Troubleshooting Mic Problems In Sound Reinforcement
by Bruce Bartlett, Crown International
Suppose you're listening to your PA mix during a worship service or concert. Something doesn't sound right. Maybe the choir mics are feeding back, an acoustic guitar sounds boomy, the person at the lectern sounds filtered, or you can't hear the piano. Let's assume that the mixer and speakers are working normally and are set up correctly. That leaves the instruments, microphones and their placement as the sources of the problem. How can you pinpoint what's wrong, and how can you fix it? This guide offers some tips on solving mic-related sound problems. Read down the list of "bad sound" descriptions until you find one matching what you hear. Then try the solutions (during a rehearsal) until your problem disappears. I wont mention EQ fixes because were focusing on mic issues.Before you start, check for faulty cables and connectors. Also check all control positions; rotate knobs and flip switches to clean the contacts; unplug and plug-in connectors; clean connectors with a cleaning fluid such as DeoxIT from www.caig.com.
FEEDBACK (the sound system squeals when you turn up a microphones volume)
- Place the mic closer to the sound source. For example, mike a choir about 18 inches from the front row and 18 inches over the head height of the back row.
- Ask the person being miked to get closer to their microphone or to talk more loudly.
- Use a noise-canceling (differential) mic, either hand-held or headworn (Figure 1).
Figure 1. An example of a noise-canceling headworn mic, the Crown CM-311A
- Use a directional mic (cardioid, supercardioid, or hypercardioid) instead of an omnidirectional mic.
- The more mics that are on at any one time, the poorer the gain-before-feedback. Mute unused mics. Use only two or three mics on a choir if possible.
- If the mic is a cardioid, aim its dead rear at the floor monitors unless its a choir mic (Check out the mics polar pattern in its data sheet.)
- If the mic is a supercardioid, its least-sensitive region is 125 degrees away from the front. Aim the null at the floor monitors (Figure 2) unless its a choir mic.
Figure 2. Aiming the null of a supercardioid polar pattern at a floor monitor.
- If the mic is a hypercardioid, its least-sensitive region is 110 degrees away from the front axis.
- Aim the null at the floor monitors unless its a choir mic.
- Consider using an automatic feedback suppressor
If the mic is a condenser type, be sure that phantom power is switched on at the mixer for that mics input channel.
- Replace the cable.
- Replace the mic.
- Check the snake channel for that mic.
- Be sure the mic and its cables are plugged in.
- If the mic has a switch, be sure it is turned on.
MUDDY (EXCESSIVE LEAKAGE)
If the sound is distant or muddy due to leakage between mics, try the following:
- Place the microphones closer to their sound sources.
- Spread the instruments farther apart to reduce the level of the leakage.
- Use directional microphones (such as cardioids).
- Pick up the electric instruments direct.
- Use clear Plexiglass panels around the drummer.
- Deaden the stage acoustics (add absorptive material).
- Filter out frequencies below the lowest note of each instrument
MUDDY (LACKS HIGHS)
Is the sound muddy because the high frequencies are weak? Does the mix have a dull or muffled sound? Try the following:
- Use microphones with better high-frequency response, or use condenser mics instead of dynamics.
- Change the mic placement. Put the mic in a spot where there are sufficient high frequencies. Keep the high-frequency sources (such as cymbals) on-axis to the microphones.Make sure the mic user is singing or talking into the front of the mic.
- Use small-diameter microphones, which generally have a flatter response off-axis.
- If a guitar or drum set sounds dull without amplification, consider asking the musician to replace guitar strings or drum heads.
MUDDY (LACKS CLARITY)
If your mix sounds unclear in general, try these steps:
- Consider using fewer instruments in the musical arrangement.
- Mute unused mics. The more mics that are on, the muddier the sound.
DULL KICK DRUM
The kick-drum sound you're getting might be flabby instead of punchy. These suggestions might help:
- Listen to the kick drum without amplification and make sure it sounds punchy. Maybe it needs a new head, a harder beater, or retuning.
- Remove the front head and damp the kick drum with a pillow or blanket.
- Mike the kick drum next to the center of the head near the beater.
- Use a wooden beater if it suits the music and the drummer allows it.
If you hear the gritty sound of distortion, try the following:
- If the mic preamp is clipping, increase the input attenuation (reduce the input gain), or plug in a pad between the microphone and mic input.
- If you still hear distortion, switch in the pad built into the microphone (if any).
