Wireless mics are an essential part of documentary filmmaking. The mics are small and easily hidden from view and the wires are minimal. Sometimes on reality or news shows you get a peek at the metal mound protruding out of the backs of people’s clothes. Those are lavaliers and they are handy. They are used often by wedding videographers since shotgun mics aren’t super awesome at picking up the vows from clear across a church. You don’t see them as often in movies because people have to actually wear them (and that is distracting) but for sit-down interviews, or most TV applications, they’re great. If you’re just starting to get into any of these fields, read up on frequency blocks – you’ll impress (if mildly) your sound tech.
What are Frequency Blocks?
There are a finite number of frequencies that are allowed to be used in different parts of the world. The frequencies are a range of hertz units that are divided into “blocks” and certain countries cover certain blocks and not others. For a quick refresher, hertz measurement is the number of waves that pass/vibrate per second. So different blocks cover different ranges of frequencies. For example, Block 22 is universally understood to cover 563200000Hz – 588700000Hz (or, more commonly denoted in MHz: 563.200 – 588.700).
Why Do Frequency Blocks Matter?
There are a lot of people out there trying to use wireless systems. There has to be a certain amount of organization and regulation to keep everything from interfering with each other all of the time. So a certain number of blocks have been deemed OK for lavalier microphones to operate over. In the United States, as of this writing, anything Block 26 to Block 31 is reserved for folks like cops and companies who offer wireless broadband. They get approximately 698MHz-806MHz all to themselves.
What About the Rest of Us?
So long as your wireless lavs aren’t operating at or between 698MHZ and 806MHz then you don’t need permission or a license from the FCC. Note, again, that I am only talking about U.S. regulations here. Also note that none of this applies to XLR mics or shotgun mics. You just plug-and-play with those and start recording. Lectrosonics has a nice chart showing the frequency ranges and what channels they belong to.
Why Bother with Lavs if They’re Regulated?
Odds are, your lav isn’t. Unless you have some really old wireless mics, your lav is probably legal for use. Also, while the law protects public service frequencies from interference, they don’t really return the favor. The FCC does not protect the rest of us from interference with our lavs from other devices. Good thing each block has many frequencies to operate at. If you are ever experiencing interference from another signal on the frequency you are using, change it. Note also that when working with multiple lavs you can keep them on different channels but you should use them in the same frequency range to prevent problems. Here’s a much better guide on working with multiple channels and ensuring proper operation.
How Do I Change My Lav Frequencies?
For the Sennheisers, typically you’ll find the setting to change transmission frequencies and channels under the Main Menu, then Advanced, then Tune. Find out more than you’ll ever need to know via their site. On the Lectrosonics, there is an access door you flip open with two switches inside – one for channel and one for operating frequency. More on that subject can also be found on their site.
If you’re the paranoid type, here’s a full list of no-no devices straight from the horse’s mouth. I reiterate that this is U.S. regulation only. If you’re the planning type, here’s a Frequency Finder for snagging an open channel. Lastly, if you’re filming an event and using lavs for sound (common at weddings), get friendly with the sound technician (just how friendly is up to you). Often they can tell you exactly how to set your lavs in conjunction with their own audio gear for interference-free sound.