This question made my brain hurt so I had to make a video to explain it to myself. The answer is NO for the common subwoofers we encounter in the field that are closed in the back and have ports in the front. The answer would be YES if the sub were open in the back, but then the coverage pattern would be a figure-8, not omni.
- Why do you polarity invert the rear sub in a cardioid array?
- Phase Wheel
- FIGURE 4.12 Summation zones from Sound Systems: Design and Optimization by Bob McCarthy
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I have a question for you: If I grab this speaker and flip it around so it faces the rear, does that polarity invert the signal?
Think about it for a second. A student asked me this recently and it made my head hurt a little bit. And I realized that I had some competing ideas in my head about how speakers work versus how instruments work. And so I wanted to just talk about that for a few seconds in case other people are having sort of if this question makes your brain hurt, then hopefully this video will help a little bit.
Here’s why I think this question makes your brain hurt. I asked this question in my student community, and you can see most people are saying no, but twenty five percent said yes. I just posted this question a while ago on YouTube and I only have five votes and it’s only an hour ago. You can see there’s still some division as to what’s going on with this question. Maybe I didn’t ask the question correctly. So maybe the question is confusing.
But but let’s look at this. So, um, let’s see. I have this video over here from Alex to find out where he’s talking about making a snare drum. And I think this is maybe where this idea comes from, because one of the first things we learn about making drums is that if you would like to get the nice sound of the snare on the bottom of the snare drum, and you’re going to put a microphone down there and you’re going to put it equidistant like this, then you need to do a polarity inversion on the input channel where that microphone comes in.
Otherwise you’re going to have problems. OK, so we all learned that a long time ago and we are familiar with this idea. I like this. I found this video of a slow motion snare drum. We’re familiar with this idea that when the top head has struck up here, then the bottom head also goes down at the same time. And so we have these equidistant microphones that are receiving opposite pressure. And that’s why we need to polarity invert them so that they’ll go in the same direction when they get some together in our console digital audio workstation.
I have another image here from Sound on Sound. So here is a recording of those two microphones. And if I draw on this, it should be pretty easy to see that where we have a peak here. Then at about the same time down here in the bottom snare mic, we have a peak, but in the opposite direction. Now we start thinking about how this relates to loudspeakers and we think, oh, it’s the same thing, right?
We’ve got positive pressure going in this direction and we’ve got, um, negative pressure going in the other direction. I should probably use different colors anyway. And this is, I think, kind of what we’re thinking. And here’s just the same picture. But just the parts exploded out, positive pressure, negative pressure. So if I go back into my simulation here and I take a look at the speaker and I look at it at this microphone, then what do we expect to see?
Well, I’m thinking that there’s going to be a positive peak going up. OK, let’s measure that. Let’s zoom in and let’s do let’s reset delay and let’s store that. So I feel like my expectation was met. I expected a positive peak. So now if I have this idea that if I flip the speaker around and now at that same microphone, the peak should go down, then the same thing would be true if I left the speaker the same way, but just measured behind it.
Right. So I have a microphone back here so we can just switch microphones. So I’m at my one in one hundred and eighty degree microphone and I’ll hit predict and I’m kind of expecting to see it go down. Right, because this is my idea. I flip the speaker around, but we can see that the speaker is totally weird now, right. Because we’re not getting as much high frequency information back there behind the speaker, but we still have a peak to look at and it’s going up.
So that’s weird. You may be wondering why I don’t have this perfectly in the center, and that’s because that it takes a little bit longer for the sound to travel around the speaker and get to this microphone. And so I had to offset a little bit so I could put the microphones at the exact same position, three meters and negative three meters. But I just move the speaker. OK, but now you’re thinking, OK, well, that doesn’t work with high frequency drivers.
But surely subwoofers, which we know are omnidirectional sounds going forward. Sounds going all sounds are going everywhere. So let’s test that. Let’s get rid of this X 40. I’ve got a 750 here. Let’s measure the 750 at our front microphone. Let’s autoset delay, zoom out, we’ll store this and let’s do the same thing, let’s just switch to the rear microphones. We’re going to go to the trouble of actually flipping the speaker around.
Oh, same problem. Yes, it’s arrived a little bit later. And that’s why I had to change the position a little bit so that I could line the peaks up perfectly on top of each other. But same polarity. And if you like to look at phase, we can do that.
