SB 240 speaker kit – three-way floorstanding speaker with double bass
After the SB 36, an extension of the legendary SB 18, was also well received, we have now developed the ultimate floorstanding speaker based on the SB 18, which now leaves nothing to be desired in terms of bass reproduction.
Development
While the SB 36 simply got a second woofer of the same type as used in the SB 18, we went a different way with the SB 240, without compromising the excellent mid-high reproduction of the SB 18. For the SB 240, we simply used the SB 18 as the mid-high unit and added a good pound of bass in the form of two additional larger bass drivers.
In order to maintain the basic character of the SB 18, we deliberately did without a smaller midrange driver. After all, the SB Acoustics drivers are impressive because they work so well together in the SB 18, so we have once again incorporated the tried and tested genes of the SB 18 into a new project: The SB 240 also offers the same sound perfection as the SB 18 and the SB 36 with a further increase in bass pressure - and bass response. The development goal was to design a speaker that not only offers more bass pressure like the SB 36, but also more bass response.
Casing
In principle, there were two possibilities for the cabinet construction: Either an additional subwoofer for the SB 18, or a completely new design. For several reasons, we decided on the second option of a completely new floorstanding speaker.
By the way, existing SB 18s make good surround speakers, and if you prefer the solution of separate subwoofers under existing SB 18s, you can modify these building instructions accordingly and just add two bass cabinets. Since all that's needed is a couple of 75 cm high "stands" with a net volume of 100 litres, we've decided not to include detailed instructions for this, since we've decided to build complete SB 240 floorstanding speakers that are well suited as main speakers in a home cinema set-up with the SB 16 centre and the SB 18 as surround.
However, we scored a small own goal: Instead of one speaker, we had to build two: The SB 18 is integrated into the SB 240 with its own cabinet, but this time with a net volume of 11 instead of 18 litres. Another advantage of the new design is that it takes up less space.
Since slim floorstanding speakers are easier to accommodate than wide ones and there is usually enough room at the top, we decided to make the speaker a little narrower and taller and to place the tweeter between the midrange and the woofer. This results in a better sound dispersion for the listener, as the tweeter sits at about ear level when you are sitting on the couch yourself. In addition, the dispersion pattern is more favourable (compared to placing the tweeter at the very top) when walking around the room and listening to the music standing up.
Building a presentable enclosure takes time, of which we always have too little, despite good organisation. While the actual cabinet construction is usually child's play for us thanks to CNC-milled boards, it is not so easy to get the surfaces done properly "in a hurry", so we went to the DIY store once to get inspired whether there might be an easier way than a high-gloss lacquer finish ... In the DIY store we found foiled furniture boards measuring 120 x 40 cm that looked quite similar to beech wood.
The employee in the wood department cut five of the boards half a centimetre narrower on each of the long sides (instead of sawing them off by a total of one centimetre). This left us with open pores on the edges, so to speak, in which the joint glue holds quite well. The DIY store employee divided one of the boards into four sections of 39 x 26.2 cm for the tops and bottoms of our floor-standing boxes. For the fronts and backs we had the man at the saw cut four more boards of 120 x 30 cm black MDF.
He cut the 26.2 x 10 cm cabinet stiffeners and the boards for the internal mid-high frequency chambers from a piece of chipboard. The entire wooden material, including the cutting, cost us 130 Euros. The result was a beautiful, professional-looking loudspeaker cabinet with little effort.
Don't forget! One mat of Sonofil folded once fills the upper chamber, two more mats are rolled up slightly and placed behind it in the bass compartment and under the reflex tube and one behind each of the two bass drivers. Of course, the mid-high chamber has holes in it, which we will close later with hot glue after the cables for the crossover have been fed through.
Chassis
The drivers have already been discussed elsewhere, so we'll briefly list the most important data here.As in the SB 18 and the SB 36, the tweeter is the extremely inexpensive SB26STC-C4, which is free of ferrofluid for best detail fidelity and outperforms many drivers twice as expensive.
The SB17NRXC35-8 bass-midrange driver, tried and tested in the SB 18, is used as the midrange driver, which is now available in a thoroughly neutral black without the grey patches of the predecessor model. The driver parameters have remained the same or practically identical within normal tolerances.
For the low frequency range, we chose two SB23NRXS45-8. Despite the width, the chassis in this floorstander is comparatively slim.
Tuning
The most important thing in a multi-way speaker is always the crossover. So we went to the measuring room. Two different crossover topologies, both with good sound results, presented themselves. While we had largely followed SB Acoustics' suggestion for the original version of the SB 18 and only deviated from it in the "remastered" version, we went our own way with the SB 36. We were able to get by with fewer components without any loss of sound quality.
Our experience with the SB 36 led us to use this model as a basis for the construction of the SB 240. For the extension with two basses, the original crossover of the SB 18 would have had to be changed anyway, if only because of the changed cabinet dimensions. It was therefore questionable whether components of the "old" crossover could be used at all. So we started with an open mind to give the SB 240 the right euphony.
Wir starteten mit der Neubesetzung, also den beiden Bässen. Diese wurden parallel geschaltet und in eingebautem Zustand mit dem Mikrofon in Ohrhöhe gemessen. Es zeigte sich ein bis 600 Hz ganz ordentlicher Frequenzgang mit einer deutlichen Senke bei 1.200 Hz. Schuld an dieser Welligkeit ist der unterschiedliche Abstand der Chassis zum Mikrofon bzw. eben auch zu den Ohren des Musikliebhabers, also würde in diesem Fall eine Messung zwischen den beiden Tieftonchassis recht wenig bringen. Denn Frequenzweichen entwickelt man nun einmal für die Ohren und nicht für die Füße bzw. Knie.
