Click here to purchase the entire book in PDF format. 5-channel Surround In the case of 5.1 surround sound playback, we are actually assuming that we have a system comprised of 5 full-range loudspeakers and no subwoofer. This is the recommended configuration for music recording and playback[Dolby, 2000] whereas a true 5.1 configuration is intended only for film and television sound. Again, all loudspeakers are assumed to be equidistant from the listener and at angles of 0$^\circ$, $\pm$30$^\circ$ and with two surround loudspeakers symmetrically placed at an angle between $\pm$100$^\circ$ and $\pm$120$^\circ$. This configuration is detailed in ITU-R BS.775.1[Union, 1994] (usually called ``ITU775'' or just ``775'' in geeky conversation... say all the numbers... ``seven seven five'' if you want to be immediately accepted by the in-crowd) and shown in Figure 10.27. If you have 25 Swiss Francs burning a hole in your pocket, you can order this document as a pdf or hardcopy from http://www.itu.chwww.itu.ch. Note that the configuration has 3 different loudspeaker apertures, 30$^\circ$ (with the C/L and C/R pairs), approximately 80$^\circ$ (L/LS and R/RS) and approximately 140$^\circ$ (LS/RS). Figure 10.27: Recommended loudspeaker configuration for 5.1-channel surround listening[Union, 1994]. How to set up a 5-channel system using only a tape measure It's not that easy to set up a 5-channel system using only angles unless you have a protractor the size of your room. Luckily, we have trigonometry on our side, which means that we can actually do the set up without ever measuring a single angle in the room. Just follow the step-by-step instructions below. Step 1. Mark the listener's location in the room and determine the desired distance to the loudspeakers (we'll call that distance $X$ ) Try to keep your loudspeakers at least 2 m from the listening position and no less than 1 m from any wall. Step 2. Make an equalateral triangle marking the listener's location, the Left and the Right loudspeakers as shown in the figure on the right. See Figure 10.28. Figure 10.28: 5-channel setup: Step 1. Measure an equilateral triangle with your L and R loudspeakers and the listening position as the three corners. Step 3. Find the halfway point between the L and R loudspeakers and mark it. See Figure 10.29. Figure 10.29: 5-channel setup: Step 2. Find the midpoint between the L and R loudspeakers. Step 4. Find the location of the C speaker using the halfway mark you just made, the listener's location and the distance $X$. See Figure 10.30. Figure 10.30: 5-channel setup: Step 3. Measure the distance between the listening position and the C loudspeaker to match the distances in Step 1. Step 5. Marks the locations for the LS and RS loudspeakers using the trangle measurements shown on the right. See Figure 10.31. Figure 10.31: 5-channel setup: Step 4. Measure a triangle created by the C and RS loudspeakers and the listening position using the distances indicated. Step 6. Double check your setup by measuring the distance between the LS and RS loudspeakers. It should be $1.73 X$. (Therefore the C, LS and RS loudspeakers should make an equilateral triangle.) See Figure 10.32. Figure 10.32: 5-channel setup: Step 5. Double check your surround loudspeaker placement by measuring the distance between them. This should be the same as either surround loudspeaker to the C. 7. If the room is small, put the sub in the corner of the room. If the room is big, put the sub under the centre loudspeaker. Alternately, you could just put the sub where you think that it sounds best. Room Orientation There is a minor debate between opinions regarding the placement of the monitor configuration within the listening room. Usually, unless you've spent lots of money getting a listening room or control room designed from scratch, you're probably going to be in a room that is essentially rectangular. This then raises two important questions: Should you use the room symmetrically or asymmetrically? Do you use the room so that it's narrow, but long, or wide but shallow? Most people don't think twice about the answer to the first question - of course you use the room symmetrically. The argument for this logic is to ensure a number of factors: The coupling of left / right pairs of loudspeakers to the room are matched. The early reflection patterns from left / right pairs of loudspeakers are matched. Therefore, your left / right pairs of speakers will ``sound the same'' (this also means the left surround / right surround pair) and your imaging will not pull to one side due to asymmetrical reflections. Then again, the result of using a room symmetrically is that you are sitting in the dead centre of the room which means that you are in one of the worst possible locations for hearing room modes - the nulls are at a minimum and the antinodes are at a maximum at the centre of the room. In addition, if you listen for the fundamental axial mode in the width of the room, you'll notice that your two ears are in opposite polarities at this frequency. Moving about 15 to 20 cm to one side will alleviate this problem which, once heard once, unfortunately, cannot be ignored. So, it is up to your logic and preference to decide on whether to use the room symmetrically. Figure 10.33: Two rectangular rooms of identical arbitrary dimensions showing the maximum possible loudspeaker distance for an ITU775 configuration. Notice that the loudspeakers can be further away when you use the room ``sideways.'' The second question of width vs. depth depends on your requirements. Figure 10.33 shows that the choice of room orientation has implications on the maximum distance to the loudspeakers. Both floorplans in the diagram show rooms of identical size with a maximum loudspeaker distance for an ITU775 configuration laid on the diagram. As can be seen, using the room as a wide, but shallow space allows for a much larger radius for the loudspeaker placement. Of course, this is a worst-case scenario where the loudspeakers are placed against boundaries in the room, a practice which is not advisable due to low-frequency boost and improved coupling to room modes.