I’ve been into stereo equipment for most of my life, but to some degree had fallen out of it for awhile before recently diving back in with more vigor than ever. I tell you that to say this: I’ve spent months researching, reading, and scribbing down numbers and formulas given by others, as well as my own living room and office experimentation.
Everything that follows is the result of firsthand experience and the teachings of other more knowledgeable people from exhaustive forum dives etc. on my part.
I’ll be listing a good summary for this topic in what I feel is a logical progression for someone else trying to learn. If you encounter a section you’re already knowledgeable about, feel free to skip ahead.
It’s natural for anyone getting into speaker systems to wonder whether a smaller or larger subwoofer is better for their setup.
“Do I need a huge subwoofer, or can I get away with a smaller one?”
Basic Principles of Making Bass
As with any type of speaker, a woofer creates bass by vibrating at various frequencies — in this case much lower ones than midrange and tweeter speakers.
As the frequencies go lower, the corresponding wavelengths get bigger and longer. The speaker cone must move slower and deeper to generate these waves, and it requires significantly more power to produce these frequencies well than higher ones.
This is often why music and home theater enthusiasts include subwoofers in their systems rather than rely on the woofer in their main speakers. The main speakers’ woofers are usually also trying to produce midrange frequencies, and all that big cone movement from the bass can muddy up the sound quality of the midrange. Also, many receivers/amplifiers don’t deliver enough power to adequately drive deep bass notes.
That or the speakers themselves aren’t big enough or powerful enough to handle it if the amplifier did provide that much power. (This can be true even for larger floorstanding speakers.)
The biggest advantage of a subwoofer, consequently, is that it’s powered separately from the rest of the speakers, and that it’s only trying to handle frequencies under 150hz. It’s not trying to handle midrange frequencies as well, so its larger mechanical motion doesn’t interfere with anything as long as the subwoofer is built well enough to handle this motion cleanly.
Bass is all about sound pressure.
The kind of bass you commonly think of for home theater systems that creates that rumble in explosions or action scenes, or the fierce thump of a kick drum or warm bass guitar in music — these all require an adequate sound pressure level (SPL) to be created by the speaker in order to experience.
This is especially true for that pants-rattling bass many are after either in their cars or for movies in their living rooms.
Since a speaker creates sound by vibrating back and forth rapidly, the larger the cone is the more air it can move with each of these back and forth cycles.
Generally speaking, this means two things (all other things being equal):
- A larger woofer will produce more sound at the same wattage than a smaller one because of how much more air the cone moves.
- This is why efficiency ratings on larger subs in a given product line tend to be higher in larger subwoofers than their smaller counterparts.
- A larger woofer doesn’t need to move as far back and forth at a given wattage to produce the same sound pressure level as a smaller subwoofer.
The concept of sound pressure is directly affected by the air space of the room you’re listening in. It’s also why you can often get away with using a small subwoofer in your computer room/office but not your living room. The office is likely a smaller room, so it’s easier for the motion of a smaller cone to create enough air movement in that air space to seem adequately powerful.
If you have a large living room, it’s also why an 8-inch or 10-inch subwoofer would probably also sound anemic in that space unless it’s an extremely high powered 10-inch. (Or unless you’re using several smaller subwoofers… see below.)
Subwoofer myth: Smaller subwoofers are cleaner and “tighter” than larger ones.
This often repeated myth is a holdover from many years ago when speaker manufacturers would use the same size speaker magnets for their entire product lines, regardless of woofer size. So you’d end up in a situation where a big 15″ woofer was using an undersized magnet — the same size as the 10″ woofer in the same lineup.
Or, this statement can be true in cases where it’s cheaper to make a smaller woofer, such as an 8″ or 10″ model, than it is to make a larger cone. This is because as the cone moves harder back and forth it has a tendency to want to flex, something that’s exacerbated by a bigger, heavier cone.
In short, you could say that it’s often more expensive to make a high quality large subwoofer than it is to make something decent sounding that is small.
However, if a larger subwoofer is built well, it can be just as “fast” as a smaller one. After all, being able to accurately reproduce a 50hz signal means moving back and forth 50 times per second. If a speaker can do this properly, it’s indeed moving “fast” enough. Saying that one speaker moves “faster” than another doesn’t really make sense if both of them can properly play the same note.
If the larger subwoofer has a magnet big enough to generate the proper motor force on the voice coil, and if the voice coil has the right balance of strength, control, and heat dissipation for a lot of motion, the larger and comparatively heavier cone can still be properly controlled.
This is why some of the most respected high-end subwoofers out there tend to be 12″ and 15″ cones. (SVS, Hsu, and even DIY brands like Dayton Audio)
Reason being? If the subwoofer is built well enough to control the cone cleanly, it will almost always move more air than a smaller one, resulting in punchier, more powerful-sounding bass.
