Surround Sound Calibration

Hey everyone, it is now my second week of being back at SAE after my three-week break. Over the break I worked on a project for the Make-A-Thing game jam that was a Plants-vs.-Zombies kind of thing where your character fended off a bunch of slimes that advanced along a grid. I worked with a fellow audio student Brenden to make the sound effects, which were for the most part pre-recorded samples, except I did record some squelchy sounds with wet dog food in a plastic jug and de-reverbed them for the slime jump and attack sounds, which I synced to video captures given to me by the animation student. This project unfortunately didn’t eventuate but it was a good learning exercise all the same.

For my first blog post of the trimester, I would like to cover surround sound calibration, which we were fortunate enough to learn in SAE’s Avid S6 studio. Calibrating your speakers for surround sound is incredibly important. If you don’t calibrate your speakers properly, then the audience could experience imbalance and a comb-filtering effect due to phase problems (caused by variation in distance to the sweet spot in the studio, incorrect angular placement of speakers and unbalanced speaker amplitude). For proper calibration, you need:

  • 5 monitors (Left, Right, Centre, Left Rear, Right Rear), plus your LFE (low-frequency effects) subwoofer
  • Tape measure to measure the distance from your sweet spot to each monitor
  • Protractor or compass to measure the angle of the speaker’s position relative to the sweet spot
  • An SPL (Sound pressure level) meter (though there are phone apps that do a similar job to a industry-standard one)
  • Microphone stand or similar to attach your SPL meter to
  • Tape to help attach your SPL meter to the stand
  • Signal generator to send pink noise through each monitor

It is important that you use pink noise, and not white noise, as unlike white noise, pink noise generates an even distribution of the frequency spectrum, which is exactly what you want to measure with your SPL meter.

The process involved with calibrating your studio starts with pinpointing the spot in the room that you want to mix from. This is called your sweet spot, and can either be directly in front of your mixing console or a little further back, depending on the acoustics of your room. If your room emphasises different timbres at different sides of the room such as wooden surfaces at the back and concrete surfaces at the front, then you want an even distribution of both at your sweet spot.

 

Surround Calibration for ITU Standard

ITU Standard for Broadcast Studio calibration (International Telecommunications Union, 2012)

After you have chosen your sweet spot, you then want to set up your SPL meter on the sweet spot, at the height your ears would be at when mixing. This should also be the height at which your monitors are emitting sound from. You should then proceed to placing your monitors and your LFE emitter according to the sweet spot and your centre speaker.

Broadcast studios and theatrical mix environments have different standard for measurement and angular placement relative to the sweet spot, but for this blog I am just covering the broadcast standard (ITU) which dictates that front speakers should be placed 30° from the centre speaker, and that the rear (or surround) speakers be placed at 100°-120° from the centre speaker (measured from your sweet spot). Generally you would want to place your LFE subwoofer directly below your centre speaker, but due to spacial limitations in the studio, a lot of systems do not do this, and undertake a separate procedure for calibrating their LFE. When you are placing your speakers in this way, it is also important to make sure that the speakers are placed in an equidistant fashion, meaning that all the speakers are the same distance away as each other from the sweet spot. It should be noted that 5-speaker equidistance is only relevant to broadcast studio calibration (ITU specification), and theatrical calibration uses the dolby standard instead, where the front 3 speakers are equidistant and the rear (surround) speakers are not.

Once you have placed your speakers appropriately, you then generate pink noise through each speaker one-by-one and measure the amplitude with your SPL meter. It is important that this pink noise is emitted discretely through each speaker, and not through multiple speakers simultaneously. For broadcast studios (again, for ITU standard), your 5 monitors should be measured to 75db, with your subwoofer at 4-10db louder. In theatrical mixing spaces or for Dolby specifications, your monitors should all be measuring at 85db, with your subwoofer at 4-10db louder than that. Make sure you’re using a C-weighting on your SPL meter, as this is the weighting that is closest to the Fletcher-Munson equal-loudness contour, which a way of measuring the typical frequency response of the human ear. Without using this weighting, you’re likely measuring the SPL as if the human ear had a perfectly flat frequency response. If a speaker is not emitting the correct SPL, then you need to adjust the amplitude on the speaker itself.

After completing all of these steps, you should have a surround sound studio calibrated to the ITU broadcasting studio standard and you will be ready to start mixing in your surround sound setup.

Reference

International Telecommunications Union (2012). Multichannel sound technology in home and broadcasting applications. ITU-R. Retrieved from https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BS.2159-4-2012-PDF-E.pdf

 

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