Driver behaviour, pistonic or ocillation?

[Overkill Audio]

Mass on a spring.

Introduction.

The word "Pistonic" is generally used to describe cone surface behaviour or cone break-up behaviour.
I believe this has distracted us from the real problem i.e. the whole cone / former / coil /spider and surround are all oscillating out of control…. like any mass on a spring should…while the surfaces of the beautiful titanium, ceramic, alloy, diamond, paper or latest fad "unobtainum" cones, domes, ribbons and panels remain "pistonic"!

I believe It's much more important how fast a cone STOPS than how fast it starts!

I first published this theory in a far more comprehensive article on my Overkill Audio website back in 2005 when I was manufacturing high end speaker systems based around the Manger driver, a unique "Bending Wave" driver with a flexible polymer diaphragm.
Back then I had an obvious commercial interest in promoting the benefits of the Manger driver over conventional (i.e. non Bending Wave) drivers.
Now I am in the Fire safety business I have no commercial audio related bias at all and I am happy to share my experiences and the few audio "secrets" that I discovered over the 7 years it took to develop the Ovation, Encore, Angel and Finale loudspeakers.

Below 300Hz or 400Hz the Manger needs to be crossed over to "pistonic" drivers.
In this thread I would like to take a close look at the fundamental operation of "pistonic" drivers, specifically in the 20Hz to 1 Khz range.
Most of my time was spent concentrating on the 20Hz to 500Hz band as I always crossed over to the Manger at 300Hz to 400Hz but 90% of what I discovered below 500Hz applies equally all the way up to 20Khz.

The Problem.
The problem with so called "pistonic" drivers is this, they are never pistonic!
They can not function like a piston i.e. like the alloy pistons in an engine.
All cone, dome, ribbon, panels, stats, co-ax, and compression drivers are actually all "Oscillation" drivers i.e. a mass on the end of a spring. Not a fixed rod with a finite mechanical range of forward and backwards motion.
Let's examine our audio pistons …..
Imagine looking side on at a clear plastic tube containing a ping-pong ball, this ball has a coil of copper wire wound round its circumference. The ball rests in the middle of the tube. There is an elastic band with one end fixed to the ball and the other end is fixed to the end of the tube.
In the middle of the tube there is a ring of magnets around the outside of the tube. Let's assume the magnets are powerful and generate a perfectly symmetrical magnetic field around the ping pong ball and the elastic band (spider and surround) is also 100% linear in its operation. (Big assumptions but pistonic theory needs all the help it can get!)

All this model lacks is the paper cone around the ping pong ball, but that brings in aerodynamics which I'll touch on in a separate post.
We pass a short burst of electric current through the wire, an impulse (to simulate a drum strike) NOT a constant current, and hey presto the ball shoots forward inside the tube!
BUT, after the ball runs out of forward momentum, the ball then shoots back (pulled by the elastic) and OVERSHOOTS its starting point! It then OSCILLATES out of control until all the kinetic energy / potential energy is slowly (compared to the initial transient acceleration) dissipated. The important bit is "compared to the initial transient / acceleration". i.e. The decay time is much greater than the rise time, by an order of magnitude in many drivers!
Music signals are all about transients, amplitude and ….Timing!
All real ( not signal generators and sine waves) sounds, musical or snapping twig etc, are formed by an initial transient of air pressure and then decay back to ambient pressure.
That's the only way we can hear any sound at all, binary on /off, compression or rarefaction of the air, everything else is just decay of air pressure back to ambient. That’s worth a second thought, all the subtle texture, harmonics and audible decay from a piano, cello, harp, drum or any sound at all, is just a very short spike in air pressure, followed by a reduction in air pressure back to ambient. Simple, elegant, beautiful, and yet Oh so hard to reproduce…!!!

