Ole Witthøft

Revolution integrates woofer and cabinet principle /1

What can reasonably be called a revolution in a technology that has been around for decades? The Q113 Revolution has an answer. We're stepping out where we might not be able to, trying to discover something new and at the risk of failing.



Pierced. It's clear that the design has been optimised to allow large volumes of air to pass smoothly and unhindered between the back of the membrane and the interior of the enclosure.


Truth be told, there are significant details of this construction we don't fully understand. This is not due to the incompetence of the engineers involved in the process or the equipment and software used. It's all Best in Class, but we're in a corner of technology where practical experience is very limited. At the same time, computer programs can't simulate exactly how things work in this kind of engineering, so we do a bit of guesswork. Of course, that's also why it's exciting. Here in Part 1 I review the things we are sure of and in Part 2 you can read about the more uncertain elements.


A brief look back

In the early 00's System Audio A/S introduced a compact loudspeaker, which we called SA2K. It became known among sound and music lovers as one of the best small speakers on the market, and the design itself introduced a number of new ideas for the first time. That's all in the past, and the Loudspeaker Q113 project sets out to create a successor to the old master. I have chosen to develop two successors to begin with. They are created from two different mindsets and they will compete against each other at a later stage. We call them Evolution and Revolution. The whole development process is presented on ing.dk, where readers' opinions, experiences and practical help are actively used in the process. Today's topic is the bass speaker for the Q113 Revolution. The speaker that will try to rethink the compact HiFi speaker and shake up the traditions.


Precious bass speaker, made in Denmark

The bass speaker for the Q113 Revolution is based on the Danish Scan Speaks 15WU Illuminator, which in the standard version alone is e
one of the most expensive 5½-inch speaker units available. The engineers have made various changes to the design so that the small woofer is now tailored exactly to our purposes. Hear the wild details here.

The engine could hardly be stronger

The magnet in the motor system itself is made of neodymium, the strongest and most expensive material available for magnets in loudspeakers. Normally ferrite magnets are used, but this magnet has to be so powerful that it would hardly be safe to use ferrite. The magnet must also be small enough to allow air to move freely between the back of the diaphragm and the inside of the cabinet. There must be no compression, and neodymium has the perfect combination of high magnetic strength and small size. Add to that the need for this powerful motor because the speaker's voice coil is driven in a somewhat inefficient manner. It's underslung.



The difference between overhanging and underhanging. On the left, the overhung voice coil, which is the most common.
A tall and relatively heavy coil in a narrow magnetic field. The underhung coil is just the opposite. The coil is low and light, but it is in a wide magnetic field.


Light coil in large magnetic field

In order for a speaker cone to move and thus create sound, the voice coil through which the music signal travels must be in a magnetic field. In short, there are two ways of dimensioning the magnet and voice coil in relation to each other. You can have a tall and relatively heavy coil in a narrow magnetic field. This is by far the most common principle and here the motor system is said to be "overhung" because there are voice coil windings outside the magnetic field. The underhung principle follows almost the reverse recipe, where a narrow and light coil is in a wide magnetic field. The advantage of the underhung principle is that the voice coil is well surrounded by magnetism, regardless of where it is in the magnetic gap. This results in linear motor strength and thus low distortion. Conversely, the underhung motor requires a very powerful magnet because a large potential of motor power must be available at all times. Muscle costs in a loudspeaker. Motor power is used to minimize distortion, but a moving system that wants to move is also needed. We've got one of those!



Separate. Top right: dust cap, diaphragm, centring disc, chassis, voice coil, sealing ring and bottom left: magnet system.


Titanium keeps the mechanics in check

The movements of the voice coil must be transmitted efficiently to the diaphragm via the cylindrical shape on which the coil is wound. In the context of the Q113 Evolution, I mentioned how the voice coil shape must not be inducible, because it acts as a brake on the movement of the diaphragm and thus a limitation on the dynamics of the music. Here the problem was solved with fibreglass, but in the bass speaker of the Revolution a material is needed that is both non-magnetic and very strong, because it has to be made so thin that the magnetic gap is not filled unnecessarily with material. The choice here is titanium, which is as strong as steel but weighs about 40% less. There is reason to believe that even the slightest movement of the voice coil will be transmitted to the diaphragm and become sound.


