I was just about to purcahse a Sennheiser G3 wireless kit, when I noticed there's three different models available - each with a different frequency spectrum. Why is this this important, and should I be looking at any one in particular?
Sweetie is essentially correct. There are two reasons why they come with different set freqs:
1. Pretty much all countries/regions as far as I'm aware have laws which govern the division of the wireless radio spectrum. Bands restricted for use to say commercial air traffic, the military, taxi companies, commercial radio stations, etc. Of course, how each country/region divides the spectrum isn't exactly the same, so a wireless kit set to one particular frequency may be legal in one particular region/country while being illegal in another.
2. The band of legal freqs (for private radio broadcast) for a particular region can get pretty crowded depending on the locale. A major metroplois for example is likely to have many taxi and couriers companies operating in the same band of freqs which wireless mics are allowed to use. In this case, it's useful to be able to choose a radio freq (in the allowable band) further away from particularly crowded areas of the allowable band, to avoid break through.
The more expensive wireless kits have selectable freqs rather than being factory fixed to one freq and I believe some have such a wide range of selectable freqs they can be used pretty much anywhere in the world. If you always film in one particular location or within one region then a fixed freq system, obviously fixed to an appropriate freq, would be the cheapest practical option. In certain major cities or if you film in different countries/regions, then a switchable freq system might be the better choice.
The best advise regarding production sound always tends to come back to hiring an experienced local pro PSM. They will know not only the legal freqs for a particular region but from practical experience will know which freq is the most reliable.
One of the funniest experiences I had (many years ago) was editing the dialogue for a scene shot in London. Due to too much noise in the boom, I auditioned the wireless lav track which went something like:
Female character: "Peter please don't, you know what it does to me when you touch my neck like ..."
A second of interference and then: "Oscar four four, what are you riding, a donkey? What's your pick-up ETA?"
This actually gave me some great ideas, then I saw my career flash in front of my eyes and decided to use a different take!!
G