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Sonarworks Reference 3

Headphone Calibration Plug-in [Mac OS/Windows] By Sam Inglis
Published May 2015

Sonarworks Reference 3

Can Sonarworks’ calibration plug-in help to make mixing on headphones less of a compromise?

Following our recent cover feature on control room design, one reader took to the SOS forums to ask whether it is ever cost-effective for home studio owners to spend thousands on acoustic treatment. Why not simply buy a good pair of headphones, and cut the room out of the equation entirely?

All other things being equal, there are plenty of reasons why good speakers in a good-sounding room are desirable. Long periods spent working on speakers are less fatiguing and less isolating than prolonged headphone use. It’s easier to pick up the warning signs of excessive volume that can cause hearing damage when you’re working on speakers. And there are some crucial mix factors that are notoriously difficult to get right on headphones, such as vocal levels and reverb treatments. My own experience is that a mix that sounds good on speakers will nearly always translate well to headphone listening, but that the reverse is not a given by any means!

Having said that, mixing on headphones is simply a fact of life for many SOS readers, whether it’s because we don’t have the space and budget for a control room, or because we’re mixing on the road. I’ve used quite a few pairs of ‘phones over the years, and am often surprised at quite how different from one another two supposedly ‘flat’ headphones can sound. And while it’s true that familiarity with any monitoring system enables you to learn its quirks and compensate for them, I often feel that there are ‘blind spots’ to which you never fully acclimatise.

For instance, quite a few of the studio ’phones I’ve tried have a noticeably ‘scooped’ frequency response, which exaggerates the high and low frequencies at the expense of the mid range. Even when I know this is the case, I find it undermines my confidence in making decisions about the bottom end, and in evaluating potential mix problems such as excessive sibilance or cymbal wash. Treble boost can easily mask problems in the mid-range, and most closed-back designs also suffer to a greater or lesser extent from a boxiness in the low mids, which can make it really difficult to know whether a ‘tubby’ bass sound is a real problem, or whether overhang from the kick drum is really masking something else.

The Works

This is where Latvian start-up Sonarworks come into the frame. Over the years, we’ve reviewed a number of systems that are designed to measure the frequency response in your control room, and apply corrective equalisation to flatten out a loudspeaker system. Sonarworks’ Reference system can do that — and we hope to test the loudspeaker version, with its supplied measurement mic, in a future issue — but, uniquely, it is also designed to correct the frequency response of headphones.

The actual correction is done using a plug-in called Sonarworks Reference, which is available in all major native formats for Mac OS and Windows, including 64-bit AAX for Pro Tools 11. To make it work, you need an authorisation code and a calibration file in Sonarworks’ proprietary ‘.SWHP’ format, which specifies the way in which the frequency response of your headphones deviates from flat.

There are two ways of obtaining the relevant calibration file. Where Sonarworks have been able to measure a sufficient number of pairs of the same headphone model, they have created generic calibration files that record the average deviation for that model. At the time of writing, these average calibration curves are available only for eight common headphone models, including AKG’s K701 and K712 Pro, and Sennheiser’s HD600 and HD650. So, if you own one of the models on this list, you can simply buy the plug-in, download the relevant averaged calibration file, and set to work.

The other option is to have a calibration curve specially created for an individual pair of headphones. You can send your own headphones to Latvia to have them measured at Sonarworks’ HQ, or they can supply the plug-in with a pair of headphones that have already been measured. For the purposes of this review, they supplied a pair of Sony MDR-7506 closed-back ‘phones along with the individual calibration file derived from their measurements.

Making Corrections

The central area of the plug-in is a graphical plot of amplitude against frequency, which can be used to display a number of different curves. The ‘Before’ curve shows the deviation from a flat response of the uncorrected headphones. The ‘Correction’ curve is, as you’d expect, a mirror image of this, while the ‘After’ curve shows the response of the corrected ’phones. This is normally pretty close to that of the ‘Target’ frequency response, albeit with a tiny amount of ripple at the low end — the Target response is flat by default, but this can be changed, as we’ll see. You can also display the phase response of the equalisation, and ‘Limits’, which displays the maximum amount of correction that is theoretically possible at any given frequency.

