Pistonic or uncontrolled oscillation...?

Loudspeaker marketing guys love to talk about how their latest designs act like “perfect pistons” and claim their latest “unobtainium” coated cone is more rigid than a stealth bombers wing yet lighter than the air it moves….Blah, blah blah…!

All this marketing hype is at best irrelevant and at worst a smoke screen to try and hide the truth….The vast majority of cone and dome loudspeaker drivers are incapable of acting like pistons and are in fact bouncing around out of control when reproducing music or sound effects!

The word "Pistonic" has been hijacked by the loudspeaker industry & the marketing guys now use it to describe the surface of the cone in an attempt to distract us from the real problem…. In conventional drivers the entire cone / surround / voice coil former and spider behaves like a mass on a spring and more often than not is simply oscillating out of control.

All loudspeaker drivers are bound by the laws of physics, they are simple mass on a spring devices ie cone / dome suspended by a surround and spider. They do not function as pistons....Internal combustion engines have pistons, loudspeakers do not!

Let's examine our so called “audio pistons”:

Imagine looking side on at a clear plastic tube, sealed at both ends and containing a Ping-Pong ball, this ball has a coil of copper wire wound around its circumference. The ball rests in the middle of the tube. There is an elastic band with one end fixed to the ping pong 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 (spider and surround) is also 100% linear in its operation.

We pass a short “on / off” burst of electric current through the wire, an impulse (to simulate a drum strike) not a constant current, just a brief impulse 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 back and forth 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.

It's not how fast a driver starts that counts, it's how fast it stops… 

This extended decay time is known as “cone ringing” and it produces the most insidious form of distortion…Time domain distortion or ghost echoes.

Music signals are all about transients and timing (See the John Watkinson article) …Not sine waves and signal generators. Real sounds such as musical instruments, vocals or snapping twig etc, are all formed by an initial transient of increased air pressure and then decay back to ambient air pressure.

Our ear / brains ability to “decode” this compression or rarefaction of the air pressure and decay back to ambient pressure is the only way we hear any sounds.

That’s worth a second thought; all the subtle texture, harmonics and decay that add so much to the emotional content in our music are just fleeting spikes in air pressure, followed by a reduction back to ambient air pressure. Simple, elegant, beautiful, 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 wave of the air in front as it is propelled forward by the electrical impulse and simultaneously it is being pulled back 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 being carried past the start position by its own momentum. This behaviour is 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….They are a long way from being pistonic!

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 the vast majority of conventional loudspeaker drivers fall apart, displaying gross time domain distortion.

Now start to factor in some real music....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 does 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.

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.

In order to reproduce sounds accurately, 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 millionths of a second) is generally accepted as detectable by most people.

Now when you look at the measured decay times of pistonic drivers you see the scale of the problem. A typical dome tweeter takes nearly 3 milliseconds (3 one thousand of a second), a good bass / midrange driver takes 10 milliseconds and a large bass driver takes around 150 milliseconds to settle from a 100-dB impulse signal.

Custom Install Audio have developed a new range of flat coned Rapid Energy Decay (RED) loudspeaker drivers which are an order of magnitude ahead of conventional cone drivers….Sub 1ms settling time over a huge bandwidth, see our "SM 15" PDF flyer.

In summary, it's not how fast a driver starts that counts its how fast it stops!