. Rigid cone drivers are more difficult to work with, same as ported enclosures, TL enclosures, AP membranes, ribbon tweeters, horns, and many other examples. For this reason, they get bashed more because most people are incapable of using them to their fullest potential. For that reason, there are more "forgiving" drivers created than ones focused upon superior performance. Take a look at the newer audio design goals and you'll see that most companies are focusing their efforts upon fullrange and "extended range" drivers. I feel that these drivers exist solely to cater to people that are afraid of crossovers. 6-7 octaves from a midbass driver may sound impressive, they ALWAYS sacrifice a ton of performance to get it. In car audio, this can see more use considering the variety of installation options that have to be considered, but the funny thing is that car audio manufacturers aren't doing this and home audio manufacturers are.
Keep in mind that this is rather generalized, not all rigid drivers are the same, nor are "soft coned" ones. Companies differ in composite technologies and manufacturing methods, because of this performance varies by a large amount.