I'm talking about best position to put PA speaker while me (as a band that consist of lead vocal, 1 backing vocal, an acoustic-guitar (plugged), a bass, and an acoustic-small-drum.) play in a small room (like cafes, or small restaurant), where I can't get any feedback from the speaker.
Should I put it on the side of the stage, or behind me?
The standard for big gigs is for the PA speakers to be in front of the performers, pointing at the audience. So as not to block the view, they're typically in front and to the side, or above the stage and pointing downward.
The point of this is that the audience gets loud music, while the musicians are in a quieter area, where they can hear the music more clearly (loudness really screws up your perception of pitch and even timing) and microphone feedback is easier to control.
If necessary, performers can have quieter monitor speakers. See: In live performance, what is a "monitor"
For smaller gigs, things might be different. You might all be playing quietly enough that feedback isn't a problem even if the PA speaker is behind you. You might be able to hear enough leaking through the back of the PA speaker to use that as a monitor. You might want to play so loud that you can't contain the PA sound -- since it'll bounce back to you off the walls wherever you put the speaker.
Your other defence against mic feedback is to use a directional vocal mic held very close to the lips.
The most important part is that no microphones are directly pointing towards any of the speakers. This means that the monitors should be in front of the band, facing towards them, and the audience speakers should be in front of the band (at the sides, so that they're not in the audience's view) and naturally facing away from the band.
The monitors' positions are not that critical when you're using pointed microphones. For instance, the drummer's monitors are often places beside and behind the drummer as well, which works as long as none of the drum microphones are pointing directly to the monitors.
At one of the venues I play, the room is long and thin, with the stage area in the middle of a long side.Not ideal,but there it is. The p.a. speakers are unusually placed at 2 metres height, level with each side of the stage, pointing towards it.Yes, it sounds a weird set up, but it works. The feedack is minimal, as the mics are not facing the speakers, which act as foldback as well as p.a.As they are flown, there are no stands to get knocked over, but often they could be safe in an ad hoc situation that could copy this.
Otherwise, as others have said, lacking foldback, which is often used unnecessarily, put the p.a. speakers at the side of the stage area, level with the frontmost mics, so that sound can be heard on stage. I try to get the cabinets up as high as possible, in order to send the sound over the audience's head, also it puts them out of line with mics.
If you have an eq, turn the 100 and the 250 to -12. It works for me and it sounds great too.
The first duty of a PA system is to enable the performance by letting the musician hear himself. Then you need to consider what sort of gig it is. Do the audience want it loud and 'in their face' or do you? Strategic positioning of even a single speaker can achieve either. If you have two, even better. Forget stereo, run 'one for me, one for them'. In some situations you can forget about the 'one for them'. I was at a job recently where my friend was providing background music on keyboard/vocals. The front row had his speakers in their face, he complained he couldn't hear himself. Everyone would have been happier had the speakers been pointing just at him. Feedback isn't necessarily a problem with a good microphone and moderate volumes. Don't do anything silly like using compression on the vocals though!
Positioning is not the only thing that'll help you out here. I've set up stages with speakers behind the performers, facing out into the room, it can be done.
There's a lot of factors which can cause feedback, including the resonant frequency in the room, quality of your equipment, etc. So for any answer to this question you'll have to experiment a little in the venue to find the right solution for you?
Have you tried playing more quietly? I know it's a radical suggestion, but it really can help. This solution may require ego-checking on the part of the band.
You say you're in a small room? How small? Do you really need PA in a cafe? It's worth thinking about.
Always try and locate your feedback before doing anything with the desk, usually you can do this by taking down a channel when feedback is sounding. If the feedback stops, you've found it's source. There may be more than one sound source (e.g. a mic and a guitar) which is feeding back.
There's quite a lot you can do with EQ to mitigate feedback, which is why big, expensive desks have big, complex equalizers. If you can find where the feedback is, try cutting a particular band to see if this stops the feedback. Usually feedback comes in at a single frequency, if you can cut that frequency, you might get more head-room without feedback. Remember, with EQ, a cut always sounds sweeter than a boost.
If you have a graphic equalizer on the master (So, not on a specific channel, going out to the mixer. You could try 'ringing out'. Set the stage up, but have no performers on it. Put every input up, then take up the master until you are close to feedback point. Put up one band on the graphic EQ, if it starts triggering the feedback, put that band lower than the others. Do not let the feedback get very loud or go on for long. Take the master level down if you need to.
You mentioned an electro-acoustic guitar (an acoustic guitar, plugged in), these are often the source of feedback. You might want to consider a 'feedback stopper' or 'soundhole plug' (they have many names) such as this one. http://www.gear4music.com/Guitar-and-Bass/Planet-Waves-Screeching-Halt-Soundhole-Plug/KJN
You also want to look into any effects you're using.
Sorry for the long post, but feedback really isn't just about positioning. Any sound tech will tell you they spend a lot of time fixing it, in a wide variety of ways.
I highly recommend using a digital Mixer for any gig - Behringer has some great stable and reliable gear with the eq you need just for this issue - namely the feedback and room tuning concern.
When a singer asks for more me - pull up the eq on thier channel and overlay the RTA and look for thier "dominant" frequency - and as long as it isn't the frequency you reduced for anti-feedback purposes then give them a little mid to high mid in thier monitor.
Then go to the graphical eq for the mains and pull down that specific frequency just a little.
I have also found that if there are guitars and other instruments in the monitor of the vocalist in question try and eq around the vocals - if you boost slightly for more me then pull those frequencies out of the guitar and keys - your mains will probably sound better as a result as well with that little trick.
I am a firm believer that a band should try and rehearse the show in a location that simulates the club environment as much as possible - even go as far as setting up and rehearsing in a club you play at on a slow night where you can stop and eq various channels on the digital mixer to optimize the sound of the band. Once the settings are in place the digital Mixer will remember them and sound check will go much smoother next time.
