You can Tuna Fish, but can you tune a sound system?

You can find this article and many others in my Kindle Book, Assorted Audio Essays.  Also available on Barnes and Noble and iBooks store.

People refer to this as tuning the room, but we will 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.

Guerilla tuning or “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.

We’ll start with the setup:

Start with a wired microphone on a stand center stage. It is important to use a wired microphone rather than using a wireless microphone.  It is not advised for this procedure as wireless microphones are not as linear (input = output) as a wired microphone.  The 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).  In this exercise we will be manipulating the output equalization of the console,  or an external equalize.  We are looking to manipulate the last EQ before the main speaker’s amplifiers.  Flatten any output equalization and bypass any compression.  The path must be a clear as possible. Now you are ready to carefully route the microphone to the speakers and quietly verify the signal is flowing where you expect it to be.

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 begins 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/.

Figure 1 Speaker pointed at the center of a room, in this case showing uneven coverage.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 5 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.

Figure 2 Speaker pointed at the back of the room, showing more consistent coverage.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 the speaker is all in the audience and not bouncing off of the walls.  Generally speaking, point speakers to 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.