Yes, transverse waves propagating through the cone of course result in cone resonances - unless the cone is resistively terminated (preferably at the originating end as well as the periphery, i.e. surround), matched to the mechanical impedance of the transmission surface. (And also, smearing at the higher frequencies, given that velocities will be significantly lower than that of sound in air - at least for traditional cone materials.)
I suspect this may be the reason for preferring low Mms - the weakly resistive termination provided by the usual types of cone surround is more effective for flimsier diaphragms, and hence "better control" as regards transient response. (The cone presents a lumped mass only at the lowest frequencies.)
The behaviour of large cones is of particular interest to me, as a fan of OB. Another method of achieving good behaviour in large cones is to make them elliptical. It's a pity those (cheap) 13"x9" EMI drivers are long gone now, as they probably had a lot of promise in this regard (as demonstrated by one of Peter Baxandall's projects):
https://keith-snook.info/wireless-w...ld-1968/Low-cost High-quality Loudspeaker.pdf
I suspect this may be the reason for preferring low Mms - the weakly resistive termination provided by the usual types of cone surround is more effective for flimsier diaphragms, and hence "better control" as regards transient response. (The cone presents a lumped mass only at the lowest frequencies.)
The behaviour of large cones is of particular interest to me, as a fan of OB. Another method of achieving good behaviour in large cones is to make them elliptical. It's a pity those (cheap) 13"x9" EMI drivers are long gone now, as they probably had a lot of promise in this regard (as demonstrated by one of Peter Baxandall's projects):
https://keith-snook.info/wireless-w...ld-1968/Low-cost High-quality Loudspeaker.pdf
Could you elaborate? In most materials speed of sound is significantly higher than in air, as far as I know. And whose preference for low Mms you are referring to?
That would be for longitudinal waves in bulk material, but not for transverse waves in a diaphragm (unless it has very high rigidity relative to density) - and the OP referred to his subjective preference for low Mms in his blog article, so I was offering an explanation.
The most important aspect anyway is to control the transverse waves by using resistive termination, which is NOT what you usually will find in the average midwoofer - as they're mostly designed for large excursion and with a low loss surround for better performance at low bass frequencies. As I said, I'm interested in drivers for OB, which means for upper bass and midrange you want a large driver with low cone mass and damped surround - which may be why those drivers aimed at PA work can be useful.
The most important aspect anyway is to control the transverse waves by using resistive termination, which is NOT what you usually will find in the average midwoofer - as they're mostly designed for large excursion and with a low loss surround for better performance at low bass frequencies. As I said, I'm interested in drivers for OB, which means for upper bass and midrange you want a large driver with low cone mass and damped surround - which may be why those drivers aimed at PA work can be useful.
You can’t really compare such speeds I think. And resistive termination, while it helps mitigating effects of break-up, isn’t beneficial for the pistonic movement of the cone. You even could argue the break-up itself is not desirable, as it introduces nonlinearities. Or that the break-up should be damped as much as possible in the cone itself, such as still is unsurpassed in PP cones (what happened to them…).
What we can hear of such non-desirable behavior is of course another discussion.
What we can hear of such non-desirable behavior is of course another discussion.
Not - as you see that the membrane bulge, its in a bending wave mode...
Pistonic means what the name indicates - the membrane is completely stiff and keeps an unaltered shape through the whole motion - like a... piston 🙂
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That's why I said that modern midwoofers are usually compromised by having to accommodate large excursions.
Of course "break up" is undesirable, but it's the result of transverse waves propagating from the cone centre to the periphery, where they are incorrectly terminated.
Using a lossy material for the cone could be very useful - the result being that lower frequencies would tend to move the cone as normal, while the higher frequencies would propagate outward, but diminishing progressively in amplitude. Net effect would be the effective diameter of the cone would reduce as frequencies increase. Which could be useful if well controlled.
But regarding how existing drivers sound, all I'm saying is that the lighter cones may exhibit better "transient response" because they're easier to terminate.
Well, if you think about it, no membrane is truly "pistonic" - that would imply infinite propagation speed for transverse (bending) waves. It's like saying that there's zero delay from one end of a piece of coax to another - it's just that, for low frequencies, in terms of the timescales involved, the delay is in effect zero. But at higher frequencies, you start to notice the delay, because it may present itself as an appreciable difference in phase.
For the case where the diaphragm is below its first mode resonance, I believe that force (and thus deflection) propagate through the diaphragm at the speed of sound in the diaphragm.
Given a cone angle of about 30 degrees, the speed of sound within the diaphragm needs to be about 1.7x the speed of sound in air to keep up with the sound propagation out of the center of the cone.
At low frequencies relative to the size of the driver, the speed of sound within the diaphragm is not an important factor. Only when the 1/2 wavelength becomes comparable to the depth of the cone does the speed of sound in air, and in the cone material, become an important factor.
So for most stiff cones, when they are operating below the first mode resonance, they are close enough to an "ideal piston" that we can analyze the situation from this perspective.
Does this make sense to you?
j.
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Just to continue with the coaxial cable analogy, because it's quite useful, it's well known that you get standing waves in coax (or any transmission line) when it's incorrectly terminated with a resistor which doesn't match the characteristic impedance of the cable. So it is for a driver's cone, the "break-ups" being standing waves associated with termination mismatch. In this case, it's mechanical impedance of course.
Yes, and that holds true for all frequencies of course. And if the cone is correctly terminated, there won't be any resonance, and the deflection will ripple through the cone until it's absorbed by the damping offered by the termination.
That should be the aim. However, as I said earlier, there may be problems getting the correct termination if the driver has to provide large excursion in order to handle the lowest frequencies.
I think the "weakly resistive" termination of the cone surround and also the mechanical resistance of the spider is a major component of the equation. The observed increase in transient response and clarity of low mass cones is benefitted by lots of factors obviously.
Higher frequencies travel shorter distances with less force transversely across a smaller often curved diaphragm often with a perimeter voice coil operating in a pistonic nature. I wonder if the supposed "issue" of phase delay associated with lateral movement of a wave in this instance is any more of an issue than with low frequency waves.
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Yes indeed, and that's why I make the distinction between the need for large excursions and the need to correctly terminate. The two requirements tend to work against each other.
The OP cited an ancient 12" driver as having a sweeter sound than most similar modern drivers, and this is what prompted me to speculate as to why - resulting in the discussion about lighter cones and/or lossier surrounds.
The air itself can damp vibrations, assuming that the cone material is very light. And horns can increase the apparent mass (across the effective frequency band) by aligning the air next to the cone into a column that has to be pushed longitudinally.
If I recall correctly pushing a small column of air is an easier task for a driver especially once oscillating continuously when coupled to said tube, than pushing a full room worth of air. Combined with a venturi effect producing phase plug, a major increase in sensitivity is achieved compared with a direct radiating cone in air. If our voice box operated in free air, an open baffle per se, it would be far more of an energetic chore to speak and sing presumably.
Go learn about acoustic impedance. Air load on a cone becomes quite impressive at higher frequencies. And about the amplitude of these transversal waves which to you are not break up modes. I’m under the impression you are addressing cones in break up mode. Please don’t continue the discussion about lighter cones being better without a proper physical backup. And there is no such thing as ‘sweet sound’. Sorry for being a little blunt.
How do you quantitatively define resonance in a cone? It is generally accepted that a cone doesn't significantly resonate when the frequency response is smooth and the waterfall doesn't show any unusual behaviour... but it sounds like you have a different understanding?