So let's play some music through our ping pong ball speaker!
First up one single drum strike. Bang, the ball shoots forward causing a compression of the air in front of it (yes it does have to move through the air and yes aerodynamics do come into play!) it is propelled forward by the electrical impulse and simultaneously it is being retarded by the elastic band (the spider, surround and suspension) which is now storing this kinetic energy as potential energy. So when the ball reaches the end of its forward motion it is now pulled backwards by the elastic band, and will overshoot its original starting point (carried past the start position by its own momentum) it behaves exactly as the laws of physics dictate, a mass on the end of a spring. It will oscillate!
Back and forward until it dissipates all the energy in the elastic band.

So instead of the one clean drum strike you get one clean strike followed by a slowly diminishing number of ghost echoes. Time domain distortion of the worst type.
This is the inherent mechanical failing with all pistonic drivers. Some display gross errors (heavy coned bass drivers) some marginal and some almost undetectable at low SPL's playing simple acoustic music.
But at life like SPL's and with more complex music 99% of all pistonic drivers fall apart, displaying gross ghost echoes or time domain distortion.

Now start to factor in real music i.e. before the driver has stopped oscillating from the first strike, bang here comes another and another and now a double bass and look out here comes the piano…! You get the picture, it’s the compounding of errors one on top of the other which really do the damage.
The wider the bandwidth the driver is covering the worse the problem gets. When a bass / mid driver is trying to simultaneously reproduce a 70Hz piano note, a 700Hz vocal note and a 1,700Hz violin note it must be able to start and stop cleanly from its marks i.e. the zero energy point where the spring (surround / suspension) is not exerting any push or pull force on the driver.
However full range or large bandwidth drivers do have the benefit of little or no crossover to mess things up. The eternal speaker design compromise of bandwidth Vs crossover complexity.

Playing music demands that the cone not only starts in the shortest possible time, playing music demands that the cone also STOPS in the shortest possible time.
The actual start and stop (rise and decay) times must by definition be shorter than the minimum time domain errors detectable by the human ear. There is ongoing research and much debate on this subject but the figure of 25 uS (25 one millionth's of a second) is generally accepted as the threshhold detectable by most people.
Now when you look at the measured decay times of pistonic drivers you see the scale of the problem. A really good dome tweeter takes nearly 30 milliseconds (30 one thousands of a second), a top class (ATC dome midrange) takes 190 milliseconds and a big bass driver takes around 480 milliseconds (nearly half of 1 second!) to settle from a 100 dB impulse signal.

So in summary, it's not how fast a driver starts that counts its how fast it STOPS!

All is not lost, we can still get great results from pistonic drivers but the more we can minimise driver excursion the better, my best solutions will follow in another post...!!!
Please now feel free to defend the pistons of the world!

All the best

Derek.

[markaudio]

Hi Derek,

An interesting read and some of your insight has merit. However, the issues relating to driver design and their electro-mechanical properties is more far complex that you suggest.

Sadly I haven't got time to go into more detail but as a driver maker, I can state the following.

1 - There's some merit in your thinking around "how fast the power-train of the transducer stops". It is reasonable to infer that drivers with power-trains that stop and change direction quickly and easily may acoustically perform better than those that don't.

2 - There's much less merit in the rest of your submission when its related to the new low mass and ultra-low mass driver technologies now entering the market. Advances in cone and damping design, materials and motor technologies have significantly improved the performance of long throw drivers in recent years. These developments particularly apply to more wide band and full range units. Yes, drivers with large mass power-trains can exhibit the anomalies you describe but usually these units are limited to particular frequency ranges, mitigating the effects you describe.

3 - Human perception and interpretation of sound is very complex. It's also totally subjective. The reliability of the empirical data has long been a hot subject of debate within the hard sciences while the soft sciences often remain critical of the most of the research.

Overall, your submission is interesting, but it over-simplifies the properties of electro-acoustic devices.

Don't get me wrong, I think it's good to have these debates. But I worry when conclusions are made about the performance of certain types of drivers on a limited perspective drawn from the very complex interplay between sets of physical, electrical and mechanical properties.

All the best,

Mark.

