On The Plus Side

Regardless, level restoration of at least 3 dB in the rear of the room is required, the most common argument in favor of a delay loudspeaker, in order to place the entire audience within 6 dB of level variance or less.

Figure 5 offers a section view of this approach. The mains have been titled slightly down to limit their coverage to a 2:1 range ratio, reaching all the way down to the second row. A delay loudspeaker will take care of the remainder of the audience, piggyback-riding on the mains.

Figure 6 demonstrates the essence of this article and the often overlooked necessity of delay loudspeakers if re-aiming or absorption is not a viable solution. A single loudspeaker at a grazing angle of incidence, placing the audience in or near the propagation plane with respect to its reflected sound of a non-absorbent rear wall, can only maintain its level dominance in the first half of the audience.

Beyond that milestone, path lengths approach each other, resulting in strong reflections. We can’t expect the main loudspeaker to reach the last row and magically avoid the rear wall. When we get closer to the rear wall its presence increases. On our dual-channel FFT analyzer, this will manifest itself as a decrease in coherence (Figure 7), a metric for SNR.

Contrary, the forward-positioned delay loudspeaker has a considerably different geometrical relation to that very same rear wall. Its increased down-tilt angle enters angular attenuation into the off-axis reflected path (Figure 8). The reflected trajectory traveled a longer distance compared to the direct sound and on top of that suffered an additional penalty at the start.

Therefore, the delay loudspeaker will exhibit improved D/R (direct-to-reverberant ratio) that benefits the compromised main loudspeaker while simultaneously restoring level and reducing tonal variance.

The angle of incidence of the delay loudspeaker with respect to the main loudspeaker will determine the rate at which main and delay tear apart in terms of time (Figure 9). Evidently a properly delayed delay loudspeaker placed in-line with the main speaker will remain time aligned over distance, while delay and main opposing each other will create a stalemate situation.

When choosing the correct position for the delay loudspeaker(s), the coverage, level and tonal variance should be carefully balanced against the improved D/R of a more forward position in exchange for reduced “synchronicity.” If room treatment is not an option (again, Figure 6), then delay loudspeakers are a viable alternative for improving D/R in the most vulnerable part of the audience.

Rules Of Thumb

To insure that the output of the delay loudspeaker is as inconspicuous as possible, make sure:

1. It is equally loud as the main, at most.
2. It arrives on time. The Haas (or precedence) effect requires a differential system to detect an offset in arrival times. A system we do not possess in the vertical plane contrary to the horizontal plane. Over-delaying will result in an audible and measurable degradation, less efficiency, and artifacts ranging from strong tonal coloration to possibly discrete echoes, depending on the program material.

   

   Figure 9.jpg

   DISCLAIMER: If big temperature swings are to be expected, affecting the sound speed and consequentially time alignment, delay times must be revaluated. If temperature swings can’t be accounted for, the relay line offers better trade-offs.

3. It sounds the same. Typically, there is way less air between delay loudspeakers and the audience than there is between the main(s) and the audience. The main loudspeaker has suffered a bigger HF penalty than the delay loudspeaker. The latter, therefore, should be made equally dark. Alternatively, the output of the main could be made brighter as long as it doesn’t make things worse in the front of the audience.

   

   Figure 10.jpg
4. The low-end is shelved or ultimately even cut. Most mains will have suffered a substantial amount of low-frequency buildup in the back of room. Shelving out some low end in the delay loudspeaker will reduce tonal variance and simultaneously reduce LF backwash for the audience in front of the delay loudspeaker.Figures 10 and 11 show the differences between both approaches with boundaries enabled. Notice the reduced comb filter in the back of the audience.

In conclusion, properly deployed delay loudspeakers will:

• Restore level (which isn’t always required)
• Decrease tonal variance
• Improve D/R
• Reduce comb filtering
• Improve SNR, coherence, and intelligibility

Based in The Netherlands, Merlijn van Veen is a consultant specializing in sound system design and optimization, and he’s also a noted audio educator.