The EQ display is flanked by detailed input and output level meters, with an output level attenuator. This can be set to adjust itself automatically, taking into account the settings of the other parameters, so as to avoid clipping. If you hit the large blue power button at lower right rather than bypassing the plug-in within your DAW, the level adjustment is retained, so you can properly compare its corrected and uncorrected output.

Beneath the EQ display are various controls that allow you to specify what sort of target response you want to work to, and how it should be arrived at. The options are divided into two pages labelled Calibrate and Simulate, though the distinction seems fairly arbitrary. The three options on the former page set the target to a flat response, a user-defined bass boost or ‘tilt’ curve, or a choice of four ‘Predefined’ curves, which are averaged from the responses of different selections of hi-fi system. Switch to the Simulate page and you’ll see four preset slots, each of which can be loaded with one of the supplied target responses measured by Sonarworks. At the time of writing, there are only six of these: two pairs of high-end open-backed headphones, a pair of Beats ’phones, some high-end consumer headphones, Yamaha NS10s and a ‘French hi-fi’.

There’s also an Advanced settings page. The most important control here is a three-position slider that sets a trade-off between phase response and latency. In the default Min position, the plug-in imposes minimal latency (about 1.1ms at 44.1kHz), but significant phase shift will be visible in the Filter Phase curve, and perhaps audible. At the opposite extreme, the plug-in operates in linear-phase mode throughout, but incurs latency of 63.4ms at 44.1kHz. The middle position represents a compromise where, presumably, linear-phase processing is used for the mid and high frequencies, and a small amount of phase shift is accepted below 100Hz or so. In this mode, latency is about 20.4ms at 44.1kHz.

The other controls in the Advanced section relate to the possibility of damaging your loudspeakers by trying too hard to correct things, and as such, are inactive in headphone mode. The control set is completed by a dry/wet mix knob, and a handy mono monitoring option.

The Numbers Game

Sony’s humble MDR-7506 headphones are transformed by Sonarworks’ calibration process.Sony’s humble MDR-7506 headphones are transformed by Sonarworks’ calibration process.Although I am a fan of other Sony ’phones such as the MDR-7509 and MDR-7520, the 7506s have always struck me as painfully bright and splashy. Perhaps Sonarworks chose them partly to showcase the difference their product can make, because a glance at the Before curve showed a frequency response that was +6dB at 3kHz and a stonking +12dB at 10kHz! Interestingly, too, significant differences were visible between the responses of the left and right earcups. I assumed that this was down to random manufacturing variations, but was surprised to see that similar differences were present in the averaged response for this model. Sonarworks say the consistent asymmetry is almost certainly down to the presence of a cable entry on the left cup.

The audible effect of flattening the 7506s’ frequency response is every bit as dramatic as you’d expect. In fact, I’m not exaggerating when I say that the Sonarworks Reference plug-in completely transforms them. In their raw state, these headphones are at best marginal as a monitor system for making mix decisions. With Sonarworks equalisation applied, they are a different animal altogether.

The best way to get a feel for what this plug-in can do is to set it up for a flat response in Calibration mode, then hit the blue power button a few times to perform level-matched A/B switching between processed and unprocessed sound. What’s remarkable is not only how the tonal balance changes, but how the mix balance changes with it. When you switch off the calibration, elements such as the hi-hat leap to the front of the mix, and other things are masked almost completely. With some material, the correction also makes a noticeable improvement to the stability of the stereo image.

Out Of The Box

When I’ve been forced to mix on headphones, I’ve found Focusrite’s VRM Box to be an invaluable aid. This, for those who’ve never encountered it, is a USB headphone amp that incorporates emulation of loudspeaker listening within an acoustical environment, providing simulations of everything from pricey monitor speakers in a treated control room to a flat-screen TV in a bedroom. Unfortunately, it seems that the future might be shaky for Focusrite’s VRM technology. There has been little development since the launch of the VRM Box four years ago, and it is not officially supported in Mac OS 10.10 ‘Yosemite’. With that in mind, I was intrigued to see whether the Simulation aspect of the Sonarworks Reference plug-in could fulfil the same role.

