Click here to purchase the entire book in PDF format. Bass management Once upon a time, people who bought a stereo system bought two identical loudspeakers to make the sound they listened to. If they couldn't spend a lot of money, or they didn't have much space, they bought smaller loudspeakers which meant less bass. (This isn't necessarily a direct relationship, but that issue is dealt with in the section on loudspeaker design... We'll assume that it's the truth for this section.) Then, one day I walked into my local stereo store and heard a demo of a new speaker system that just arrived. The two loudspeakers were tiny little things - two cubes about the size of a baseball stuck together on a stand for each side. The sound was much bigger than these little speakers could produce... there had to be a trick. It turns out that there was a trick. The Left and Right channels from the CD were being fed to a crossover system where all the low-frequency information was separated from the high-frequency information, summed and sent to a single low-frequency driver sitting behind the couch I was sitting on. The speakers I could see were just playing the mid- and high-frequency information... all the low-end came from under the couch. This is the concept behind bass management or bass redirection. If you have a powerful-enough dedicated low frequency loudspeaker, then your main speakers don't need to produce that low frequency information. There are lots of arguments for and against this concept, and I'll try to address a couple of these later, but for now, let's look at the layout of a typical bass management system. Figure 10.25: A typical monitoring path for a bass-managed system. Note the 10 dB boost on the LFE channel. Figure 10.25 shows a block diagram for a typical bass management scheme. The five main channels are each filtered through a high-pass filter with a crossover frequency of approximately 80 Hz before being routed to their appropriate loudspeakers. These five channels are also individually filtered through low-pass filters with the same crossover frequency, and the outputs of the these filters is routed to a summing buss. In addition, the LFE channel input is increased in level by 10 dB before being added to the same buss. The result on this summing buss is sent to the subwoofer amplifier. There is an important item to notice here - the 10 dB gain on the LFE channel. Why is this here? Well, consider if we send a full-scale signal to all channels. The single subwoofer is being asked to balance with 5 other almost-full-range loudspeakers, but since it is only one speaker competing with 5 others, we have to boost it to compensate. We don't need to do this to the outputs resulting from the bass management system because they five channels of low-frequency information are added, and therefore boost themselves in the process. The reason this is important will be obvious in the discussion of loudspeaker level calibration below.