A loudspeaker for Anders

Not quite a ProAc clone

My dear friend Anders decided one day to build a pair of good loudspeakers. He scanned the web and found several descriptions of ProAc clones claimed to be really good. All of them use the Scan-Speak 18W/8535 driver and he really wanted to copy one of those DIY projects.

As Anders already owned two Scan-Speak 18W/8542 drivers acquired for a previous project, I suggested we try to use them in the same kind of ProAc clone boxes. If he was prepared to construct the boxes, I promised to help design and tune his new speakers. This is how it started.

The Scan-Speak 18W/8535 and 18W/8542 drivers are pretty different. The 18W/8535 has cast frame, carbon fiber filled paper cone, which is impregnated and coated to damp the resonance normally associated with stiff cones. A low loss linear suspension, especially the rubber surround, combines dynamic linearity with low coloration. Free air resonance frequency is stated at 26 Hz and Qts at 0.38. The driver is designed to be used in vented boxes, 15 to 40 liters, and is known to produce a very deep, uncolored and listenable bass end.

The 18W/8542 has a cast frame, an impregnated paper cone with inverted dust cap, foam surround, 42 mm diameter voice coil and a ventilated pole piece. The resonance frequency is stated at 30 Hz and Qts at a low 0.22. This driver is designed for somewhat smaller vented boxes, 10 – 20 liters in volume. It seems that the driver is most appreciated for its very natural midrange reproduction. Variants of this driver were used in the British ProAc Studio series speakers.

Being aware that the 18W/8542 driver is different from the 18W/8535 used in the ProAc clone projects, I suggested Anders to build boxes of the same size as the ProAc clones, i.e. vented boxes of approximately 34 liters in volume. Should the project fail, we could always fall back to the ProAc clone by acquiring two 18W/8535 drivers.

Since the 18W/8542 is designed for smaller boxes, we can do some fun with the 34 liter box volume. A traditional Butterworth QB3 box for the 18W/8542 will be 16 liters in volume, tuned to 47 Hz and have an f3 of 50 Hz. Moving to an Extended Shelf Bass (EBS) tuning of the 34 liter box will, however, result in a box tuned at 35 Hz and an f3 of 37 Hz – or even lower.

My previous experience with EBS tunings is that they can be quite surprising. My own bass speakers, based on Audax 8” drivers in a box volume of 72 liters, give substantial output well below 25 Hz.

So, this will be the basis of my design. We should remember though, no matter what magic we include, the 18W/8542 driver is, and will always be, a 6.5” driver – meaning that low bass at high pressure levels will never be an outstanding feature of this speaker. The purpose of the EBS tuning is instead to give a very deep, clean and enjoyable bass.

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