Also consider this: As a cone begins to near its maximum excursion distance (moving about as far back and forth as it can without bottoming out) it will create distortion. Higher quality woofers will do this with less distortion, but distortion is mechanically inevitable.
Because a bigger woofer moves less at the same wattage as a smaller one, and because it generally has larger surround material (around the cone) it can move further when driven hard, a bigger cone can potentially play with less distortion than a smaller one when driven hard.
This is why for many audio enthusiasts in various forums the opinion is essentially: Get the biggest subwoofer you can afford and can fit in your space/car.
Using multiple smaller subwoofers
Many sound enthusiasts opt to use multiple smaller subwoofers rather than a single big one.
Generally people do this for these understandable reasons:
- More woofers means more overall power output potential (from multiple amps or more speakers to take the load given by one big amp), ultimately meaning more sound can be created with each of the woofers moving less than they would have alone. This means cleaner sound and less potential distortion.
- Room/design constraints sometimes make trying to fit a gigantic subwoofer into an area impossible. It’s much easier to find room for a couple 10″ woofers.
- Multiple subwoofers can help overcome “room nulls” that are common. (See below.)
A bass null is basically a space in your room where the positive (or upward curve of a frequency wave) sound wave collides with an opposite negative wave of the same frequency. The result is that they cancel each other out. You can notice this by walking around a room with a subwoofer playing. Some areas the bass will seem extremely powerful, and then you’ll hit an area where it may not even sound like the subwoofer is on.
Often this is caused by room harmonics. Floors and walls rattling and vibrating, or even reflecting too many sound waves back into the room. A common method of overcoming bass nulls is installing sound dampening material on walls and corners, which reduces sound wave reflections (and therefore nulls).
Using multiple subwoofers can also help with this because the second subwoofer placed elsewhere in the room can create different waves that prevent or overcome nulls, resulting in a more consistent sound throughout the room.
What you should know about subwoofer size differences and multiple woofers (caveats)
The idea behind using multiple smaller drivers makes sense. But it’s less linear than it may seem because of the way circular area works.
For instance, a common thought is, “I can use two 8-inch woofers and that gives me roughly equivalent cone size to one 15 inch.”
This makes sense at first because 8+8=16, right? But remember that the formula for the area of a circle is πr^2. Put more simply, multiply pi (~3.14) by taking the radius (half the circle’s diameter) squared (or multiplied by itself).
If we do this for an 8-inch woofer, we have: (3.14) x (4×4) = 50.24
So for two of them, we’d have a combined surface area of 100.48 square inches.
Compare that to a 15-inch woofer: (3.14) x (7.5×7.5) = 176.625 square inches
You can see that you’d actually need 3x 8-inch woofers to even come close to the surface area of a 15-inch, and even then the 15 would still have more area. You’d be safer going with 4, but then you’d have to consider the costs of buying four 8-inch woofers and whether you have enough space for that many of them.
The same applies for 6.5-inch woofers vs. 12-inch woofers, something I’ve played with firsthand as well and the math supports reality.
The subwoofer’s enclosure and build quality play a bigger role in “tightness” of sound than cone size.
As we talked about above, it’s totally possible to hear a big 15-inch or 18-inch subwoofer that sounds muddy and think it’s because of its mass. But there are just as many muddy-sounding 8-inch woofers out there.
The key is, is it a well-made product or not?
I added a spare Kicker Comp VR 12-inch subwoofer I had laying around (powered by a separate 150W amp) to my desk speakers in my office. The Logitech Z623 desk speakers had their own 6.5-inch subwoofer, but the addition of the Kicker made a huge difference.
And the Kicker, despite being 12-inches, was both louder and tighter sounding because it’s a solid middle of the road subwoofer versus the pretty cheap one inside my desk speaker system.
A box that is too big can make the subwoofer sound muddy or extend too much on low notes. A box that’s too small can constrain cone movement and also cause ringing sounds or resonance, making the sub sound boomy. It may also not sound loud enough.
Also, if the box walls are too thin it can allow too much box vibration. THis also causes resonance and can contribute to a muddy sound. Many cheap pre-fab boxes are made from 0.5 inch MDF, which is only adequate for smaller speaker systems. For a subwoofer playing more than 100 watts and hitting deep notes, you should stick with at least 0.75 inch MDF.
Thinner than that just isn’t strong enough for the level of pressure happening inside the box. Some people even go full-on 1 inch think MDF for this reason. It results in a very heavy box, but it’ll be built like a tank (especially with internal bracing).
If you’ve been using a good sub in a cheap box and haven’t been impressed with its sound, don’t jump straight to thinking you need a better subwoofer. Chances are it could be your box ruining the sound, and taking that same subwoofer out and installing it into a better box might surprise you how much better it suddenly sounds.