Introduction

NHT was founded in 1986 with the notion that great sound reproduction doesn't have to break the budget. Someone recently mentioned to me that “NHT has religion”, and after hearing its hyper sophisticated Xd DSP speaker system at CES 2005, I couldn't agree more.

The new SW12 powered subwoofer visually complements the NHT Super Audio loudspeaker lineup, and is designed to offer good performance in mid size rooms. Its smaller brother the SW10ii is suitable for smaller rooms.

The Design

The SW12 is a bass reflex design, with a front-firing 12” driver, a single front-firing port (3” inner diameter, 4” flare), and a 24 Hz enclosure tuning frequency. The driver features a butyl rubber surround, an aluminum cone, a stamped steel basket, and a large magnet structure.

The NHT design studio should get an award because the SW12 is flat out gorgeous. The black piano gloss finish is perfect, and with the black anodized woofer cone, it looks equally good with the grille removed.

The cabinet design is an interesting example of subtle non-symmetry. On the front face, the top and bottom edges are rounded, and the side edges are square. Conversely, on the rear face, the top and bottom edges are square, and the side edges are rounded. This makes for a very modern and pleasing cabinet geometry when viewed from any angle. Even the injection molded plastic grille is stylish, with rounded top and bottom contours to match the cabinet face, and a cool aluminum NHT logo at the bottom.

The interior of the cabinet is lined with Polyfill, and is well braced. The woofer wiring leads are wrapped in foam, and the heavy amp is encased in ABS plastic and secured to the cabinet with 12 Allen head screws. I was very impressed with the quality of construction and attention to detail.

There are no permanent feet on the bottom of the cabinet, which is fine for carpeted surfaces. NHT supplies four flexible rubber stick-on feet for placing the subwoofer on smooth, hard surfaces. Don't attempt to slide the subwoofer around after the feet have been mounted, or they will peel off.

The Owner's Manual is quite comprehensive, with detailed descriptions of all features, a lot of diagrams, and great set-up and fine-tuning advice.

Amplifier Description

- Class G 250 watts continuous.
- On/Off Main Power Switch
- On/Auto (slider switch)
- High Level Screw Post L/R Input Block
- Low Level L/R RCA Filtered Inputs
- Low Level RCA Unfiltered Input
- Low Level RCA Unfiltered Output
- Low Pass Filter (40 Hz - 140 Hz, 2nd order)
- High Pass Filter (23 Hz, 2nd order)
- Gain/Volume
- Phase Control (0/180 slider switch)
- Boundary EQ Switch (3-way slider switch)
- Adjustable Line Voltage (115/230)
- Detachable Power Cord

The control layout is logical, and the slider switches and rotary knobs have a quality feel and smooth operation. The high level input block only has screw-down binding posts, rather than the more typical five-way binding post. The auto-on circuit initializes easily at low volumes. If no signal is present, there is a 20 minute delay before the amp shuts back off.

The boundary EQ features consists of a three way slider switch with –3 dB, 0 dB, and +3 dB settings. This allows the user to optimize the subwoofer frequency response depending on location. NHT recommends the –3 dB setting for corner loading, the 0 dB setting for mid-wall loading, and the +3 dB setting if the subwoofer is placed farther away from any walls.

On the Bench

Set-Up: All objective tests were conducted outside away from any reflective structures, with the microphone on the ground, facing the subwoofer, at a distance of 2 meters from the width and depth center point. I used the low level unfiltered input (which bypasses the low pass filter), and set the phase control to 0⁰  (this resulted in a non-inverted impulse response).

Frequency Response: An short-duration (about 0.5 seconds), digitally synthesized logarithmic sine sweep was used to evaluate the quasi-anechoic frequency response of the subwoofer. The SW12 frequency response measured 27 Hz - 130 Hz ± 3 dB.

Peak Dynamic Output: This test measures how loud the subwoofer can play across its pass band with a short-duration signal representative of typical music and movie transients. Short duration frequency response sweeps were conducted at progressively louder (2 dB increments) levels until dynamic compression was noted.

In the graph shown below, the yellow curve represents the maximum uncompressed dynamic output of the SW12, with 106 dB at 90 Hz, and 103 dB at 30 Hz. The next higher curve (orange) showed dynamic compression in the 30 Hz -80 Hz bandwidth, and the test was subsequently terminated.

Power Compression: This test measures how loud the subwoofer can play across its pass band with a longer duration signal. A sustained bass passage in an action movie can create excessive heat in the voice coil, reducing longer term output capability. Power compression was evaluated with a slow reverse sine sweep from 100 Hz down to 10 Hz. Sweeps were conducted at progressively louder (2 dB increments) levels until power compression was observed.

