One of the most common questions posted on the Home Recording Forums is "Should I buy a USB microphone?". As with so many things, the only honest answer is "Maybe, but it depends".
The attractions of USB mics can normally be summed up with two statements:
1. They're cheaper than buying a conventional mic, XLR cable and audio interface.
2. It's easier to use a "plug and play" device rather than setting up a complicated interface.
...but, are those two statements true?
Lets have a look at how things work.
All microphones are analogue devices. They convert the sound waves in the air to a very small electrical current. To use the output of a microphone on a computer, the signal has to go through two stages:
First, the extremely low voltage produced by the mic (it'll be measured in thousandths of a volt) has to be amplified to something more useful. Most electronic gear is set to work at what is known as Line Level which is up nearer to one volt and a mic signal has to be brought up to this level before it is fed into other gear. This amplification is handled by a microphone pre amp--pre amps have a big effect on your sound since they have to add a lot of gain without adding unwanted noise or distortion.
Second, to feed into your computer, this analogue signal has to be converted from a varying voltage into a string of digits: zeroes and ones, basically "on" and "off" signals.
With a conventional microphone, these two processes are normally handled in one box known as an audio interface or a sound card. The input from the mic first goes into a pre amp circuit that raises the levels then the signal is sampled and converted to digital. The output of the interface is then fed into your computer, generally via USB or Firewire but there are still some internal cards that can plug stright into the PCI bus on a desktop.
In a USB mic, the signal still has to go through these stages but they're all built into the body of the microphone. So, is this a good idea?
Well, there are some pros and cons to consider:
Cost: Yes, many USB mics are cheaper but they still need all the same electronic stages as a conventional mic--so the eventual quality is often compromised. There's no reason why they need to be bad quality--indeed, Neuman make an excellent mic with a digital output that costs more than $8000--but MOST manufacturers aim their USB mics at an entry-level market and don't put a USB output on their good stuff.
Ease of Use: It's not always as simple as you think to use a USB mic, especially when you get serious about recording. Some things to consider:
-Drivers: Drivers are the set of instructions used by a computer to use a piece of hardware. To keep them "simple", most USB mics just use the generic drivers in your Computer. These are MME drivers on a PC and Core Audio on a Mac--they're non-specialist drivers designed for things like Skype phone calls and playing back Youtube videos. They're not made for serious recording and tend to have more buffering (and therefore latency) than something more specialist. Most audio interfaces came with dedicated drivers using a system called ASIO, designed specifically to keep latency as low as possible.
-Monitoring: An audio interface is a two-way device. You plug in a microphone into the input to feed the signal into your computer--but the sound from the computer is also fed back into the interface to drive your monitor speakers or headphones. This normally lets you create a monitor mix of both the mic input and pre-recorded tracks from your computer in the interface hardware, avoiding all the issues of timing and latency. Most USB mics are one way devices only, with no provision for monitoring, forcing you to use whatever built in sound card you have to listen to your stuff. This can lead to problems, like:
-Signal routing: Depending on the mix of Operating System and DAW software you have, some combinations are difficult (or even impossible) to set up so you can monitor on one audio device (your inbuilt sound card) while recording on a different one. This can lead to situations where you can't hear what you're recording when tracking--or, even if you can, latency can make it almost impossible to track properly.
-Expandability: That's with a single mic but, as you get more into recording, it's very common to want to use more than one mic at a time. Maybe you'll want to sing while playing your acoustic guitar or maybe you'll want to try recording your guitar in stereo. Again, depending on your combination of OS and DAW software, it's often impossible to use two USB mics at once and, even if theoretically possible, it's frequently a very tricky set up. The "complicated" audio interface route is often much easier.
-Upgrades: As you get into recording, you rapidly realise that there's no "one size fits all" mic you want to use for everything. A longterm disease technically known as "Flubent" (but commonly referred to as Gear Acquisition Syndrome) frequently sets in. With a conventional interface, you can swap and change mics as often as you want (and can afford); with a USB mic, every new mic has to be another USB device until you relent and get an interface. It's probably cheaper and more flexible to go the interface/XLR route from day one.
-Cable Length: The maximum cable length for USB 2 is five metres. This might be enough--but for serious recording, the position of a mic in your room can make a big difference to the recorded sound. It's often a good idea to get as far as possible from computer fan and drive noises and in a spot with a nice acoustic. Five metres, particularly if you want to avoid trip hazards, can be limiting.
So, back to the first question: "Should I buy a USB mic?". Well, you should think long and hard about it. If you have very specific, simple needs where ultimate quality is not an issue, you KNOW you will only ever need one mic and you KNOW you won't want to expand/change in the future then MAYBE a USB mic could be a good solution for you. For example, I have a friend in the UK who does daily recordings of newspapers for use by the blind--I recommended a USB mic to him. It's a non-commercial use so quality just has to be okay and he has no interest into getting into complicated recording. He doesn't need to monitor music tracks and perform along with them--so, for all these reasons, a USB mic was fine for him.
However, if you're a musician with the normal needs for monitoring and likely desire to improve your recording techniques over the years, think long and hard before investing in a USB mic. A conventional XLR mic and interface is almost certainly a better long term investment.
And, if you DO still decide to go with a USB mic, look for the rare models that include things like monitoring--some of the Blue range for example. I'll leave it to others to suggest specific mics if they want since I don't actually own any USB mics.