The one rule: sing so you can hear the person next to you
Okay, so we've done the warm-up thing and we've found your section, roughly. Now I want to give you the one rule that actually matters more than any of that.
Sing so you can hear the person next to you.
That's it. That's the whole rule. If you can hear yourself over everybody else in your section, you're too loud. If you can't hear the person on either side of you, same problem. The goal is that your voice disappears into the group a little, and out of that disappearing, something bigger comes out the other side.
Here's the thing though. Most choirs get this exactly backwards. People think a good choir is a loud choir, or that singing well means being heard. So everybody pushes a little harder to be heard over everybody else pushing a little harder to be heard, and pretty soon the whole room is straining and nobody sounds good, including the people who were trying hardest. I've been in choirs like that. It's exhausting to sing in and it's not even pleasant to listen to. Mildly contrarian opinion, but I don't think I'm wrong here: blend matters more than volume. The person straining to be heard over the section is almost always the problem, not the solution.
What this actually sounds like
When it's working, you get this blended sound where individual voices sort of melt into each other. You can't pick out who's singing what. That's not a flaw, that's the whole point. A choir isn't twenty soloists standing in a row. It's one instrument built out of twenty people.
When you're doing it right, singing next to somebody should feel a little like you're matching them instead of competing with them. Same vowel shape. Same volume, roughly. If they back off on a phrase, you back off too. If they lean in, you lean in. You're listening at least as much as you're producing sound, probably more.
How to practice this at home, alone, which sounds impossible but isn't
You obviously can't practice blend by yourself in your kitchen. But you can practice the listening half of it, and that's most of the skill.
Put on a recording of a choir, any choir, doesn't matter the piece. Pick one voice part and try to track it through the whole recording without losing it in the texture. Then try to lose track of it on purpose, let your ear zoom out to the whole sound. Practice moving back and forth between those two modes, zoomed in on one part, zoomed out to the whole. That's the exact skill you need in rehearsal. You're doing that same zoom in and out constantly, hearing your own line clearly enough to be accurate, and hearing the room clearly enough to blend into it.
The other thing you can do at home is just get comfortable singing quieter than feels natural. A lot of people, especially if they grew up singing solo in church or in the shower, have one volume, which is "enough to fill the room." Practice singing a hymn or a favorite song at maybe sixty percent of your normal volume and see if you can still keep the pitch solid and the vowel clean. That's a different muscle than loud singing. It's worth building on purpose.
The why behind it, because there's always a why with me
The reason this matters so much has to do with air and humidity, actually, which is going to sound like a detour but stick with me.
I spent an entire Saturday a while back trying to make a decent baguette. Kept failing. Crust would set too fast, inside wouldn't finish proofing, whole thing came out dense and sad. Took me most of the day to figure out it was our air. It's so dry up here that the crust forms before the interior's caught up. You have to work around the environment you're actually in, not the one the recipe was written for.
Singing in a group is the same principle. You're not singing in the room you'd sing in alone. You're singing in a room full of other instruments, other sets of lungs, other pitches trying to land in the same chord as yours. If you sing like you're alone in your car, which is a great place to sing loud and I fully endorse it, you're going to work against the room instead of with it. The environment changes what the right technique is. Blend is just adjusting your technique to the actual room you're in instead of an imaginary solo one.
A real caution, nothing dramatic
If you're pushing volume to be heard over your neighbor, you're also probably straining your voice a little, and at this elevation with this dry air, that catches up with you faster than you'd expect. If your throat feels tight or scratchy by the end of rehearsal, that's usually a volume problem, not a stamina problem. Back off before it turns into a hoarse Tuesday.
Before next time
Try the recording exercise once this week, just five minutes with headphones on, tracking one part and then zooming out to the whole. You don't need to tell me you did it. You'll just notice it in the room next Tuesday.