I had a series of back and forth emails with a fellow audiophile yesterday regarding the LFE levels on my calibration and alignment disc. He wrote to me and asked if I had reduced the level by 10 dB as recommended and required for home theater systems. I double-checked the tones of the “AIX Records Calibration and Demonstration Disc” by playing the test tones in my studio and reading the levels. It turns out that I didn’t attenuate the LFE channel levels. And he believes that I’m violating the technical requirements of a proper home theater setup. I disagreed and tried to explain why…he was not convinced. I spent so much time trying to answer his questions and explain the use of the LFE and subwoofer that I thought I would discuss the topic today.
Before we get into the nitty gritty this topic, let me first start with the concept of stereo panning. Trust me it’s relevant. There is a rotary knob on recording consoles called a panner. Whenever the “stereo bus” or main mix outputs are engaged, the panner allows the engineer to distribute the audio signal coming through a particular input channel anywhere in the Left/Right stereo field. Imagine the track is a cowbell. During the mixing process the engineer can pan the bell to the extreme left or right side. These special cases would place all of the amplitude of the cowbell in a single speaker.
However, if the mixer decides that the cow bell belongs in the center of the stereo mix, he or she will use the stereo panner to move the cow bell so that it appears to come from the between the left and right speakers. This is called “phantom center”. We hear the sound coming from the center but in fact, it’s coming from the left and right speakers with equal signals level…but not the same signal level that it had when it was panned to the extreme left or right side. Why has the amplitude changed? Has it been lowered or raised?
It’s been lowered by 3 dB when the image is panned to the center because now there are TWO sources for the sound. And when a sound is output from two speakers, the level of the two signals combines and increases by 3 dB. So the panning circuits built into the channel inputs of the console strips has a smooth attenuation of 3 dB when panned to the center position. Look at diagram below:
Figure 1 – An illustration showing how a panner attenuates a signal when in the center.
Engineers don’t have to worry about the center “build up” because the electrical engineers designed the circuits to avoid the problem.
More to come…