There are currently only a few systems available that allow monitoring of fully immersive 3D audio on headphones. In my Ambisonics tutorial I used the Waves NX VST plugin together with the Waves NX Bluetooth tracker. Currently, this is still the best option out there but there are new technologies emerging which might change that in the near future. One very promising approach comes from high end headphone manufacturer Audeze: the Audeze Mobius.
In this post I want to talk a little bit about these headphones, how they can be used in the immersive audio production process and why and how I decided to fine tune them. To be perfectly clear: this is not supposed to be a full review of these headphones (which can be found elsewhere), my interest is if and how they can be used for monitoring 3D audio during immersive audio production.
About the Mobius
The Audeze Mobius are headphones that combine 3D head tracking and spatial audio processing with closed back planar magnetic drivers. Simply put, they integrate the functionality of the Waves NX plugin (with a little caveat, more about that later), the Waves NX Bluetooth tracker and the latest high end audio driver technology into a fully self-contained turnkey solution.
If you never had any headphones that utilize planar magnetic drivers, the first thing you will notice is how absolutely gorgeous they sound. The Mobius are specified to reproduce sound from 10 Hz to 50 kHz and that is something you hear immediately, particularly at the low end which sounds full without being overly dominant. And while the Mobius cannot compete with the higher end models from Audeze in terms of high-end audiophile quality, they are most certainly some of the best sounding headphones in their price range on the market today. But this is not the main selling point of the Mobius. Where they really shine is the integration of 3D head tracking.
I am not going into too much detail on how the Mobius function and what you need to do in order to take advantage of their head tracking capabilities. Let me just say that the accuracy of tracking along with the internal audio processing is remarkable. There is no noticeable latency and the resulting 3D audio impression sounds completely natural. Unfortunately, however, the Mobius currently only support 7.1 audio out of the box. I have been in touch with the developers at Audeze and they told me that full Ambisonics support is planned in the near future through a firmware upgrade (they apparently have a functional prototype in the lab), but as of the date of this writing, 3D audio is limited to 7.1 (and therefore technically not really 3D).
This essentially means that if you want to monitor immersive audio you either need to use the Mobius with Waves NX plugin and head tracker (which is somewhat pointless) or you need to accept the fact that you are losing all vertical information. Fortunately, humans are not really set up to perceive vertical audio information particularly well anyway, so this restriction is a lot less problematic than it sounds. However, it is something to be aware of before you turn to the Mobius for 3D audio production.
To use the Mobius’ internal spatial audio processing with Ambisonics audio, you therefore first need to convert the Ambisonics audio output into 7.1 format before you send it to the Mobius. I can recommend two solutions which are both good choices for this purpose. One is to create a custom 7.1 decoder using the free IEM Plug-in Suite. This is fairly easy to do and produces excellent results (did I say it is free?). The second option, which produces slightly better results but is significantly more expensive, is the O3A Decoder – 7.1 by Blue Ripple Sound. Both decoders work extremely well. The IEM decoder has the additional advantage that it can be used as a standalone application. There is no need for a separate plugin host which makes it extremely convenient to use.
Either way, I found that even though you are losing the vertical information, the Mobius are a viable alternative when used with one of these decoders. The 3D impression they provide is about as seamless as you would expect. And it is also important to point out that Waves NX can only handle 1st order Ambisonics. This means that while Waves NX does provide vertical audio information which the Mobius does not, the horizontal resolution for the Waves NX system is lower than what you can achieve with the Mobius. Depending on the particular application scenario you are working on, using 7.1 instead 1st order Ambisonics might therefore be a trade-off worthwhile considering.
I intend to create a little video tutorial (provided that time permits) to demonstrate my immersive audio monitoring setup with the Audeze Mobius. There are a couple of minor things to be aware of when creating a custom decoder with the IEM Plug-In Suite (such as the channel ordering or the proper use of additional virtual speakers in order to optimize the decoder) and this is better explained through a video. Stay tuned.
A Second Pair of EARS
This brings me to the final part of this post. And before you read any further I need to make you aware of two things: 1) the Mobius sound great right out of the box and 2) the following is not based on scientific measurements. It is somewhat subjective and needs to be taken with a pinch of salt. With that out of the way, here it goes.
