Last month, we discussed some of the issues that plague rooms with poor acoustics, as well as some ideas on how to deal with them. An issue closely related to room acoustics is distribution of sound, which can be hindered by room shape or interior design, even in rooms with favorable acoustics.
Some sound distribution issues are obvious. For example, a theater may have a bar in the lobby outside, and closed off from, the seating area. In such a case the PA system is not going to penetrate the walls of the theater, so a smart installer will feed the output of the mixing desk to a set of small speakers in the lobby, allowing patrons who are purchasing an adult beverage to hear what they’re missing while they wait on line. Similar arrangements work for venues that might have a restaurant area outside the main room for patrons who prefer to hear a show at a softer volume or possibly watch a video feed.
But what about areas of the main room that, for one reason or another, do not allow direct sound from the PA system to reach listeners? Poor distribution of sound throughout a venue can take many forms, but it’s relatively easy to correct.
Don’t Jump Off the Balcony
When you’re mixing underneath a balcony, direct sound from the PA may not reach your ears for two reasons. Either the face of the balcony reflects sound from the PA back to the stage, or the dispersion pattern of the cabinets does not “fit” under the balcony where your mix position is located.
As a result, your mix is very loud to the folks up front, and you don’t realize it. EQ may help improve intelligibility a bit, but may worsen the sound quality for the folks up front. Under-balcony fills make the PA system more present to listeners who get limited sound from the main PA system because they put the listener in the near- to mid-field. Here’s where we have to add a bit of science.
Let’s suppose we decide to add under-balcony fills to a theater. Sound from the main PA typically reaches only the first 5 to 10 rows of seats that are under the balcony, so we’ll have to hang those fill speakers under the balcony a few rows back. This places the fills physically closer to listeners than the main PA speakers. Recall that sound travels very slowly, roughly 1,100 feet per second, or about a foot per millisecond (this varies somewhat with temperature, elevation and humidity). Can you hear a delay of a few milliseconds? Most people can, and audio engineers perceive such differences as comb filtering, phase issues or — in some cases — flanging.
Next, let’s suppose that the distance from the main PA system to our fill speakers is 60 feet, and our listener is seated directly in front of the fills. If we feed the exact same signal to the main and fill speakers, sound from the main PA arrives at the listener’s ears roughly 60 milliseconds later than sound from the fills — simply due to the fact that the mains are farther away. It takes that amount of time for sound from the mains to travel across the room to the location under the balcony where our fills are located. I think in the 1950s they called that slapback echo. Audible? You betcha.
Problems Create More Problems
In solving one problem, we’ve created another: we solved the issue of distributing sound to areas under the balcony where the main PA can’t reach, but the added delay results in time-smear and echoes. Luckily, the solution is relatively easy: delay the fill speakers by the amount of time it takes for the main PA to reach the fills. To accomplish this task, we do not use a digital delay that one might use to create echo effects (though I suppose it would work). In the old days, there were, in fact, delay lines specifically for the job, but DSP has rendered those obsolete. This is why you find output delay on drive rack processors.
Fig. 1 shows a simplified example. Stereo output from the mixing console feeds a 2×4 drive processor. Two of the outputs feed the main speakers, and the other two outputs feed the under balcony fill. The outputs for the main speakers have a delay applied to them so they can be time-aligned to the fills. Recognizing that it is easier to measure the distance between the main and fill speakers, most drive processors allow you to set the delay in feet instead of milliseconds. Simply measure the distance and set the delay to match. This is the same way that delay towers at large festivals are processed so that the audience does not hear the towers first and the mains later.
Another common distribution problem is that the main speakers are not hung high enough (or may not have a sufficient dispersion pattern) to reach the balcony, so some venues employ a center fill cluster. This might be a single cabinet or a cluster of cabs hung at a height that reaches listeners in the balcony. A center fill can be fed from the L/R bus from a (mono) center output on a console if the console provides one. Careful aiming and attention to cabinet dispersion is required to make sure that sound is not simply hitting the front of the balcony (or back wall) and bouncing back to the stage.
Let’s Get Intimate
Many smaller venues (i.e., dinner theaters) promise an “intimate experience” for patrons by providing tables directly in front of the stage. While this may be great for seeing the band, it’s generally a crummy place to hear the band — they are too close to the performers to be covered by the house PA. As a result, listeners hear mostly what comes off the stage, which we all know could be great, or very odd, especially when a band has abandoned wedge monitors and on-stage guitar cabinets. Thus, the reason for the front fill. Front fills can be smaller “standard” PA cabinets, or (preferably) a series of low profile wedges across the downstage edge, aimed at the audience. In some cases, the front fill is hung from above and aimed down at the front rows.
Dialing in a front fill can be trickier than under-balcony because you may not want to send the entire mix to a front fill. For acts that do not use vocal wedges but have loud guitar amps, a patron sitting in front of the guitar amp hears nothing but guitar. So you may need to dial only vocals into the front fill from an aux send or possibly a matrix (sound check required here). This concept also holds true for large stages where the left and right mains are so far apart that listeners in the middle don’t hear direct sound from the PA.
