Cone Material Discussion
IntroductionI'm posting this discussion to test a theory that I have that deals with working with and filtering metal cone drivers. The comparison will use the Seas CA18 and Seas L18 which are basically identical except for the cone material. The L18 uses an aluminum cone and the CA18 uses a coated paper cone. I've worked with several different aluminum and paper cone drivers and have my opinion of their sound in general. I will try to explain some of the differences that I hear when working with these two different cone materials and what I attribute them to. Most of the discussion involves harmonic distortion and how higher odd order products in metal cone drivers can excite the higher frequency resonant modes of the cone. I will present examples of the distortion curves for each driver and give my summary on the comparison. I'll discuss how this harmonic distortion can affect the sound and ways to tune the crossover for a more natural, smoother sound. Remember, these are all just theories based on my experience and limited knowledge. Some of this stuff might be obvious and some of it might be wrong. I invite you to contact me if you disagree with any of my theories.
CA18 versus L18The plot below shows the harmonic distortion measurement for the L18 and if you move your mouse over the plot you can see the distortion of the CA18. These plots are taken from John Krutke's site, www.zaphaudio.com, where there is a detailed comparison between several drivers. These plots give the 2nd through 5th order harmonic distortion levels. Harmonic distortion occurs when a higher frequency harmonic is produced by the fundamental tone. For example if the fundamental tone is 1kHz, then the 2nd order harmonic would be 2kHz, the 3rd order harmonic is 3kHz and so on. The plot tells you what the sound level of that tone is at each frequency relative to the fundamental "1st order" tone. The lower the harmonic level, the better.
What I want to focus on in these plots is the distortion in the mid to upper midrange region. The L18 has a cone resonance which results in a large peak in the fundamental response (not shown in the 1st order harmonic but shows up in the frequency response plot which isn't shown). This resonance is excited by lower frequencies when there is a multiple of the fundamental frequency that coincides with the resonance mode. Thus, there are peaks in the 2nd, 3rd, 4th, and 5th harmonics as you go down in frequency. The 5th order harmonic is the most threatening because it is located lower in frequency (around 1.5kHz) and is pretty large. The 3rd order harmonic is also a problem but is higher in frequency so you can filter most of it out if you have to.
The CA18 has a much smoother distortion response with very few peaks. In the midrange region there is much less high order distortion but you have a little bit more 2nd order distortion. I think I can attribute most of this to the cone material if you read further. Another advantage of the paper cone is that the low order distortion is lower in the bass to midbass region.
Intro to Cone MaterialsI'm going to attempt to try to explain why these two different types of cones have such different distortion profiles. We must consider a few basic properties of cone materials including stiffness and damping. An aluminum cone is very stiff with very little damping. A paper cone is less stiff but has a bit more damping. Now the stiffer cone will resonate at a higher frequency (think of a guitar string... the tighter, stiffer string plays a higher note than a looser string). However, since there is very little damping it tends to have a high Q resonance (sharp, tall peak) while a softer, better damped cone will have a low Q resonance (wide, small peak) that will show up lower in frequency. These resonances also have to do with the speed of sound as it travels through the cone material and sound travels faster in aluminum hence the higher frequency resonance.
If we examine the distortion properties of the paper driver you'll notice that it tends to have less high order distortion in the midrange but slightly more low order distortion. The ear senses different types of distortion in different ways. Second order distortion is known to have a very smooth, melodic sound and your ear can tolerate high levels of 2nd order distortion sometimes without even noticing it because it can be so smooth sounding. Third order distortion is a bit more harsh but can add a sense of detail to the sound. Odd order distortion in general sounds more harsh but is often perceived as more detail. As you go higher in order it is easier to detect the distortion at lower levels. So based on the plots above the 5th order distortion would be the easiest to detect and the harshest as well. The L18's aluminum cone has some strong odd order harmonics that show up in the upper midrange region and my theory is that those harmonics can make the driver sound harsh or even misconstrued as more detailed. The paper cone has more 2nd order distortion in this range which would give it a smoother sound with less artificial detail (more like a tube amp).
How They SoundI've used several paper and aluminum cone drivers in the past and I know that it isn't good to generalize based on a small sample set of drivers but the analysis is so consistent that some generalizations are warranted. All of the aluminum cone drivers that I've used have had a very neutral sound with a lot of detail which made them very analytical sounding. Depending on how the crossover was tuned they could sound harsh at times. Paper cones that I've used tend to sound smoother with a little less detail and some can have a warmer presentation relative to the aluminum cone drivers. I've found that a different type of crossover response shaping is required for each cone material to get a nice sound and I'll discuss this in what follows.
Adjusting Crossover For Cone TypeI've found that it sometimes helps to do a bit of response shaping when working with aluminum cone drivers. It is best to cross as low as possible so the passband is below the point where the high order harmonics show up (less than 1.5kHz for the L18). However, this isn't always possible so what I've found is that it is best to tune the crossover to get a smoother sound. If you go with a flat response some of the high order harmonic distortion can excite the resonance mode of the driver and give an added edge to the sound (that shouldn't be there). This edge shows up in the stopband of the midwoofer filter so there is nothing you can do to reduce it because if you were to filter the midbass or tweeter more in this region you will lose the sound that should be reproduced in that region. I've found it is better to stop it at the source by attenuating the range where the fundamentals are located that excite that resonance range. It really is a balancing act but I've found if you attenuate the top end of the midwoofer response just a little bit (less than 2 dB) then you can get a smoother sound with less edge.
I've found that paper cones are much easier to work with because you don't have to worry about high order distortion as much. I've found that a flat frequency response with a paper cone midwoofer can sound fine with very little adjustment although sometimes the lower midrange can be a bit warmer relative to an aluminum driver with the same response.
ConclusionsSo if you haven't guessed already, I prefer paper cone drivers over aluminum cone drivers in general. I think my analysis explains why but probably has a few flaws and a few things I might have overlooked. My main belief is that odd high order distortion is a bad thing especially in the midrange region because it can add harshness to the sound that is often perceived as added detail and clarity. Even low order distortion is much nicer sounding to my ears and is less noticeable. In general solid state amps can tend to have more odd high order distortion and tube amps often have more low (2nd) order distortion especially when driven into clipping. I welcome any comments or corrections regarding this analysis.