When I first put on the Mobius I noticed that something was very slightly off at the higher midrange frequencies. The type of music I listen to (mainly Trance and EDM) has a tendency to push things at the 2-4 kHz level (sometimes referred to as the “clarity” range). When a set of headphones emphasizes that range (usually for making voice communication sound as clear as possible), there is the potential that music starts to sound harsh. In case of the Mobius it was not very dominant. If you are not an audiophile and/or are not listening to this type of music, you would very likely have not even noticed any harshness. But it was enough for me to want to investigate.
I had recently bought a MiniDSP EARS device which is essentially a fake head with two microphones inside silicon ears. I primarily got it to be able to record the audio that is produced by the Mobius for some video tutorials I am preparing. As the Mobius processes all audio internally, recording the resulting audio with microphones inside the ear cups is effectively the only way to demonstrate its functionality. And the MiniDSP EARS device is perfect for that purpose. But its intended primary use is actually for measuring the frequency response of headphones. So I decided to give it a go and measure the Mobius in order to see if there is anything happening at the higher frequencies which I might be able to correct with an EQ.
However, the MiniDSP EARS is a very low cost measuring system and accurately measuring frequency responses of headphones generally requires expensive specialized high end equipment. It is therefore expected that the resulting MiniDSP EARS measurements will not be particularly accurate. Additionally, any perceived frequency response depends significantly on the placement of the headphones on the listener’s head as well as the particular physiology of the listener’s hearing system and thus is impossible to quantify accurately. But even with all these limitations, the EARS device can be expected to provide useful information for fine tuning the sound experience to someone’s individual needs and/or liking as long as the results are put into context.
EARS Measurements and Mobius Correction
For measuring the Mobius I used the Rational Acoustics Smaart Di2 audio analysis system along with the Waves Tract auto-correction EQ. I found this to be the most convenient approach to analyze and fine-tune a speaker system. And it works equally well for measuring headphones through the MiniDSP EARS device. I did a couple of measurements based on slightly different headphone placements, both with 3D audio turned on and off. The internal EQ of the Mobius was always set to “flat”.
The following screenshot shows one of these measurements with the red curves being the 4 individual measurements for one headphone placement for left/right as well as 3D on/off. The yellow curve shows the consolidated correction curve suggested by Waves Tract.
Different headphone placements gave slightly different results, especially at the lower frequencies. Overall, I found that the consolidated measurement below 2 kHz was essentially flat. But there was a consistent bump around 3.2 kHz and some lack in frequency content between 5 kHz and 10 kHz. In almost all of my measurements I also had a very curious spike at around 5 kHz. But this seems to have been an artifact of the MiniDSP EARS measurement, the room in which I was measuring in, or the measurement system itself and I therefore decided to more or less ignore it.
An interesting side result of these measurements was that the Mobius appear to perform almost identical with 3D on or off which indicates that the internal DSP that handles the audio spatialization is exceptionally well executed. The sonic balance with 3D turned on or turned off is essentially the same.
The next step was to create a correction curve. When fine tuning the Mobius it is important to remember that because of their 3D functionality, left and right side have to be treated equally. Having separate correction curves, which is common when correcting regular headphones, does not make sense in case of the Mobius. I ended up using the consolidated correction curve suggested by the Waves Tract system as a starting point and adjusted it to my liking with the use of (my own) ears.
The following screenshot shows the correction I settled with. It primarily consists of a 4dB cut at 3.2 kHz and a 2dB boost at 5.5 kHz. Additionally I decided to give it a slight low shelf boost below 100 Hz and a high shelf cut above 12 kHz in order to follow more of a Harman target instead of a flat target curve. But this is really a personal preference.
The bump at 3.2 kHz was consistent with my impression of some harshness in certain types of music and the fine tuning took care of that. Overall, I am quite happy with this correction. But it might not work for everyone in every situation. If you are looking to fine tune your own Mobius, only use this as a starting point and adjust it to your particular needs and preferences using your ears along with your experience. If you have any questions or comments, feel free to drop me a note.