Test Bench: Five Mid-Price Subwoofers
The primary performance requirements for a good subwoofer are that it have smooth frequency response over its operating range and that it be capable of producing strong output down to 30 Hz or (preferably) below without obvious distortion. I measured frequency response and bass limits for each subwoofer with it set to maximum bandwidth and placed in the optimal corner of a 7,500-cubic-foot room. In a smaller room users can expect 2 to 3 Hz deeper extension and as much as 3 dB greater sound-pressure level (SPL). The frequency-response measurements are ground-plane taken at a distance of 0.5 meter - far enough to allow full acoustical summation of the outputs from all radiating elements (drivers, ports, and passive radiators). I used the same basic setup for testing the function of operating controls as well.
The 10% distortion threshold we use to establish bass limits is not arbitrary. It is based on research by DLC Design indicating that at 10% distortion a loudspeaker will still sound clean, but that driving it harder still causes distortion to rise exponentially. In other words, when speaker reaches the limit of its linear operating range, distortion doesn't creep up to 11% or 12% but instead jumps quickly to 25% or 50% and shortly thereafter to 100% or more, causing an obvious deterioration in sound quality. We also consider a speaker or subwoofer to have reached its limit when port turbulence, mechanical suspension noises, amplifier clipping, or protection circuits create sounds or noises that are not program-related.
We supply each subwoofer's bass-limit data in several forms to help you get a handle on its performance. Most basic is a single-number figure of merit, the average maximum output from 25 to 62 Hz - the heart of the range subwoofers are expected to reproduce. But modern digital media can carry high-amplitude signals down to much lower frequencies, well into the infrasonic range below the nominal 20-Hz limit of human hearing. Remember the last time you "felt" a thunder report, an approaching train, or a door slam in a stairwell? Bone conduction is a major contributor to acoustical sensation at extremely low frequencies (and even 20 Hz, cleanly and strongly reproduced, is more an experience than simply a sound). So we also report the lowest frequency that each subwoofer can deliver without excessive distortion. And because the average figure can be distorted by output that is high at the upper frequencies but falls off rapidly below, we also report maximum output at a number of individual frequencies between 16 Hz (or the subwoofer's lower limit) and 62 Hz.
The last thing I measure is the actual acoustical effect of operating controls and the interaction between controls. For example, the settings marked on a crossover-frequency control often do not exactly match the acoustical turnover frequencies that actually result. And sometimes changing the crossover-frequency setting also affects the output level. With some of these subwoofers, I found that reducing the crossover frequency extended the apparent low-frequency response. This is largely a function of crossover/level-control interaction, however, and does not reflect any improvement in dynamic performance or bass limits. A subwoofer's dynamic capability is a determined by cabinet design, size, and tuning, amplifier power, and maximum linear driver excursion. Low-frequency EQ, such as this, won't relax any of these constraints.