Can someone explain this feedback problem?

[ktedrow]

We had bad feedback problems last night playing outdoors on an open-air patio. Feedback was primarily through the mains (stage monitors off). Nothing much in front of us for reflections. We did have a wooden wall behind us and a glass wall to the left of us (we were tucked in a building cutout at the corner of the patio). There was a roof above us extending to approx 8 feet past the mains. The left main speaker (and sub) was very close to the left glass wall. Our mains are 12" yamaha DXR. We also have a 15" sub as base for one of the mains. We route a mono signal to the mains through the sub's crossover.

The worst feedback mic was the drummer's vocal mic who was furthest from the mains, in front of the back wall.

We aren't a loud band. Our volume allows customers to converse comfortably.

So how were we getting feedback from the mains with nothing in front to reflect off of?

To be honest, we've been having feedback problems at most of our venues. I just thought that this one, being outdoors, would be feedback-free.

Any ideas?

 

[lat]

Glass can be pretty resonant. Did the glass wall go all the way back to the drummer?

 

[ktedrow]

Glass can be pretty resonant. Did the glass wall go all the way back to the drummer?

Yes. About 10 ft to his left.

Just so I understand, you're saying the glass is resonant, as in the glass surface is vibrating, as opposed to reflecting.

 

[lat]

Well, both I guess... If the stage left speaker was close to the glass, it might resonate enough to add a feedback hum, but glass is also very sonically reflective. The overhang could've been a problem too. Also, the overall proximity of the mains to the band might have had some effect. Just throwing out some possibilities... Approximately what frequency was the feedback at?

 

[ktedrow]

Resonating frequency was at a pitch of F#3, 185Hz.

Doing some googling on speakers I'm finding that my mental picture of sound radiating only from the front of the speaker is not a good model. That lower frequency radiates in all directions, even for a closed back speaker. I'm having trouble finding info detailing what frequency you start to see back radiation.

 

[two fingers]

Try turning the mains up, and individual channel gains/volumes down.

 

[ktedrow]

The speakers were relatively close to the band, maybe 4 ft forward and 4ft to the side of the frontmost mics.

 

[lat]

Yeah, that could've been the glass at that frequency. But, you've been having ongoing issues, so there may be another underlying cause.

 

[ktedrow]

Try turning the mains up, and individual channel gains/volumes down.

Interesting. I would expect overall loop gain to be the only thing that matters for feedback. I do have the mic preamp gains up pretty high, to where I see clip light often, although I never hear any clipping in the vocals. In fact I felt the vocal quality was better with the higher gain, which is why I set the gains high. Maybe this is something I should look into.

Mixer is soundcraft ui16.

 

[ktedrow]

Yeah, that could've been the glass at that frequency. But, you've been having ongoing issues, so there may be another underlying cause.

One of our worst venues for feedback has a similar layout, wall on our left (not glass) with the mains close to the wall. Next time I'll try moving the left main around the corner (forward and to the left), to where the main is not visible to us.

 

[LowFactor]

With the glass wall beside you and a wall behind you and the ceiling and floor it is acting like a big parabolic reflector. With the drummers vocal at the back where a lot of energy is being focused. You do get a lot of low mid energy of the back of most PA cabinets. Rolling right down the glass wall into the drummer vocal. 185hz must have been the resonant standing wave of the space you were in. Either notch that frequency or sometimes you can reverse the polarity on that channel to get rid of it.

 

[Wasnex]

Resonating frequency was at a pitch of F#3, 185Hz.

Doing some googling on speakers I'm finding that my mental picture of sound radiating only from the front of the speaker is not a good model. That lower frequency radiates in all directions, even for a closed back speaker. I'm having trouble finding info detailing what frequency you start to see back radiation.

Your on to something here. At 185hz the sound will wrap around a relatively small speaker. Some speakers post their dispersion pattern by frequency in various published documents. I checked out the DXR12 owner's manual, and unfortunately it does not contain this sort of info.

The corrective tool is EQ and mic technique. The vocalist should be very close to the mic. This causes proximity effect, which is a bass boost. To compensate for the bass boost you turn to down the bass on the channel strip and use a high pass filter if you have one. Also, if you have a 31-band graphic EQ, you can notch out any problematic frequencies.

 

[Wasnex]

Interesting. I would expect overall loop gain to be the only thing that matters for feedback. I do have the mic preamp gains up pretty high, to where I see clip light often, although I never hear any clipping in the vocals. In fact I felt the vocal quality was better with the higher gain, which is why I set the gains high. Maybe this is something I should look into.

Mixer is soundcraft ui16.

Having your gain up too high anywhere in the signal path can increase the tendency for feedback to occur. Why? High gain results in compression and introduces harmonics through distortion. Sine the feedback is occuring at such a low frequency I don't think the introduction of harmonics is the problem, but compression could be part of it.

 

[Wasnex]

Either notch that frequency or sometimes you can reverse the polarity on that channel to get rid of it.

Flipping the polarity can be helpful. A related idea is to change the distance between the mic and the speaker(s). Since the problem is frequency dependent, it is also wavelength dependent and therefore phase dependent. The feedback occurs where the sound from the speaker enters the mic close enough to "in phase" that a feedback loop is formed. By changing the propagation distance from the speaker to the mic, the sound enters the mic at a different phase angle. This may shift the problem frequency or eliminate the problem altogether.

 

[Zooberwerx]

I've had problems before with overhangs / awnings (patios, port-a-stages) when it comes to feedback. The absolute worst was bluegrass band who used a single center mic...it would set up a howl with little provocation. Even with the gain set correctly and notched, the sonofagun still picked up the compressor noise from a beer concession 100+ feet away.

Riis

 

[musicman7722]

Was the kick being miked?

 

[Wasnex]

The absolute worst was bluegrass band who used a single center mic...it would set up a howl with little provocation. Even with the gain set correctly and notched, the sonofagun still picked up the compressor noise from a beer concession 100+ feet away.

Riis

There is only so much you can do with relatively quiet instruments standing 2-4 feet from a mic. The mic responds primarily to the loudest sound in it's pickup pattern, whether that be a mandolin or a beer cooler compressor. Consider what happens when bleed from the speakers approaches the SPL of the original source instruments in the mic's pickup pattern. Yep feedback!

 

[five7]

Gating the drum mics will help tremendously and a directional mic on the drum vocal.

 

[ktedrow]

Was the kick being miked?

Yes

Gating the drum mics will help tremendously and a directional mic on the drum vocal.

All drums were gated. Beta 58 vocal mic.

Drum gating was a breakthrough for us about 6 months ago. Before gating we were having a horrible problem with low freq feedback through resonating kick drum head. Gating worked like magic.

Gawd how I hated it when that low freq feedback would start, and get really loud really fast.

 

[ktedrow]

Also, if you have a 31-band graphic EQ, you can notch out any problematic frequencies.

Our mixer (soundcraft digital) has feedback suppression built-in but I've been afraid to use it in an actual gig. I tried it out at home and it's nerve-wracking being on the edge of feedback for so long while ringing things out. And I'm not very proficient with it.

If I was convinced that feedback suppression is really effective I would give it a more serious shot. But my experience at home was that it didn't help much because there wasn't one or two frequencies that stood out as being worse than other frequencies. But maybe this was an artifact of ringing out in a living room (small, carpets and drapes) as opposed to a venue. I also worry that with a lot of notches the vocal quality suffers.