In a previous video, I showed you what happens when you wire two speakers with opposite polarity. This will build upon that concept, but let’s do a quick recap in case you missed that episode.
A speaker’s polarity is determined by the connection to the amplifier. If you wire both speakers the same way, they will work together, and the energy from each speaker will sum to create a louder signal for the listener.
Here in the DAW, I have two copies of a signal. When I play one at a time, the signal meter reads -18 dB…
And when I play them together, the signal meter reads -12 dB (an increase of 6 dB).
But let’s say you wire one speaker with correct polarity (positive to positive and negative to negative) and one speaker with inverted polarity (positive to negative and negative to positive). Now the speakers will work against each other. When one speaker creates a positive pressure change, the other creates a negative pressure change, resulting in a cancellation.
I have the two copies of the signal again, this time with one copy inverted. Each copy still reads -18 dB on the meter.
But when I play both copies together, they completely cancel each other out.
While this is helpful for understanding the mechanics of constructive and destructive interference, wiring speakers with opposite polarity rarely results in a COMPLETE cancellation in practice.
Let’s demonstrate that by playing a stereo signal through your speakers. I’ll start with the same polarity in both the left and right speaker – then I’ll invert the polarity of one of them…
If you’re listening on headphones, the effect is quite odd because none of the signal from the left side reaches your right ear and none of the signal from the right side reaches your left ear…
In speakers, the signals will cancel when they interact acoustically, but you’ll notice that they don’t COMPLETELY cancel, as they did in my electrical demonstration…
That’s largely because my electrical demonstration was 1-dimensional, while the acoustic space you’re listening in is 3-dimensional, which makes things a lot more complicated to predict. I think another demo would be helpful here…
I’ve got two speakers and a microphone. When I play a signal out of the left speaker, we see -18 dB on the microphone meter.
The same thing happens when I mute the left speaker and play the signal out of the right speaker.
These two speakers currently have the same polarity. So what do you think will happen when I play both speakers together?
It results in a greater signal level, but it’s not exactly 6 dB because there are now other factors at play. More on that in a moment. What do you think will happen when I invert the polarity of one speaker?
The combined level is less than the level of either speaker alone, but the signal doesn’t completely cancel out as it did in the electrical demonstration.
One reason for this is that we are hearing indirect sound that has reflected off of nearby surfaces and back to the microphone. Another reason is that the mic may not be perfectly aligned. Maybe it’s slightly closer to one speaker than the other, which creates a slight time offset between them, causing the signals to partially cancel rather than totally cancel.
Watch the meter as I deliberately move the microphone closer to one speaker…
Remember – each speaker alone was at about -18 dB on the meter, but now the meter level increases (even though the speakers are wired with opposite polarity). How is that possible?
A moment ago, we saw that the waves could partially cancel when there is a slight time offset between them. At a certain point, with a large enough time offset, the waves will actually sum together.
And if you keep delaying one signal, they will eventually totally cancel again.
When two copies of a signal arrive at a different time, there will be phase interference. This is a big challenge in the world of sound system design because it means people sitting in different locations throughout the room will experience the sound differently.
If the speakers are wired in the same polarity, the signal from each speaker will work together for the person in the center of the room (who is an equal distance from each speaker). But the speakers might work against one another from the perspective of the person sitting off to one side.
Unfortunately, it gets even more complicated when dealing with a signal containing many frequencies (like music). That’s because each frequency has a unique wavelength, which means the same time offset will result in different levels of interference at different frequencies.
In the DAW, we can visualize this. Instead of two copies of one frequency, let’s make two copies of a group of frequencies.
Right now, they all align in time. But if I shift one group of frequencies back in time, some frequencies sum together and some cancel out. The amount of phase interference depends on the wavelength relative to the time shift.
Listen to the effect this has in the physical world when I play many different frequencies through these speakers and move the microphone…
In closing, understanding how multiple speakers interact offers key insights into improving audio quality. Keep these principles in mind to enhance your sound setups. You can learn more about this effect, called “comb filtering” in this post.