So here we go at our zero degree microphones store and here’s our rear microphone, and it’s exactly the same. So what’s going on here? I think the confusion is that sound is not coming out of the rear of the subwoofer the way you are imagining it like this or like this or like this. OK, this is kind of what we’re imagining. And that might be true if this were not closed in the back. So we might be tempted to think that positive pressure coming out here and then negative pressure coming out of here.
But that’s not really happening because it gets here and it says, I can’t get out this way. So then it goes over here and out here and then actually comes out of a port somewhere. And then maybe then it goes around the speaker and and that’s why it takes a little bit longer to get to that rear microphone. But you have experienced an open back driver and where you have experienced that is with an open back guitar cabinet. So it’s pretty common for a guitar amplifier to be open on the back in many times in studio sound, we will mark the front and the back.
And you have the same situation, right? You need to polarity, invert that rear microphone. I can’t insert a guitar amp into MAPP XT, but I have tried to make a simulation to play with this. So it looks like these two speakers are really far apart. But just imagine that this is a big guitar amp and that this speaker up here is the going to stimulate the forward pressure of that single speaker. And this, uh, back here is going to stimulate the rear pressure.
So if I play these both at the same time, this is the kind of pattern that we would get with an open back guitar. And it. Right, similar to a microphone that receives on two sides that’s open on the back. You’re going to get this figure eight pattern guitar amplifier figure eight pattern of coverage, subwoofer, not a figure eight pattern because it’s not open in the back. And the way I built this, in case you’re curious, is I just turn this guy on.
Let’s look at him at the zero degree microphone. And store, and then I just inverted the polarity at this guy and pushed him back far enough so that his peaks would still line up with the peaks of the other guy so that we’d have a lot of. I wanted to make a really dramatic example. Right. OK, in case this is all still confused you, there’s one way that always works for me. Any time I’m ever getting confused.
And I’m like, oh, this still doesn’t make sense to me. You can always insert a gradient array in MAPP XT. The next question that I found people often get to once they realize that flipping a subwoofer around does not invert the polarity, then they say, wait, but isn’t that how you build a cardio subwoofer array? You flip the sub around so that it inverts the polarity? That’s not right. You flip the sub around to create delay.
So I have another video called Why Do You Polarity Invert the rear sub and a cardioverter way where I suggest you watch it. If you haven’t, it’s just a nice step by step illustration of how an inline gradient cardio subwoofer already works. But right now, I’m not going to go over that, but I am going to show you one. So here I’ve got the same subwoofers, but now they’re creating cardio and subwoofer so I can do a prediction at eighty hertz and we’re too zoomed in to really appreciate it.
You can see that we’ve got some action going this to the front and a cancellation going to the rear. And so just keep that in mind because the inverted gradient stack is the exact same principle, just smush together so that it uses less real estate. So here we’ve got two processing channels. Right. Let me expose that for you. So here’s my gradient in line, forward gradient in line rearward and I’ve got normal polarity and reverse polarity. So you always need two processing channels, one with the polarity inversion and the delay.
So let’s compare that then to an inverted gradient stack. And this is really fast in case you ever get confused in your and you’re like, wait, how does this work? You can open up map sixty just right click choose insert gradient flown subwoofer. And when you do that, take note of two important things. Uh, number one, we have two processing channels and we are achieving a polarity inversion not by flipping the sub around but with an electronic polarity inversion.
Number two, we can automatically apply the processing that we need, namely the delay directly into our process. So make sure you check this and make sure you choose the right channels. OK, so I’ve already inserted this, so I’m not going to do it again. But that’s what’s going on here. We have two speakers facing forwards, one facing to the rear, and it’s the one that’s facing to the rear that has the delay and polarity inversion.
So over here you can see I’ve got gradient forward, gradient rear and this is my stack. And if I do a prediction, we’ll see that we get summation to the front and cancellation to the rear. So just to summarize, flipping a sub around does not invert its polarity because the back is closed. And the way we achieve this polarity inversion in our gradient subwoofer arrays as through an electronic polarity inversion, not by this change in orientation. All right.
Let me know what questions come up for you about this and let me know if you have any suggestions for me. I’m always trying to improve my own understanding of these principles. Thanks.