oligoon Posted March 28, 2009 Share Posted March 28, 2009 Ok guys - I'm fairly new to this so go easy on me :D Just done a gig at the local school hall, which I've done a few times before. the acoustics are pretty nasty with lots of refelctions from the end wall. however this time I took longer with my setup and had a few more bells and whilstles. I've got equalisers for the monitor and house mixs and spent an hour with the A&H acoustic analyser software removing the feedback frequencies. however even at this stage I noticed really early onset of feedback, particularly on the house side, which the frequency notching helped - but did not stop. The speakers were positioned about 5 foot in front of the lines of the mics, angled slightly (~15 degrees) into the centre. mics were 3 off SM58, 1 off sennheiser 865, the SM58's seemed to be worse culprits. drums were miced (kick,snare & overhaeds) plus 2 SM57 on guiotar amps (Bass DI). main speakers beyma CP600 on top of PDN15 driven by chevin amps. monitors are pair of 150W anyhow had a miserable evening trying to control the feedback - one father (professional sound engineer) gave me a right bollicking - I asked him for some contructive advice - but he said sort it at sound check. 7 bands to do in 3 hours and they always arrive on stage with no leads, not tuned etc & why wasn't he doing the gig (always unpaid)? ended up virtually with nothing on the monitors to try to control it - but was still bad. one thing I did notice was that as soon as they stopped playing the feedback got much worse - but I guess this is the compressor releasing? anyway - feels better to have got that lot off me chest - but anyone got any suggestions as to why this was? Link to comment Share on other sites More sharing options...
cedd Posted March 28, 2009 Share Posted March 28, 2009 Ditch the compressor, always a good start. Make up gain is a nightmare for that kind of thing. One local venue I do is particularly bad and it's all down to mechanical feedback through the floor and mic stands. Take the mic out of the stand and hold it - no problems. Might be a cause? Particularly if it's low end feedback you're getting. As to a solution, shockmounts? Foam mats? Speaker mounting? Whilst an SM58 isn't the best for feedback rejection, at least it's consistently bad. I'd say that if it's your house mix giving you problems, it's probably something a little more involved than plain old feedback, especially with the distances you've quoted. Which channels were feeding back? Was it everything? You could potentially have built a feedback loop into your FOH setup without it ever being the mics feeding back. I had this when using a spec analyzer on my laptop fed from an aux, whilst also playing back music from it. Howl around was loud, painful and avoidable if I'd thought a little before lifting the fader. Constructive criticism during a gig is one thing, but if what you say is true then a "right bollicking" probably isn't the best form of help. Yes soundcheck is the place to sort it, but we've all been there and know there just isn't the time at school type gigs. Final thought;Your monitors may be a touch under powered, how hard were you pushing them? I always find (somebody will tell me the science behind it) that a smaller speaker being pushed hard is far more likely to feed back than a larger one not working hard at all. Link to comment Share on other sites More sharing options...
Shez Posted March 28, 2009 Share Posted March 28, 2009 anyway - feels better to have got that lot off me chest - but anyone got any suggestions as to why this was? one thing I did notice was that as soon as they stopped playing the feedback got much worse - but I guess this is the compressor releasing? Inappropriate use of compression will always push you closer to feedback. Winding them back a bit might have helped. Difficult to guess at other things without actually being there.I wouldn't get too hung up on spending ages trying to eq to what a simple analyser says - they have their uses but they don't tell you the whole story. In general, make cuts on the graphics but avoid boosting. What were the most commonly feeding back frequencies? 3k15 is often a favourite for SM58s IME. Link to comment Share on other sites More sharing options...
Dmills Posted March 28, 2009 Share Posted March 28, 2009 Actually, your work with the RTA might not have helped any either! The problem with simple minded spectrum analysers is that they cannot differentiate between a peak or dip due to a system response issue (IOW a frequency based problem) and a time based problem....It you try to eq a time based problem flat then it will generally both sound horrible and causes a much worse problem elsewhere in the room. You should be leaving the equalisers flat except for a few (3 or 4 occasionally 5 later) notches (And no boost) until you gain a lot more experience. I second loosing the comp, evil things for adding gain back in when you don't need it. I just wonder if something might have been picking up an induction loop? That can cause comedy feedback issues. Of course, some days almost everyone gets a gig where it all goes pear shaped. Regards, Dan. Link to comment Share on other sites More sharing options...
