How to audition a studio vocal microphone
The worst way to do a "vocal mic shootout"
You've seen this photo on Instagram: six or 10 condenser mics crammed together as close as possible. It looks cool! But it won't tell you anything useful about how the mics sound.
If you have six nice vocal mics, by all means, cram them together and post the photo to social media. That's a great way to flex your mic locker, although any audio engineer in the audience will think "I sure hope they didn't sing into that!"
Condenser microphones are sensitive to positioning. A microphone that is directly in front of your mouth will sound different than one that is four inches to the side, or above or below. So even if you had six of the same model microphone, and you put them all in a cluster like the photo above, those six identical mics would sound different.
The best way to audition vocal mics in the studio
Step 1: Disable all effects.
You might have spent months cooking up a complex signal chain for your old vocal mic: EQ, compression, overdriven transformers in the mic pre, etc. Presumably that complex processing recipe (whether it uses analog outboard gear or digital plug-ins in your DAW) makes your old vocal mic sound great.
There's practically no chance the same recipe will complement the new mic. (And if it does, it means the new mic sounds just like the old one, and you should return it to get something different!)
Therefore the only fair way to compare microphones is to use a simple signal chain, ideally with a neutral (rather than a 'character') preamp.
If you don't like the new mic without a ton of processing, it's probably not the right mic for you anyway. So, audition it in its natural state, without all the seasoning and "sauce" that you might normally add.
Step 2: Test two mics at once -- no more!
Put your current favorite microphone on a stand, at the proper height for vocal recording. Put your first "contender" mic next to it. Position the two mic grilles as close together as possible without allowing them to touch. Sometimes it is helpful to mount one of the microphones upside-down, so that the shockmounts do not artificially separate the mics.
Plug the two mics into identical preamps. Adjust the gain so you're getting a roughly equal signal level into the DAW from both mics. (Some microphones will have higher output. You will normalize the track levels later anyway, but it is good practice to get the two mics relatively close now, using the preamps' gain controls.)
Why test only two? Because both mics can be equidistant from the singer's mouth. That means both mics are hearing the same performance -- which would not be true if you try to squeeze three or more microphones into that space.
Step 3: Record one verse and chorus.
There is no need to record the entire song; you would not have time to listen back to the whole thing anyway. Choose a short section of the song that is representative of the volume and energy level of the piece, and which the singer can perform reliably without strain.
Step 4: Match levels.
In the DAW, use a gain plug-in to set both tracks to the same RMS level. Then listen back to each track in the context of the mix.
If you do not match track levels, the louder track will always sound better.
The mic you prefer in isolation might not be the one you prefer in the context of the mix! Unless you are recording an acapella piece, you should choose the vocal mic based on playback with the music.
Step 5: Pick your favorite of the two mics.
Keep your pick from this test on its stand. Replace the other mic with the next contender.
Your "favorite" mic will change from day to day, from one song to the next, and of course from one singer to the next. That is why we preach Sonic Diversity, and why we manufacture affordable mics with different voicings.
Step 6: Repeat.
For each mic pair, choose your favorite, then replace the second mic with the next contender. Do this until you are out of time, or out of alternative microphones.
Whatever mic is still on the stand is apparently the best of the bunch for this particular session.
Why this process works
It is efficient. Imagine if you recorded the full song every time: the singer would burn out, and you'd need hours to track and more hours to listen back, second-guessing yourself at every step.
It gives real-world results. If you cram six microphones together and sing one time, you might pick your favorite, but that chosen microphone probably won't sound the same when it is the only microphone on the stand.
The process is not "all or nothing." It can be interrupted at any point, and you would still be reasonably sure you have the "best" mic already on the stand. For example, if you have six mics to compare, but you run out of time after the third take, your favorite from that pairing is the best of four. You do not have to finish the entire process to get actionable information.
Credit for the two-at-a-time technique goes to Ronan Murphy!