John: The hollow sound you get when there is sufficient delay to cause flanging and audible nulls at longer wavelengths.

AlanH: What are some common problems with system design and/or tuning that you see being made by system engineers and/or installers?

John: The basics, really. What I teach the most is gain structure and equalization. It amazes me that it is necessary for virtually all sound systems, yet it’s not even taught in NICET certification. And the gain structure part is so easy, with Pat's (Brown) piezo trick especially.

Moderator: What is NICET?

John: It’s our industry's first attempt at getting certification for low-voltage technicians that, hopefully, will be a recognized standard like a licensed electrician.

Tucci: A big misconception that is prevalent here is that EQ is bad, that phase gets all messed up when you EQ. Can you address the topic of amplitude and phase being intrinsically tied together?

John: There are basically two problems with speakers as moving-piston band-pass filters. The first is non-minimum phase problems, which are largely time-oriented. These have been addressed, in part, by EV with their Ring Mode Decoupling developments, which can’t be equalized.

John (continued): Likewise, misalignment at crossover is another example of non-minimum phase problems. They cannot be equalized. Minimum-phase problems are resonances, not time oriented. The change in amplitude (not being flat) is the other side of the coin from the change in phase. They are linked.

John (continued): It is these problems that a minimum-phase filter set (equalizer) can fix, so when you EQ to flat, the phase is also corrected. The hi-fi idea that they don't use EQ because of messing up the phase is bull. The phase needs fixing with the amplitude. And, by the way, the EQ on hi-fi speakers is built into the crossover network doing the same thing at speaker level that a standard EQ would do at line level.