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How does comb filtering happen between 2 drivers, the woofer & tweeter?

Dennis Foley December 15, 2014 No Comments
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In last week’s Google hangout I was asked quite a few questions on comb filtering so below I have included a video of the discussion and a transcript as hopefully this will be useful to further your understanding of this room acoustic issue.

AD: A few question on the comb filtering videos again, that seems to be a popular topic.

DF: It needs to be a popular topic because people have been living with it too long and I’m glad people are questioning this, I’m glad we’re raising the consciousness of people because it’s really a nasty thing. I mean just put a piece of foam on your console and listen to your monitors, just put a blanket, put a towel ,I don’t care put anything that’s got sound absorption qualities that will kill that reflection and listen.

So all that distortion off your console is in your mix, I’m going to go out on a limb here, and it’s not that big of a limb, and saying if you’re not producing quality recordings that people are buying or that your mixes aren’t translating, you’ve got too much distortion in your mix and I’ll bet you one of those distortions and there are many is that bounce off that console. I’m going to bet that’s the case.

AD: Alrighty, alright so this one, this first one is from our old friend Nelson out in the Philippines. Can you share some insights into how comb filtering between two drivers, woofers and tweeter and two tweeters mainly for speaker DIY’s because this happens a lot too in my experience in speaker DIY’s?

DF: Well it’s a form of distortion that the speaker designer has to work with. Now he can compensate for that through crossover design and electronics but he can’t compensate for it too much. So you’re right it is a form of distortion but it’s a form of distortion that all speaker designers try to deal with.

You ever seen a speaker that has no cabinet? There’s just a speaker in a hollow or in a tube, Gallow Electronics they make those spherical speakers, they’re just in round cabinets. There’s a designer’s answer to, it’s called diffraction, that diffraction affect off the speaker cabinet face. So Nelson’s correct, it is an issue. Unfortunately it’s an issue that all speaker designers have to deal with so they just work around it, they just work around it.

If you took the cabinet away from a speaker that you like to listen to, you wouldn’t like it anymore. If it was just the speakers floating in mid-air boy wouldn’t that be nice? If it was just the speakers floating in mid-air and you had no cabinet on your favorite speaker, you wouldn’t like it because the total sound that comes out of that speaker is a result of about five things: the drivers, the cabinet size, the cabinet construction, the wiring, the crossovers, the internal film material, all of those, it’s like painting a picture with different mediums you use oil, acrylics, watercolors. Use all the tools that you can to create a sound so that final sound that you’re hearing, it comes out of the speaker is a combination of many compromises that the engineers had to do.

Now ready for this? Then you take that speaker which that engineer spent years and years trying to figure out all those compromises to get the sound quality you wanted and you put it into a small room, yikes.

AD: Alrighty I don’t know if people know but Dennis doesn’t like big speakers in small rooms.

DF: Nope, it’s not only me. Anybody that understands. See I deal with distortions every day from people and from rooms. And the people distortions I can’t do anything about because those are based on norms and values and belief systems and I want no part of that but the bottom-line is room distortions once minimized and we prove it here every day in our studio we have a $400-electronic front-end with blown speakers in a $150,000-room and there’s very few distortions in our rooms and people sit in our chair and cry.

So what does that tell you? That the beauty of music, the emotion of music, the thing that we all need, seek and want and got to have is there but it’s all these distortions getting in the way and when you take those distortions out wonderful things can happen and that’s what it’s about, that’s why we keep buying new speakers, that’s why we keep buying new amplifiers, that’s why we keep buying new cables, that’s why we keep turning up the volume.

We want more but you’ve got to get the things out of the way, you got to remove the accidents off the freeway so you can drive to your goal quicker and that’s what we have to do. So let’s don’t add more distortions by putting our speakers too close to the wall, let’s don’t add any distortions by putting a large speaker in a small room, more is not better believe me it’s not. Consumerism is not that great when it comes to room acoustics, be very careful there is a balance. It’s all about balance.

I’ve had too many cups of coffee this morning!

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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