Shakin' it with the Subpac S2 Headphone Subwoofer

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Over the years I've tried a number of headphone subwoofer gadgets. Most didn't work well at all. For instance, I had fun reviewing the Skullcandy Crusher; the sound was surprisingly good and the subwoofer bit worked...sort of. It was much better than the previous Snoop Dog version, but it still tended to deliver bass that was somewhat one-note—the Sensation 55 bass driver frequency response was narrow delivering, sensation, only between about 50Hz and 130Hz.

I've also checked out the Woojer body-attachable sub-woofer. I can't speak to the 2.0 version about to be released, but I found the initial model difficult to attach to my body firmly enough to deliver reliable bone conduction. The sensation was also centralized where the device was positioned on my body, and tended to be distracting for me—felt like a couple of mice duking it out on my chest.

The Subpac S2 ($329) is a whole 'nother story. This fairly large (17" tall x 11" wide x 2" thick) tactile transducer straps to the back of a chair with a three-point strap system. It also includes a small control box (4.75"x3.2"x0.825") that is permanently attached to the Subpac via a short coiled cord. It has a belt clip on the back allowing you to secure it to a chair arm using the wide Velcro-closure strap that is included. Subpac power is provided by an external power supply with a six foot cord. A five foot 3.5mm to 3.5mm TRS cable is included to provide the input to the Subpac controller from your audio playing device. The driver itself is a electromagnetically actuated proprietary in-house device.

There's probably no way around it, but the ergonomics of the Subpac 2 are a bit...inconvenient. With the power, audio-in, and headphone cords, I do find cable management a bit messy. And the Subpac two inch thickness does take up seat room on the chair. I found that straight back chairs (my diningroom table chairs) seem to work best—I tend to sit a bit on the edge of my chair when typing at the computer. In a rolling office chair it seemed to take up too much room to be comfortable for long term listening—however, office chairs do sometimes have a forward/back slide adjustment under the seat. I'd consider both these problems in the "minor annoyance" category.

Mitigating the above problem is Bluetooth capability and an internal recargable battery. Using these two features with only the headphone cable remaining allows you mobility in your chair. Battery life is claimed at 8 hours. Personally, I'd just stick with the wires.

Normal use of the product is to take the headphone output of the player device to the "line-in" of the Subpac controller—"headphone in" would probably be better terminology. On this page, Subpac claims, "The SubPac passes audio through to the headphones without adding or subtracting signal." I did Ohm out the pass-through and found what looked like capacitive coupling (meter showing a slowly increasing resistance). However, turning the power off does turn off the sound at the headphone output. I did call in for clarification and was told that the audio is capacitively coupled, and the audio does pass through an active device, but no frequency shaping is intentionally applied. Additional detail on the internal design is proprietary.

Using the HiFiMAN HE400S—which has a bit of sub-bass roll-off—and the Subpac set to minimum volume, I did find the Subpac would slightly alter the sound, seemingly reducing the level of the sub-bass. Here's a page addressing use of the Subpac for audiophiles addressing best practices for audio performance. Basically, they suggest splitting the headphone out signal prior to the Subpac controller. I tried it both ways and was markedly more satisfied with the sound not using the pass-through of the Subpac controller—it seemed to me the integration between the Subpac and headphones was a bit more coherent, but it might simply have been the slightly better sonics on the headphones when driven directly.

A passive split after the headphone amp might degrade the sound if the input impedance of the Subpac presents a difficult load. I did measure the DC input impedance as 9.9kOhms. Assuming it doesn't have a wonky reactance over the audio range, the 10kOhm input impedance shouldn't affect the sound on the headphones when using a passive splitter after the headphone amp. Using a CEntrance M8, which has two parallel headphone jacks, I heard no difference when removing and replacing the Subpac connection. If you don't have two jacks on your amp I'd recommend using a headphone splitter—something like this.

This is where I would normally begin talking about "sound quality" in a review, but the term really doesn't fit; let's call it "experience quality." The highest quality experience I've had with sub-woofer aided headphone listening was long ago when I had a regular sub-woofer set up with a pair of AKG K1000 ear speakers. They had notoriously poor low-bass performance that was pretty much erased when I had a sub in the room. Of course, that somewhat defeats the purpose of private listening with headphones. One note of caution here: When using the Subpac with a hard-backed chair on my hardwood diningroom floor, the bass can be heard under me in the den downstairs; when I get off the chair, it got much louder downstairs! Just a warning not to let it run when you're not seated and have downstairs neighbors.

While the Subpac doesn't quite deliver the quality listening experience of a sub-woofer in the room, it does a far better job than any of the other headphone solutions I've tried...far better. I found the Subpac 2 particularly effective with rock, hip-hop, electronica, and other contemporary music with significant sub-bass content and compressed kick drums. The palpable percussive impact did significantly add to the listening experience. On the other hand with acoustic music like jazz, I felt that continuous tones—let's say a tuba whole note—didn't integrate with the sound as well as percussive notes had. Continuous low tones seemed more to deliver a buzzing sensation in the back not as related to the sound heard as the percussive sounds.

Like all sub-woofers in my experience, a gentle hand setting the level just right—not too loud, not too quiet—allows you to forget about the effect and just listen to the music with satisfying umph. No doubt, bassheads might want dial it up more than I do— and no worries there, this thing goes up to 11. I also found I had to re-adjust the Subpac level with the music. Sometimes, once set, the level might be right for an entire album, but listening to playlist of varying content would have me re-adjusting for pretty much every track. I didn't find this bother—I'm a geek and don't mind knob twiddling—but some might find the need to readjust the Subpac often tiresome.

There does remain some lack of integration with the Subpac, I have two thoughts: First, it would be nice to have an adjustable frequency cut-off. Currently the Subpac rolls-off at 130Hz; I feel the urge to lower it to about 80Hz to get rid of the "buzzy" bass sensations. I'd love to see this added to future models. Second—and there's nothing Subpac can do about this—the Subpac doesn't provide the bone conducted low frequencies to the skull that would be heard in a speaker based system. I would guess this might account for some of the loss of integration between the visceral vibration and the sounds heard through the ears.

Gamers and Other Uses
I spent a bit of time on the phone with Darin McFadyen, Subpac's artist liaison, and health and wellness manager, talking about my impressions of the device from an audiophile's perspective. But I also spent some time inquiring about other uses for the device. He said Subpac was working closely with gamers and game developers to understand and grow use of the device. He claimed gamers were able to improve their gameplay with the Subpac as it did provide another channel of information, but said also that game manufacturers are very interested in the additional dimensions the device would provide. Think jumping in a tank and driving down the road being able to feel the vibration of the tank and the steady beat of the treads hitting the ground. Or having a hand-grenade explode near you and being able to feel the concussive blast viscerally.

Subpac is also finding use in the life of deaf people. Subpac is working with a number of organizations that endeavor to provide music listening experiences for the deaf. In that case they open up the frequency response of the Subpac to 500Hz and find that deaf people are able to "hear" the music. Feel Harmonic is one of the organizations they work with.

Summary While it didn't provide an absolutely seamless transition between visceral and audible bass, the Subpac S2 did a far better job of delivering bass support for the headphone listening experience than any other device I've used to date. Percussive low frequency information (kick drums, rapidly plucked electric bass) were enjoyably enhanced and integrated quite well with my listening experience. But continuous tones (organ, bowed bass) felt a little "buzzy" and disconnected from the listening experience by comparison.

I reckon the Subpac S2 might not be worth the bother for your average headphone enthusiast, but if you're a basshead or gamer you might find it just the ticket. I'll happily recommend this device for low note lovers.

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