Two-way Floorstanding Speaker SB36 for more Bass Pressure
The two-way SB 18 is already a legend and already available "remastered". What's missing is comparable euphony with even more bass for larger rooms and blacker music. If you have the space anyway, the SB 36 with the same drivers and double the bass in a floorstanding speaker is a good choice. We'll be showing two different versions of this speaker.
Development
The SB Acoustics drivers are particularly impressive because they work so well together in the SB 18. And yet, some of our readers have a larger listening room with plenty of space for a pair of floorstanding speakers, so the perfect harmony can be spiced up with a bit of bass. So we took the proven genes of the SB 18 and put them into a new project: The SB 36 two-way floorstanding speaker.
It offers the same perfect sound with even more bass. The development goal was to design a speaker that, thanks to its additional bass power, can perform at its best in any style of music.
Cabinet
Even though it is often enough emphasized that the biggest speaker is often the better one, the footprint in real living rooms is usually tightly limited, often by the (wife). Thus, we opted for a slim double bass system, which theoretically offers a simple solution to our "more bass reproduction desired" problem. To make this work in practice, there are a few factors to consider with double bass systems:
One or two enclosure chambers?
Does each bass need to play in its own volume to work at a precisely defined volume? Despite minimal measurable differences, we opted for a common housing with a common bass reflex tube and arranged it for the greatest possible flexibility in placement on the front - once as a tube and in the second housing example as a channel.
At this point we can immediately clear up a prejudice or an often disappointed hope regarding such double bass systems: The second bass chassis does not lead to a deeper bass even in the doubled cabinet volume, even if this is often perceived as such in a direct comparison: The larger diaphragm area moves more air, which then reaches the listener as a stronger pressure wave and is therefore also perceived as more powerful. By the way, the abbreviations SB 18 and SB 36 refer to the internal volume of the two speakers in liters, so in the case of the SB 18 not the chassis diameter.
What is the best driver arrangement?
Especially popular in the loudspeaker DIY scene is the so-called D'Appolito arrangement of two woofers (midrange) each above and below the tweeter with a special crossover. The result of two midrange drivers running in parallel with a tweeter in the middle simulates a point source with excellent spatial imaging, for which full-range speakers are also praised. However, since this point source has to be at ear level, such a speaker becomes quite high, otherwise the different travel times result in wavy frequency responses and sonic colorations. Therefore, we decided to place the two woofers below the tweeter and relieve the lower one from the midrange signal to avoid interference. This circuit is often referred to as a two-and-a-half-way box. Advantage: The decoupling frequency of the lower woofer can be used to tune the low-mid range, but more on that later.
| Holzliste in 19 mm MDF | für ein Gehäuse: |
| Front/ Rückwand/ Seiten | 4 Platten 95,0 x 22,0 |
| Deckel/ Boden | 2 Platten 18,2 x 22,0 |
| Frästiefen: | |
| Bassmitteltöner | 6 mm |
| Hochtöner | 3mm |
| Bassreflexrohr: | IT BR70/HP ungekürzt |
When building the cabinet, we like to follow the Bauhaus motto "Form Follows Function"; anyone is welcome to add additional flourishes. Four identical boards per cabinet as front, sides and rear wall, as well as two matching lids and shelves form the basis of the construction. Due to the slim construction, we were even able to do without internal bracing. How it works more beautifully (and considerably more elaborately) is shown by the alternative with a cabinet rounded off at the back and a bass reflex port at the bottom. Unfortunately, we do not have any photos of this elaborate woodwork, but only of the construction of the standard version.
Gluing six wooden boards together using joint glue should already be routine for many readers. However, for clarification, here is the sequence:
One side serves as the base for the construction and the front panel, the back and the two shelves are glued on there, the second side is put on as the lid at the end.
Caution: Wipe off any freshly escaping glue directly with a wet rag. This makes it easier to sand the cabinet later, because glue residue prevents an even surface when painting later.
The glue is always applied to the cut edge and the individual boards are aligned flush, in this case we set them apart from the adjacent boards with an additional shadow gap milled in.
For this work we bought a flush milling cutter with a triangular degree years ago, which can be seen in action in the FirstTime 10 article in the February 2009 issue. The milling of the chassis cut-outs is also illustrated there, so we will spare ourselves detailed explanations at this point. Cutting the chassis is not a must, but it looks and sounds good.
