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Sound Advice | By Jason Pritchard You Can't Tuna Fish But can you tune a room? Tips for ringing out a room. People refer to feedback elimination as “tuning the room,” but let’s reserve that language for discussions surrounding acoustics, not sound systems. We can’t actually tune the room with the sound system, but we can manipulate the sound system to work within the room. “Ringing out the room” is used for quick feedback elimination on sound systems with only a couple of speakers. This technique can be used when the presence of feedback is prohibiting the sound system from generating the levels necessary to get the job done. It can quickly help to provide a few extra dB of gain before feedback. This isn’t the same thing as tuning a system for flat response or coordinating multiple sources to act as one but rather a quick fix. Ringing Start with a wired microphone on a stand center stage. It is important to use a wired microphone rather than using a wireless microphone. Wireless microphones are not as linear (input = output) as a wired microphone. That nonlinearity will make the following process much more difficult. Plug the microphone into a console channel and ensure that any channel equalization is either turned off or set flat. Also be sure to bypass any dynamics processing (compression, expansion or gating). Flatten any output equalization and bypass any compression. In this exercise we will be manipulating the output equalization of the console, or an external equalizer. Either way, we are looking to manipulate the last EQ before the main speaker’s amplifiers. The path must be a clear as possible. Now you are ready to 22 October 2010 • www.stage-directions.com
Figure 1. Speaker pointed at the center of a room, in this case showing uneven coverage. Figure 2. Speaker pointed at the back of the room, showing more consistent coverage. carefully route the microphone to the speakers and quietly verify the signal is flowing where you expect it to be. There are a couple of different types of equalizers one might encounter in a sound system. The typical filter in an equalizer has three parameters: Gain (+ or -), a center frequency, and a width parameter, which is referred to as bandwidth or sometimes Q. A graphic equalizer is a series of filters which have fixed frequency and bandwidth parameters. Most often seen in a 1/3 octave spacing configuration, a third-octave equalizer provides about 30 filters, which gives the engineer reasonable, although not absolute, control over the signal. Graphic equalizers are relatively easy to use as two of the three filter parameters are fixed, leaving gain as the only moderated parameter. Parametric equalizers provide precise control over all three filter parameters. Parametric equalizers are much more difficult to use for quickly ringing out a room, and are more often used for system tuning and calibration. Mixing console channel equalizers are often parametric equalizers. The ringing out procedure consists of the iterative process of slowly raising the level of the microphone until you can just hear the first frequency begin to feedback, then controlling the feedback by varying the position of the input fader while identifying the frequency of the feedback and adjusting the output equalizer to eliminate the ringing. Every cut one makes with an equalizer is effectively a reduction in gain. This exercise is designed to maximize gain, so only adjust the equalizer the minimum amount necessary to stop the ringing. Identifying the ringing frequency can be a challenge. For a little ear training have a look at the “Simple Feedback Trainer” (http:// sft.sourceforge.net/). Carefully raise the level of the microphone. If the same frequency begins to feedback again, reduce that frequency further with the equalizer. If a different frequency begins to ring, identify and reduce that frequency using the equalizer. Repeat this process until you have either achieved enough gain to comfortably operate the system or until you have identified and eliminated about five frequencies. At that point the system is probably reaching the breakeven point between problem frequencies and overall gain reduction. The idea is to maximize gain before feedback. If too much cut equalization is applied, it becomes the same thing as simply turning it down. the speaker is all in the audience and not bouncing off of the walls. For the vertical axis, generally speaking, point speakers toward the back of the room, not to the middle of the room. This will produce a more even response throughout the space. Speaker coverage patterns have shapes, try and make those shapes complement the shape of the room. Figure 1 shows a speaker on a stand pointed at the center of the room. Notice the increase in level at in the third row. In Figure 2 the speaker is pointed at the back of the room, notice that the sound pressure levels are more even for each row of the audience. This method isn’t a substitute for a more extensive sound system tuning and calibration, but sometimes ringing out the room is all that is required to get through the gig. The process can also be used as a tool to help the engineer understand the limits of the sound system. Extra Notes There are a few other items of which to be aware before attempting to ring out the room. It should go without saying, but the physical relationship between the microphones and speakers is very important. Putting microphones in front of the speakers is almost always impossible, or at very least frustrating. A microphone should maintain several feet of distance behind or to the side of any speaker. When pointing the speakers, be aware of the walls. Sound reflects off of hard surfaces. Try and point the speakers so that the pattern of www.stage-directions.com •October 2010 23
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