Patch bays are one of the oft overlooked items in an audio rig. Either you have them and you know all about them or you've never seen one and they're terrifying when you encounter one. Chances are if you have them you also simultaneously love and hate them but more on that later. So let's get in to why people have them and the different ways to use them.
There's two versions, hard wired and fully patchable. TT bays tend to be hard wired. They save space in the studio and have the added benefit of being more secure. If all the connections on the back side are soldered in place, there's a few less things to go wrong. For the rest of us though, bays with jacks on both sides make more sense because it's easier to change things around should the need arise.
Before we go any farther there's a concept called normalling that we need to cover. Your garden variety patch bay will have two rows of jacks on the front and the back. The top row on the back is connected directly to the top row on the front and the same goes for the bottom row. Normalling is an internal connection that allows the top row on the back to go directly through to the bottom row if nothing is plugged in on the front side.
Let me lay it out like this. If the top row is inputs coming in from the stage or the live room and the bottom row is going out to the inputs on your console or DAW you want to use full normalled bays. They let those snake lines on the back top row go right to the inputs on the back bottom. But then if you need to move snake input 7 to mic input 13, you just grab a short patch cable and make that connection on the front. If the bay was not normalled then you'd have to use a patch cable for every single connection.
Half normal bays are a little bit different. You can either buy a bay this way or some allow you to open them up and flip the cards around to half normal some or all of the channels. In this configuration you still get normalling through on the back side but when you plug into the front it doesn't interrupt the signal. You can use this function as a split to send signals to separate processing. You can also use it to monitor a signal. It can be an easy way to use a small analog mixer for zero latency monitoring in an otherwise all in-the-box studio. Just jack in to the channels you want to monitor, they still find their way into the DAW, but they can also be run to a small mixer to break out cue mixes. This is not done without penalty though. Ground loops are more easily created and there will be some slight signal loss.
With that said there's yet another way to send signal to multiple places. You can set up a series of channels as a "mult" by connecting them on the back side. Whatever you plug into one jack on a mult will be split out and show up on the rest of the jacks. These are typically set up to mimic a Y-split cable so you can use them as a one input, two output patch. Typically if there are any unused channels in a bay some of them will be set up as mults.
Now that we've got the basics down there are just a million things you can do with a bay. At the simplest you might set up a small home studio with just the snake and the DAW inputs but that type of routing is typically pretty easy to do right in the computer. So what about routing some of the hardware outputs to a bay so you can patch in outboard gear. That way when you bud loans you that sweet vintage mic pre you don't have to climb around behind everything. You can just patch it in to a vacant slot on the bay, jump from one of your outs to that slot and then jump it back in to an input as a return.
Patch bays aren't just for the studio though. Some installed systems have them. In my own setup at work I've got every line from the snake coming in, every input to the console, as well as every channel and group insert, the group and aux outputs and all the I/O from my outboard gear. When I need to strap a compressor across a channel or a group, all I have to do is bend down, insert a couple short patch cables and I'm done. No need to worry about what type or gender of cable, that was all sorted out at the time of installation.
I've also added a bay or two to a live rig to avoid having to buy a split snake. With a couple short studio snakes that are XLR on one end and TRS on the other, you can bring snake inputs into a half normalled bay. Then you can route one to a monitor desk and one to front of house. Or in a space where there's only a desk at FOH you can use a similar setup to feed an Aviom type monitoring system. When you've got twenty-four or more channels coming in and only eight or sixteen available on the musician's mixes you've got some work to do to shrink it down. With a patch bay though you can have the direct outs from the console, as well as the aux and group outs on there and patch them in as needed. Stereo drum group, check. Lead guitar, check. Key group, check. Easy as pie, no crawling behind gear and simple to swap as things change from week to week.
You do need to take some care though, live or in the studio. It's easily possible to make connections that will create an unholy mess. Feedback loops are the least of your worries though. With all the connectors being the same type and size, you could easily connect a channel input with phantom power on it to the output of a piece of gear that absolutely does not want to see forty-eight volts DC coming at it. Or fry a ribbon mic. Or fry the phantom on the console by shorting the cable against something that's grounded.
There's also the failure issue to consider. Up at the top of the post when I mentioned the love/hate relationship most people have with their bays, this is what I was referring to. A 1/4" connector can sit in a bay undisturbed for twenty years and work fine. Then one morning it'll just up and quit. Anyone who works with cables knows that they can be finicky beasts. So just keep in mind that everything you run into a bay has a potential point of failure at every jack and plug along the way.