If any of you have trouble getting your head around what I mean by a good positive control, here's an example. Once, at a DIY speaker meeting, a guy proudly presented his 2-way speakers that were mainly of his own design. The drivers were very expensive, a SEAS Excel 6½" mid-woofer and SEAS Excel Milenium tweeter. He also told us, proudly, how he used only teflon-insulated silver wires, silver solder, very expensive capacitors, and expensive flat-wire inductor coils within the cabinets. He assured us that all those materials significantly improved the sound of the speaker.
When he demo'ed the speakers, someone (Dennis Murphy) politely suggested that he might have mistakenly wired the tweeter with polarity reversed from the intended design. The builder doubted that. Later that same day, the speakers were measured acoustically, and they were indeed mistakenly wired. Those acoustic measurements looked similar to this other 2-way design:
Wired with the correct polarity
View attachment 39863
And with the tweeter with reversed polarity
View attachment 39864
The difference looks pretty large on these graphs, but at roughly 2.5 kHz, were surprisingly easy to overlook. The take home lesson: If the builder could not hear the difference between the correct and incorrect wiring polarities, what can we make of his claim about the improved sound qualities of the silver wires and other expensive crossover components? They must have been much larger than the suckout at 2.5 kHz. That is a positive control – in this case, an unintended positive control. The guy's understandable late night mistake undermined his claims about silver wires and exotic crossover components.