Musings: Subwoofers

Subwoofers might well be my favorite type of loudspeaker. What can I say, I like loud explosions. I also like a team player, which the subwoofer is the king of in the land of audio.

First and foremost, what exactly is a subwoofer? A subwoofer is a loudspeaker dedicated to producing the range of 80Hz – 20Hz, give or take a few Hertz. Relatively simple beasts, most subwoofers consist of a big driver (8”+), a cabinet of some type (sealed, vented, horn, etc.), and an amplifier. Subwoofers without a built in amplifier are referred to as passive.

So here’s the deal. You have your speakers, an A/V receiver, and a subwoofer. The subwoofer takes a load off things by taking over those very low frequencies that the relatively piddly drivers in the rest of your speaker struggle with. This happens via an active crossover in your receiver. A high pass filter attenuates frequencies below a given point for your main speakers, usually 80Hz, while a low pass filter attenuates frequencies above that point for the subwoofer. This process also saves a bit of juice on your receiver’s power amps as well. If only subwoofer setup was a one size fits all proposition...

My rig isn’t anything extraordinary; it’s fronted by a pair of KEF R500s and a R200c. That’s a pair of 5.25” woofers per speaker for those who don’t feel like Googling right now. Sans subwoofers, they’d get into the 40Hz-50Hz range with decent authority, but I’d be missing the truly deep stuff. Add in a pair of subs, and suddenly I have a rig capable of getting below 20Hz with real power, AND I’ve crossed over my main speakers at 120Hz. What’s special about 120Hz you ask?

1. A 120Hz crossover allowed for a good blend with my speakers, i.e. no nasty peaks or dips in the response around the crossover point. Fun fact, outside of finding a place to set your sub down, integration may be the most important step to getting good bass. Make sure you take the time to get it right, and yes, that means taking measurements.

2. 120Hz was thankfully low enough to avoid localization issues. Having a pair of subs helped there too. Theoretically, multiple subwoofers should also help with consistency across multiple seats. I’m selfish, so I wouldn’t really know / care. What I do know is that it also takes work and measurements to integrated multiple subwoofers properly.

3. By crossing at 120Hz vs 80Hz, I’ve taken an even bigger load off my speakers. While a difference of 40Hz doesn’t seem like much, running a peak at a given level @ 120Hz takes less than half the excursion from a given woofer vs @ 80Hz.

So, what makes a good subwoofer? Here’s where things can go nuts very quickly, so I’ll only say that a good subwoofer is the one that meets your needs in terms of output quality and quantity, size, finish, and price. While mostly self-explanatory, one thing the uninitiated seem unaware of is the relationship between output quantity and size. This goes back to Hoffman’s Iron Law: you can have a small speaker, you can have one that digs deep, and you can have one that is reasonably efficient. You can’t have all three at once.

Hoffman presents a problem for subwoofers. Obviously, we want our subs to dig deep, so the battle lines are drawn between enclosure size and efficiency (the amp power needed to achieve a certain level of output). Making things worse, most people view subwoofers as an afterthought; a one cubic foot enclosure is about all some people can handle. This makes getting adequate output an expensive endeavor, as it requires copious amounts of amplifier power along with a meaty driver that can take it all. My advice, be smarter. You don’t have to be like the guys with multiple refrigerator-sized monstrosities in their living rooms; however, if you want real bass in your own home, you need to budget space accordingly.

As for where to buy, thankfully there are a number of purveyors of high quality subwoofers at reasonable to not so reasonable prices. Vendors I’d recommend include SVS, Hsu Research, Rythmik, Power Sound Audio, JTR, and Funk among others.

If you want to learn more about subwoofers, or if you're now interested in some reviews, check out Audioholics.