Diagram H is scary stuff at first glance, because I’ve used one diagram to illustrate several related points. There’s a lot of information in there, so let me break it all down into some manageable blocks for you, starting at the top and working downwards. Hopefully, you’ll recognize the gray block with the sliders on it as my attempt to draw a graphic equalizer. Even if you didn’t recognize my drawing, you’ll have seen similar devices everywhere from bass rigs to hi-fi systems. This is no ordinary graphic though: this one has the full 31 bands typically used to compensate, or ‘tune’ the output of a Sound Reinforcement rig to the acoustics of a venue. Because there are 31 bands, they are close enough together to allow problem frequencies to be tuned out without unduly compromising the overall sound balance. In fact, this format of equalizer is so universally recognized, the frequency centers from 20Hz-20kHz are set out in an ISO (International Standards Organization) specification. I’ve set those frequencies out in numbers below the graphic of the equalizer, with the octaves highlighted in yellow. Below this, there is another block showing where these frequencies sit in terms of ‘sub-bass’, ‘bass’ etc. In an advanced 5-way rig, these general labels would correspond directly to the speakers that these frequencies would be coming from (with some deliberate overlap at the crossover points). You might wonder, if this is such a useful format, why doesn’t every channel of a mixing console have a 31-band graphic? Well, one reason is that 31 bands take up an entire 19-inch rack case, so our channel strips would be unworkably big. As it turns out, for individual channel EQ, we don’t need access to all 31 bands. What we need is to be able to tune one or two bands to the frequencies we find useful on that particular channel.