[Calvin]

Hi,

Derek assumes a driver with open connecting ports, which is not the case in reality. The idea completely lacks the influence of the electromagnetic motor which supplies for drive and damping.

jauu
Calvin

[Overkill Audio]

Hi Mark and Calvin,

Mark,

Thanks very much for your reply and as you correctly point out this is a very complex area and I have indeed simplified the issue.
Hopefully we can explore the new low mass and ultra low mass technology you alluded to in more detail when you have some spare time.
As a driver manufacturer you are in the best position to solve any problems I (or anyone else) identify.
The human ear / brain mechanism is the most complex of all, worthy of its own post for sure!

Calvin,

If I understand your point correctly, you are pointing out that I have not made any allowance for the Bl force acting on the voice coil to control the oscillation.
Yes exactly! This is my point!

The impulse (drum strike) is a brief acceleration force which sends the cone on its way, then BEFORE the cone stops its forward motion, the impulse stops (the drum beat transient stops) but the cone continues on its way under its own momentum!
Does this make more sense, its such a counterintuitive point that I do have trouble describing the effect!

Thanks again for opening this much needed debate.

All the best

Derek.

[Rudolf]

Derek,

the same laws of physics that keep any loudspeaker from working like an ideal piston are keeping the drum set from working the way you are describing it:

"So instead of the one clean drum strike you get one clean strike followed by a slowly diminishing number of ghost echoes. Time domain distortion of the worst type."

Your "clean" drum strike makes the drumhead (which is making the sound, not the stick!) behave almost exactly like the loudspeaker that has to reproduce it. Those clean steep rising and stopping transients you are talking about simply don´t exist in music made with real instruments, but on oscilloscope displays only.

Any decent loudspeaker with a moving mass that is lower than the moving mass of the musical instrument has no difficulty whatsoever to reproduce the music made by that specific instrument.

Are you sure to really understand how the sound of a drum is generated in time and how it decays? What the drum stick does, is to punch a local bulge into the drumhead. Everything else, which makes up the actual drum sound, is distortion and resonating of a piece of hide that is worse than you ever would allow a loudspeaker cone to act. So why making infinitely greater demands on the reproducing instrument than the sound originator meets?

Rudolf

[Overkill Audio]

Hi Rudolf,

Thanks for your reply and the points you make, if I may address them in order.

Your quotes : "the same laws of physics that keep any loudspeaker from working like an ideal piston are keeping the drum set from working the way you are describing it:"

And

"Your "clean" drum strike makes the drumhead (which is making the sound, not the stick!) behave almost exactly like the loudspeaker that has to reproduce it. Those clean steep rising and stopping transients you are talking about simply dont´to exist in music made with real instruments, but on oscilloscope displays only"

((1) If I read your points correctly we need to examine exactly what is happening with our drum strike in order for me to more clearly disagree with you!

Imagine a round, flat, calm pond of water.
Now drop a stone into the centre and observe how the ripples spread out in a SIMPLE circular pattern.
Now look at the COMPLEX interference pattern develop over the whole surface when the ripples reach the edge of the pond and are reflected back in towards the centre. Multiple peaks and troughs all over the surface.
This is exactly the same as our drum strike (assuming we hit the drum in the centre) and this is also how the human ear drum works, (and the Manger driver!!) NONE OF THIS IS PISTONIC!!
This is all " Bending Wave " motion.
(The mechanical / biological / electrical function of the ear / brain is fascinating in its own right and will make a great thread for some Audiologist if there are any on the forum. )

All sounds only have two elements : (a) an initial sharp increase in air pressure (compression) followed by a reduction (decay) back to ambient air pressure. So all sounds do indeed have those impulse / step response graphs you see on computer screens or test equipment. These graphs are the sonic fingerprints of rapid changes in air pressure or "sounds" as we like to call them!

