In a recent thread, Steve announced if you played the stereo mix of the Kinks' song Waterloo Sunset, you could hear the backing vocals come from well outside the speakers and maybe even behind you. Further, he stated that you could use this song to set up your speakers properly. I want to explore the technical side of this effect. I have heard this effect many times in the past. Sometimes it appears on individual recordings like Waterloo Sunset. More consistently, there were two manufacturers in the 1980s and '90s who built hardware which exploited this effect. Carver's Sonic Holography circuitry and Polk Audio's Stereo Dimensional Array (SDA) speakers both created this effect on many additional recordings. I owned a stereo store that was a big Polk Audio dealer and I both sold and owned SDA speakers. I had a pair of Polk SDA CRS speakers in a basement home theater in 1984. The SDA effect worked great with movies. This was the era of Dolby Surround, with my sources all analog: VHS, Beta and Laserdisc. Getting sound to the rear speakers was difficult and anything I could do to supplement my surround effects helped. Mostly, though, what I named the Waterloo Effect was used with music. Let's look at how it works. Do you know how three dimensional the sound can be through headphones? That's because with headphones, the left ear only hears the left speaker and the right ear hears just the right speaker. Many of us assume this also happens with our stereo speaker set-ups but it doesn't. Instead, with speakers, not only does the ear nearest to the speaker hear the sound but so does the ear on the other side of your head. This is why the Waterloo Effect is more accurately called interaural crosstalk. The brain uses the differences between information heard by both of your ears to locate where a particular sound is coming from. This is fine when you listen to a family member at the dinner table. Even with your eyes closed, you can locate them across the room. But with stereo speakers, advocates of Carver's Sonic Hologram circuitry and Polk's SDA speakers say this interaural crosstalk smears the stereo image. To create the Waterloo Effect, sound from each channel is fed into the opposite channel. It is flipped 180° out of phase, delayed a bit and attenuated in the highs. That's because the sound has to travel further to the opposite ear and has less high frequencies. With the song Waterloo Sunset, the out of phase information is mixed in. Carver's Sonic Hologram allowed you to add it with a preamplifer or an add-on box. Polk Audio did it physically, with a cable running between the two speakers. They sent the out of phase signal to a special set of midrange drivers on each loudspeaker. Called the dimensional speakers, they were on the baffle about 8" away from the mains, nearly the width of the human head. That's how they got the delay necessary. Whatever the process, the out of phase information is then transmitted by the opposite speaker. If done right, being 180° out of phase, it cancels the sound that is heard by the opposite, or "wrong" ear. What we now have is the same thing we have with headphones. The left ear only hears the left speaker and the right ear just hears the right speaker. People who owned this technology in the 1980s and '90s often describe it as fun. It may not be accurate but it can be spectacular when done well. There are companies who are still messing around with interaural crosstalk. Q Sound is the largest, with their Q Surround technology. Another is Ambio4You. And whatever you think of these aftermarket ways of defeating interaural crosstalk, Waterloo Sunset sounds very cool when played on properly set up speakers. Later in this thread I want to discuss Steve's assertion that you can use Waterloo Sunset to set up your speakers correctly. He's absolutely right but now is not the time. This post is way too long as it is.