Two thin membranes in a sandwich

The perfect loudspeaker membrane is infinitely light, rigid and free of self-resonance. And while we're at it, it also has to be cheap and environmentally friendly. That last point was the first decision made by readers here at Q113. We were given the opportunity to make speaker cones in beryllium and I asked the engineers for their opinion. A very interesting conversation ensued, but beryllium was dropped. You can replay the debate here. Without beryllium, we can now make an exciting woofer after all. In essence, Scan Speak's diaphragm is an inventive sandwich, consisting of two very thin and rigid diaphragms joined together. Each diaphragm is embossed with a pattern that looks most like a trefoil. The embossing forms an elevation on the surface of the membrane and by rotating one membrane 60 degrees relative to the other, a very small and well defined gap is created between the two membranes. In this gap Scan Speak applies a kind of damping material. They don't say what substance is used to bond the two membranes together, but it's no secret what they want to achieve.



The original Scan Speak 15WU in 8 ohms. A nice frequency response, where one can immediately get over worried about the strong peak at 5-6 kHz. We take this as a good sign of a stiff diaphragm.


A compromise between stiffness and damping

A relatively soft and well damped diaphragm is every loudspeaker builder's wet dream, because it gives a really smooth frequency response and it is, all things being equal, easier to make a crossover work when its circuitry should not be too complex. A relatively soft and well-attenuated diaphragm has its drawbacks, however. The speaker's ability to reproduce the fine details and nuances of music is reduced when the diaphragm is soft. We may be damping resonances in the diaphragm, but we're also lulling the music into a pleasant, delicious haze that prevents the performers from appearing alive in your living room.
Here the rigid membrane represents a different compromise. As long as a membrane moves like a piston, rigidity is far preferable. Even the slightest movement of the voice coil causes movement of the diaphragm, and a stiffer diaphragm reproduces more of the details of the music. That's what we want to optimise, but stiffness also comes at a price. At a given point, the diaphragm will stop moving like a piston controlled by the voice coil. It will start to oscillate on its own, the so-called self-resonances, and in that part of the tonal range the speaker is useless.
You can see from the measurement of the woofer that at about 5 kHz there is a peak of about 10 dB. This is the self-resonance of the diaphragm material. It tells you that the diaphragm is reasonably stiff and that it is very likely to work like the piston we want, up to about 2-3 kHz. This is good news, if we manage to tame the resonance at 5 kHz with the speaker's crossover. And one last thing. We have quite good experience with the sound of wood fibre cones, so of course we try that in the Q113 Revolution as well. No one has tried that before on these woofers, Scan Speak tells us, and it just adds to the excitement.



The modified woofer for the Q113 Revolution. Now with 4 ohm voice coil, 20% stiffer centring disc and wood fibre cone. The dip above 1 kHz is gone, but the peak at 5 kHz is more pronounced.


The bass speaker uses our own centring disc

Like the woofer for the Q113 Evolution, Scan Speak has a rather unique centring disc that the engineers have played with to make it just right for the Q113 Revolution. Scan Speak's design is that the centering disc should not act as a brake when the music signal requires the diaphragm to really move. Many woofers are protected by their centring disc, which gradually slows down the diaphragm as the music signal gets louder, acting as a limiter on the dynamics of the music. Scan Speak has reduced the braking effect by making the waves of the centring disc progressively louder and wider. Compared to the standard version, the centring disc on the woofer of the Q113 Revolution is 20% stiffer, resulting in a slightly higher resonance frequency and a Qts of 0.32. You can see the specs at the bottom left of the Scan Speaks measurement. The motivation for making this change stems in part from the interesting conversation that arose in the comment sections following this blog post. In addition, the engineers chose to think of the speaker's bass response as a whole, because it quickly became clear that it couldn't just be used in a bass reflex cabinet, but that a collaboration between the woofer and two passive units must be established. More on that headache in the next section. Finally, get a sense of what's in store.



Openness. There are no small closed cells where air can stand and be compressed, acting as a brake when the moving parts actually move. That's a good sight for a speaker builder.


A stroke length of an insane +/- 9 mm!

In a good bass speaker, the diaphragm has a linear travel of +/- 3 mm. The Q113 Evolution bass speaker delivers +/- 6.5 mm via its large overhung motor system. This bass speaker for the Q113 Revolution has an underhung motor system, a voice coil with a diameter of 42 mm and a stroke length of +/- 9 mm. Probably no other 15 cm speaker unit is capable of achieving this. It should be said in fairness that speaker manufacturers do not have a standard for how the stroke length should be measured. Scan Speak calculates it here to the point where the magnet has 85% of its strength left. Next time it's about the result of an engineering feat where we step out into deep water. It's about the passive devices that assist the woofer in reproducing the deepest bass.
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Ole Witthøft
Ole is the founder of System Audio. His 3 greatest passions are music, design and technology. Every day, Ole is working on some kind of projects, and you find him in the workshop, in the production, behind a computer or on one of his many presentations around the world.
Revolution integrates woofer and cabinet principle /1 | SA

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