Asheville, North Carolina ‘s Home Theater and Audio specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Overlap

Let’s get back to our basic subwoofer crossover today.

The purpose of the built in crossover is to make sure the subwoofer doesn’t play music higher than a specified frequency; that frequency specified by the user via a low pass filter control. Remembering the term ‘low pass’ means to pass the low frequencies, this seems simple enough to choose what point you want to have the music to not go through the subwoofer as the frequency goes higher.

For our example we’ll continue using 40Hz as the highest the sub is allowed to go. From there, we’ll assume it’s rolling off at 12dB/octave (remembering an octave is a doubling of frequency). This means our music is 15dB down at 80Hz (-3dB for the set point and -12dB because we’re at 80Hz, twice that of 40Hz). Let’s also assume our main speaker rolls off at the same rate as our subwoofer, 12dB/octave.

We want to set the subwoofer’s highest frequency somewhat below the lowest the main speaker goes. We want to do this because we’re expecting – depending – on a bit of overlap, meaning the sub and the main speaker produce some of the same frequencies in music at the point where they crossover – the top of the sub’s range, the bottom of the main speaker’s range. If both were set at exactly the same frequency, we’d likely get a bump up in sound as the top of the subwoofer will add to the flat response of the main speaker.
So, imagine our main speaker starts rolling off at 60Hz. Setting the subwoofer somewhat lower, at 40Hz, then makes sense because as the one is going up and getting softer in volume, we’re getting far enough away from the crossover frequency to minimize the overlap.
Using your main loudspeaker’s specifications is a good starting point to set the subwoofer’s maximum frequency on the low pass filter. But listening, playing around with the sub’s settings will be needed to get the smoothest transition at the point of overlap.

Asheville, North Carolina ‘s Home Theater and Audio specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Whoa, skippy!

My, my, there’s a lot of opinions out there. Thank goodness for that! I suppose a few hornets have been riled up by my comment of not rolling off the bottom of your main loudspeaker to accommodate the sub. I suppose that needs a little more explanation.

First, let me repeat an oft mentioned ‘Paul’s Rule’. The goal of any loudspeaker system, or for that matter any music system, is to disappear. There’s no higher compliment given than when people sit in Music Room One and ask where the sound is coming from because, ‘surely it’s not coming from those big speakers’. Those ‘big speakers’ disappear and that’s the whole point. Same with a subwoofer. The greatest compliment to any subwoofer installation is that it is invisible.

Think of food. The individual ingredients should be invisible if they were prepared properly. You don’t want to say to a chef ‘I can sure taste that basil’.

So the idea of a subwoofer is to augment the performance of the main speaker and then do so in a way that never draws attention to itself.

Now, on to the main topic, to roll off the bottom of the main speakers or not. I say NOT.

Why would you want to roll off the bass from the main speakers? Because of an effect known as Doppler distortion. Simple to understand, Doppler distortion describes that property of an object, like a speaker driver, to do two (or more) things at once. I know … that’s not entirely accurate … but it’ll do. If a slow moving woofer is also making higher frequencies at the same time as it’s slowly woofing, then those higher frequencies are being modulated by the lower frequencies causing a type of ‘muddiness’ or ‘muddledness’ to the sound. Better to have the woofs woofing and the tweets tweeting, never the two to meet. That’s the reasoning behind removing the requirements for bass from the woofer and assigning it to the subwoofer.

But here’s the thing. The methods used to remove that bass from the main speaker (yet another crossover filter) add more trouble than they remove. That’s my thought on the matter.

You know what? Your main speaker was designed, I am assuming, from someone competent enough to make it all work. Leave it alone. Augmenting it with a sub is smart. Changing its functionality is not.
Thank goodness I don’t have an opinion on the matter.