Johnno Posted March 28, 2009 Share Posted March 28, 2009 How big is your school hall that you have to mic a drum set???? I did two concerts last week in our acoustically reflective hall - 60ft plus stage x 50ft x 18ft - and wanted to kill the drummer. And the guitarists. And the music teacher on keyboard. All were too loud for the singers to be heard at max gain before feedback. Don't you just hate musicians? Link to comment Share on other sites More sharing options...
wilkee Posted March 29, 2009 Share Posted March 29, 2009 It may just have been a back line too loud sort of thing, impossible to tell without being there. I always use comps on vocals but sometimes you have to just back them off to get the extra couple of db's that can make all the difference. Tony Link to comment Share on other sites More sharing options...
Killyp Posted March 29, 2009 Share Posted March 29, 2009 How big is your school hall that you have to mic a drum set???? I did two concerts last week in our acoustically reflective hall - 60ft plus stage x 50ft x 18ft - and wanted to kill the drummer. And the guitarists. And the music teacher on keyboard. All were too loud for the singers to be heard at max gain before feedback. Don't you just hate musicians? Never let musicians control their own level, especially guitarists. If it's the same as any gigs I've done with my school though, the drummer will hit the cymbels too hard, tickle the drums, you won't be able to hear the bass player and the guitarists will push their amps up to 11. Link to comment Share on other sites More sharing options...
Rob_Beech Posted March 29, 2009 Share Posted March 29, 2009 Johnno is not necessarily suggesting leaving them with control. He's stating that the level required to fill a room of that size is achievable without micing the backline. Whether they are balanced well is not what he's trying to get across. The issue is the items ON THEIR OWN are too loud, so micing isn't going to help, unless you can rip their faces off with the monitors until they give up and turn down. Link to comment Share on other sites More sharing options...
oligoon Posted March 29, 2009 Author Share Posted March 29, 2009 thanks for the replies and support guys - makes me feel better anyway! in answer to the queries:- 1. The mics feeding back were any of the 3 SM58 and the 865. any would feedback and got worse the more you had on - so I tried to keep only the main vocal mic up and the rest muted or much lower. 2. The floor of the stage is polished hardwood over 12" of concrete so is pretty solid and plus the feedback was as bad when holding the mic. so I dont think its a vibration through the floor issue - but will check. 3. It is possible that I had set up some sort of loop in the mixer - but was playing incidetal music from an ipod not the PC and have checked setup again of mixer when I got home to make sure I had no other loops (also had taken out all reverb from the mix). 4. Monitors are 2 off 150W Celestion 12" monitors driven from Chevin A500 and 2 off Beyma 15" coax 350W driven by Chevin A350. The former might be a tad underpowered - but the latter were exceptionally powerful. But feedback was still an issue with monitors out. 5. Will take out compressor from loop next time - ta for tip. 6. Feedback frequencies were 125, 250 2k5 badly and 400 & 1k2 to a lesser extent. 7. I'm not sure I understand the issue with the time based or sound based issue with the analyser so obviously need to undertand this further - but thanks for pointer. 8. I only had frequencies notched on the graphics -no boost. 9. Induction loop - is this mic cables picking up from mains or speaker cables and/or transformers? Or something different - anyway its always a possibility as the cabling always ends up a bit all over the place - although I never run power/speaker cables & signal cables next to one another - but inevitably they have to cross. Or is this the thing for deaf people? I don't think there is one installed at the school but will check. 10. Hall sound very similar dimensions maybe 80ft plus stage by 40ft by 22 ft - reason I miced the drums was to get a bit more definition to the kick, snare and overheads - to be honest that worked well - until the last drummer who took animal from the muppets as his inspiration! but then the guitarists (one of whom was using a 4x12") also went up to 11 - it was as much as I could do to get the vocals audible let alone ledgible! I do agree that it would be great to not let the musicians control their own levels - but how is that possible? Even if you DI everything then then can still play with the controls on the instruments. and plus the guitars wouldn't sound right. it would be great to have someone on stage who really knew the amps and was strong enough to not let the muso's bugger around with them - sadly I only had a year 7 (11year old) on stage, coz no one else would help :) One final thought occurred to me - it is easily possible that the installed PA which has radio mics was all left on after an assembly on Thursday morning. I'm not sure how this system works but it could easily account for horrible results. Will check whether it was on when I go back to school to get rest of gear on Monday early. Of course I should have checked this before the gig - but with all the other stuff I had to deal with I forgot about it until 3am Friday night unable to sleep after having done a crap job! Anyway thanks again for the help. Link to comment Share on other sites More sharing options...
