Look at the mic, speakers, and room as a system. No one thing is solely responsible. A little knowledege and common sense can probably help a lot.
There are a few things that contribute to monitor feedback. In a basement, I'm betting mic and speaker placement is your biggest problem.
1) Mic/speaker placement
2) Mic/speaker frequency curves
3) Amp/mic gain
4) Room factors
Feedback is one or more freqs coming out of a speaker, going into the mic, getting amplified by the speaker, going into the mic, getting amplified, lather, rinse, repeat, feedback! Looking at the mic, speaker, and room as a system rather than separate things will help you understand why things feedback, and help you get rid of it the best you can.
OK, look at the factors one by one. The basic ideas apply to all monitor systems. And remember, monitor systems are a compromise. Sometimes space or equipment limitations mean you need to work with what you have. Knowing the basics will let you improvise to make the best of any situation. Like if you don't have an eq you can use, you gotta get best placement, and that's what you get.
1) Mic/speaker placement
The basic idea is to get the speakers to shoot as close as you can into the null points of the mic. That is the place where the mic is least sensitive. For a 58, which is a cardiod mic, that is basically the back. Sounds coming in the back will be rejected due to the pattern of the mic.
Since you can't really have the speaker right in front of your face, put it on the floor in front of you, facing up at an angle, and try to picture a line going from the tweeter to the connector at the end of the mic. Just get it somewhere in there, don't worry too much.
If you place it off to the side, the speaker shoots at the mic somewhere other than the null point. You are going to get less gain before feedback.
With a cardioid, if you are using two monitors, don't angle them around you, even though it looks cool. Put them right in front, pushed up next each other so they point more into the null spot of the mic.
Way more gain.
Make sure the mic is horizontal. Tilting it down makes it more sensitive to the monitor. Tilting it up makes it more sensitve to reflections from low ceilings.
2) Mic/speaker freq. response.
As you raise the gain of a system, peaks are going to be the first things that feed back.
The 58 has a peak around 3K. In an otherwise perfect set-up, 3k is going to feedback first, as it has more energy than the other freqs.
If your speakers have a peak there too, 3k will feed back even faster. Sinve you can't do any thing about your voice, you have to manipulate the system.
Ideally you smooth out the monitor/mic combo, then adjust to taste.
A graphic EQ is the tool of choice for evening out monitor systems for most people. If you can, run one before the amp driving your monitor. I foyu are a home reccer, maybe you have a little parametric, or something. Anything is better than nothing. The idea is to be able to cut freqs that feedback.
It takes practice to find the balance between the eq on the channel and the graphic to get the most gain and best sound, but here is a basic method:
Get your placement set. Turn on your system. Without speaking into the mic tturn it up until you just hear feedback. Turn it down. Set the volume just below where it fed back. The graphic should be set flat first. Go through the sliders, raising each one slowly and bringing it back down to zero until you find the one or ones that you heard feed back. Cut each one twice as much as you boosted them to get feedback. Raise the gain until you hear more feedback, and repeat the process. Eventually you will get enough volume without feedback, or reach a point where you are cutting so much you might as well have just turned the whole thing down.
Turn it down. Talk into the mic, slowly turn up the volume. See where you are at, and adjust for tonality. You may need to cut another freq or two, as now your voice is part of the system. Talk and play with the graph if you have a problem,
If you are playing an instrument that you need in the monitors, now is the time to plug it in and turn it up, check for feedback and tone. You don't have to start from scratch, adjust the eq as you add in each thing that might feed back.
Hint: guitars feed back in the low end first. 80 hz and 150hz are the usual problems. Just roll off a lot of low end on the channel to start with. Same with vocals.
3) Amp/mic gain
Another balancing act. Generally people turn the monitor amp up all the way, then turn up the monitor send for each mic after they get the volume in the house. In a small room, if your monitor amp is cranked, you may not be able to control the volume of the send very well, as just cracking it open makes it feedback. Play around till you find a good compromise between signal level and volume.
4) Room factors
Low hard ceilings reflect sound from the monitors back down towards the floor and the mic. A wall right behind you reflects. Being in a corner is a bad case all around.
Play around, you can probly make it a lot better.
