User manual SABINE THE DIGITAL DELAY ADVANTAGE

[. . . ] 6 - Application III: Synchronizing the signals of a far-throw and short-throw loudspeaker. 7 CALCULATING DELAY TIME USING DISTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Visit Sabine online at www. SabineUSA. com THE DIGITAL DELAY ADVANTAGE Why Digital Delays?The most intelligible sound occurs when two people speak face to face. The sound is loud and dry and the direction of the sound aligns with the speaker. [. . . ] The most obvious advantage is that the distance to the closest and most distant locations in the audience is almost equal, so most listeners hear similar levels of amplified sound. Center clusters also offer two other advantages regarding the visual imaging. Studies have shown that people can detect even small horizontal changes in the direction of a sound source, but vertical shifts are much less noticeable. This suggests that the sound from center-cluster speakers is more likely to be visually aligned with the performer than loudspeakers placed on each side of the stage. All those in the audience who are closer to the performer than the center cluster will hear the direct sound from the performer before they hear the sound from the loudspeakers. This makes the sound seem to come from the performer, not the loudspeakers. (See the Precedence Effect below. ) Comb Filter Distortion Many who took high school science may remember ripple tank experiments where waves are generated from two separate point sources. The waves from each source combine to form visible interference patterns. In some places, the wave crests and troughs are in phase so they combine to make a larger wave. In other places the crests are out of phase, so the crest of one wave source is canceled by the trough of the other. Ripple tank experiments show the interference patterns are strongest when the amplitude of the waves from each source is equal. A similar interference occurs in sound systems when a signal is delayed and mixed back into the original signal. These interference patterns are called COMB FILTERS because their frequency response plots look like the teeth of a comb (see Figs. For example, when the program is played through two loudspeakers, the loudspeaker that is farther away interferes with the closer loudspeaker. Comb filters are also created when a performer is picked up by two microphones, one closer than the other. You even introduce comb filters by mixing digital effects back into the "dry" signal at the mixer's effects loop. Fig. Reducing the amplitude of the delayed signal reduces the comb filters' effect. ! Calculating Comb Filter Frequencies The reinforcement and cancellation frequencies depend on the delay time (the time difference between the arrival time of the original signal and the delayed signal). The frequency of the first cancellation occurs at 1/(2 x t) Hz, where t = the delay time in seconds. 3: Comb filters get closer as delay time increases. Comb Filter Amplitude If the original signal and the delayed signal are the same amplitude, the reinforced frequencies increase in amplitude by 6 dB, while the out-of-phase frequencies cancel completely to - dB. " Comb filters cause a lot of problems. The frequencies that are reinforced are prone to excite feedback, while the out-of-phase cancellations make the program sound thin and over equalized. Try this simple experiment to hear what comb filters do to your sound. Fig. 4: Comb filters noticeably affect your sound. Stack two identical full-range loudspeakers as shown in Fig. Correcting Comb Filters Comb filters are inevitable to some degree in every live sound system, and they cannot be corrected with equalization. [. . . ] It is almost impossible to perfectly align the stacked speakers mechanically, so comb filter distortion becomes a problem in the area where the levels from both speakers are equal. The same thing happens with speakers mounted on the right and left sides. Fig. 7: Aligning far- and short-throw speakers (the level from both speakers is equal). is louder is louder A & B at equal levels It is impossible to remove comb filters with equalization, but a Sabine digital delay eliminates them in short order without affecting the spectral balance for the rest of the audience. Carefully adjust the digital delay so that the signal from both speakers arrives at precisely the same time. [. . . ]