Using our tried and tested Wagner W 660, we sprayed three lengths of Hornbach acrylic PU clear lacquer onto the bare wood without primer, and sanded by hand using a 240 grit sanding sponge. Considering this little effort, the final result looks surprisingly high quality.
Chassis
The SB17NRXC35-8 is the bass driver used in the SB 18. Even though the bass-midrange driver is now in a neutral black, i.e. without the grey spots of the predecessor model, the parameters of the chassis have remained the same or practically identical within normal tolerances.
Once again, the incredibly inexpensive SB26STC-C4 tweeter, which is free of ferrofluid for the best detail fidelity, is used, and its sound characteristics are better than those of many drivers twice its price
Frequency Switch
Apart from a strong ripple above 900 Hz, the frequency response of the two SB 17 mid-bass drivers connected in parallel looks quite decent. However, these are only the result of the aforementioned time differences of the sound waves from the two woofers to the microphone. Sue disappear immediately when the lower woofer is kept from the midrange by means of a large coil.
While we largely followed SB Acoustics' circuit suggestions for the first SB 18, we gave free rein to our creativity for the SB 36 and added a low-pass filter for the upper mid-bass driver (which lets low and mid frequencies through, it is not a low-bass filter). This consists of a Corobar coil with a superimposed Audyn MKP-Q4 capacitor that cuts the peaks above 3.5 kHz and behind it a smooth electrolytic capacitor parallel to the (upper) bass-midrange driver.
The small tweeter also requires the same number of components as the two large woofers. Its level is lowered with two Mox-4 resistors as voltage dividers and protected from too low frequencies with a slope of 12 dB/octave below 2.8 kHz by means of a capacitor and a small air coil. The sum frequency response has a small dip at 3 kHz, which we don't owe to the crossover, but to the narrow baffle, since the high frequency signals radiated all around are no longer reflected in this range.
Tuning
The perfect tuning of the crossover can be seen in the addition of both amplitudes at 0, 30 and 60 degrees respectively: In total, all sinks and overshoots are almost balanced. To illustrate the circuit, we placed the components once on the crossover plan and then glued and soldered them only on the special kit PCB. This is a neat thing to do.
Even though the crossover was completely redesigned, we still had in mind when tuning to preserve the good qualities of the SB 18. With the dimensioning of the coil connected in front of the lower woofer, the fundamental range can be "adjusted" so that the (male) voice reproduction is thinner with a larger coil and fuller with a smaller coil. If you want to test this effect, you can simply unwind the existing coil a bit for a test or buy a larger one for a different sound tuning and then dimension it appropriately by hand. But be careful: coils are quite robust, but the winding can come loose if handled incorrectly and thus make the coil unusable. With capacitors and resistors, it is important not to break any supply wires while experimenting.
Our crossover has a special feature that may be important to some readers: Three additional, non-separating components serve as impedance correction for the operation of these speakers on tube amplifiers, which react very gruffly to an odd impedance response. These can be added if flexible, even later.
The need of insulation material is comparatively high with this construction manual with five bags of Sonofil. The separate insulation plan shows how it can best be accommodated.
Listening session
The listening session immediately brought it to light: Actually, we could copy the sound description of the SB 18 into this construction manual without further ado and add the note "even more powerful".
However, this does not describe the whole truth. Even though it is only the bass reproduction that has gained in foundation, the whole SB 36 seems a lot more "grown-up" than the small SB 18.
The tweeter proved to be excellently adapted to the double bass construction. Calmly and unobtrusively, it contributed to a great overall performance.
The excellent SB 18 now finds its limits in comparison to its bigger sister, the SB 36, which, with equally good midrange and treble characteristics, provides an additional plus of foundation to make even larger rooms sound more powerful and (seemingly) with more depth in any music genre, without ever losing the overview in the bass range.
For example, the slim floorstanding speaker clearly reveals that Leonard Cohen sang his famous "Suzanne" forty years after the studio recording, "Life in London 2008" as if he had clearly gained in contour. Wait a minute: would have? Well, sometimes playing CDs in the Kerpen allows impressive time travel through music history ...
For two and a half hours we could not detach ourselves from this grandiose musician and his long matured voice. Decide for yourself whether it is the Leonard Cohen, or the SB 36, that fills any musical enjoyment with such emotion that the listener can hardly detach himself from it.
Conclusion
The SB 36 is more than just a duplicate of the SB 18: it is more powerful and more grown-up, with the same good reproduction of detail and space. Compared to the SB 18, it loses none of the fabulous imaging that made this inexpensive speaker so famous.