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Subpac M2X

Tactile Subwoofer
Published May 2018
By Mike Crofts

Subpac M2X

Low frequencies are the weak point of nearly every home and project studio — could this unique tactile subwoofer give you the edge?

There are various ways headphone listening can mislead you at mixdown, one of the most commonly cited reasons being that cans can’t deliver the same physical sensation of bass that you get when cranking up a speaker system. Your trouser legs don’t flap about. The kick drum doesn’t give you that satisfying thump in the chest. The contents of your lava lamp don’t emulsify. In response to these concerns (well, the first two, at least), the manufacturers Subpac have developed a number of ‘haptic’ transducers to help silently recreate the tactile effects of low-frequency playback. There’s clearly a consumer market for this kind of thing amongst immersive-audio film/game enthusiasts and anyone who actually finds Beats headphones a bit anaemic, but how does the concept fare when you’re more concerned with making critical mix decisions, and less with shaking your pacemaker to destruction?

Let’s Get Physical

The Subpac technology comes in several different forms: there’s the S2, which straps the vibration driver to the back of your studio chair, and the M2, which straps it directly to your back like a rucksack. As a self-righteous posture freak in possession of a ridiculous bouncy swivel stool, I chose the backpack version and found there was no shortage of adjustment options to fiddle with while suiting up: a chest strap, two shoulder straps, an elasticated waist strap (I can’t imagine what they’re trying to imply about their clientele there), and a pair of what the user manual splendidly refers to as ‘back side straps’! (A vibration device with back side straps? Honestly?! You might as well write Julian Clary’s scripts for him...) The straps are no sniggering matter, though, because the exact way the M2X contacts your back makes a good deal of difference to its subjective response.

All the device’s sockets and controls appear on a box a little larger than a pack of cards, which is hard-wired to the haptic drivers via a 20cm cable and can be clipped to the lower straps at a choice of locations. The box’s line input and headphone output sockets are both designed to accept stereo mini-jacks, and the adjacent centre-detented (but otherwise uncalibrated) Intensity control determines how strongly the vibration pack responds for a given input level. The colour of the main power LED indicates not only the battery and charging status of the M2X, but also whether the input signal is clipping the device’s internal electronics. It’s possible to send audio to the system wirelessly over Bluetooth instead, and a second status LED is associated with that.

Whether you choose to feed in your mix via the included 1.5m mini-jack cable or over Bluetooth, the control box’s lack of input/output level controls means that adjusting the signal level at source is the only way to vary the headphone listening volume and avoid clipping the M2X’s electronics. Although this causes no practical problems with most consumer-grade headphones, with higher-impedance studio models you may find yourself exceeding the M2X’s input headroom even at fairly moderate headphone volumes. The manufacturer acknowledges this issue, and suggests that independent headphone preamps be used for the Subpac and your headphones in this case — a setup which also makes it easy to toggle the M2X experience on and off via its power button without having to upset the Intensity setting.

It took me a little while to adjust the M2X’s strapping for what seemed like the most representative response, but despite my best efforts I felt there remained something of an emphasis around the 35-50Hz zone. Moreover, even with the M2X pretty firmly secured by all the available means, I noticed a good deal of variation in the response according to my exact posture, simply because the driver pack is not as flexible as my back — and I’m far from a contortionist, I can tell you! As a result I felt compelled to sit stock still whilst making critical low-end judgements, and I did find this became rather uncomfortable over longer mixing sessions, especially during the mix-referencing process. It’s easy to forget how important normal small-scale movements in your studio chair are when it comes to keeping your back and shoulder muscles from locking up. (On the upside, the necessity of taking breaks meant that I never got close to draining the Subpac’s eight-hour battery life between charges, which is good news.)

Another crucial variable is how you set the Intensity control, and while the manufacturers do provide a downloadable calibration file for this purpose, their setup process is so subjective that I reckon you may as well simply play things by ear (or should it be ‘by spine’?). While anyone with the slightest sense of adventure will surely crank the intensity to maximum within about 10 seconds of firing up the Subpac, once the novelty of rearranging your internal organs has worn off it’s actually most useful as a mixdown tool when treated in a similar manner as a subwoofer. In other words, it seems to work best when it’s perceived as a fairly natural extension of what you’re hearing in the headphones, rather than as a distractingly sub-mungous special effect. Besides, the transducer’s vibrations felt better controlled and more precise to me at conservative intensity levels, which also made low-end sonic characteristics easier to make sense of subjectively.

Fixing It In The M2X

For my tests, I twinned the M2X with my own familiar Beyerdynamic DT880 Pro headphones, feeding both from an Aphex Headpod multi-output headphone amplifier. Despite feeling rather like some kind of sheepish Dad’s Army SWAT team member, I quickly became accustomed to the action of the haptic transducer, and it took only a few hours before its internalised physical sensations had began to dovetail reasonably well perceptually with the ‘in the head’ low end of the normal headphone listening experience.

