Measuring the World’s Most Awesome Subwoofer

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A 24-inch woofer + 1,800 watts = ???

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Brent Butterworth

As I was admiring one of Pro Audio Technology's 18-inch subwoofers during a visit to the company about a year ago, company founder Paul Hales surprised me when he told me the model I was looking at wasn’t the largest sub the company makes. “We also have one with a 24-inch driver, for really large installations,” he said. Figuring it might be the most powerful subwoofer I’ve ever encountered, I immediately asked Hales if I could return to run CEA-2010 maximum output measurements one of the 24-inch subs -- model number LFC-24SM, weight well over 300 pounds, price about $10,000 -- the next time he had one on hand.

I finally got my chance today. I figured it’d be easier for me to make the drive from my home in northern Los Angeles to Pro Audio Technology's HQ in Lake Forest, Calif., than to ship the subwoofer. So I packed up all my measurement gear, including with my 15-inch handmade measurement reference subwoofer, and headed down for southern Orange County.

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Pro Audio Technology LFC-24SM: The Backstory

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Brent Butterworth

While I was setting up for the measurements, I asked Hales why his company makes such a huge subwoofer, and what they do with it.

“It’s for very large home theater installations, and people who want very clean, loud bass,” he replied. “Right now we’re putting two of them into a home theater that’s really more like a small commercial cinema, with stadium seating for about 80 people. Ideally you put a couple of these up front and a couple of smaller subs in the rear to smooth out the bass response in the room.”

The LFC-24SM employs a single 24-inch driver in a quad-ported cabinet. Hales designed it to work with his company’s amplifiers, which have extensive digital signal processing (DSP) built in to tune the response. “The one we’re using today is a new one, a prototype that puts out 6,000 watts into 2 ohms,” he said. “The driver in this sub is 8 ohms, so we’re getting about 1,800 watts out of the amp.”

Subwoofer enthusiasts may be surprised to learn that despite its size, the LFC-24SM has minimal response below 20 Hz. Why not use the big driver to get subsonic response? “Our design objective was to reproduce the LFE [low-frequency effects] band as effortlessly as possible,” Hales explained. “We have a high-pass subsonic filter that attenuates the signal below the box tuning frequency, which is at about 22 Hz. That minimizes distortion and protects the driver.

“Part of the reason this sub has such high output is that the driver sensitivity is 99 dB at 1 watt/1 meter. You can’t make a driver that goes to 8 Hz and has good sensitivity and reliability.”

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Pro Audio Technology LFC-24SM: Sound

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Brent Butterworth

Indeed, I was amazed as I ran the measurements to see that from my safe distance about 20 feet away, the LFC-24SM’s driver barely seemed to move until I got down to 20 Hz, the lowest measurement frequency. With most of the subs I measure, I can easily see the driver moving even from 20 feet.

What also surprised me was how clean the LFC-24SM sounded while I was doing the measurements. Most of the subwoofers I measure sound like they’re about to tear themselves apart by the time they reach a high enough level to hit one of CEA-2010’s maximum distortion thresholds. The LFC-24SM sounded precise, well-defined and unruffled throughout almost the entire measurement session, only starting to sound a tad strained when I got down to 20 Hz. Usually, the only distortion harmonic that broke the CEA-2010 distortion threshold was the third harmonic; there’s a good chance that was the amp, not the driver, reaching its limits.

(Am I getting too technical with this? Read my CEA-2010 primer to learn more about this fascinating and essential measurement technique.)

So without further ado, here are the measurements ...

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Pro Audio Technology LFC-24SM: Measurements

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Brent Butterworth

                            CEA-2010A           Traditional
                             (1M peak)              (2M RMS)
40-63 Hz avg        135.5 dB                126.5 dB            
63 Hz                    135.2 dB                126.2 dB
50 Hz                    136.0 dB                127.0 dB
40 Hz                    135.4 dB                126.4 dB
20-31.5 Hz avg     130.5 dB                121.5 dB
31.5 Hz                 133.6 dB                124.6 dB
25 Hz                    131.4 dB                122.4 dB
20 Hz                    123.7 dB                114.7 dB

I did the CEA-2010 measurements using an Earthworks M30 measurement microphone, an M-Audio Mobile Pre USB interface and the freeware CEA-2010 measurement software developed by Don Keele, which is a routine that runs on the Wavemetrics Igor Pro scientific software package. I calibrated my measurements to the response of Pro Audio Technology's warehouse by first measuring my 15-inch reference sub in the space, comparing that measurement to a measurement I took in a park with 50+ feet of clearance in every direction, then subtracting the warehouse measurement from the park measurement to create a correction curve.

These measurements were taken at 3 meters peak output, then scaled up to 1-meter equivalent per CEA-2010A reporting requirements. The two sets of measurements presented -- CEA-2010A and traditional method -- are the same, but the traditional measurement (which most audio websites and many manufacturers use) reports results at 2-meter RMS equivalent, which is -9 dB lower than CEA-2010A reporting. Averages are calculated in pascals.

To put the LFC-24SM's performance in perspective, the most powerful sub I can recall measuring to date is the SVS PC13-Ultra. By the CEA-2010A reporting standard, the PC13-Ultra averages 125.8 dB from 40 to 63 Hz and 116.9 dB from 20 to 31.5 Hz, and it delivers 114.6 dB at 20 Hz. Thus, the advantage for the LFC-24SM is +9.7 dB average from 40 to 63 Hz, +13.6 dB from 20 to 31.5 Hz, and +9.1 dB at 20 Hz. Of course, the PC13-Ultra costs $1,699 and is a fraction of the size of the LFC-24SM.

Hales also did a quick check with his SPL meter (seen above). He asked me to run a 60 Hz sine wave, then did a measurement at 1 meter at what he considered the highest maximum safe level. You can see the result above. This is with a continuous tone; CEA-2010 delivers higher numbers because it uses 6.5-cycle burst tones that are closer to the nature of the bass content of real music and movies.

I guess there may be more powerful subwoofers out there -- I've seen a picture of live sound guru Bob Heil next to a 36-inch sub once, and I once stumbled upon a sub in a Vancouver, B.C. speaker repair shop that had, as I recall, two JBL 18-inch pro woofers pushing a 30-inch front radiator in an isobarik enclosure. But somehow, I think it's highly unlikely I'll ever measure CEA-2010 numbers as high as I got from the LFC-24SM. Now I just need to figure out how to fit this thing in my listening room. Maybe if I get rid of the couch ....

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