That audio just isn’t right! (Troubleshooting, Part 1)24 July 2014
InfoComm International’s guide to the fundamentals of troubleshooting problem sound
Technology has moved the audio visual industry in leaps and bounds over the last few decades. We have gone from an analogue industry that would require multiple racks of equipment to a digital one that can now make adjustments and alterations wirelessly as our tablets communicate to the signal processer controlling an entire venue. Unfortunately, one of the things that has gotten lost in this transition, when it comes to audio, is the ability to listen and recognise where in the signal the problems are occurring.
With the evolution to digital signal processors (DSP), all of the audio processing elements are now located in a single device. This does make the installation, control, programming, and manipulation much easier for the systems, but can make the technician’s task of troubleshooting a system much more difficult.
Think of all the possible issues that could happen with an audio system: an audible hum, feedback, unintelligible audio, imbalance of multiple signals, distortion, or the audio professional’s worst fear – silence. Each of these instances has a cause with potentially multiple resolutions, the key is to understand which issue is present by listening before attempting to resolve it through the DSP programming of a system solution.
In this first of two parts, we look at hum, feedback and unintelligible audio.
An Audible Hum
In the case of an audible hum the cause will most commonly be a ground loop in one or more of the signals. The primary goal in this instance is to determine where in the signal flow the hum is taking place. Is it present when the output level is nominal and all inputs are down? Does it happen when just a single input level is raised? Isolating the location of the hum is the first stage in resolving it. After the location has been identified the common solution is to insert a balancing transformer between looped devices or life the ground at one of line level balanced cables. Make sure that you are not lifting the ground on microphone cabling. (Incidentally, Hum is also a tiny village in Bosnia, near the Croatian border, where the main picture was taken.)
Feedback might be the most common problem heard from clients. People without experience in audio may be familiar with the term and have a tendency to use it as the description of all audible problems, even if it has nothing to do with the actual issue. Audible feedback occurs because the audio being projected from the speakers is getting picked up by the microphone creating the feedback loop. In order to resolve this when the problem occurs there are three options. The first is to decrease the gain level of the microphone so the only audio that is picked up is closer to the microphone element. The second is to physically relocate either the microphone or the speakers in relation to one another. The third way to resolve feedback is to understand that it will be one particular frequency that is caught in the loop and, using a parametric or graphic EQ, turn that specific frequency down. This is very similar to decreasing the gain level, only at one specific area in the frequency spectrum.
Another consideration with feedback is in the selection of the equipment. Each microphone will have a specific pickup pattern determining the direction it will receive frequencies. An omni-directional pickup pattern will gather audio from all around the microphone element while a uni-directional pickup patter will focus towards one direction. The omni-directional microphones can be more susceptible to feedback because they are listening to everything while the uni-directional microphones will reject audio that isn’t in front of the element.
The ability to hear and comprehend the audio coming out of the PA system is the primary role of the audio professional. If that audio is unclear the message, music, or information cannot be received by the audience. Getting intelligible audio starts with proper gain structure in a system and understanding what each of your signal processing elements does to manipulate the audio signal.
Level: The level can have one or two gain adjustments. The trim or sensitivity allows for much larger increases or decreases in audio level, while the fader adjustment is more for refining and fine tuning the audio level of that specific audio channel.
EQ: The EQ stage provides the ability to increase or decrease specific frequencies of that independent signal. For example, if you are working with a voice and want to turn down the lower frequencies to emphasize speech intelligibility you would turn on the low cut filter or high pass filter. The most common types of EQs are graphic, which adjust certain frequencies with a fixed bandwidth, parametric, which adjust specific frequencies with a variable bandwidth, or shelving, which adjust all frequencies above or below the determined shelf frequency.
Compressor: The compressor is a useful tool when working with signals that have large variations in level. It gives the audio engineer the ability to set a certain threshold that dictates when audio level goes above that threshold it will be turned down. This allows for input levels to be set higher to accommodate for lower signals while not getting distortion when a louder signal is present.
Limiter: The limiter works very similarly to a compressor, but as opposed to reducing volume when the set threshold is crossed, it does not allow any level above that threshold to pass.
Gate: The audio gate works to help prevent unwanted signals from being heard. By setting a threshold for level, the audio engineer determines that unless the signal reaches that volume, the signal path will not be opened. This allows for the prevention of ambient noise in the area as well as things like comb filtering where multiple microphones might be picking up the same source.
Matrix: The matrix in an audio system allows the engineer to determine which audio signals will go to which outputs. This can be a handy tool for signal management because you probably will not want voices going to the subwoofer channel, or you can to ensure that your stereo signals are going to the proper left and right channels. Additionally, if you were to be managing multiple mixes for the PA, a recording device, or remote feed, this would be an ideal location to adjust the balance of the signal levels to each output independent of the others.
These are the basic audio elements that are commonly encountered. Understanding what each audio signal processing element is and how it operates will assist in determining where in the signal flow issue might be occurring.
(Continues in Part 2, coming soon)