There’s no question about this system’s low-frequency reach, and I can well believe the 5-130Hz system response stated in the manual, because the M2X didn’t let even the most subterranean bass elements in my reference collection fly under its radar — a feat beyond the capabilities of most real-world project-studio loudspeaker systems, in my experience. More impressive, however, was how it tracked the envelope characteristics of low-frequency sources, presumably partly on account of its comparative immunity to the speaker/room resonance artifacts that commonly afflict the time-domain response of small-studio monitoring setups. As a result, the M2X is exceptionally useful for musical styles featuring a strong kick drum, because it really highlights the low-frequency punch, sustain and decay characteristics of this instrument that often need to be so finely judged when competing with commercial releases. Although spectrum analysers and spectrographic meters can help give an idea of a mix’s low-end frequency composition, they’re rarely much good at revealing time-domain attributes, and cash-strapped EDM producers frequently struggle with mixing kick drums simply because their speakers and room acoustics conspire to turn the low end into one long temporal smudge, preventing them hearing what’s going on. For those people the Subpac will likely feel like manna from heaven.

I learnt to be more circumspect about drawing other conclusions from the haptic information, though. In particular, the response never felt linear enough for me to make truly dependable low-end balance judgements, for example when trying to even out bass notes with EQ and automation; or when balancing kick drum against bass; or when comparing low-end levels between different mixes. Granted, decisions became more reliable where the low-end elements under comparison happened to occupy very similar frequency ranges, but for the most part I found myself relying heavily on metering tools and the low frequencies in the unadorned headphone sound for corroboration. To be fair, though, the Subpac’s low-end spectrum seemed no more lumpy than that of many budget loudspeaker monitoring environments I’ve encountered, and it nonetheless increased my insight into the balance to a small but useful degree.

What also grew on me over time was the realisation that I wasn’t able to perceptually separate the different low-frequency components of the haptic information in the same way I can when working with speakers. So, for example, the balance between a bass line’s fundamental and its first harmonic is clearly discernable on a full-range speaker system, but I found it almost impossible to glean such data from the Subpac’s physical vibrations. I’m guessing that this may be because human skin and musculature can’t resolve frequency information in the same detail as the inner ear, but that’s probably not the only reason, given that some of the haptic transducers’ output must inevitably travel directly to the cochlea via bone conduction.

With a little experience, I learnt to distinguish broadly between different low-frequency ranges in isolation, but layered bass sources quickly coalesced into a single clotted stream that seemed to resist detailed inspection. Obviously you could mitigate this somewhat at mixdown by giving each bass instrument channel individual attention in solo, but that dodge doesn’t cut the mustard at the all-important referencing stage.

One important ramification of all this is that I don’t think a Subpac will bail you out if you’re using headphones that have poor low-end performance, because you’ll want to rely at least as much on the headphones as on the haptic device when it comes to making low-end balance decisions. By the same token, the low-end reach of your speakers will dictate the extent of your low-end balancing power if you choose to use the Subpac to supplement those (see ‘Subpac As Subwoofer?’ box).

Good Vibrations?

Although the M2X is clearly a bit of a curate’s egg as far as mixing’s concerned, the real question is whether its good points are worth the price. In this respect, it’s first worth remembering that mixing exclusively on headphones presents a number of potential difficulties that the Subpac won’t alleviate at all: the stretched stereo picture without inter-ear crosstalk; the overemphasised dynamics and transients from drivers placed right next to your ears; the unvarying listening perspective (loudspeakers offer many different perspectives depending on where you stand); the highlighting of low-level details because of reduced room reflections and background noise. (There’s also a tendency for people to listen too loud on headphones, although the Subpac’s sense of low-end reinforcement does help reduce that urge somewhat.) Even if a haptic device delivered everything a subwoofer could, headphone mixing would still remain an exercise in compromise, and it would be unrealistic to expect the Subpac to be some kind of silver bullet solution to the whole problem.

If you’re working on music that trades less on low-end tightness and definition, or you have occasional access to full-range monitoring to troubleshoot low-end time-domain issues, then the Subpac becomes a less compelling proposition in my view, because it doesn’t expand your low-end mix-balancing capabilities much beyond what can be achieved using a freeware spectrum analyser. I might also add that the Subpac only provides haptic feedback for one person at a time, so if you want to work collaboratively and impress visiting clients, then you’d probably do better to save up for a full-range loudspeaker system.

That said, if you’re trying to produce kick-heavy music like hip-hop or EDM primarily on headphones, then I reckon the M2X’s low-end extension and response speed are easily enough to justify its cost. Certainly, any loudspeaker capable of providing that kind of information would set you back several times as much money. Although there remain some low-end mix decisions I’d have reservations about trying to nail down using the Subpac, there’s no doubt in my mind that the information it provides increases the mixing power of headphones enough to justify the investment for hard-hitting genres.  

Subpac As Subwoofer?

Although the Subpac clearly seems to be targeted at headphone users, why not use it to supplement your speaker system, in lieu of a subwoofer? Well, I wondered the same thing, so I tried it out with a couple of different pairs of small two-way nearfield speakers, but I can’t honestly say it floated my boat.

There were two problems, both of which were as much about psychology as anything. Firstly, within that kind of listening context I found it really hard not to treat the M2X instinctively as if it were an actual subwoofer, so I kept getting distracted by unhelpful haptic balance impressions, rather than focussing on the Subpac’s strengths (ie. its low-frequency reach and time-domain information). Secondly, I struggled to perceptually connect the ‘in the body’ bass sensation with the externalised loudspeaker phantom images, which added to my confusion. What I mean by this is that the haptic device always seems to be a separate thing distinct from the loudspeakers, rather than combining into a holistic mix impression as you’d expect with a real subwoofer. Somehow, this is much less of an issue with headphones, perhaps because both the haptic sensation and the headphone phantom image are, in a sense, internalised.

Published May 2018