I designed the ScanSpaker as a mid-sized but high-end two-way speaker using Scan-Speak drivers. This floorstanding speaker uses the Scan-Speak D2905/990000, a 1″ soft-dome tweeter, and the Scan-Speak 18W/8545-00 mid-woofer, which has a 17-cm diaphragm made out of carbon fibre and paper.
As the ScanSpeaker name suggests, all the drivers are made by ScanSpeak. I used the 8545 woofer (17cm diameter) and the 9900 tweeter (aka «The Revelator», 28mm dome). These are very good, but expensive drivers. The 8545 midwoofer has the reputation of very smooth reproduction of voices. This wasn’t true in my speakers at all until I added impedance-compensation for the impedance-rise caused by the voice-coil. The 8545 has a strange “bump” at about 600–700 Hz. It doesen’t look like a membrane-resonance at the waterfall-diagrams so it can be corrected with an LCR-network. The 9900 tweeter has a special frontplate which is supposed to improve lobing behaviour. I heard and read rumors suggesting that this special frontplate design causes a slight resonance somewhere between 10–20 kHz, but I couldn’t see it on my measurements. The dome diameter is 28mm and the resonance-frequency is at 530 Hz, so the Revelator goes lower than most other tweeters. The 9900 has a reputation to acts a bit like a «diva» when designing the crossover, which I attribute to the very pronounced impedance peak at resonance.
I used a bass-reflex construction for the ScanSpeaker. I put two bass-reflex channels in the bottom of the case, «aiming» at the floor. This works because the speaker stands on spikes at about 2 cm above the floor. The internal volume of the case is about 18 liters. The front is 18.8 cm wide which is about the minimum possible with the 8545 woofer. An ideal case does not vibrate at all – the driver’s membranes are the only thing that should move. To get a «dead» case I built the case from 18 mm particle-board (inside) and 10 mm MDF (outside). Enclosure vibrations are further dampened by a layer of ceramic «bathroom» tiles and a layer of «Hawaphon» on the insides of the enclosure walls. A layer 10 mm felt and some wool dampen internal sound resonances and eflections in the case. In addition, I installed a piece of cardboard at the top of the case with some foam glued to it. The cardboard is angled at about 30° relative to the face plate, which helps to avoid standing waves along the long axis of the enclosure.
3. Crossover network
The final crossover filter networks for the midwoofer and the tweeter are shown in the figures below. The initial design started out by calculating theoretical parts values, which were then optimized by acoustic measurements and by ear. As mentioned above, the 8545 woofer frequency response shows a slight «bump» at about 600–700 Hz. This is compensated using an LCR network. Also, for proper operation of the tweeter filter, the impedance peak of the 9900 needs to be compensated. This is achived by an L-pad in between the tweeter and the filter network. The L-pad is also matches the tweeter SPL to that of the woofer.
The parts values for the midwoofer filter are:
- L1 = 1.8 mH
- L2 = 15 mH
- C1 = 10 µF
- C2 = 15 µF
- C3 = 3.9 µF
- R1 = 5.6 Ohm
- R2 = 4.4 Ohm (tot. resistance of LCR including L2)
- R3 = 1 Ohm
The parts values for the tweeter filter are:
- C4 = 3.9 µF
- L3 = 0.82 mH
- R5 = 10 Ohm
- R6 = 8.9 Oh