> Dynamic mics generally reject room noise better than condenser mics …
This is absolutely a misconception, and it’s what I was talking about in the last sentence.
Think about it this way—how would a microphone distinguish between room noise and other types of noise? The answer is simple—it can’t, but microphones do have polar patterns which allow them to reject noise based on the direction of the noise, and you can combine that with good microphone placement to achieve good rejection of unwanted noise.
(I encourage people with both types of microphones to do an experiment. You can set up the mics identically by swapping them out and adjusting the gain so that the signal level is the same for both. You can use something simple like a pink noise file playing on your phone, 10cm away from the mic, as a reference sound source to adjust levels. It turns out that different mics with the same polar pattern mostly record the same background noise, at the same volume! Keep in mind that there are lots of minor variations in polar patterns that will throw this off.)
This misconception is a stubborn myth. Whether your room is treated has no bearing on whether you want to use a condenser or dynamic mic. These days, improvements in mic technology have really narrowed the gap between both mic types.
When we talk about noise floor, we can talk about it in terms of SPL equivalent, and you’re right, dynamic mics typically have a lower noise floor. But the noise floor on a typical microphone is going to be very, very low—something like 5-20 dB SPL equivalent. This means that the noise you get on recordings will be dominated by other factors—mostly, the acoustic noise in the room, and the noise from the microphone preamplifier you use. My (very quiet) room is about 32 dB SPL, which is much higher than mic noise. Microphone preamp noise will be much higher for dynamic microphones, because you typically need more gain to get usable levels, and the noise in the first stage of the preamp is amplified. For this reason, you’ll typically see higher amounts of noise when using a dynamic microphone, even though the microphone itself is producing less noise. This is why people complain about the SM7b—they’re really complaining because they bought an SM7b and found out that their microphone preamp has too much noise and not enough gain to use the SM7b effectively.
If you do have problems with room noise, switching microphones is not an effective way to help out (assuming you already have a decent cardioid microphone). What does help is to place the microphone close to your sound source (but not so close you get exaggerated bass), and if the noise is coming from a specific direction, point the microphone’s null (the opposite end of the mic) towards the noise source to cancel it out. You are equally trying to make the signal louder and the noise quieter when you place a mic.
(If this comment sounds rehearsed, it’s because I feel like I’m on a mission to bust this particular misconception.)