As shown in the graph below, the SW12 held steady up to the orange curve,
with 105 dB at 90 Hz and 102 dB at 30 Hz. The next higher curve (blue)
showed 1 dB of power compression at 30 Hz, but otherwise looked linear.
The next higher curve (green) showed more severe power compression, and the
test was subsequently terminated.

The fact that the power compression levels almost matched the dynamic output limits indicates the woofer has good thermal power handling and heat dissipation characteristics.

Total Harmonic Distortion (THD): Harmonic distortion occurs when multiples of the fundamental signal are produced due to non-linear driver behavior. A subwoofer with low THD will sound clean and distinct, especially at the deepest frequencies. THD was evaluated with sine waves (about 5 seconds duration), and was limited to approximately 10% unless otherwise noted.

Clean output from the SW12 was better than average in the important 20-40 Hz bandwidth, and very impressive from 50-80 Hz.

Frequency (Hz)	SPL (dB)	THD (%)
20	79.1	10.1
22	84.9	10.4
25	89.0	10.1
32	100.6	9.8
40	101.3	10.7
50	107.5	10.4
63	108.1	10.6
80	108.7	10.3

Bandwidth Linearity: Bandwidth linearity is calculated by dividing the average distortion-limited SPL by the maximum distortion-limited SPL, and expressing the result as a percentage. A score of 100% means the subwoofer exhibits perfect clean output linearity across a given bandwidth. The SW12 performed very well, scoring 90% or better for all test bandwidths.

Bandwidth (Hz)	Average SPL (dB)	Bandwidth Linearity
20-80	97.4	90%
22-80	100.0	92%
25-80	102.5	94%

Phase Response and Group Delay: A sufficiently large and abrupt phase shift may cause the perception of time smearing at the affected frequencies. Group delay is used to quantify this phenomenon, and was calculated at select music note frequencies. The approximate audibility thresholds are based on extrapolations of existing group delay audibility studies. Group delay from the SW12 remained below the approximate audibility thresholds at each tested frequency.

Musical Note/Frequency	Group Delay (ms)	Approximate Audibility Threshold (ms)
F2 / 87 Hz	2.9	15
G1 / 49 Hz	8.1	25
C1 / 33 Hz	25.6	35
A0 / 27 Hz	40.5	42

Impulse Response: A phase switch setting of 0⁰ on the SW12 resulted in a non-inverted impulse response. The impulse response (black line) shows transient overshoot and system ringing for about 40 ms. This is a normal performance for a bass reflex subwoofer.

Spectral Decay: Spectral decay was evaluated to the –35 dB level, relative to the test volume. Several minor system resonances were noted in the 25 Hz - 120 Hz bandwidth. None of the resonances exceeded –25 dB in amplitude, and they all dropped below the test floor after about 210 ms. This is a typical performance for a subwoofer, and these resonances won't be audible under normal use.

In-Room Frequency Response:

The SW12 was placed near the front left corner of my 2,000 ft³ room. For digital bass management, the main speakers were set to “Small” with a crossover frequency of 80 Hz. The digital bass management circuit imposes a 2nd order high-pass filter on the speakers, and a 4th order low pass filter on the subwoofer.

The in-room frequency response was measured at the primary listening position (about 11 feet from the subwoofer) with the main speakers and the subwoofer operating. Setting the subwoofer phase to 0⁰ provided the best results, with no obvious cancellation at the crossover frequency.

The in-room frequency response showed a pronounced peak in the 50 Hz - 75 Hz region. This peak is the combined result of room acoustics and the asymmetrical filters in the digital bass management circuit. I used my Rane PE-17 parametric equalizer (PEQ) to eliminate this mid-bass peak. As expected, room gain helped to improve deep extension, with the SW12 holding flat to about 22 Hz, and then sharply dropping 10 dB by 20 Hz.

I experimented with the boundary EQ switch on the SW12. This feature starts to affect the frequency response below 60 Hz, reaching full effectiveness in the 30 Hz - 40 Hz bandwidth. I settled on the 0 dB setting, and the final in-room frequency response was 22 Hz - 100 Hz ± 3 dB (purple line).

With Movies

I played several action-oriented DVDs, evaluating the SW12 for dynamic impact, sustained output capability, audible artifacts (muddiness, compression, port chuffing, cone cry, rattling), and deep extension. My overall subjective home theater ratings for the SW12 are provided in the table below:

Evaluation Criteria	Rating (1-5)
Dynamic Impact	3.50
Sustained Output	4.00
Audible Artifacts	2.00
Deep Extension	3.75

Provided below are my listening notes from Blade II in DTS-ES 6.1 Discrete. To give readers an idea of how loud the SW12 can comfortably play in my mid-size room, I increased the master volume until the subwoofer started to exhibit audible artifacts on the most demanding passages, and then I backed off a few dB. I monitored the sound pressure level at the listening position with a B&K meter set to C-weighted fast. The SPL peaks listed are straight meter readings, with no correction factors added.

