Before shopping for a baby monitor, it's important to know that there are two different types of audio baby monitors, analog and digital, and each has their own advantages and disadvantages. Which one you choose can depend on your budget, the size and location of your home, and what other kinds of electronics you use at home regularly.
Analog Baby Monitors
Analog baby monitors are usually inexpensive, and they're widely available, so you're likely to be able to find one that meets whatever unique baby monitor need your family has.
Analog baby monitors generally use the 49 mHZ or 900 mHz frequency, and it's a little like a radio channel in that you might get a lot of static and interference from other things nearby that are trying to use the same channel. That classic baby monitor hissing sound is typical from analog monitors, particularly the cheapest models.
There are plenty of electronic gadgets that share the standard analog frequencies, so analog baby monitors often come equipped with several channel options that you can change manually to find one that's clear, or the monitor may have an automatic channel scanner that tries to minimize electronic interference. Some analog baby monitors use the 2.4 GHz frequency, which may offer a little less interference.
A second problem with analog baby monitors comes if you live in close proximity to other people. When your baby monitor is competing for frequency space with cordless phones or other baby monitors, those signals are going to cross paths.
The result could be that someone else hears your baby crying in the middle of the night, or you get to hear the neighbors' awkward fights. If you're concerned about the security of the signal, be aware that it's possible, and even easy at times, for others to hear what's happening inside your home with an analog monitor.
Analog baby monitors, in general, have a weaker signal and can't be used for long distances like some digital baby monitors can. Most can handle walking around the average home, though, so unless your home is very large, this may not be a problem.
Digital Baby Monitors
Digital baby monitors tend to cost more than analog baby monitors, sometimes by quite a lot. Digital baby monitors still operate on a frequency, often 2.4 GHz, (though you won't often see frequency listed on the package, strangely enough), but the signal is encrypted so that others can't hear what's happening in your household when those signals cross paths. Although the connection is secure in that no one can eavesdrop intentionally or accidentally, that doesn't mean you won't have any trouble with interference. Many parents report that their digital baby monitors wreak havoc with wireless routers, though those reports have tapered off in recent years. Others have trouble with cordless phones causing noises on their digital monitors. Most video baby monitors today have digital audio.
DECT Digital Baby Monitors
The only way to have a secure connection plus a nearly interference-free guarantee is to purchase a DECT digital baby monitor.
These baby monitors use a newer, less common 1.9 GHz frequency that doesn't seem to be prone to interference. It's encrypted for security and generally stays totally quiet until baby makes a sound. Today's DECT digital baby monitors often have high-sensitivity microphones with ambient noise filters, so you can hear baby breathing but won't hear a fan or air conditioner running.
The final consideration with digital baby monitors is whether or not the pulsed signals are harmful. The type of signal coming from the digital baby monitor is slightly stronger, and different, than that of the analog baby monitor. This isn't a solid reason to stay away from digital baby monitors, though, due to the lack of evidence of harm plus the normal positioning of a monitor far enough from baby that it isn't an issue.
According to scientists speaking to Forbes in 2015, there's no plausible way for radio frequency (RF) waves to cause cancer "without re-writing the laws of physics and biology."
You should never place the baby monitor right next to your baby, anyway, regardless of the type of signal it emits, as it is not designed to be touched or chewed on by a little one. Also, all monitors with cords must be placed well out of baby's reach to avoid the strangulation hazard. Parents who are concerned about the effects of baby monitor radiation should also make sure no cell phones, cordless phones, or wifi devices are in their home.
Digital, Analog, or DECT?
Unless you live in a household that is totally free of other electronic devices, it's possible that you'll get some sort of interference with both digital and analog baby monitors. You may need to try several types to find the one that works best for you. If you're not up for the trial and error, the one sure thing in baby monitors is DECT digital technology, but be prepared to pay for it. Only a few DECT digital baby monitors are available now, and most start around $100.