Wireless microphones. They've changed the way performers perform. They might be one of the biggest game changers in modern music performances. They also might be the biggest source of grief for audio engineers and technicians in the field. There's lots of white papers about RF mics, interference, and in the last couple years, the white space issues, and the sale of the 700 MHz spectrum.
I'm not going to write about all of that, but I do want to give some practical knowledge and tips that can save you grief in the future when using wireless mics.
First, let's keep in mind that with most things, you get what you pay for. A cheaper RF mic is going to be more prone to issues in the field than a nicer pro level model. The big difference in price with RF mics is going to be the ability to have more active mics at one time without interference, and also the ability of the receiver to be selective to a narrower bandwidth of frequencies, to prevent interference troubles. Think about future needs and what environments you will be using the mic in.
Second, if you have mics that operate in the 700MHz range, get rid of them. Put them on Ebay with a disclaimer that you can't use them in the US, and sell them internationally. You can't be in that spectrum anymore, and you will start getting interference from all sorts of devices.
Ok, now to the big issue that goes unrecognized when using multiple mics at the same time. Using one mic or two doesn't usually present much problems. As soon as you introduce a 3rd mic to the system, you are prone to experience intermodulation interference. What is that you ask? In a nutshell, here it is:
When you have two transmitters and receivers within close proximity to each other (basically the same stage, venue, etc), they will interact and produce RF harmonics in the airspace. Several harmonics are introduced, but the odd harmonics (3rd, 5th, 7th order) can create interference strong enough to be picked up by wireless receivers. The 3rd order harmonic is the one to watch out for the most. Sometimes the 5th can cause issues, but not typically. So, how do you know what the harmonics are? The math for that is actually pretty simple. Take the frequency of Mic A, subtract that number from Mic B (assuming it's a higher freq), and then subract that difference from A, and add it to B, and you know the 3rd order intermod harmonics. Here's an example:
Mic A = 605MHz
Mic B = 615MHz
The difference is 10 MHz, so the intermod frequencies will be 595MHz, and 625MHz.
If you only have two mics, not a big deal, but remember, if there's a TV signal present in the space, at a strong enough amplitude, you will get intermod artifacts from it. If you add a 3rd mic to the system, you have to be careful not to put it on one of those frequencies. You also have to find out the intermod freqs of all the combinations of mics (A/B, A/C, B/C). That math just got a little more complicated. The more mics you add, the more intermod freqs you will have.
So many times I see people set up their mics on fequencies right in a row...501, 502, 503, 504...WHOA! big issues with that setup. Lots of issues will arise.
What's the solution?
Most brands now come with mics programmed with "banks" of frequencies that are pre-programmed to not interfere with mics running on the same bank. So bank "A" might have twelve channels on it, where you can use twelve mics of this brand, all on the same bank, on these pre-coordinated channels. This feature works pretty well for the most part, but you can't take into account TV interference, or other wireless systems. There is something more you can do:
Software. Let your computer do the math for you. The major RF mic manufacturers have free software available to download and it will do the math. You tell it what zip code you are in for TV station reference, you tell it what model mics you have and what frequency ranges they are, and it will give you groups of frequencies to choose from to set up your mics.
I'm partial to the Sennheiser 300 series mics and IEM systems. The big reason is at their moderate price point, they offer network ports on the receivers and transmitters. Plug them all into a network switch, plug in your laptop, and you have some powerful tools at your fingertips. The software will let you do a frequency scan, so you can see the interference that is in the frequency ranges you are using. Using that information, the software can automatically allocate what frequencies the microphones should operate on. You press OK on the assign button, and presto - all your receivers are automatically set to the right frequency. All you have to do is sync up your transmitters and IEM receivers. Very easy.
What if you have different brands? Well this gets a little more complicated. Most of the software has a place where you can tell it where known RF mics are being used. So I would coordinate Brand A using their software, and then tell Brand B software what frequencies you're using for Brand A. It's not going to be bullet proof, but it's a start. The other solution is a great piece of software from Professional Wireless, called the IAS software, which can take into account different brands of RF equipment, and take into account local TV issues in the area. It does cost a few hundred bucks, but think about how it can pay for itself if you're getting repeat business because your RF chops are solid.
Since I've learned of intermod, and the problems it creates, I've taken more time to plan RF mics on installs, and setting up RF mics and IEM's when I'm out with the band.
You can dig deeper into the science of RF mics and intermod, a Google search will yield many results.
I'd suggest getting the software that works with your mics, and playing around with it before your next event. Wireless mics will become less of an enigma, and will give you clean, quick results that will give you more time to tackle the other challenges in you day. Have fun, and good luck!