Speakers are meant to create air-pressure changes.
Let’s take one speaker. Let’s use the most popular size in car audio – the 6.5” round speaker.
That speaker can play an amazing range of notes. It can play from pretty low in the bass – 80 Hz for sure, maybe even lower in some cases – and it can play up to roughly 2kHz or so before it starts to roll off from the listener’s perspective in a car door.
But one 6.5 just isn’t loud enough. What do we do?
We can give it more power. But each time we double the electrical power we send to the speaker, we only get 3dB more output – and 3dB isn’t that much. It’s noticeable, but it’s not impressive.
We need more cone area – another speaker. So now we have two speakers playing the same notes. Double the cone area gets us a potential 3dB increase.
Power and Cone Area
Now let’s give that added speaker another amp channel, same as the first. We have doubled the electrical power of the sound system. That’s another 3dB! That’s 9dB we’ve gained! Now, we are getting somewhere.
However, now we have a few potential problems.
First off, we had better connect the speakers in proper polarity! If we accidentally get one connected backwards, we will cancel out a lot of the potential sound. If a speaker is connected in reverse polarity, the output is 180 degrees out of phase relative to the other speaker. When one speaker cone is moving out, the other is moving in, and the air-pressure increase we are looking for doesn’t happen.
Two speakers wired in polarity
Secondly, we must be cautious about where we place the speakers relative to the listener and to each other. If both speakers are the same distance from the listener. If the two speakers are different distances from the listener – if the path lengths the sound must travel are different – then the sounds don’t arrive at the listening position at the same time. If the sounds don’t arrive at the listening position at the same time, they may not be in phase any longer. They may be misaligned.
Depending on the path-length difference, and the note being played, we may inadvertently cancel out the note almost completely. That’s not supposed to happen – adding another speaker was supposed to make our system louder!
In home audio, we can just move one speaker to solve the problem (we may not want to, but we can). In car audio, we used to try to do that by building speakers into the floor, to make the path lengths closer to equal. It’s a lot of modification, and nowadays we have other tools we can use to address this problem – but first we have to know it’s happening.
Four Causes of Cancellation
When do car-stereo systems suffer from this problem of driver cancellation? There are four chief causes:
- Front and Rear speakers. The rear speakers in most audio systems are there to help us play the system louder. In older cars, the rear speakers were the larger ones, so they could play louder and they could play more bass. In modern cars, that’s rarely true any longer. Regardless, having front and rear speakers play the same notes causes this problem, when the rear speakers are farther away.
2. Speaker drivers on adjacent sides of a two-way crossover filter, such as a tweeter and a midrange. Wait, I thought crossover filters existed to prevent this? Well, all crossover filter networks have a transition band – a band where the high-passed driver and the low-passed driver are both contributing to the sound. Depending on the slope of the crossover filters used, this band might be wide or it might be narrow, but it exists. Speaker system designers have to ensure that the filters selected don’t introduce such cancellations (especially after the high-passed and low-passed speakers are installed at different distances from the listening position!) This is most commonly noticed at the subwoofer-to-midwoofer crossover point, where cancellations cause a loss of bass (as well as revealing that the subwoofer is in the back of the car).
3. Left and Right speakers. This isn’t just about stereo. A stereo system plays the same sounds from both left and right speakers when the recording engineer wanted to place some sound in the center of the imaginary stage, midway between the speakers. But if you play a mono signal, both left and right speakers still play the same sounds at the same time – this isn’t just about stereo! When that happens, and the left and right speakers are different distances from the listening position, we get cancellations which ruin our stereo illusion – as well as reducing system volume at several frequencies.
4. Reflected sound arriving at the listening position later than the direct sound from the speaker. This is a fact of life in a car. Glass is highly reflective for sound, as is dashboard material. We can’t eliminate reflections in a car, even if we want to. Fortunately this is the least critical of the four causes of cancellation. The farther a sound travels, the more attenuated it becomes, and the less cancellation it can cause.
What do we do?
Solving #1: For years, my goal was to not use rear speakers – but that prevented me from getting the benefit of the added cone area and added amplifier power. So now I use them, but I use either delay (for a one-seat system) or phase equalization to manage rear-speaker-caused cancellations.
Solving #2. My solution to this is to always use Linkwitz-Riley 4th-order 24dB/octave crossover filters. Then, I just have to worry about distance-related cancellation, not crossover-filter-induced cancellation.
Solving #3. For the past decade or more, delay has been the standard fix for this problem. However, it only works for one listening position. Upmixed systems don’t rely on the same sort of summed signal creating the phantom center, so OEMs have started using them more often over the past 10 years or so. Phase Equalization can also fix this – at least partially – and Bose (among others) has been using this for at least 20 years. This is why most DSP UIs ask you for the distance you want to delay the sound – because delay caused by distance can be offset by introducing delay on the other channels, to get everything in sync. (Some DSPs ask you for the path-length difference for that speaker, and they should really make that fact clearer).
Solving #4. Don’t worry about it. We’re good. Some people want to put midranges in A-pillars to change the reflective properties, and that’s fine – but we aren’t eliminating them, we are only changing them. Reflections will still happen.
Precision, Distance, and Frequency
Some installers have tried to impose high degrees of precision onto the management of cancellations. They’ve tried to use laser distance measurement, or complex formulas, to measure the various distances. Fortunately, we don’t need that degree of precision. As the table below shows, to reach 180 degrees of cancellation at 80 Hz, the path-length difference has to be 82 inches. That’s a long distance for two subwoofers! “Well, isn’t there partial cancellation before we get to the 180 degree point?” Sure, there is, but not at 1% or 10% of 83 inches! At 90% of 83 inches, sure, there’s cancellation! The good news is, you need to be close to 83” of path-length difference for cancellation to be come a big problem.
As you move to shorter and shorter path-length differences, the initial cancellation frequency moves higher and higher. Those shorter path-length differences are found in cars more often!
But once we get to really small path-length differences – an inch or two – we couldn’t possible hold our heads still enough – we would have these problems all the time! It turns out, we do have these problems all the time – we have just learned to ignore PLD-caused cancellations above some frequency. What frequency? Some researchers say 1500 Hz, some say 3000, some say 6000.
That’s why, when we have two subwoofers 2” apart from each other, and we wire one backwards, we get almost complete cancellation. At the frequencies involved, a 12” path-length difference (technically, the voice coils of 2 10” woofers would be about 12” apart in this scenario) is so close to 0 that they might as well be coexisting in the same space. So don’t overthink subwoofer distance – close gets you 98% of your benefit.
But once we have a 6.5 and a tweeter, and one is 2.5” farther away than the other, and we use a crossover point at 3000 Hz, we just put those two speakers 180 degrees out of phase with each other at that frequency. That’s a big deal. Now if one is 5’ farther away, they are back in phase at 3000. If one is 7.5” farther away, they are back out of phase at 3000. Above a certain frequency, the higher we go, the less sensitive we are to these problems.
Once we start thinking of DSP less as a tool for prissy audiophiles, and more as a tool for cancellation management, we can use it to sell more speakers, sell more amplifier channels, and earn more business from happy clients.