AndyL Posted March 29, 2009 Share Posted March 29, 2009 9. Induction loop - is this mic cables picking up from mains or speaker cables and/or transformers? Or something different - anyway its always a possibility as the cabling always ends up a bit all over the place - although I never run power/speaker cables & signal cables next to one another - but inevitably they have to cross. Or is this the thing for deaf people? I don't think there is one installed at the school but will check. Induction loops are used as a means of transmitting sound to the hard of hearing. They can be picked up using the T switch position on a hearing aid or with a dedicated induction loop receiver. Essentially an induction loop is just a loop of wire, usually layed in or on the floor of the room, directly driven by an audio power amplifier - there are more sophisticated layouts, but essentially that's how they all work - so that the audio current flowing in the wire creates a strong audio induction field in the room. Sometimes the amplifiers used can be quite high powered, and as they can essentially be driving not much more than a short circuit (there should be a series resistor if the loop resistance is low) the current and hence the field strength can be quite intense. Quite often the feed to the loop amplifier is just an ambient microphone somewhere in the room. The field from the induction loop can often get into any audio or equipment in the room by various means including dynamic microphones and mic cables, any unbalanced wiring, which can lead to unexplained feedback problems amongst other things. And of course on pfl it just sound like more room ambience.... I have also once encountered a problem with a relatively high powered amplifier driving into wiring that should have had speakers on the end, but which due to 'punter intervention' was actually driving lots of short circuit speaker cable instead inadvertently creating a really intense induction loop. The restaurant staff swore blind that everything was working ok even though about 75% of the speakers in the place weren't working. Of course they had just been turning the volume up to compensate.... Link to comment Share on other sites More sharing options...
oligoon Posted March 30, 2009 Author Share Posted March 30, 2009 andy, thanks for info about induction loops - I will check all my cables, speakers etc to ensure there are no shorts! On the topic of the installed PA - one of the radio mics had indeed been left on and was picking up through the installed PA system - which would account for the horrid feedback. bloody annoyed with myself that I didn't sort it out as I can remember noticing some pick up on the input meters of the installed PA rack, but had not thought anymore of it - doh. Ah well - live and learn I guess. Link to comment Share on other sites More sharing options...
Matt Riley Posted March 30, 2009 Share Posted March 30, 2009 I think installed PA is probably your killer. I had something similar with a speech only system in a church, which we had to turn off during the music bit. The other thing is placement, placement, placement. Sometimes it is stuff as simple as moving the mic so it is horizontal on the stand instead of at 45 degrees to the floor. Also experiment with monitor placement. M Link to comment Share on other sites More sharing options...
Killyp Posted March 30, 2009 Share Posted March 30, 2009 Not just speaker placement, but the speaker's dispersion itself is something to consider. Were the speakers horn-loaded, or just simply a pair of drivers screwed to the baffle of the box? If the latter, then you'll find they offer next to no rear-rejection and hence will give you far less gain before feedback... Link to comment Share on other sites More sharing options...
AndyL Posted March 30, 2009 Share Posted March 30, 2009 andy, thanks for info about induction loops - I will check all my cables, speakers etc to ensure there are no shorts! Ah, I didn't explain that very well, sorry. It wasn't your cables I was suggesting needed checking. The restaurant with the short-circuit-wiring story was supposed to explain how what should have been a permanent "PA" installation in a basement bar/restaurant had inadvertantly become an induction loop instead. The only load on some of the amplifiers was the cables which should have fed speakers, but instead, where customers had decided they would prefer things a bit quieter, the ends of the wires had been twisted together behind the speakers, so some of the amplifiers were driving short circuits down long lengths of wire. The level of interference (crosstalk) it was causing to the sound system in the venue upstairs was quite remarkable. Trying to persuade the restaurant management that there was any reason why they should let the people 'from upstairs' look at their sound system - which according to them was working just fine - was quite a challenge... Anyway, glad you have found the source of the trouble. Link to comment Share on other sites More sharing options...
Chris Adam Posted March 30, 2009 Share Posted March 30, 2009 One thing I'd suggest is, don't dwell on it, but learn from it. You've been given a lot of good advice here, which is really what the"professional sound engineer" father should have done, instead of being a knobber. Make sure you give yourself plenty of time before the gig next time, to check all of the little things that you remembered in hindsight. Or, if possible, get a day to yourself to set it all up and get your head around it. That way, if you have any more questions, you can ask them beforehand.Thing is, we've all been there - standing behind the desk thinking "where the f**k is that noise coming from", so I'm pretty sure you'll be doing all you can to not go through that again. Link to comment Share on other sites More sharing options...
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