Jared Enters Stairwell, 0:01:15-19, 97-98 dB:
This scene features a quick burst of deep moody bass without much masking sound in the higher registers. I heard some port chuffing from the SW12 on this scene.

Blood Donation Room, 0:02:11-55, 93-96 dB:
The SW12 dug down to faithfully reproduce the ultra deep bass note that menacingly builds before Jared attacks the vampires. Few subwoofers can effectively convey a sense of dread and foreboding on this scene, and the SW12 pulled it off nicely.

Motorcycle Crash, 0:07:40, 105-107 dB:
Each bounce of the rear motorcycle tire was deep, distinct, and clean. Only a subwoofer with low distortion in the lowest octave can move the room from the ground up on this scene and the SW12 played it convincingly with no muddy overtones.

Extended Fight Scene, 0:09:38-0:10:42, 106-107 dB
Compared to my reference subwoofer, the body blows lacked some visceral impact, but otherwise had good definition and depth. The full-auto gunfire bursts were staccato and distinct.

Door Closes, 24:52:60, 101-102 dB:
I also heard some port chuffing on this scene. Again, if port chuffing occurs, it is always the most noticeable on passages without much other masking sound.

House Of Pain, 0:38:14-0:39:00, 109-110 dB:
The SW12 really pounded the room on the techno music sounding deep, powerful, and clean. It played at nearly 110 dB for over 45 seconds with no signs of thermal power compression or other distress - very impressive.

With Music

I evaluated the SW12 on several music selections for balance, definition, pitch, and coherence. This subwoofer sounds very good on all types of popular music. It lacks the ultra-deep extension required to play 17 Hz organ music, but otherwise digs sufficiently deep even for the synthesized bass often present in techno and industrial music. Because it has lower than average distortion in the lowest octave, it exhibits good definition and pitch when reproducing bass musical instruments. My overall subjective music ratings for the SW12 are provided in the table below.

Evaluation Criteria	Rating (1-5)
Balance	3.75
Definition	3.75
Pitch	3.75
Coherence	3.50

Provided below are some listening notes from a few CDs.

1) Rumours - Fleetwood Mac, Warner Brothers Records 1977, 2001 High Resolution DVD-A. The electric bass in Dreams had good definition and pitch. The kick drum in opening of The Chain sounded tight without overhang.

2) Underworld Soundtrack - Lakeshore Records, 2003. Renholder's Now I Know contains extremely deep (about 23 Hz) synthesized bass. Most subwoofers ignore bass this deep, but the fundamentals were clearly within reach of the SW12. If I pushed things too hard, the SW12 started to falter and I could hear some port noise, but at moderately loud volumes the SW12 definitely delivers the goods on this demanding music track.

3) Go! – Dexter Gordon, Blue Note (Capitol) Records, 1962, 1999. Few classic jazz aficionados would disagree this is Gordon's finest album. Then budding bassist Butch Warren anchors the famous composition "Cheese Cake" with perfect rhythm and lively bass lines. The SW12 filled in the bottom end nicely, never sounding heavy and blending well with the mains, preserving the delicate timbre of the upright bass.

4) Horse Of A Different Color - Big & Rich, Warner Brothers Records, 2004. Rollin' features a throbbing and aggressive bass guitar track, and the SW12 rocks out well on this track, sounding rich and resonant with good pitch on the bass lines. At the 2:35 mark, there is a surprising deep synthesized bass effect, and the SW12 dug down and cleanly hit the fundamental with authority.

Conclusions

The NHT SW12 is exceedingly stylish and handsome, with excellent construction and immaculate fit and finish. The innovative boundary EQ feature will help users fine tune the low end response for various boundary loading and room placement options.

This subwoofer has a flat frequency response, legitimate in-room extension to 22 Hz - 23 Hz, and better than average distortion-limited output, which definitely has a positive impact on pitch, definition, and clarity in the lowest octave. These qualities help the SW12 to perform well on both music and HT applications, with occasional port chuffing at loud playback levels being my only reservation. At more moderate playback levels, the SW12 impresses across the board and without qualification.

The NHT SW12 definitely makes my recommended list, and anyone shopping for a sub in the $700 range should give it a hard look.

- Ed Mullen -