I regularly read forum posts on ProSoundWeb questioning the necessity of adding delay loudspeakers to sound reinforcement systems.
The default argument in favor of them appears to be level restoration, while the default objections are budget, sightlines, timing issues, distortion of the sonic perspective, logistics, and labor.
However, there is a serious but often overlooked advantage of deploying delay loudspeakers, bordering the effectiveness of absorption, that go beyond plain level restoration.
The venue shown in Figure 1 originates from an actual PSW post and is a small house of worship. The person who posted it was seeking input from the forum community regarding the best approach for designing a suitable sound system with limited means.
The walls and ceiling of the venue in Figure 1 are constructed of drywall (a.k.a., gypsum or Sheetrock) offering little to no mid- and high-frequency absorption. The stage and floor are covered with carpet. Typically, larger room volumes result in longer reverberation times unless the increase in size is accounted for by adding extra absorption. In this case. the volume is sufficiently small enough to get away with low absorption. And that doesn’t even consider the typically beneficial effects of audience absorption.
That said, audience members sitting near low absorbent boundaries at moderate to far distances to a sound system are expected to suffer from strong reflections at near identical levels. If direct (as the crow flies) and reflected trajectories approach each other in path length, relative level offsets decrease and the frequency response ripple inherent to comb filtering becomes worse. Each time direct and indirect cancel each other out, all that’s left is background noise. Signal-to-noise (SNR) ratios degrade and intelligibility suffers.
There are three common ways to deal with this:
- Steer clear of the offending boundaries with the main system by aiming the loudspeakers differently, and without missing the very audience members we’re trying to serve who are located just in front or next to said boundaries. In practice, this presents a conflict of interest and is virtually impossible, especially at low angles of incidence.
- If we can’t avoid striking those boundaries, then absorb or scatter the sound on impact. Either approach will probably affect cosmetics in some way and is also likely to change the acoustics of the venue, which might be at odds with other applications like unamplified events that benefit from a certain amount of natural amplification and reverberation.
- 3) Deploy delay loudspeakers and exploit their directional properties by careful positioning and aiming in an attempt to effectively “bypass” boundaries. Before we look at the latter, let’s start by considering the default argument in favor of delay loudspeakers, namely level restoration.
Coming Up Short
A section view of the venue (Figure 2) shows a 5.6:1 range ratio (15 dB of level variance from front to back) for a loudspeaker placed downstage at the “highest” possible position. A single loudspeaker, however, can only correct a range ratio of 2:1 at most (from on-axis at 100 percent relative distance to off-axis 50 percent closer). Even if we’re willing to accept 6 dB level variance, this leaves us 3 dB short at the back of the audience.
At least four to five loudspeakers configured in an asymmetrical coupled point source or a “dash” array (a line array of six loudspeakers or less) would be required to deal with this kind of asymmetry in the vertical plane. Both solutions are beyond the scope of this article, as well as the available real estate and budget.
Another disadvantage of a single loudspeaker/main-only approach is tonal variance. The low-frequency transducer of a typical loudspeaker is incapable of introducing any directivity because it’s producing wavelengths that exceed its own diameter several times, rendering it immune to rotation. This is contrary to the mid and high frequencies, which can be controlled very well by a proper constant directivity horn, allowing us to direct the sound where we want it to go.
Use Front Fills
Figure 3 shows a single 50-degree loudspeaker aimed at the rear of the room. The front to back level drop in this part of the spectrum is 9 dB (15 dB of range minus 6 dB of angular attenuation), and it’s overshooting the beginning of the audience. The latter issue affects only a minority of the audience and is best dealt with by a local solution, e.g., front fills.
Figure 4 depicts the low end of the same loudspeaker. Its lack of directivity and inherent immunity to rotation result in a 15 dB loss. There’s only distance at play and no angular attenuation because there’s no coverage angle to begin with in this part of the spectrum. Room gain favoring low frequencies, by accumulated reflections over distance (also known as LF buildup), is likely to decelerate the LF loss rate.
That being said, if left unaccounted for, different loss rates result in tonal variance. Should the result in the back of the venue be too dark (the rule and not the exception) because we simultaneously suffered from HF losses by air, the delay loudspeaker provides an additional bonus by restoring only those frequencies that are missing. This reduces the spectral tilt. Just be mindful that the hi-hat can’t be traced back to the delay loudspeaker, focusing attention on its location.
