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.
HSU VTF-3 MK3 Turbo
The VTF-3 is the largest of the subwoofers in this roundup, especially when its optional Turbocharger attachment is installed. Removing the Turbo extension reduced maximum output about 2.3 dB from 25 to 62 Hz and 10 dB at 20 Hz, with no useful output at 16 Hz, and plugging one of the ports took the average output down another 2 dB. What sounded like internal suspension noise or port turbulence was often audible when the subwoofer was driven into overload during bass-limit testing, and with the crossover bypassed there was an out-of-band artifact at 250 Hz as well. With the Turbocharger attachment installed (the primary test mode) the VTF-3 MK3 does an honest 16 Hz — a rare feat. Crossover control frequencies matched dial makings more closely than is typical.
In the lab
Frequency response (at 0.5 meter)
Turbo: 24 to 92 Hz ±2.3 dB
Both ports open: 24 to 85 Hz ±1.6 dB
One port open: 26 to 85 Hz ±1.7 dB
Crossover bypass: 26 to 124 Hz ±1.9 dB
Bass limits, with Turbocharger (lowest frequency and maximum output with limit of 10% distortion at 2 meters in a large room)
| Frequency | dB SPL |
| 62 Hz | 109 |
| 50 Hz | 109 |
| 40 Hz | 105 |
| 32 Hz | 107 |
| 25 Hz | 95 |
| 20 Hz | 93 |
| 16 Hz | 83 |
Average maximum output, 25 to 62 Hz: 105 dB SPL
Maximum output: 109 dB SPL at 50 Hz
Bass Limit: 16 Hz at 83 dB SPL
Crossover
| Control Setting | Measured Frequency | Level Change |
| Bypass | 124 Hz | – |
| 90 Hz | 92 Hz | - |
| 60 Hz | 60 Hz | +3 dB |
| 40 Hz | 37 Hz | -1 dB |
| 30 Hz | 36 Hz | -2.8 dB |
Low-pass slope: 18 dB per octave
(Note: Reducing the crossover frequency also extended measured low-frequency response very slightly — about 2 Hz typically.)
Klipsch RW-12d
This subwoofer has a classic dynamic characteristic, with high output at 62 Hz that falls off by 14 dB per octave below. It also delivers a true 20 Hz, however. Marked settings for the crossover-frequency control were accurate only in the middle of the dial.
In the lab
Frequency response (at 0.5 meter)
Flat: 35 to105 Hz ±2.1 dB
Punch: +3.6 dB from 35 to 100 Hz
Depth: + 2.6 dB below 75 Hz
Bass limits, Flat setting (lowest frequency and maximum output with limit of 10% distortion at 2 meters in a large room)
| Frequency | dB SPL |
| 62 Hz | 115 |
| 50 Hz | 112 |
| 40 Hz | 110 |
| 32 Hz | 107 |
| 25 Hz | 98 |
| 20 Hz | 86 |
Average maximum output, 25 to 62 Hz: 108 dB SPL
Maximum output: 115 dB SPL at 62 Hz
Bass limit: 20 Hz at 86 dB SPL
Crossover
| Control Setting | Measured Frequency | Level Change |
| Off | 105 Hz | – |
| 120 Hz | 94 Hz | - |
| 100 Hz | 82 Hz | - |
| 75 Hz | 77 Hz | - |
| 50 Hz | 70 Hz | -2.7 dB |
| 40 Hz | 60 Hz | -5 dB |
Low-pass slope: 18 dB per octave
Outlaw Audio LFM-1EX
The Outlaw subwoofer had exceptionally uniform maximum output capability — within about 1 dB from 32 to 62 Hz — and could also deliver a true 20 Hz. It could almost reach 16 Hz at 89 dB SPL, if not for audible intrusion of its protection circuit. There were out-of-band artifacts at 250 and 400 Hz when the crossover was bypassed. The crossover-control markings were off considerably, but the LFM-1EX we tested was a prototype, and prototypes are often quirky; production units may be better in this regard.
In the lab
Frequency response (at 0.5 meter)
Bypass: 26 to 160 Hz ±3.8 dB
Max Output: 34 to 103 Hz ±2.2 dB
Bass Extension: 34 to 103 Hz ±2.2 dB
Bass limits, Bass Extension mode (lowest frequency and maximum SPL with limit of 10% distortion at 2 meters in a large room)
| Frequency | dB SPL |
| 62 Hz | 108 |
| 50 Hz | 108 |
| 40 Hz | 108 |
| 32 Hz | 109 |
| 25 Hz | 96 |
| 20 Hz | 96 |
Average maximum output, 25 to 62 Hz: 106 dB SPL
Maximum output: 109 dB SPL at 32 Hz
Bass limit: 20 Hz at 96 dB SPL
Crossover
| Control Setting | Measured Frequency | Level Change |
| Bypass | 160 Hz | – |
| 180 Hz | 103 Hz | - |
| 120 Hz | 66 Hz | - |
| 60 Hz | 71 Hz | -3 dB |
| 30 Hz | 58 Hz | -5.6 dB |
Low-pass slope: 18 dB per octave
(Note: Reducing the crossover frequency also extended measured low-frequency slightly, by as much as 6 Hz at the lowest setting.)
