Room Calibration

A large part of our AUS230 unit has included surround sound concepts, including both room calibration for surround and mixing in surround. It has been an interesting experience to listen critically to the difference between stereo and surround, I’ve discovered just how capable surround sound is of creating depth and space using panning and effects.

In a class we ran through a simple room calibration, it was simpler than I had previously expected. Firstly we took an SPL metre and set it to “C” frequency weighting, which is the standard for room calibration. SPL metres work by responding to the frequencies that humans are most sensitive to, mainly between 500Hz and 8K. There is also “A” frequency weighting, the difference with “C”, is that it won’t filter of any low frequencies, where “A” will. The diagrams below best depict the difference between the two.

“C” will not have any major effect on the reception on the lower frequencies, which is the main reason it is used for room calibration. We placed it on a mic stand at the speaker monitors level, then got an XLR cable (in place of an actual tape measure) and used it to measure the distance between the centre cone of each of the speakers. Lining them up to as similar a length as possible from the SPL metre which was listener’s “hot spot”.

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With all of the speakers at a similar distance, we then, one at a time, sent pink noise through them. We used pink noise as more proportional across all frequency bands to the human ear, which is what the SPL metre is simulating. To the ear, it sounds like it has more of a tone, as opposed to an aggressive hissing noise. The aim at this point was to then go around each speaker and play the noise through them, turning up the speakers output until they all read 75 dB SPL or as near as possible. 75 or 85 db SPL are fairly standard figures for working in a surround sound environment, it can depend on the studio and the engineers preference. What was interesting about this part of the calibration, was that even though the speakers were the same distance from the SPL metre, some output levels were not resembling the same level from the SPL metre. Some even had noticeably different character. This was a very eye opening moment, as I learnt how this can influence our mix drastically.

c-dms1-sound-meter-front-on

Surround Sound Mixing in Pro Tools (main differences from stereo)

On campus we are lucky enough to have Digidesign C24 console, offering the ability to mix very easily in surround 5.1. Firstly, whether there are I/O settings saved or not, the Pro Tools session I/O needs to be set up to 5.1 surround, so that there is control over all 6 of the speakers. Luckily in our C24 studio, there is a saved setting for this that can be easily opened. If there isn’t already, we would need to create a new 5.1 output path in the Pro Tools I/O.

2015-08-04-io-setup-01-output

There are a couple of ways of routing/sending tracks through to specific speakers in a surround mix in Pro Tools. The first and possibly most common way is to make the output of every track, the full 5.1 bus path that goes directly to a Submix Aux. This would imply that any effects Aux tracks are run in parallel to the audio tracks. A great advantage to this method, is the fact that it means every track has the full 5.1 panning ability, and furthermore you can use “divergence” knob in the panning parameters (also with other tracks depending on the output path) to control the spread or width of that signal in the mix. Say for example you wanted bass to be panned centre, however didn’t want it to be exclusive to the centre speaker, you could increase the divergence to spread it out amongst the Left and Right too. Personally I find this the simplest and most effective way of working the mix as the only variable is the panning, and it’s therefore much less confusing with the I/O routing.FMPLoop.gif

The other way, and many people use this way also, is to route the output of the track to individual speakers. Such as sending the bass or vocals out of the centre speaker. A way to roughly combine the two methods would be to create a designated Aux track for each speaker (L, C, R, Ls, Rs, LFE), and send the specific audio tracks to the certain speaker auxiliaries, thus removing the need for panning. From there we would route the output of each of those Aux tracks to the same 5.1 bus path, which would then go to a 5.1 Submix track.

One of the main capabilities of the 5.1 setup is being able to send things to the rear speakers to create a load of space and depth. To create this effect, most of what gets sent to the rears is reverb and delay, which is sort of simulating the surrounding reflections from the environment that the human ear picks up on that create give space and depth to whatever it is that we’re hearing. One way of doing this in Pro Tools is to create a left and a right aux tracks with a surround 5.1 delay or reverb plug-in and send audio via buses to them, then panning to rear left and right.

One very effective way of creating distance and depth with these rear speakers is to use the 40/20 technique (similar to the Haas Effect). Firstly we bring up a delay plug-in on each on each of the delay aux tracks and bring up the pre delay on one of them to 20ms, and the on other bring the pre delay to 40ms, both with no repeats or feedback. Combined with the dry signal that is coming from the front speakers, this can make the sound far bigger and wider and straight away it give the impression of the theoretical position where you may be standing in the room, which can vary upon the amount of pre delay used and whether or not you may have reverb dialled etc.

There are other panning considerations now that 5.1 offers many more options, one helpful function is the FMP (Follow main pan) function for bus sends, this means that the bus send will follow the same panning as the panning on the actual track. As each send has a full 5.1 panning control, it gives a lot of options about where the effect can be sent. Say I wanted to send a vocal that had been panned hard left for the first half of a song and that automated the tracks panning so that it would move to hard track for the second half (hypothetically), and i wanted the reverb to remain with the vocal. I would activate FMP and the bus to the reverb would automatically follow the vocal tracks movement. For the 40/20 method above, you would not use FMP, as you would often want the vocals to be up front in the mix, but the delay to be panned to the back.

I hope to have more opportunities to explore these techniques while I’m still studying at SAE, especially with how to fold a 5.1 mix to stereo and vice versa.  

References

Frequency Weightings – A-Weighted, C-Weighted or Z-Weighted. Noisemeters.com. Retrieved 2 November 2016, from https://www.noisemeters.com/help/faq/frequency-weighting.asp

Hays, T. (2009). ROOM LEVEL CALIBRATION 101 (1st ed., pp. 1-2). Technicolor Interactive Services. Retrieved from http://twvideo01.ubm-us.net/o1/vault/gdcaustin09/slides/Hays.Tom.Room%20SPL%20Calibration%20101.pdf

Kuehnl, E. (2015). Setting up to Mix in Surround in Pro Tools. YouTube. Retrieved 2 November 2016, from https://www.youtube.com/watch?v=sP9w50WyF0E

Kuehnl, E. (2015). Surround Track Routing in Pro Tools. YouTube. Retrieved 2 November 2016, from https://www.youtube.com/watch?v=dk707yTNqA8

Monitor Wizard | Sound On Sound. (2016). Soundonsound.com. Retrieved 2 November 2016, from http://www.soundonsound.com/techniques/monitor-wizard

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