The amount of air pressure increase and the rate of decay are the only two variables which differentiate all sounds.
That is well worth repeating " The amount of air pressure increase and the rate of decay are the only two variables which differentiate all sounds. WOW!
A snapping twig or piano, air pressure and decay. There is no more.
So when our drum head impacts the drum skin, bang, a short sharp compression wave with a high peak (= to say 120dB) is sent directly to our ears. As the initial ripple in the drum skin spreads out and reaches the edge of the drum most of the energy is reflected back in towards the centre of the drum causing a series of complex and diminishing secondary ripples which in turn send out a corresponding series of air compression waves at much lower SPL's until all the energy is dissipated and all falls silent.
If you look at the step response of real sounds (not loudspeaker driver / crossover / electronic systems ) you will see exactly this peak and slow decay pattern. ( The Manger website used to show some of this stuff.)

Your Quotes : " Any decent loudspeaker with a moving mass that is lower than the moving mass of the musical instrument has no difficulty whatsoever to reproduce the music made by that specific instrument. "
AND
" Everything else, which makes up the actual drum sound, is distortion and resonating of a piece of hide that is worse than you ever would allow a loudspeaker cone to act. So why making infinitely greater demands on the reproducing instrument than the sound originator meets? "

This is simply not accurate. I can see why you say this as the true nature of sound is so counterintuitive but I hope my explanation is beginning to make more sense now.

Thanks again for your reply.

All the best
Derek.

[tinitus]

Is there a special reason fore posting here at "fullrange-forum"

I thought this issue was about woofers

edit,
Scott, I wonder how many read your post before deleting it
Well, I did
But I can understand why you deleted it

[Rudolf]

Originally posted by tinitus
Is there a special reason fore posting here at "fullrange-forum"

Sorry Derek,

I did not see that you wanted to discuss this in the context of fullrange drivers. From my point of view a concise answer would only apply for a 2- or more-way system. For a fullrange driver I too see problems to reproduce natural transients.

Rudolf

[Overkill Audio]

Hi Tinitus,

I know my theory will meet with strong opposition but I thought that all the Manger guys will at least understand where I am comming from!
The reason I posted this on the Full range section is that the ultimate full range driver is the Manger driver and by using it we only have to use conventional drivers below 200Hz to 400Hz depending on SPL reqirements.
Also if enough people examine my theory here it may well be safe to venture out into the big bad world of mainstream loudspeakers....!
I will be happy if more poeple discover the Manger driver if they read this thread and decide to at least experiment and minimise the risk of using "pistonic" drivers, if I am right we will all benefit.
There are ways to greatly reduce the "Oscillation effect" which I will post later. First I hope enough people at least enter the debate!
We are all on the same side, better sound at home!

There is no such thing as a stupid question, only stupid answers
so asking "could 99.99% of all drivers be fundamentally wrong" is my question and maybe a few good answers like those from Mark, Calvin and Rudolf will lead to some minor of major driver design improvements.
Change (improvements) scares some people and excites others.

All the best

Derek.

[Scottmoose]

I decided after posting I really didn't want to get involved, so I deleted the offending article. Now I've been rumbled (curses ), I suppose I'll have to resurect it.

Paraphrasing as best I can, as I read this thread, I couldn't help but reflect on the irony that, irrespective of pistonic or bending wave behaviour, all the implementations of Manger drivers I have heard to date have been mediocre in their transient response, and downright miserable in their dynamic headroom. IIRC the first example I heard was crossed at 450Hz; not sure about the others -it's been a while. Either way, I well remember that first one doing an acceptable, if uninspiring job on undemanding material, but falling apart in a depressing fashion when presented with anything more demanding, especially when the wick was turned up.

Unfortunately therefore, based on what I've seen & heard to date, I've been forced to conclude is that they are a solution in search of a problem. No, 'conventional' dynamic drivers are not perfect, and I doubt anyone would claim they are. Having said that, they haven't exactly been doing a bad job since the 1920s, while electrostatics & ribbons have been wombling along quite nicely since the late 1950s onward too. YMMV as always of course -perhaps I'll hear a pair at some point that will cause me to drastically revise my opinion. I think I can understand the appeal, but they're just as much a niche product as any other wide-band driver, and based on what I've heard so far, not one I could live with.