As an installer, you understand that there is good surround sound and there is bad surround sound. Anthony Grimani explains how to achieve a quality audio experience for your client.
One of the most exciting parts of a good cinema experience are those surround sound effects that can so totally immerse you in the action that you completely forget who you are, where you are, and what time it is. However, you may have noticed that the process doesn’t always work quite right. Why is that?
You may think that creating a surround soundfield should be as easy as putting four speakers in a square around you, adding a fifth centre speaker, and Bob’s your uncle. You will, indeed, hear sound sources at the five speaker locations but not really anywhere else. The phantom images that you should hear between pairs of speakers playing the same sound will not come through clearly outside of the frontal soundstage. The way we hear sounds in front of us is just different from how we hear to our sides and to our rear. Our ears are screwed onto our head with directional horns pointing forward, and our brain is used to interpreting the sounds that hit our ears in very specific ways that don’t match the soundfields generated by a square arrangement of speakers.
The phantom menace
While you can create a very believable phantom sound between the two front left/right speakers, realise that you can’t do the same for phantom images between the left and the surround left speakers. You don’t believe me? Just try this little experiment. Play a stereo source over your two front speakers and place yourself in the apex of the triangle between the two speakers so that you get a clear phantom image between the two. You may want to choose program material with a mono voice or some such. Then, turn your head 90º clockwise so that you are now facing the right wall, keeping the middle axis of your head at the same place as before. Do you hear a phantom image coming from between the two front speakers now? Remember how clear it was when you were facing the speakers; is it that clear now? Move your head back and forth a bit along the left/right axis of the room and notice how the image collapses to the nearer speaker almost immediately, but it never really forms a virtual image between the two! Continue this experiment by turning another 90º clockwise. The left/right speaker pair should be behind you now. What do you hear? In some cases, you may hear a bit of a phantom image behind you; in other cases, it may actually flip around and sound like it’s in front of you! How bizarre; two speakers are playing sound behind your head, and the image is coming from in front of your head. In the domain of psycho-acoustics, this effect is known as a front-back reversal.
What to do with all this mess? You want to achieve the potential of a 360º soundfield with the ability to place sounds anywhere around that circle, but speakers in a circle around you can’t do it! You can get the five cardinal directions of the speakers to come through OK, but those pesky phantom images between the speakers don’t work right. By the way, extensive research at the BBC came to this same conclusion and decided that the old-school “quadraphonic” layout of four speakers in a square just wasn’t right. It took them a lot longer, with tons of data, and some very pretty graphs!
Industry rules of thumb
The scientific community has sometimes recommended using what is now know as the ITU 775 Surround Layout for 5.1-channel Systems (see Figure 1). It is certainly better than having four speakers all at 45º from your seating position. However, it may or may not always work well for achieving 360º accuracy and imaging. In some rooms, the 110º angle to the surround speakers is too little to create a good front/back contrast, and in all cases, it continues to be hard to get that ideal phantom image between the fronts and surrounds if you use all the same front-firing “direct-radiating” speakers. Even when it does work for the seat right in the middle of the room, it often falls apart when you move one seat off to the left or right because of proximity effects to the nearer speakers. Crikey! A home cinema has more than one seat after all!
Necessity is the mother of invention
Enter four inventions to make the surround field work better:
1) First Invention: Tuning! Plan on adjusting surround speaker locations in each and every room to get the image to work right. This means being prepared to move the surround speakers around a bit, as well as adjusting the delay times in 0.5ms increments (15cm) and the levels in 0.5dB increments. Yes, I know not all surround decoders offer that level of precision. So pick one that does!
2) Second Invention: Bipole or Dipole speakers. By driving more of the reflected soundfield of the room areas around the speakers, you can fill in the uncertainty holes in the phantom images to the side of your head. I will be the first to admit that it will never yield as tight a phantom image as two speakers in front of you, but it certainly beats the unconvincing and inadequate non-images produced by direct speakers to your sides.
3) Third Invention: Back speakers using surround EX or 7.1-channel formats like Dolby TrueHD and DTS-HD. By using a pair of Back speakers behind you at, say, 160 to 170 degrees from the font centre speaker axis, you can create a fill-in in the back areas of the room and move the Side speakers forward a bit to between 90º and 100º from front centre. Now you can get a true image to the side of your head, and the Side speakers have an easier time creating phantom images at 45º from front and rear centre.
4) Fourth Invention: Acoustical Tuning and Enhancement Devices. The right combination of sound absorption and sound scattering devices can help tie together the seven loudspeakers into one cohesive 360º sensation. By using some damping along the side and back walls, along with 2D scattering towards the front half of the room and 3D scattering towards the back half of the room, you can fill in the soundfield and create a very satisfying surround experience for several seat positions in the home cinema room. When correctly installed, the acoustical modules will control the sound waves generated by the Front, Side, and Back speakers and will help generate the sensation of phantom images all around you. The diffuse scattered energy will fill in the holes, and the wall damping will reduce the confusion between actual speaker sources and correlated wall reflections. You will finally get to enjoy sounds seamlessly creeping along the sides of the cinema as they pass you on their way to the back of the soundfield. Very exciting, very convincing, and very perfect for a horror movie!