Introduction to 5.1 surround sound

If I were to ask someone what a 5.1 surround sound system does to a home theatre, I would expect the reply to be something along the lines of, "It makes the sound come from behind you as well as in front of you". Even though this isn't wrong, there is a lot more that goes into it than just 'sound coming from behind you'. The number 5.1 means that there is 5 full range speakers and 1 LFE (Low Frequency Effect) speaker that has 1/10th frequency range of the other speakers (if the full range speakers have a frequency spectrum of 20 - 20,000Hz, the LFE would have a spectrum that is 1/10th of that; 20 - 2000Hz).

Placement for a system such as this is critical, the speakers consist of: left front, centre front, right front, left surround, right surround and a seperate LFE channel. Not only must they be set up in the appropriate order, but to effectively calibrate a room for surround, the speakers must also be of equal distance from the listener and all be producing the same dB level. There is even specific angles that these speakers should be directed to, right down to the degree (Appendix A).

Appendix A: speaker placement for 5.1 surround

The numeric figures seen above are the degrees apart in relation to the centre speaker. Front left and front right must be 60 degrees apart from one another (30 degrees either side of the centre speaker) and left and right surround must be -110 and +110 degrees from the centre. The LFE does not need to be placed in a specific location due to it only expelling low frequencies, meaning it does not have directionality; the sound source of the LFE can't (for the most of it) be pinpointed to a specific location.

Notice how none of the speakers are facing each other?

This is strategically done because if two (or more) speakers are equal distance apart, expelling the same dB level and are directly facing each other, when the sound from the individual speakers reach the centre point between them it will cause Phase Cancelation and Comb Filtering. In short it means that overall clarity will be drastically affected and has the potential to completely ruin the sonic experience of 'surround sound'.

How is the sound separated in the speakers?

When mixing engineers produce audio for film in 5.1 surround, there tends to be a few things that are common with all mixes. One great example of that is only putting spoken dialog in the centre speaker, this is done because if the film is to be overdubbed in a different language to the original recording, the mixing engineer who is replacing the vocal recording only needs to change the audio coming from the centre speaker and not the rest of the mix. Another common technique is applying most (if not all) of the reverb in the surround speakers, this adds to the effect of being engulfed in an immersive sonic experience; having the natural reverbs come from behind you as though the source audio is bouncing through the space that is behind you. everything else is mostly mixed in stereo (Left front and right front) apart from the occasional flying bullet passing by your head or an explosion that is happening behind the camera.

Exit Sign Syndrome, what is it and how does it relate to surround sound?

Exit Sign Syndrome can happen in many different circumstances, it essentially means when you are focussed on something (a conversation, movie, video game etc.) and all of a sudden you become disassociated with said thing that you were just entirely focused on. Imagine being in a movie theatre and all of a sudden there is a car crash off camera and it's clearly audible from the speakers behind you, you turn your head because you could have sworn you head a crash coming from behind you. When you turn your head from the noise, you're brought back to reality by realising you're actually in a movie theatre and not in the story of the movie. That's why mixing audio in surround (particularly for film) has to be done with a lot of attention and care, it falls back on the mixing engineer if viewers attention is being taken off of a movie and on to the soundtrack completely.

There are so many production styles that can be done to optimise surround mixing and I am very excited to expand on this concept and it's techniques, but we're going to have to save that for another blog. Keep your eye out for more updates on my blog about extra tips and tricks to help you achieve the best possible outcome in your mixes.

Till next time,

Jordan.