Why Do I Need A Subwoofer?

Responsibility for all frequencies below your crossover threshold fall to the subwoofer. For most speaker systems that means everything under 80Hz is coming from the heavy box on the floor. It might not sound like a lot, but due to the way that octaves work (going up an octave is a doubling of frequency), the same number of notes fall between 40Hz and 80Hz as fall between 10.24kHz and 20.48kHz (half the range of human hearing in terms of frequency). Humans can hear approximately ten octaves total, and two of them fall entirely under 80Hz, so a good subwoofer will help you enjoy that last 20% of the range (though most music doesn’t dip below 30Hz or so).

Obviously if you don’t have a subwoofer then you don’t have a crossover, and your lowest frequencies are going to be the extent of what your front speakers can handle. Generally this is not going to be below 40hz unless you have massive floor-standing speakers, so you miss out on the last octave entirely. A subwoofer is also purpose-built for low frequencies, whereas if you force the low-end on your fronts they’re going to have significantly less output, greater distortion and noticeable volume falloff as they approach the lower extremes of their range.

Built For Power

Subwoofers have comparatively huge drivers and meaty amplifiers, as deep bass requires a lot of energy and air movement to produce. They’re generally the only speaker in your setup that is active and therefore is not receiving its power from the receiver, due to the wattage they require. A large subwoofer can draw as much as 4000 Watts for brief periods, more than double what a conventional 15-amp household outlet can provide continuously without tripping the breaker(15 Amperes * 120V = 1800W). This means it’s generally a good idea to have your subwoofer run on a different outlet circuit than the rest of your equipment.

Dinosaur Footsteps and Driver Size

Subwoofers are great for music as they let you hear instruments like the bass guitar in all their glory. However, it’s probably even more important to have a good subwoofer for home theater, as all the explosions and dinosaur footsteps are very low-frequencies. Where music is fairly lenient in terms of the lower-end extension of your subwoofer (rarely reaching below 30Hz), home theater setups can benefit greatly from extra low-end extension (the ability to hit lower frequencies).

One of the main factors in extension and output is driver size, AKA the diameter of the speaker frame containing the cone and driver. For example, a 10” subwoofer might have low-end extension down to about 30-35Hz and then begin to see drastic volumes fall-offs as you try to push it deeper, whereas a similarly designed 12” subwoofer might make it down to 20-25Hz assuming its amplifier is beefy enough to power it. There are many other factors that go into this, so a larger driver does not automatically equal a better subwoofer, as this excellent article from SVS’s website explains.

Anything below 20Hz is known as infrasonic, or outside the range of human hearing. These are the sounds that are so low that you can only feel them, the real chest thumpers. Generally only large high-end subwoofers delve much below 20Hz.

Sealed VS Ported

Another major factor in extension and output capabilities is the design of the subwoofer. There are many variants, but one of the most important design decisions is whether a sub is ported or sealed. Ported subwoofers have up to three ports or holes in the speaker enclosure, meaning that air can flow in and out as the speaker driver does its thing. This greatly reduces the energy required to create louder and deeper tones by making the tuning frequency lower and allowing the ports themselves to reinforce the output of the driver.

Though they can’t match the output of a ported model at frequencies below 30Hz, there are a number of reasons to opt for a sealed subwoofer. One slight benefit is that since there are no ports there isn’t any port noise as air rushes in and out. They also tend to be significantly smaller and lighter than their ported brethren, making them easier to transport and setup, as well as allowing them to blend into the room rather than commanding attention. This is an excellent feature for when your less-than-enthused spouse has yet to agree to the addition of a subwoofer to your living room. They also tend to be cheaper due to their smaller size demanding fewer materials and components.

Another benefit is that sound from a sealed model can be tighter and more dynamic through transients than ported subs. This makes them ideal for music, where there is hardly ever content below 30Hz and the sound often changes on a dime. Though it’s ‘common knowledge’ that ported subs are better for home theater and sealed subs are better for music, a quality subwoofer of either type can be excellent for both music and movies. What it really comes down to is your use case and the room that you have to work with. If you want to hear more on the subject, here’s a great explanation video from SVS.

Multiple Subwoofers

Many people advocate for multiple subwoofers, but not for the reasons you might think. Your average room doesn’t need more than one subwoofer in terms of output, but there’s another less obvious benefit to having more than one. Due to the longer wave-lengths of low frequency sound it is highly probable that standing waves form in your listening area, essentially layering the room with peaks and valleys in terms of the sound level. By adding a second subwoofer in a strategic location you can cause superposition of the standing waves at off-set phase, averaging out the sound level to near even across the room. From there you can simply adjust the settings in your receiver to get the output level you desire.

What’s the Damage?

Speaking of quality, the budget market for subwoofers is rather depressing. Due to their large size, internal amplifiers and built-in Digital Signal Processing (DSP) software, subwoofers tend to be the most expensive speaker in your setup. There are some low-cost options, such as the $200 BIC F12, but for the most part if you buy a budget subwoofer it’s going to be a noticeable compromise. To get something quality it’ll run you anywhere from $400-800, and higher end subs can easily be $2000. The major exception to this rule are DIY subwoofers such as the “Ultimax 15”, available as a kit for those without tools to cut and clamp MDF “(u/DZCreeper). These can provide exceptional sound for the money if you’re willing to put in the effort.

Subwoofer Brands

The subwoofer is the only speaker that doesn’t really have to match the timbre of any of your other speakers, so you can pick your brand based on quality rather than matching it to the rest of your setup. SVS is my personal brand of choice, though they command a bit of a price premium for what you get due to their excellent standing.

Other Internet Direct(ID) brands like HSU, Rythmik and PSA create products with great reputations as well, though I haven’t personally tested anything from them. Many of the online subwoofer suppliers have generous trial periods so that you can audition it with your setup before committing to the purchase. I know from personal experience that SVS has a full customer bill of rights to be read on their website, with a 45 day free return period, all shipping paid, included. Just be warned, once you pull the trigger on a subwoofer you won’t want to return it.