SVS SB12-Plus
The SB12-Plus is by far the smallest of the subwoofers in this review. It's long been said that in drag races there's no substitute for engine displacement. More cubic inches mean more power. To a large extent that's true of subwoofers as well. Larger cabinets and larger drivers tend to deliver deeper, stronger bass. The SB12-Plus could produce 88 dB SPL at 25 Hz, but not without distortion and audible limiter action. (From 40 Hz down, protection-circuit effects were audible when the sub was driven into overload.) It also produced out-of-band artifacts at 260 and 400 Hz when the crossover is disabled, but since that setting would be used only when an external crossover were in place (in a receiver, for example), this shouldn't be a problem. At the top of its range, and down to 32 Hz, the SB12-Plus had impressive output capability, however, especially for such a relatively tiny subwoofer. (To get an idea of its performance relative to that of more comparably sized, though somewhat smaller, subwoofers, see Tiny Killer Subs.) It also sports an impressive array of operating controls, though some may require care to use effectively. The crossover-frequency control markings were fairly accurate, and the control exhibited only minor level interaction.
In the lab
Frequency response (at 0.5 meter)
Crossover Disable: 34 to 116 Hz ±2.1 dB
Bass limits (lowest frequency and maximum SPL with limit of 10% distortion at 2 meters in a large room)
| Frequency | dB SPL |
| 62 Hz | 110 |
| 50 Hz | 107 |
| 40 Hz | 97 |
| 32 Hz | 102 |
Average maximum output, 32 to 62 Hz: 104 dB
Maximum output: 110 dB SPL at 62 Hz
Bass limit: 32 Hz at 102 dB SPL
Crossover
| Control Setting | Measured Frequency | Level Change |
| Disable | 116 Hz | – |
| 120 Hz | 108 Hz | - |
| 80 Hz | 70 Hz | +2 dB |
| 40 Hz | 54 Hz | - |
Low-pass slope: 24 dB per octave
(Note: Reducing the crossover frequency also extended measured low-frequency slightly, by as much as 10 Hz at the lowest setting.)
PEQ Function
The parametric equalizer was relatively difficult to use. The actions of individual controls seemed to depend on the settings of others. Q (filter sharpness) was adjustable from 0.17 to 0.5. Full cut fell between -5 and -9 dB, depending on other control settings. And though the frequency setting was relatively close to indicated, certain actions might be confusing to unsophisticated users. For example, a full cut set to 20 Hz and Lo Q just cut overall response below 50 Hz.
Room Compensation
Small: Cut response above 50 Hz by 4.6 dB
Medium: Cut output bellow 80 Hz by 1.3 dB
Large: Cut output below 80 Hz by 2.1 dB
Velodyne DLS-5000R
This subwoofer's operation was pretty straightforward. It can deliver 110 dB SPL or better from 32 Hz up, but its response appears to be cut off sharply, by design, below 25 Hz. Control function is reasonable, and the remote control is fun.
In the lab
Frequency response (at 0.5 meter)
Classical Jazz: 35 to 100 Hz ±2.8 dB
R&B: +1.5 dB from 50 to 65 Hz
Movies: +1.5 dB below 50 Hz
Games: +2.2 dB above 48 Hz, -2.2 dB below 43 Hz
Bass limits, Classical Jazz mode (lowest frequency and maximum SPL with limit of 10% distortion at 2 meters in a large room)
| Frequency | dB SPL |
| 62 Hz | 114 |
| 50 Hz | 113 |
| 40 Hz | 112 |
| 32 Hz | 110 |
| 25 Hz | 91 |
Average maximum output, 25 to 62 Hz: 108 dB SPL
Maximum output: 114 dB SPL at 62 Hz
Bass limit: 25 Hz at 91 dB SPL
Crossover
| Control Setting | Measured Frequency | Level Change |
| Direct | 100 Hz | – |
| Noon (80 Hz) | 80 Hz | - |
| 50 Hz | 52 Hz | - |
Low-pass slope: 12 dB per octave
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