Stacking Subwoofers – When and Why

We mostly see people ask about multiple subwoofers when they have a small, underperforming subwoofer. These subwoofers are usually highly reviewed by owners on Internet sites and cost very little money. Unfortunately, these entry-level subwoofers aren’t usually very good. They don’t play very low or very loud. That’s when people get an idea. If a single subwoofer isn’t doing the job, can they get a second one and put it on top to “help it out.” Well, not really. Let’s explore.

When Do Subwoofers get Stacked

You don’t see it very often, but occasionally you’ll see a stack of subwoofers. Often at concerts. If you could see behind the screen at your local movie theater, you’d likely see stacks of subs. But less often at home. Movie theaters and outdoor venues are huge. Multiple subwoofers are needed to fill these areas with bass.

But your room is much smaller. No matter the size of your room, there is a subwoofer out there that you can buy that can pressurize the space. In most cases, there is no need to stack subwoofers. You just buy two, place them appropriately, and never look back.

What Stacking Subwoofers Does

The only benefit to stacking subwoofers is simple – greater output. When you put one subwoofer on top of the other, you effectively increase the output (volume). In your room, that equates to about a 6dB increase in volume. If you have two subwoofers that can play down to 20Hz (the lowest note a human can hear) or lower (we can feel lower notes), stacking them can give you the output to fill very large spaces.

When you are trying to fill a very large space, you don’t need the lower extension of a larger subwoofer. What you need is the ability to play louder to fill the area with bass. You might be able to buy/build a subwoofer that can do it, but it is usually cheaper to buy multiple subs and stack them.

When people buy a subwoofer that is too small for their space, they start looking for solutions. If their problem was only output, buying a second subwoofer and stacking it on top of the first one could fix it. But that often isn’t the only issue. Budget subwoofers aren’t just lacking output, they have other problems. They often can’t play very low, they distort easily, and they can have other issues like port noise.

How To Know Your Subwoofer Lacks Output

If you want to know if your subwoofer lacks output, run your room setup program. If it keeps asking you to turn up your subwoofer but the volume knob is maxed, you lack output. Simple enough. But if you find yourself in this situation, you are going to want to play some sweeps. Listen to your subwoofer. Does it stop making sound and start making noise as it approaches 20Hz? Is it making clanging noises? Can you hear air whistling through its port? If so, output isn’t your only problem. And buying a second one and stacking it isn’t going to help.

What Stacking Subwoofers Won’t Do

What adding a second subwoofer to the top will never do is help the first play any lower. It can increase the volume you experience, but not the extension. A subwoofer that won’t play any notes below 35Hz, can’t be helped to do so by adding a second (or third or fourth) identical subwoofer. You can increase the output, but not the extension. The only way to increase the extension of the bass in your room is to buy a better subwoofer.

Other Subwoofer Issues

But stacking subwoofers can sometimes help with the other issues. If the noise is because the subwoofer is turned up to maximum, stacking a second on top can help. Same with port noise. Unfortunately, this is often not the case. The port noise often happens when a subwoofer is trying to play a note that is too low for it. There are ways to filter out those lowest notes, but those solutions are often more expensive than just getting a better subwoofer. And now you don’t have the lowest bass! This isn’t a great solution.

Clangs and rattles can sometimes be helped by stacking a second subwoofer. Those clangs happen because the woofer is trying to play a very loud and low note. By adding the second (or third or fourth) subwoofer on top, you’ve lowered the volume each individual subwoofer has to play. This can alleviate those driver-killing clangs.

That said, if you are experiencing those sorts of noises, you can’t assume that stacking a second subwoofer will necessarily help. A subwoofer should be designed with filters to protect the driver(s) from damaging themselves by playing notes that are too low. If you are hearing clangs, that means that your subwoofer does not have those protections. It might be that turning the volume down with help. But it also might be that trying to play certain very low notes will always result in clangs.

The Real Solution

Unfortunately, the real solution is to return (or sell) the underperforming subwoofer and buy something that has the performance you need. Sometimes that is not an option, other times people can’t or won’t get rid of their old subwoofer. So they start to think about buying a second to help it out. In nearly every case we’ve come across, it will not. This is a case where two are not better than one.