- If the mic is distorting and has no built-in pad, move the mic farther from the source or try another mic. Dynamic mics have almost no distortion.
- Try a mic with a higher Maximum SPL spec.
BAD TONAL BALANCE
If the tone quality is unnatural -- boomy, colored, dull, or shrill, for example -- follow these steps:
- Change musical instruments; change guitar strings; change reeds, etc.
- Change mic placement. If the sound is too bassy with a directional microphone, you may be getting proximity effect. Mike farther away or roll off the excess bass.
- Use the 3:1 rule of mic placement to avoid phase cancellations. When you mix two or more mics to the same channel, the distance between mics should be at least three times the mic-to-source distance (Figure 3).
- Use only one mic on a lectern, not two. Two mics picking up the same sound source create a filtered effect due to phase interference between the mics.
- Try another microphone. If a cardioid mic's proximity effect is causing a bass boost, try an omnidirectional mic instead. Caution: omnis generally feed back easier than cardioids.
- If you must place a microphone near a hard reflective surface, try a boundary microphone on the surface to prevent phase cancellations.
- Maybe too much monitor sound is getting into the mics. Aim the mics as suggested under Feedback or turn down the monitors.
Figure 3. The 3-to-1 rule of microphone placement
If you hear hiss even though your system gain staging is correct, try these suggestions:
- Check for noisy guitar amps or keyboards. Turn up the guitar and turn down the guitar amp. Maybe replace bad tubes, or replace the amp.
- Switch out the pad built into the microphone (if any).
- Use a more sensitive microphone to overridethe noise in the mixer mic preamp.
- Use a quieter microphone (one with lower self-noise).
- Increase the sound pressure level at the microphone by miking closer.
- Mute unused mics. The more mics that are on, the more noise.
If the noise is a low-frequency rumble, follow these steps:
- Reduce air-conditioning noise or temporarily shut off the air conditioning.
- If the mic has a low-cut filter (highpass filter), switch it on.
- Use microphones with limited low-frequency response.
- Maybe the low tone is feedback. Turn down any excess lows on your mixer and follow the tips under Feedback.
NOISY (BREATH POPS)
Pops are explosive breath sounds in a vocalist's microphone from the letters P, B, or T. If you hear breath pops, try these solutions:
- Place a foam windscreen (pop filter) on the microphone. Allow a little airspace between the pop filter and the mic grille.
- Place the microphone above, below or to the side of the mouth.
- Place the microphone farther from the vocalist. Caution: This will reduce gain-before-feedback.
- Use a microphone with a built-in pop filter (ball grille).
- Use an omnidirectional microphone, because it is likely to pop less than a directional (cardioid) microphone. Caution: An omni mic is more susceptible to feedback.
- Switch in a highpass filter (low-cut filter) set around 80 Hz.
- If the mic has a low-cut filter, switch it on.
- If the cause is mechanical vibration traveling up the mic stand, put the mic in a shock-mount stand adapter. Or use a microphone with a built-in shock mount.
- Use a microphone with a limited low-frequency response.
- If the cause is piano-pedal thumps, work on the pedal mechanism. Maybe stuff a foam windscreen in there.
If you hear an unwanted tone at 50 Hz, 60 Hz, or their harmonics, that's hum. It might have an edgy sound like a buzz. Assuming the hum is only on one mic channel, try these suggestions:
- Be sure that the mic-cable shield is soldered to pin 1 of the connector on both ends of the cable. Repair or replace the cable.
- Separate mic cables from power cables.
- On direct boxes and mic splitters, flip the ground-lift switch to the position where you monitor the least hum.
- In the mic handle near the connector is a set screw. Sometimes this screw is used for grounding the mic handle. Make sure the screw is tightened fully clockwise.
- Check to see whether a cable going into a guitar amp is unplugged.
- To reduce electric-guitar hum, have the player keep their hands in contact with the strings, and rotate to find a spot where the hum stops. Turn off compressor stomp boxes.
- If the XLR connector shell in a mic cable is tied to pin 1 inside the connector, and the shell contacts a metal surface on stage, that can create a ground loop which causes hum. Open up the connector and remove the jumper (if any) between the connector-shell terminal and the pin-1 terminal.
Sibilance is an overemphasis of "s" and "sh" sounds. If you hear sibilance in a vocal signal, try these steps:
- Place the microphone toward one side of the vocalist, rather than directly in front.
- Change to a duller-sounding microphone.
The next time you hear something you don't like in your house mix, the tips in this guide should help you define the problem and find a solution. Good luck!