As rock music has increasingly been fused with other genres, sometimes stages will be devoted to a specific genre, such as folk rock or reggae. Some festivals such as SXSW hold smaller concurrent concerts throughout a city. Advances in sound reinforcement systems beginning in the 1960s enabled larger and larger rock festival audiences to hear the performers' music with much better clarity and volume. The best example was the pioneering work of Bill Hanley, known as the "father of festival sound", who provided the sound systems for numerous rock festivals including Woodstock. Other examples included the Wall of Sound invented in the 1970s to allow the Grateful Dead to play to larger audiences, and mobile sound systems such as those maintained by the Merry Pranksters, which would later be important in the development of events such as reggae festivals and raves. Camping and crowd control  Many festivals offer camping, either because lodging in the area is insufficient to support the crowd, or to allow easy multi-day access to the festival's features. Festival planning and logistics are frequently a focus of the media, and while some festivals such as the historic Sky River Rock Festival were seen as logistical successes, other festivals such as the heavily commercialized Woodstock 1999 were crowd control disasters, with insufficient water and other resources provided to audiences. Many early rock festivals successfully relied on volunteers for crowd control, for example individuals like Wavy Gravy and biker groups such as the Hells Angels and Grim Reapers Motorcycle Club. Gravy in particular called his security group the "Please Force," a reference to their non-intrusive tactics at keeping order, e.g., "Please don't do that, please do this instead". When asked by the press — who were the first to inform him that he and the rest of his commune were handling security — what kind of tools he intended to use to maintain order at Woodstock in 1969, his response was "Cream pies and seltzer bottles." Other rock festivals hire private security or local police departments for crowd control, with varying degrees of success. A recent innovation is the traveling rock festival where many musical acts perform at multiple locations during a tour. Successful festivals are often held in subsequent years. The following is an incomplete list. The following is a list of some notable rock festivals that take place on a regular basis. Most are held at the same location on an annual basis. Some, like Farm Aid are held at different venues with each incarnation. For a full list of festivals, see the external links at the bottom of the page.