Cal Perkins, the audio design guru, once said in a 1986 Sound and Video Contractor article To Hum or Not To Hum; "In system interfacing, what works in theory usually differs from what works in real world applications. .... Rarely will theoretically correct approaches and cookbook methods yield the results desired, because each group of products has its unique set of interfacing problems by design and by implementation. Products A and B may test perfectly, when evaluated on a stand-alone basis; yet when connected to each other, they may hum, buzz, hiss or oscillate." Nowhere is the "real world" more real than in the transportable sound systems used in the live music industry where every day can bring a new AC power and ground source, different equipment, and a different set of interconnections and interfaces.
Sound engineers who work for touring recording or performance artists using "in-house" or rental/hire PA systems encounter every imaginable nightmare of someone else's idea of a ground scheme or no ground scheme at all in both systems and equipment. Engineers working for professional sound and equipment rental/hire companies are constantly building and rebuilding custom transportable audio systems to suit the needs of the marketplace and must navigate proper grounding daily. Installers of permanent systems are also put through this school of trial and error as they must also deal with the wide variety of equipment specified by people who have rarely (if ever) actually used the equipment professionally as the operating engineer and whose interest in the equipment is often primarily in the size of the profit margin. These are the people who suffer most and are most aware of the inconsistencies in the internal grounding designs of manufacturers, contractors and rental companies and of the difficult nature in maintaining a consistent system-wide and cohesive ground scheme. It is almost universal truth that we, as professional sound engineers cannot simply hook up a large PA system with all the AC grounds and all the shields intact and not have it buzz. This is an extremely frustrating situation for those of us who earn a living in audio and especially so for the novice engineer who did not expect this level of technological folly given the "high tech" nature of the equipment we use.
Why does this happen? When you consider that the audio manufacturing community at large has yet to settle into a consistent predictable "Pin 2 or Pin 3 hot" implementation, I wouldn't be holding my breath waiting for internal equipment grounding standards although there are indications that the situation is improving. In the wake of the June 1995 AES Convention a more encompassing understanding of grounding was set into motion that hopefully the manufacturers of audio equipment will use to make our jobs as live audio engineers a bit less complicated. Until these grounding conventions are fully realized and instituted, we will be left to deal with decades of equipment and systems with unconventional if not nightmarish grounding systems and practices.
In scanning audio magazines and sound engineering message threads online, I notice that proper grounding and troubleshooting are some of the most common subjects with many people seeking help and understanding. The biggest problems encountered by live engineers are not so much about the finer aspects of shielding with respect to RFI and EMI induction as they are about the "nuts and bolts" of getting rid of and avoiding the loud obnoxious buzzes and hums that plague the "hit and run" live music scene where every day poses a different set of interfacing problems. Even after reading technical articles on grounding, many engineers still ask, "That's great, now how do a fix this buzz?!" which reflects a general lack of available practical instruction on "hands-on" troubleshooting. Therefore, as well as grounding, I will address general hum and buzz troubleshooting which very often is NOT a result of "ground loops" but from improper connections, wiring errors and other technical mishaps.
This work is intended primarily for consumption by sound engineers working in the live audio industry. I work as a live sound engineer and use audio jargon that may or may not be understood outside of that industry. If anyone has any questions about terminology used in this work please feel free to contact me and I will either respond to clarify, or edit the item(s) accordingly. This work has been written as a web document and will be updated as needed whenever inconsistencies or errors are detected. I would highly recommend a purchase from the Audio Engineering Society web site, a copy of the "Special Excerpt on Shields & Grounds" Volume 43 Number 6, June 1995. I consider reading this collection of articles an essential component in your understanding of the technical aspect of your job as a professional live sound engineer. Also please consider it essential for a complete understanding to read the linked works referenced in main body (also compiled in the Bibliography) as they are very valuable sources of information more technical in depth than is covered within the scope of this work.
Over the years of association with the pro audio industry I have solicited, and have been subjected to, many opinions and theories on how to properly ground audio systems and the nature of “ground loops” which were often presented as oversimplified one-size fits-all representations of the problem. Many audio industry professionals I personally consulted on the subject were unable to articulate what a “ground loop” is in anything other than the most simplistic of explanations. Many of these explanations were offered with “hand wave” generalities over the specifics leaving me less enlightened. After more than 20 years of experience on the road, in the studio, on installations, and after countless discussions with industry professionals, I have acquired a practical and working understanding of audio systems grounding and offer this work as an aid to my fellow sound engineers both novice and professionals in their understanding of transportable audio systems’ grounding practices. People have been bugging me to publish this for years but it was never polished to my satisfaction and still isn't - but here it is.
Steve Guest
Special thanks to Karen Guest, Tom DiPaula, Rick Semerjian, Bob Goldstein, Paul Bauman, Mike Prowda, and Isahi Ratz for their suggestions, contributions and encouragement.