ELIMINATE FEEDBACK

THE RIGHT WAY

You have heard it many times: Somebody using a microphone with a public address (PA) system moves to a bad spot in the room or moves too close to a speaker, and a loud squeal or howl comes from the PS speakers. It's feedback. But the usual methods used to eliminate feedback are not the best methods available.

The usual method of eliminating feedback places some kind of device after the PA mixer, in the lines from the mixer to the power amplifier. But this kind of feedback prevention penalizes sound sources (e.g. electronic keyboard) that cannot possibly feed back. Better ways to control feedback will be presented here.

Here are some of the various methods used to prevent feedback:

METHODAPPLICATION LOCATIONEFFECTIVENESSEXPLANATION
Many small speakers spread around the area After the power amplifierVery Good Spreads out the amplified sound
Sound baffle between speakers and mics 1 Room acousticsVery Good Keeps speaker sound out of mics
Use headphones instead of monitor speakers 5 MusiciansVery Good Keeps monitor sound out of the mics
Sound absorbing "clouds" 1 2 3 Room acousticsGood Reduces sound reflections
Sound absorbing drapes 1 3 Room acousticsGood Reduces sound reflections
Equalize passively to cut feedback frequency 4 MixerFair Removes peaky frequencies from room
Equalize the monitor speakers 4 Mixer monitor outputsFair Removes peaky frequencies from monitors
Use pink noise and equalization to make room flat Mixer outputFair Removes peaky frequencies from room
Use compression to cut feedback MixerPoor Automatically turns down feedback
Use active equalized compression to cut feedback MixerPoor Automatically equalizes out feedback
Use pink noise and equalization to peak out room Mixer outputVery Poor Maximizes sound level at expense of quality
 
METHODADVANTAGESDISADVANTAGES
Many small speakers spread around the area
  • Does not affect sound quality
  • More even distribution of sound
  • Higher cost
  • For portable systems, more setup time
Sound baffle between speakers and mics
  • Does not affect sound quality
  • Not portable - must be built for room
Use headphones instead of monitor speakers
  • Does not affect sound quality
  • Keeps monitor sound out of the mics
  • More expensive
Sound absorbing "clouds"
  • Does not affect sound quality
  • Can improve sound quality
  • Not portable - must be built for room
Sound absorbing drapes
  • Does not affect sound quality
  • Can improve sound quality
  • Not portable - must be built for room
  • Can muffle sound
Equalize passively to cut feedback frequency
  • Can improve sound quality
  • Often included in mixer
  • Can degrade sound quality
  • Can muffle sound
Equalize the monitor speakers
  • Can improve sound quality
  • Can remove need to process main audio
  • Can cost more if not in mixer
Use pink noise and equalization to make room flat
  • Improves sound quality
  • Time consuming
  • Might not cure feedback
Use active compression to cut feedback
  • Cheapest solution
  • Causes unexpected sound level changes
  • Can degrade sound quality
  • Can get into permanent compression 11
Use active equalized compression to cut feedback
  • Better for orator than for music
  • Causes unexpected sound level changes
  • Can degrade sound quality
  • Can get into permanent compression 11
Use pink noise and equalization to peak out room
  • Intended for orator, not for music
  • Degrades sound quality

Location of the Feedback Suppression Devices:

The location where the feedback-suppression equipment is placed in the audio signal path affects how well the sound system performs.

PROPERTYPROCESSING LOCATION
MIXER OUTPUTMIXER BUS INSERTS CHANNEL STRIP INSERTSACOUSTIC BLOCKING
Method Described The device is placed between the mixer output and the power amplifier 4 The devices are placed on mixer submaster buses that acoustic sources use 4 The devices are placed in channel strip inserts or are channel strip controls 4 The devices are placed between speakers and microphones 12
Diagram
 
Feedback
processing
devices
shown in
  cyan  
mixer output
mixer bus inserts
channel strip inserts
No electronic device.

Sound absorbing or reflecting devices are placed between the speakers and the mics or are placed to keep sound from reaching sound reflecting surfaces. 1

Using headphones instead of monitor speakers keeps the monitor signals out of the microphones and acoustic instruments.

Use to keep speaker sound from reaching acoustic instrument sounding boards. 12
SOURCE MIXER OUTPUTMIXER BUS INSERTS CHANNEL STRIP INSERTSACOUSTIC BLOCKING
P
A
S
S
I
V
E		 Actual feedback source 6
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • No effect on sound
  • No change in mix
Other acoustic sources 7
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • No effect on sound
  • No change in mix
  • No effect on sound
  • No change in mix
Non-acoustic sources 8
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • No effect on sound
  • No change in mix
  • No effect on sound
  • No change in mix
  • No effect on sound
  • No change in mix
Background promo/music player 9 10
  • Reduces sound
  • Re-equalizes sound
  • Unnecessary processing
  • No effect on sound
  • No effect on sound
  • No effect on sound
SOURCE MIXER OUTPUTMIXER BUS INSERTS CHANNEL STRIP INSERTSACOUSTIC BLOCKING
A
C
T
I
V
E		 Actual feedback source 6
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • Automatic level change
  • Reduces sound
  • Re-equalizes sound
  • Changes mix
  • Automatic level change
  • Reduces sound
  • Re-equalizes sound
  • Changes mix
  • Automatic level change
  • No such device 13
Other acoustic sources 7
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • Automatic level change
  • Reduces sound
  • Re-equalizes sound
  • Changes mix
  • Automatic level change
  • No effect on sound
  • Can change mix
Non-acoustic sources 8
  • Reduces sound
  • Re-equalizes sound
  • No change in mix
  • Automatic level change
  • No effect on sound
  • Can change mix
  • No effect on sound
  • Can change mix
Background promo/music player 9 10
  • Reduces sound
  • Re-equalizes sound
  • Automatic level change
  • No effect on sound
  • No effect on sound
PROPERTIES MIXER OUTPUTMIXER BUS INSERTS CHANNEL STRIP INSERTSACOUSTIC BLOCKING
Advantages
  • Only one processor required
  • Only one processor required
  • Affects only acoustic sources
  • Affects only one source
  • EQ often included in mixer 4
  • No effect on sound
Disadvantages
  • Affects all sound sources
  • Affects all acoustic sources
  • Active can change mix
  • Many processors required
  • Active can change mix
  • Devices often unsightly

Notes:

  1. 1 This often requires a lot of tweaking.
  2. 2 A cloud is a mass of sound absorbing or diffusing material hanging from the ceiling.
  3. 3 This is often not enough to solve the problem alone.
  4. 4 Equalization is often supplied in mixers, especially digital mixers.
  5. 5 In many cases, each musician has his own monitor mix.
  6. 6 This is the microphone or other acoustic device that is actually feeding the sound back into the sound system.
  7. 7 This is a microphone or other acoustic device (e.g. acoustic guitar with pickup) that is not actively contributing to the feedback.
  8. 8 This is a device that cannot possibly pick up sound from the room. It cannot contribute to feedback.
  9. 9 This is the music player that is used to provide background music between performances. It cannot contribute to feedback.
  10. 10 This sound source should never be allowed to be altered by any feedback removal technique.
  11. 11 If this happens, another method must be used to eliminate feedback.
  12. 12 In rare cases, am acoustic instrument sounding board causes feedback. This method is the only cure for this case.
  13. 13 It would be extremely difficult to make an active version of such a device.