Subwoofer Performance: How to Know When You Have It (or Why Inches and Watts Tell You Nothing of Value Whatsoever)?
Most consumers use the “Inches and Watts” method to evaluate the performance of powered subwoofers. They assume that the bigger the driver size and/or the higher the amp rating, the better the subwoofer. Bad assumption. These two “facts” tell you absolutely nothing about how loud the subwoofer will play, or how deep it goes, or how accurate it sounds. One could at least assume that the powered sub with the largest amplifier would play the loudest, right? No way. Let us show you why: How loud a system plays is a function of the available clean power and the efficiency of the driver/cabinet system. Efficiency (and/or a similar spec called Sensitivity) tells you how much of the input power gets used to make the sound you hear. A speaker system with an efficiency rating of 90dB would sound audibly louder than another system with a rating of 87dB. Since every doubling of power gets you 3dB greater sound output, it follows that a 90dB efficient system driven by 100 Watts would play just as loud as an 87dB efficient system driven by a 200 Watt amplifier. Similarly a 90dB efficient system would play 3 dB louder than an 87dB efficient system if both were driven by the same 200 Watt amplifier. And yes Virginia, woofer size is not a good predictor of efficiency and by the way, there is no Santa Clause.
So, all you have to do is look up the Efficiency rating of the powered subwoofer, then look up the power spec and you’re all set, right? Ah, not that easy. First since powered subwoofers are self-powered closed systems, no manufacturer publishes an efficiency spec. Second, power specs are often, ahem, “exaggerated” by manufacturers. Third, how loud a subwoofer plays on average does not tell you anything about how low it goes in frequency. A competent engineer could easily design a subwoofer system that plays louder than snot all the way down to 50Hz but not do much below 50Hz. That doesn’t qualify it to be called a SUB-woofer, just a loud woofer. A woofer needs to have solid output down to at least 30Hz to qualify as a genuine sub-woofer that makes your movies and music sound like life, not a pale imitation.
CEA 2010 to the Rescue – Loud & Low
Recognizing the dilemma, the Consumer Electronics Association (CEA) formed a committee of engineers to study the problem and devise a new measurement and specification standard that consumers could use to make informed choices. Polk engineers served on this panel along with engineers from other quality minded manufacturers. They realized that what is most important is “what comes out of,” not “what goes into” the subwoofer. In 2006 they established a standard way of measuring the undistorted output of a subwoofer at various frequencies and reporting the results in two easy to digest numbers. The standard is called CEA 2010 but we call it Loud & Low because it tells you how loud and how low a subwoofer actually plays. Let’s take a look at Figure 1, the measurements of a subwoofer using the CEA2010 method. Sound Pressure Level (SPL) measurements were made at 6 frequencies: 20Hz, 25Hz, 31.5Hz (the ultra low bass range) and 40Hz, 50Hz and 63Hz (the low bass range). Then the 3 SPL numbers for each range are averaged to come up with Low Bass and Ultra Low bass numbers that the manufacturer may publish. By comparing CEA2010 numbers you will at least be able to predict how different subwoofers compare in undistorted output.
|Frequency||Max SPL (dB)||CEA 2010 Rating|
|20 Hz||90.8||103.8 dB Ulta Low Bass|
|25 Hz||107.3||103.8 dB Ulta Low Bass|
|31.5 Hz||113||103.8 dB Ulta Low Bass|
|40 Hz||118.2||120.9 dB Low Bass|
|50 Hz||121.1||120.9 dB Low Bass|
|63 Hz||123.3||120.9 dB Ulta Low Bass|
Let’s look at the CEA 2010 results of 4 subwoofers in Figure 2. Based on the Inches and Watts method of evaluation, Subwoofer D is clearly the best woofer here as it has 50 more Watts than its nearest competitor and has a large woofer. But when you look at the CEA2010 numbers you see that in fact Subwoofer D has the poorest performance! Subwoofer A with its mere 10-inch woofer and 250 Watt amplifier outperforms all of the other models and trounces the performance of Subwoofer D. Subwoofer B with its modest 200 watt amplifier nearly equals the fine performance of the more expensive Subwoofer A.
|–||Woofer Size||Advertised Power||Ultra Low Bass (20 – 31.5Hz)||Low Bass (40 – 63Hz)|
|Subwoofer-A||10″||250W||103.8 dB||120.9 dB|
|Subwoofer-B||12″||200W||102.9 dB||117.0 dB|
|Subwoofer-C||12″||250W||96.9 dB||117.7 dB|
|Subwoofer-D||12″||300W||95.2 dB||116.9 dB|
So, Is That All I Have to Know?
Wouldn’t it be great if every manufacturer published honest CEA 2010 numbers? You’d never have to get out of your easy chair. Not so fast Virginia. One limitation of the CEA 2010 standard is that it does not have the force of law — manufacturers cannot be compelled to publish the data. As of early 2007 it is unclear which manufacturers will publish the data. Polk Audio will, as always, stand up for Truth, Justice and the American Way and will publish CEA 2010 numbers on all new models. Also, CEA 2010 numbers tell you a lot but not everything. If you can accommodate any size woofer, don’t care about looks or convenience and you never listen to music, you can simply use verified CEA 2010 number to make a subwoofer choice. But for most of us there are other things to consider such as size, features, cost and sound quality. It is easy to make a huge box that goes low and plays loud but most listeners are looking for space efficient, attractive subwoofers that are easy to use as well as powerful while other listeners are looking for subwoofers that are tight, fast and detailed enough to blend seamlessly with high-end audiophile speakers in music-only systems. For all of these people CEA2010 numbers will be but one of many factors to consider when choosing a powered subwoofer. At the end of the day there is no substitute for understanding your needs and listening to the models under consideration before making a final choice.
Polk Audio, Inc.
(reprinted with permission from the author)