Room/Speaker Calibration for 5.1 Surround Sound
Regardless of what type of speaker setup being used in a mixing/mastering studio environment making sure the speakers are calibrated properly is a must. It doesn't mater how good your speakers are or how well the room is tuned if the speakers are not setup and calibrated right the mixing environment is pretty much useless. For surround mixing having speakers at different distances and outputting different SPLs will result in a bad sounding mix that won't translate well onto other speaker/room environments, which at the end of the day producing a mix that works on any system is the main requirement for mixing/mastering engineers.
What does it take to to have a professional 5.1 surround mixing environment, assuming that the room has been treated and tuned reasonably well. Firstly having speakers be identically in regards to the manufacturer and model of which can output the full audible frequency range, and one Subwoofer should be used also (Massey, 2004, pp17). If the 5 surround speakers are not identical then they will output slightly different frequency responses due to how they are manufactured and also potentially different sizes of the speakers themselves. In all mixing environments uniformity between speakers is paramount.
The International Telecommunications Union has produced guidelines for the the setup of of a 5.1 mixing environment which outlines the height, distance and radius of the speakers respective to the mixing engineer, see image below for details.
In the image above the center of the circle is where the mixing engineer would be positioned which is also referred to as the sweet spot, its called the because its the best place to listen to a surround sound recording (Massey, 2004, pp30). The distance between all of the speakers should be exactly the same measured from the center of the sweet spot, how far away they are placed will be determined by the size of the room and speakers. In some circumstances where a studio room isn't big enough for a proper surround setup the Left and Right Surround speakers can be moved closer to the sweet spot but a time delay will have to be used to compensate for the difference in speaker distance (Massey, 2004, pp31).
The speakers also have to follow the specific angle around the sweet spot, the center speaker would obviously be right in front of the engineer with the Left and Right stereo speakers spaced out 30 degrees. The Left and Right Surround speakers can be placed somewhere between 100 to 120 degrees, they can sometimes also be placed slightly higher than the front speakers and then angled down to the mixing position which comes down to personal preference. The ITU has the speakers at a height of 1.2 meters which corresponds to the average height of the engineers ears whilst mixing, this height can be changed respective to the individual engineer if necessary. This angle of for the rear surround speakers is predominately used broadcast mixing but for music mixing the angle can be from anywhere between 135 to 150 degrees. A greater angle behind the engineer for music mixing helps to produce a better rear phantom image which in most cases can be beneficially over the standard 100 - 120 degree angle (Massey, 2004, pp31,32). The Subwoofer or LFE should ideally be placed around the center speaker as to not interfere with the stereo spectrum and reduce potential phase problems, but this placement also depends on the size and structure of the room, the LFE crossover point shouldn't be any higher than 120Hz as well (Massey, 2004, pp33).
Now that the speakers are the right height, angle, and distance the next step is the make sure they are outputting the right Sound Pressure Level, to do this and SPL meter and a pink noise generator will be needed. The listening levels for each speaker are usually set between 79-85dB depending on the size of the room, the main thing is that what ever SPL you are referencing at all of the speakers should be producing exactly the same SPL, with exception to the LFE that is normally +4dB higher (Massey, 2004, pp36,37).
The technique for calibrating the speakers is reasonably simple, firstly the SPL meter need to be placed in the middle of the sweet spot at the same height to the engineers ear when mixing, the SPL meter also needs to be set to C weighted. Setup a pink noise generator in what ever DAW you are using and have it play at -20dB RMS, this signal should be played through each speaker individually whilst adjusting their amplification to reach the desired SPL. After all of the main speakers have been calibrated then move onto the LFE which will be set 4dB higher than what the main speakers have be set to, for example if the main speakers have been set to 80dB then the LFE will be set to 84dB (Massey, 2004, pp35,36).
Once all these steps have been completed the studio should now be perfectly calibrated for broadcast mixing/mastering with any mix done in the studio work on any other system that has been setup according to this ITU standard. Having industry standards like these ensures that no matter where you are mixing or where your mix is being played it will sound relatively exactly the same as what you initially intended it to be because it would be detrimental to and audio engineers career if their work only sounds good in their own studio.
Bibliography
Massey, H. (2004). Recommendations For Surround Sound Production. Retrieved from https://www.grammy.com/sites/com/files/pages/SurroundRecommendations.pdf
Multichannel sound technology in home and broadcasting applications. (2013). Retrieved from https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BS.2159-6-2013-PDF-E.pdf