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| Interference problem on a wireless mic system |
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| jmelson:
--- Quote from: Brumby on May 05, 2018, 04:12:04 am ---First thing, as has been mentioned, is that the ringing and whistling you are getting is more than likely audio feedback. --- End quote --- Simple test. Go to one of the bad spots, observe the whistling. Place your hand over the microphone. If the whistling stops, it is very likely to be audio. Many microphones are "noise canceling", or cardioid pattern, meaning they attenuate sounds from behind the mike, and only allow sounds from in front of the mike. There is no way a mike can tell whether a sound coming in the front is from a speaker or a person. So, you always want the speakers behind the mike. That is how the monitor speakers bands use work. They are placed on the floor, so they are roughly pointing at the exact back of the mikes. If a performer grabs a mike and walks around or turns away from the speaker, then the mike starts to pick it up. The only way to combat this when walking around the room is to have the mike VERY close to the mouth, so first, you can turn the mike gain down, and second, the person's head shields the mike from the speakers. Jon |
| Bassman59:
--- Quote from: jmelson on May 06, 2018, 03:28:39 am --- --- Quote from: Brumby on May 05, 2018, 04:12:04 am ---First thing, as has been mentioned, is that the ringing and whistling you are getting is more than likely audio feedback. --- End quote --- Simple test. Go to one of the bad spots, observe the whistling. Place your hand over the microphone. If the whistling stops, it is very likely to be audio. --- End quote --- Sometimes, placing your hand over the microphone exacerbates the feedback. --- Quote ---Many microphones are "noise canceling", or cardioid pattern, meaning they attenuate sounds from behind the mike, and only allow sounds from in front of the mike. --- End quote --- Cardioid (and super-cardioid and hyper-cardioid) mics are NOT "noise canceling." Like a directional antenna, the pattern has nulls, where the pickup of sound is attenuated. But it's not noise cancellation like those Bose headphones. |
| Bassman59:
--- Quote from: WaveyDipole on May 04, 2018, 09:57:25 pm ---Hi, I'm trying to help solve an interference problem someone is having with their 100v line PA system. It is in a small auditorium with a half a dozen ceiling mounted speakers in two rows and using two VHF wireless microphones. The length of the auditorium is well within the maximum range of the mics. The problem is there are several fixed spots in the auditorium where the microphone starts to ring and whistle and the mic radio signal drops significantly. The spots in question seem to be almost halfway between any pair of speakers which seems more than co-incidence. They have tried various locations for the receivers etc and nothing seems to have made any difference. --- End quote --- The problem is simply acoustic feedback. Rule #1 of microphone use: don't walk in front of the speakers with a live mic. You'll likely get feedback. Ceiling speakers are a problem if the person using the mic is walking under them. The problem is worse if the mic is an omni-directional lavaliere and it's worn on the lapel and the gain is cranked. A skilled mix person actively riding the mic channels is necessary. |
| frozenfrogz:
Is there any sort of acoustic treatment of the room / venue or just a bunch of speakers pointed towards the audience? If it is indeed acoustic feedback there probably are lots of things you can do to make the room much more enjoyable to listen to. Bass traps, diffusors, and absorbers can be built on a budget with materials from the hardware store (depending a little on how big the problems of the room are), together with proper speaker placement this works wonders :) |
| WaveyDipole:
Yes, the problem happens with just a single microphone on, although both receivers were turned on as well. I will repeat the test with one receiver turned off and also try the hand test later during the week. There is no accoustic treatment of the room that I am aware off. We have found today that careful orientation of the telescopic antenae and the receiver units seemed to help considerably. Following the manufacturers guidance to keep the telecopic whips at right angles to each other as well as placing the receiver at right angle to each other made a big difference although it did not completely eliminate the problem. I have also been advised by the sound technician that the peak light on the unit did not come on at any time, including when the problem did arise, so we do not appear to be peaking the gain. Of further note was that one of the telescopic antenae is broken and does not extend to full length, which propably does not help. There was some talk of replacing it. Is it worth trying alternative antenae, and if so, then what sort? I notice that many kits come with leads and separately mounted antenae. One other thing that might be of note is that the receiver has an A and B LED. We orignally thought that these indicate channels, but according to manual these are related to diversity and indicate which antenna is active at the time. The sound tecnicians have observed that the light usually switches from A to B just as the problem clears. From what I have seen, the light on the other receiver switches simmultaneously and they tend to stay opposite to each other, i.e. if receiver 1 is on A, receiver B will be on B and vice versa. I will do some more testing around that to confirm it. |
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