On paper, it can do something that no other 15 cm woofer can do, namely move its diaphragm +/- 9 mm. This bodes well for the possibility of getting incredible bass reproduction out of the Q113, but it doesn't happen without a struggle.
Scan Speak 15 WU. A Danish woofer that with a diaphragm stroke length of +/- 9 mm can probably move more air than any other 5 inch woofer.
Several of the readers of ing.dk have been involved in simulating and calculating how to dimension a cabinet for the 15WU woofer from Scan Speak. The starting point has been to make a bass reflex speaker, just like the project's predecessor: the SA2K speaker. The bass reproduction in the Q113 must be better than its predecessor and this is possible with the 15WU. The simulations have shown this. The loudspeaker just cannot be made as a bass reflex design.
Bass reflex. The deepest bass tones are reproduced by the tube that protrudes into the speaker cabinet, known as the bass reflex tube or port.
There is no room for the bass reflex tube
Speaker Q113 has from the start been defined as a speaker with an internal volume of 5-10 liters. In other words, a rather small speaker. This is one of the focal points of the project, because it is a challenge to test the physics and try to push the boundaries. It's interesting because it's difficult. In a bass reflex loudspeaker, the deepest tones of the music emerge in a collaboration between the mechanical and electro-acoustic properties of the woofer, the cabinet and the bass reflex tube etc., but for Scan Speak's 15 WU woofer, this results in a very unusual cocktail, where the bass reflex tube must be 30-50 cm deep inside the cabinet for it to work. There is simply no room for that when the speaker has to be small. Some might argue that you could make the tube shorter by reducing its diameter. This is true enough. However, we have used a rule of thumb that the area of the reflex tube should be about one third of the area of the diaphragm to avoid turbulence inside the tube. This can occur if the air in the tube moves faster than 5 meters per second. Here, Scan Speak's simulation program shows that even a large Ø60 mm reflector tube results in an air speed that balances dangerously on the borderline of turbulence, so we have to look elsewhere. This is heavy stuff, this!
Playback. The blue curve shows the impedance response of a new 15 WU woofer. The eigenresonance decreases by 4 Hz after the power-up (red).
Simulation. Old Thiele/Small vs. the new model. The red impedance curve shows the simulation of the impedance curve according to the old Thiele/Small model. The blue one shows the same simulation but with the advanced parameters. The green curve shows a real measurement. The advanced parameters come closest to the measurement, one might say.
We have derived our own specifications
Using a 15 WU woofer and a simple test cabinet, we have derived our own Thiele/Small parameters, because we want to verify Scan Speak's data. The specifications are good enough and we can thus rely on the simulations we have made. Furthermore, we have simulated the speaker's impedance curve at higher frequencies and can see that the more advanced parameters match reality better than the old Thiele/Small model. A brand new 15 WUs has a natural resonance of approx. 36 Hz, and after being turned on it ends up at approx. 32 Hz. We have worked with simulations of the 15 WU in the program LEAP 5, and we have shown how differently a loudspeaker works, depending on the temperature in the voice coil. Subsequently, we have decided to proceed with the optimization of our woofer based on a working temperature of 85 degrees, corresponding to approx. 50 Watt input. This corresponds to a listening level where you can hear what is going on in the music. Of course, the speaker must be optimized for this. I would have liked to show how the following is calculated, but there is no freeware tool that calculates correctly, so you have to watch here when bass reflex is thrown out.
Examples of passive devices. Loudspeakers that work by means of the pressure changes inside the loudspeaker enclosure. No power is supplied.
Q113 Revolution must have two passive units
A passive speaker does not take up space inside the speaker like the bass reflex tube does, but it does the same job. Therefore, the passive unit is a great solution to our challenges, but with both advantages and disadvantages. I will come back to those. A passive loudspeaker unit works by its diaphragm being set in motion by the pressure changes inside the speaker's cabinet. The principle is also known as a slave speaker and no electrical signal is applied to the speaker unit itself. The passive speaker's diaphragm has a very specific weight and its compliance is adapted so that the response of the passive unit improves the speaker's bass performance. In Q113, we don't just need one passive unit to relieve the active woofer. We need two. This is because large diaphragm fluctuations can occur at low frequencies and the mechanical load is effectively reduced by dividing the work between several diaphragms. We could have used one larger passive unit of say 6.5 inches and achieved a similar result, but the Q113 also has an aesthetic agenda. The loudspeaker must be narrow and therefore I use two 5 inch units instead.
Advantages and disadvantages of passive devices
On the positive side, a passive unit takes up less space than a bass reflex tube. A loudspeaker with passive units can therefore be made smaller without compromising performance. A disadvantage is that passive units cost more than bass reflex tubes. Conversely, there is no noise from a passive unit as there can be from a bass reflex tube. Yes, and then there's one more disadvantage. Not really anyone in our little kingdom has worked with passive devices in a passive loudspeaker. I've spoken to engineers who find it exciting, but they just haven't tried to design a loudspeaker with passive devices themselves. This is not because the principle is new and revolutionary, because it is not. It's just that it's not the usual way to go. Now it has to be tried, because right now the use of passive units is the only way for the Q113 Revolution Edition. If you have practical experience with passive devices in passive loudspeakers and would like to share your knowledge with the readers, I would like to hear from you.
Speaker Q113 Evolution. It hasn't gotten much publicity yet, but that's how far the Q113 Evolution has come. More about it later.
This is how far evolution has come
You may have forgotten, but the Speaker Q113 project is about two speakers, both of which could be the successor to the SA2K we once produced. Two potential successors that will compete against each other and the readers will later be involved in selecting a winner. One successor is called Q113 Evolution and is a refined, further development of the SA2K. The other is the Q113 Revolution. It is a speaker that must manage to rethink the small HiFi speaker in a bolder, fresher and more challenging way than the Evolution. Until now, it has all been about the Revolution and that's not entirely fair, because the Evolution has certainly not stood still. There's not a lot of drama in Evolution, because it's very pragmatic in its approach. The design is broadly similar to its predecessor and point by point the construction is an attempt to improve on the SA2K. Samples of the woofers have arrived and the cabinet has already been designed. Evolution is certainly not a bad principle, because you build on something that has been proven to work. I look forward to returning to it when the Revolution has been designed.
Now the revolution can be drawn
The Q113 Revolution has for the first time reached the point where it is reasonable to start talking about the speaker's applications and design. I will do that next time. I don't have a tweeter for the Revolution yet, but we are working on a solution that fits into the "innovative and challenging" mindset. The word "revolution" itself may sound a bit big, and it is not a system change the speaker will spearhead. However, it should represent a rethinking and a modernization of the traditional compact speaker that we all know.
That's why I'm looking forward to the next time I will show you two speaker designs. And I will ask you to choose the one you like best.
