Tech Page: How Low Can You Go?

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FIG. 1: In the upper graph (a), the frequency response (20 to 1 kHz) of a 5.25-inch driver is shown in red and the response of the H-PAS vent is shown in blue. The lower graph (b) shows the frequency response (10 Hz to 200 Hz) of the combined system, which is perfectly flat down to 32 Hz. The -3dB point is 28 Hz, and the -6dB point is 26 Hz—measurements that are heretofore unheard of from such a small driver.

Illustration: Chuck Dahmer

Common wisdom says that speakers must be large to reproduce low frequencies — after all, as the frequency drops, the speaker must move more air. This requires a driver with a larger surface area, which, in turn, requires a larger enclosure. Also, low frequencies require more amplifier power, which is why subwoofers often include their own dedicated amps.

Various techniques have been developed to enhance a speaker's ability to reproduce the low range, such as bass reflex (vented cabinet) and acoustic suspension (sealed cabinet), but both are imperfect solutions. Recently, a new approach was announced that promises to allow small speakers to go low like never before. Dubbed Hybrid Pressure Acceleration System (H-PAS), it was originally conceived some 30 years ago by Philip Clements, founder of a speaker company called Solus Audio/Clements Loudspeakers (

The basic idea is surprisingly simple, relying on no electronics whatsoever. Instead, low frequencies are amplified acoustically in a specially designed chamber that opens into the space outside the speaker cabinet. The system combines elements of horn loading, transmission-line loading, bass reflex and acoustic suspension — hence Hybrid in the name. The chamber's frequency response actually increases as the driver's response decreases at lower frequencies, resulting in a flat overall response from 70 Hz down to a frequency that depends on the specific driver being used (see Fig. 1). Above 70 Hz, H-PAS has little effect — the sound is much more influenced by the drivers themselves, the crossover and other factors.

Another benefit is increased efficiency. In conventional speakers, the efficiency generally declines as the frequency drops, requiring more power from the amp to achieve a given output level. In a full-range speaker driven by a single amp, this cannibalizes power from the mids and highs, resulting in a boomy, veiled sound. By contrast, H-PAS balances the acoustic and electrical impedance of the system to increase the efficiency of the speaker's electrical-to-acoustic conversion, requiring less power for the bass and thus making more power available for the mids and highs.

With greater efficiency comes more sound output with less power, as well as a wider dynamic range. Also, the driver's movement is minimized, which naturally results in lower distortion. H-PAS also includes an acoustic bass trap, which works the same way as bass traps in concert halls and recording studios to reduce low-bass distortion. Finally, this technology is completely scalable to accommodate tiny speakers with 2-inch drivers all the way up to large pro systems.

I first heard about, and heard, H-PAS at a consumer-electronics trade show in 2009, and I was impressed. The demo comprised CDs played through a pair of tall but slender prototype speakers with two 4.5-inch woofers and a 1-inch dome tweeter on top and a big port on the bottom with a scrap of paper placed inside. The bass extension on Pictures at an Exhibition played on a cathedral organ was astonishing. According to Clements, H-PAS allows those little 4.5-inch drivers to reach all the way down to 31 Hz (-3 dB at 28 Hz), which was entirely believable based on what I heard. The low notes definitely fluttered the paper in the port, but it didn't fly out; rather, it flopped around inside, demonstrating that the air was moving in concert with the driver. The drivers themselves were moving little, which helped lower the distortion far more than you'd expect from such small drivers trying to reproduce such low frequencies.

The first commercially available H-PAS speakers will appear in the consumer-electronics market this year, but I have no doubt the technology will soon find its way into studio monitors as well. I'm excited by its potential, and I eagerly await its implementation in professional products.