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Crossing the line

— By Paul McGowan —
In our continuing series on subwoofers I've chosen to break the sub down into four primary sections: crossover, woofer, power amplifier and enclosure. Today let's start to review the crossover. The purpose of a sub is to augment the low frequency performance of our stereo system. We want it to fill in the missing frequencies in music, from perhaps 60Hz down to several Hz. How high our subwoofer is allowed to go depends on the main speaker's low frequency response in the room. If you have a small set of bookshelf speakers, it's unlikely they go much below 60Hz, and in this case you'll want the subwoofer to fill in everything below 60Hz. If, for example, you have a full range floor-standing loudspeaker, you're probably going to start at a lower frequency, like 30Hz and fill in below that. But it's never quite that simple. Much depends on the type of loudspeaker you have. An electrostat or ribbon may boast 30Hz response, but it's unlikely that any such planar type speaker can move much air at those lower frequencies; despite its measured response. It's instructive to remember that the measured response published by a speaker manufacturer doesn't always match your listening experience in your room. Rarely does it match, in fact. So our subwoofer needs to take over where your main speaker leaves off. To do that, we need to make sure the subwoofer does not overlap the main speaker's output. It's a difficult task: one ending the other taking over seamlessly. In fact, it's one of the very toughest tasks we have. Most sub crossovers are fairly simple, consisting of a low pass filter and a volume control. Others include a high pass filter, variable slopes for the filters, still others offer phase control and some even have a calibration system built in. Let's focus on the medium range of controls.
And let me add something of a note here. I do NOT appreciate or recommend rolling the bottom end response of our main speakers off. I know that is common practice in home theaters but it is, in my opinion, a real no-no. Let us not hear these words of encouragement in this direction; lest I get on a soapbox and begin to preach against the evils of rolling off full range loudspeakers.
A low pass filter is the primary crossover tool we need. "Low pass" means what it says. The lows pass through and the highs do not. Get it? Low "pass", the lows pass through, the highs do not. Typically there's a frequency control, allowing the user to set how high the music can go before it cuts off. For example, if the low pass (sometimes referred to as the Low Cut) filter is set for 40Hz, this simply means everything below 40Hz gets pumped through the subwoofer, above 40Hz does not. And, how much of the music above 40Hz (in this example) gets through? Is it a brick wall? Are we to understand 40Hz gets through, but 41Hz does not? No, that is certainly not the case. This is where slope or rate of change comes into play. When you set a frequency on the low pass filter control, the result is not necessarily a sharp cutoff. It typically means the frequency selected will be -3dB down. Above that frequency the sound diminishes at (typically) 12dB/octave. What's that mean? It means the sound reduces by 12dB every doubling of frequency. So in this example of 40Hz, the sound is 12dB less loud at 80Hz and 24dB less at 160Hz and so on. We've already glazed over many folks eyes at this point. More tomorrow.

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