So haben wir eine mittelgroße Intertechnik Spule aus der Ferrobar HQ 56-Reihe vor und einen dicken Elko glatt parallel zu den Bässen eingesetzt, woraus die blaue Messkurve resultiert. Ignoriert haben wir den scheinbar frühen -3dB-Punkt bei 45 Hz, denn die Box stand bei der Messung 1,5 m weit vom Boden entfernt, im Hörraum ergibt sich durch frühe Reflexionen zum Boden und je nach Aufstellung auch zur Seite eine nach unten hin erweitere Basswiedergabe. Genau diese Reflexionen würden unsere Messung allerdings stören. Von der schweren Aufgabe der Basswiedergabe befreit, braucht der nunmehr zum Mitteltöner mutierte 7-Zöller (Frequenzgang in der Box: rot) nur im Bereich zwischen dem Bass und dem Hochtöner adäquate Pegel abstrahlen. Somit spielt er in diesem Bereich auch bei hohen Pegeln sauberer und kommt mit weniger Gehäusevolumen aus. Für die ideale Anpassung zum Bassbereich hin reichte ein großer Audyn-Q4 Folienkondensator.
Die Trennung zum Hochtöner hin gestaltete sich schon etwas aufwändiger, denn die Peaks am oberen Ende der roten Kurve mussten schon etwas nachdrücklicher in ihre Schranken gewiesen werden, um in der Summe unhörbar zu bleiben. Da half uns die Erfahrung mit der SB 36, bei der wir einen kleinen Q4 über die Tiefpass-Spule gelegt haben und die Flankensteilheit mittels eines zum Mitteltöner parallelen Kondensator so weit erhöht haben, dass die Resonanzspitzen den Sound nicht störten. Wie erwartet, zeigte diese Maßnahme auch hier den gewünschten Effekt.
Die Hochtonweiche ergab sich dann quasi von selbst, ihre Messung mitsamt eingebautem Hochtöner wurde wie immer rot dargestellt. Eine leichte Ansenkung des Pegels mittels Vor- und Parallelwiderstand aus 4 Watt Mox-Widerständen und ein klassischer 12 dB-Filter mit einem der Impedanz von 4 Ohm entsprechend großen Audyn- Q4 samt entsprechend kleiner Parallelspule resultierte im gewünschten Amplitudenverlauf (blaue Kurve).
Nun bleibt als entscheidender Hinweis nur noch zu betonen, dass sich nur bei zum Mittelhochtonbereich verpolten Bassbereich ein gerader Frequenzgang ergibt, zumindest sind dann bei der 30 Grad- Kurve fast alle Frequenzgangwelligkeiten weg. Für Röhren- Besitzer haben wir auch hier wieder eine Impedanzglättung vorgesehen, da diese aufgrund des hohen Innenwiderstandes bisweilen allergisch auf unlineare Imedanzverläufe reagieren.
Die nur bei Verwendung mit Röhrenverstärkern unerlässliche Impedanzkorrektur, ist schon auf der Weiche vorgesehen und kann später, ohne Aufwand, einfach hinzugeschaltet werden.
Box measurements
Sound
Now it was time for the sound test. For years we have been listening to our speakers with tube amplifiers and let our visitors participate. Since we sometimes use our listening studio in a hectic "in-between" way, we apparently forgot to turn down the volume and take the CD "That's Live" by Eric Burdon out of the player during the last session.
So the old classic "Don't let me be misunderstood" banged out with live concert level. Except that there was definitely nothing to misunderstand here. Compared to many other speakers, the singer here stood a little further forward, virtually in line with the SB 240s. The individual musicians were lined up neatly placed behind him and the audience's clapping hands revealed the depth of the room.
Here, old rock sounded like good old rock again: groovy, sometimes gaudy and always live. We couldn't complain about the lack of bass. Have you forgotten? That was exactly our goal!
Now it was on to the classics: Horowitz in Moscow, the famous piano concerto with the somewhat unusually restless audience and the coughing gentleman in the sixth row. The 83-year-old master, with the almost youthful agility of his fingers, offered a performance that was as grandiose as it was subtle, completed by the feathering of the piano's strings and the wonderful reverberation of the room. Not even the cold of the gentleman in question disturbed the performance. A live recording is not a studio recording, after all, and only bad concerts are deserted. Even Mozart, Scarlatti, Chopin and Liszt would not have found anything wrong in their works. Speaking of live: Eckart gave us a great pleasure with the E.S.T. CD "Live in Hamburg", a homage to Symphony 285, in which the Swedish rock trio plays "Dolores in a shoestand" for a good 17 minutes. downright breathtaking. Anyone who doesn't involuntarily tilt their foot, nod their head or even shake their whole body during this is either deaf or has neglected to close their eyes to immerse themselves in this incredible live sound. Because when we opened our eyes again after almost 20 minutes, the boxes were inexplicably still there, even though they were the ones that took us into these distant sound worlds.
Even after the third encore, our minds vehemently refused to acknowledge the connection between the music in the room and the speakers standing there, so realistic did the reproduction seem. We could literally feel the musical experience of the audience, and even suspected that they would hardly have been willing to leave after the concert. We had a similar experience during our listening session with the SB 240.
Conclusion
The development effort was worth it: the bass relief cure has done the SB18's genes good - the SB 240 offers even more playing pleasure - and the addition of two full-grown bassists offers bass reproduction of the very best. The SB 240 is thus the most "grown-up" in the SB 18 / SB 36 / SB 240 family.