Review: Sonarworks HD Reference Plug-in

Interest in the Latvian firm’s new headphone calibration software is starting to build, but is it really something to get excited about? Simon Allen delivers his verdict.

Measuring and calibrating speaker systems, particularly studio monitors, is an area of science and technology that has seen many advancements and differences of opinions.

As engineers we talk about the theoretical holy grail, which we call a ‘flat frequency response’. The principle of working in an environment that has a flat frequency response, so that our judgements aren’t influenced by anything other than the source, is a widely accepted practice. However, achieving this known ‘reference’ sound is not only extremely difficult in the real world, but it is heavily debated as to what exactly is a flat frequency response.

Assuming we have agreed upon a flat frequency response and managed to electronically capture and measure a device, how do you go about making corrections and adjustments? As we know, making adjustments can introduce secondary issues.

The Technology

Sonarworks won’t go into detail about its measurement technology or its ideas around a ‘human’ flat frequency response, which isn’t surprising. However, its approach and choice of language is certainly on the right lines. The company already addresses each debated area of monitor calibration using its own methods with much success, and now it has found a way to bring the same principles to headphone users.

Firstly, Sonarworks claims that it has heavily invested in developing technology that measures the 20Hz to 20kHz bandwidth in a way that is closer to a human ear, unlike a measurement microphone. The firm states that its method provides more realistic results. Sonarworks calls its method PAPFR (Perceived Acoustic Power Frequency Response) and makes its measurements at a normal listening volume of 83dB SPL.

While discussing its measuring technology with marketing manager Rudolfs Putninš, he explained: “The problem lies in the fact that currently there are no set standards for how to do headphone FR measurements. All of the head-microphone setups are made to measure SPL, not FR, so we had to develop our own, which we feel does a far better job”.

The company has also thought about the flat frequency response, and uses the explanation: “The default flat frequency response reference target is made to match the frequency response of flat sounding speakers, ensuring smooth translation between working on studio monitors and headphones.” Although there is no clue as to what that might be, at least we can have the confidence that this is more than just an EQ service based on theory.

So, if Sonarworks has managed to take a ‘realistic’ measurement and thought about a desired goal, what about a calibration method? In the same vein as its approach to measuring sound, the company has invested in high level mathematics and an ‘adjustable filter engine’.

This is my favourite part about the UI side of this plug-in. Due to the high-resolution EQ curve, the user can select the level of resolution and the amount of phase distortion within the EQ, to offset against latency and processing power. For example, if monitoring on headphones in a recording or performing scenario, then low-resolution EQ and a minimal phase relationship will allow for lower latency. Whatever the amount of processing you use, the plug-in will give you a gauge and a reading of the latency you are incurring.

The Plug-in Method

The Sonarworks HD Reference plug-in comes as AU, AAX Native, RTAS or VST. The principle is to run the plug-in over your headphone output from within any DAW. From there, you simply load the appropriate headphone profile file from Sonarworks. Once loaded you are able to hear the before and after of the adjustments made, and choose between linear, mixed or minimum phase filters to suit the task at hand.

The plug-in works well and doesn’t draw too much processing power. Other considerations have been taken, such as the output level post plug-in – you can choose to either monitor at a lower level so that the plug-in’s processing will never peak the output, or limit the output to avoid clipping and remain at the same audible level.

Once the plug-in knows which headphones you are using and is loaded with the correct calibration file, Sonarworks has taken this one step further by providing other targets. Besides the obvious flat frequency response, there is a selection of other devices it can simulate. For example, you can hear what your mix/master might sound like on a pair of NS10s, or a pair of Beats by Dre. Although alarming, I did find the simulations realistic.

However, there is one drawback at the moment. As this is a new product, there are a very limited number of headphone models for which Sonarworks has measured and released calibration files. Currently, Sonarworks is selling three options of headphones at varying price points with the calibration plug-in. Other calibration files exist, but are few and far between. However, if your headphones aren’t listed, then Sonarworks says you can send them to its HQ, and for only €20, they will measure, create a calibration file and return them.

Listening Experience

R?dolfs sent me a pair of Sony MDR-7506 headphones with the plug-in and calibration file to try out. I’ve used 7506s many times and already knew that they were far from my idea of ‘flat’, so thought it would be a good test. It was a shame that at the time of this review, they didn’t have profiles for some other headphones I know and use more regularly.

On first listening, I was very impressed. In fact, possibly like most engineers, I spent some time simply listening to different types of music comparing, with and without the calibration, to try to appreciate the changes. I was taken aback, the sound is vastly different with the calibration on, and it really did feel like listening to flat response studio monitors. I always knew the 7506s were very capable headphones, but now I felt I could actually do some preliminary mixing on them. If I had to be fussy, the bass response was a little too much for my taste, but easily learnt.

The issue now is that I feel I need to hear another pair of headphones from a different manufacturer with the Sonarworks calibration running. I want to know if they have a consistency for measuring headphones and if they really are able to pull nearly all headphones in-line with one another. After all, besides offering a solution to calibrate your headphones, this is a solution that is trying to bring that ‘reference’ sound across the board. I guess I will have to wait for them to continue to bring out calibration files but I am both intrigued and excited.

My verdict from hearing the response on the 7506s was better than I expected. Even if it isn’t 100% correct to everyone’s ears, it is an extremely good attempt, and a reference worth learning. What the tech does is achieve a response that is transportable and in line with working on studio monitors.


When I first heard about this calibration plug-in, I did wonder if it might have just been a simple EQ curve which was electronically calculated. Although I’m still not clear about the methodology here, it is apparent from the short explanations and what I can hear, that this is a very serious solution. It is a hard area of the market to venture into, which is why there isn’t really anything else out there quite like this. I highly commend Sonarworks for its thoughtful approach and long-term efforts in providing us with a tool that opens up possibilities on a higher level.

Besides the continual growth of supported headphones there are a couple of other features I’d like to see incorporated in future releases. Firstly, I would like to have the ability to cross-feed the left and right signals. This would add to the simulation of working on monitors in the studio. Secondly, this calibration method only runs as a plug-in hosted by your DAW. It would be an obvious move to develop an app for smartphones, tablets and computers, etc. For example, to listen to anything, you must currently run the audio from within your DAW, which isn’t ideal if you want to quickly hear a mix sent by a colleague, or a production idea from a co-producer.

Simon Allen is a freelance, internationally recognised sound engineer and pro-audio professional with over a decade of experience. Working mostly in music, his reputation as a mix engineer continues to grow.