The Simulation options allow you to emulate the frequency response of a  small range of systems, including the classic Yamaha NS10 loudspeaker.The Simulation options allow you to emulate the frequency response of a small range of systems, including the classic Yamaha NS10 loudspeaker.On balance, I’d have to say that the answer is ‘no’. Switching to the NS10 simulation, for example, certainly gives you the boxy, bass-light frequency balance of those monitors; but without VRM’s psychoacoustic trickery, it doesn’t feel like you’re listening on speakers. There’s no sense of environment around the sound source, and nor does the ‘phantom centre’ appear to be in front of you. And most importantly, the Sonarworks Reference plug-in can’t replicate the VRM Box’s most impressive trick: the ability to check how your mixes translate by instantly switching between radically different speaker and room emulations. Not that you can’t switch instantly, but with only a handful of emulations on offer and no psychoacoustic processing, it just feels like you’re swapping arbitrary EQ curves.

As a direct replacement for the VRM Box, then, the Sonarworks Reference system comes up wanting. But considered as an aid to better mixing, what this system does is actually more fundamental — and, arguably, even more useful. For, although VRM is great for checking how your mixes translate, I don’t usually leave it active whilst I’m actually mixing. VRM has a definite ‘sound’ to it, which is more like hearing a good caricature of a loudspeaker system than actually sitting in front of one. It isn’t particularly pleasant or natural to listen to.

What the Sonarworks system does, by contrast, is give you a level playing field from which to make those mix decisions in the first place. In use, I found I rarely bothered to engage any of the speaker or headphone emulations: instead, I simply rejoiced in the glorious flatness of the frequency response! Without wishing to sound too hyperbolic, I’d say that this plug-in goes a long way towards making a cheap pair of headphones like the 7506s as useful for mixing as many pricey open-backed ‘phones. The calibrated 7506s aren’t as enjoyable to listen to or as engaging as my Shure SRH1840s, but as a platform for making informed mix decisions, I’d say they’re just as effective. Not owning any of the other headphones that are currently supported, I can’t say whether it makes the same difference in those cases, but if your existing ‘phones are on Sonarworks’ list, I urge you to try out the demo. Highly recommended.

The Famous Flower Of Sound Servers

There are some potential problems associated with using Sonarworks Reference as a plug-in within your DAW, most importantly the risk of accidentally leaving it active across the master bus when you bounce down a mix. To counter this possibility, the first time you use the plug-in, it asks whether you’ve put it on a master track, and warns you of the consequences.

Running Apple’s AU Lab in conjunction with Rogue Amoeba’s Soundflower lets you use Sonarworks Reference as a  system-wide plug-in.Running Apple’s AU Lab in conjunction with Rogue Amoeba’s Soundflower lets you use Sonarworks Reference as a system-wide plug-in.Another friendly message reminds you to audition your reference tracks through the Sonarworks plug-in for comparison. However, unless your reference tracks are actually loaded into the DAW session, this is easier said than done, as not many general-purpose media player applications support Audio Units or VST plug-ins. Even if you can find one that does, you’ll need to keep a close eye to make sure that Sonarworks settings within your DAW are mirrored in your media player.

Fortunately, there are ways around this, thanks to freeware ‘sound servers’ such as Soundflower that can interpose an extra layer of routing between the outputs that your DAW sees and the physical outputs of your audio interface. By inserting Apple’s free AU Lab utility between these virtual and physical outputs, Mac users can employ Sonarworks Reference as a system-wide plug-in processing output from all active media players and DAW programs. Used like this, you’ll never risk accidentally bouncing a mix with Sonarworks processing applied, you can instantly A/B between your DAW and iTunes or similar, and you can leave the Sonarworks plug-in window permanently open. I’m not sure how easy it is to set up a similar configuration under Windows, but VB Audio’s Virtual Audio Cable ( looks a promising option.


  • Makes a huge difference to the usability of relatively cheap headphones such as the Sony MDR-7506s.
  • Very configurable, and includes the option to emulate various other headphones and speakers.


  • Needs care to avoid bouncing mixes with headphone calibration burned in, and to make it possible to audition sources from outside your DAW.
  • Not many loudspeaker emulations currently available, and without psychoacoustic processing, they don’t reproduce the experience of listening on speakers.


The Sonarworks Reference plug-in makes a difference that is out of all proportion to its modest price, and is capable of turning an unremarkable pair of headphones into a serious mix tool.


Plug-in €69; individual headphone calibration €20; Sony MDR-7506 headphones plus calibration file €179. Prices include VAT.

Plug-in $69; individual headphone calibration $29; Sony MDR-7506 headphones plus calibration file $178.