I recently went over to a friend’s house to view the new home cinema system the family had bought. It was time for a film. With the curtains drawn and the lights off, ‘Batman: The dark knight’ was fired up in the dvd player and everything was set into motion. As the explosions, car chases and satanic voices filled the room however, something was not right. all the voices seemed to be coming from my right, and anything slightly boomy managed to rattle everything in the room! I began to scan the room to see where the speakers where placed and it all started to make sense. The speakers had been crammed into the most awkward and alarming places! So I decided to investigate…
A 5.1 surround sound system comprises of 5 speakers and a sub-woofer. The 5 speakers are designed to handle the mid and high frequencies whilst all of the low frequency material such as explosions is sent to the sub-woofer.
The 5 channels…
the 5 ‘smaller’ speakers are the left, centre, right, rear left and rear right channels. If these are placed in a room correctly, they will be able to work with each other to provide a complete surround sound experience to listeners positioned in the centre of their arrangement. Of course not many houses have sofas in the middle o the living room, but it is still beneficial to set them up that way. Above are the correct angles of placement for the channels. The front speakers should be positioned around 30 degrees to each side of the listening position, with the rear channels around 110 degrees each side.
The sub is a different story… since low frequencies are not very directional, and humans find it very hard to localise low frequency sounds. So the sub should be able to be placed anywhere, in whatever direction right? NO! If the sub is placed close to any boundaries (walls and the floor), the sound produced will reflect off, complimenting the original sound as if a second source was in the room (this is an image source, created at an equal distance ‘into’ the boundary to where the sub is placed)
1 boundary (floor) gives a 3dB increase to bass volume
2 boundaries (floor and 1 wall) gives 6dB increase to things boing bang and boom.
3 boundaries (in the corner of a room) will give a 9dB increase to the original sub’s output, resulting in a very muddy sound.
These increases to bass might look like a good idea, but they don’t SOUND like a good idea! with a single sub-woofer, room modes will create some very interesting phenomena. Standing waves will occur and the volume of the bass will change depending on there you are in the room! so the guy sitting next to you might be hiding for cover while you don’t even feel a rumble. The best thing to do, is try to avoid placing the sub near walls, and if you have to… put the sub at different distances from each wall. (for example in the corner of a room… 10cm from the back wall and 22 from the side wall).
So there you have it, if you have a home cinema 5.1 system, try to space the channels out symmetrically from you, and if you find there is too much bass then move the sub away from the wall.