User manual JBL SOUND SYSTEM DESIGN

[. . . ] Sound System Design Reference Manual Sound System Design Reference Manual Sound System Design Reference Manual Table of Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 1: Wave Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wavelength, Frequency, and Speed of Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Combining Sine Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] If we reverse our path and walk back toward the source from a distance of 12 or 15 meters, sound pressure at first remains unchanged and then gradually begins to climb until, at a distance about 2 meters from the source, it has increased 3 dB above the reverberant field reading. This position, indicated by the mark on the curve, is the critical distance. The graph of Figure 5-20 is a universal relationship in which critical distance is used as the measuring stick. It can be seen that the effective transition zone from the reverberant field to the direct field exists over a range from about one-half the critical distance to about twice the critical distance. At one-half the critical distance, the total sound field is 1 dB greater than the direct field alone; at twice the critical distance, the total sound field is 1 dB greater than the reverberant field alone. The ratio of direct to reverberant sound can be calculated from the simple equation shown below the chart, or estimated directly from the chart itself. For example, at four times DC the direct sound field is 12 dB less than the reverberant sound field. At one-half DC, the direct sound field is 6 dB greater than the reverberant sound field. Remember that, although critical distance depends on the directivity of the source and the absorption characteristics of the room, the relationships expressed in Figure 5-19 remain unchanged. Once DC is known, all other factors can be calculated without regard to room characteristics. With a directional sound source, however, a given set of calculations can be used only along a specified axis. On any other axis the critical distance will change and must be recalculated. Let us investigate these two factors in some detail: first the room constant R, and then the directivity factor Q. We have already mentioned that the room constant is related to the total absorption of an enclosed space, but that it is different from total absorption represented by S. One way to understand the room constant is first to consider that the total average energy density in a room is directly proportional to the power of the sound source and inversely proportional to the total absorption of the boundary surfaces. distance from source in relation to critical distance 5-16 Sound System Design Reference Manual relationship is often indicated by the simple expression: 4W/cS. W represents the output of the sound source, and the familiar expression S indicates the total absorption of the boundary surfaces. Remembering our statistical room model, we know that sound travels outward from a point source, following the inverse square law for a distance equal to the mean free path, whereupon it encounters a boundary surface having an absorption coefficient . This direct sound has no part in establishing the reverberant sound field. The reverberant field proceeds to build up only after the first reflection. For example, if is 0. 2, only 80% of the original energy is available to establish the reverberant field. In other words, to separate out the direct sound energy and perform calculations having to do with the reverberant field alone, we must multiply W by the factor (1 - ). This results in the equation: Erev = 4W * cR Erev = 4W 1- cS () Note that the equation has nothing to do with the directivity of the sound source. [. . . ] Spkr 1 2 3 4 5 6 7 8 9 10 11 12 Distances 10 8 7 8 8 6 5 6 8 7 8 7 S 89 89 89 89 89 89 89 89 89 89 89 89 - 20 log Dist 20 18 17 18 18 15 14 15 18 17 18 17 7-19 Sound System Design Reference Manual The total level at the test position is thus 82 dB when all 12 loudspeakers are powered with 1 watt, or 85 dB with 2 watts per loudspeaker. With 20 watts per loudspeaker, we would have a comfortable 10 dB margin over our target value of 85 dB SPL. The system will be "coasting" most of the time, and a single stereo amplifier, with loads wired in seriesparallel, will suffice. A very high-end approach would be to specify two JBL 4892 Array Series systems at each corner. [. . . ]