Tag Archives: Troubleshooting

Mentalism

“Subjective” problems are still problems that have to be taken seriously.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

I’m not a doctor, nor do I play one on TV. However, I have known doctors – really good ones – and though they’ve never said this explicitly, I think they would be of the opinion that a problem in the mind IS a real problem. That is, if a patient is experiencing distress that is caused by the brain, then they are really experiencing distress. The key isn’t to tell the complainant that what’s happening to them is fake, because it clearly is not fake. Rather, if the problem truly is in the mind then the solution must be applied at the problem.

Which is in the mind.

This might not seem like something to do with audio, but you have heard that part of an audio-human’s job is psychology, right? It’s said jokingly, but it shouldn’t be. It’s true. Musical craftspersons of all types have been known to run into problems at a show that really do exist…just inside their own head.

Something just doesn’t seem right. A guitar tone feels “off.” The vocals aren’t sparking with the same magic. I liked that reverb yesterday, and now it’s awful. This DI box doesn’t work right with my instrument.

Especially for us science-oriented types, our response to this takes work. Because we’ve spent so long trying to cut through the vast piles of horse-doodie that pervade the industry, we get dismissive. “Nothing’s different, man. It’s just your imagination. Ignore it.” But they CAN’T ignore it. They’re experiencing it, or they’ve convinced themselves that they are, and that is plenty good enough for them.

This is why an understanding that audio is a service business is so important. This is why an attitude of cooperation ought to be cultivated. Some perceptual issues can’t be worked out by applying a tech-based change, but they can at least be alleviated. The salve that can be applied is a willingness to treat the mental malady as a true conundrum requiring attention. They want you to turn the knobs? Turn ’em! They want you to swap the cables? Swap ’em! They want you to drive the system into feedback (with an audience present) so that they can go after the ringing frequencies with their own EQ, and then drop the gain back to where you had it? Go for it! (This has actually happened to me, by the way.)

In the moment, taking a desired action matters greatly – even if that action is not likely to physically affect much of anything – because what you’re really applying the fix to is a person and not the PA. After all the dust has settled, THEN you can talk about whether or not the distress was objective or subjective. Rationality is a part of handling whatever bugbear there was, but rationality only works when people are calm. The important thing “in the now” is being on the same team…and proving it.

Refrain from lecturing in a crisis; The person experiencing the crisis can’t process what you’re saying.

I think I’ve proven many times on this site that I value an analytical approach. I put very little stock in “audio theater,” which is using techniques and buying gear that make you THINK a problem is getting fixed, rather than actually fixing something. I don’t advocate doing something damaging or insane just to make somebody else feel better; Sound people need to know what’s a flat-out bad idea. Diplomacy, though, is essential. When the show must go on, there’s little use in winning a technical argument. What’s needed is to get everybody to a place where they’re as happy as is physically and mentally possible.


Is The Crossover Leaky?

A lot of low-end can still get into your mains.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

Every so often, I get to chew on a question that a reader asks me directly. I kinda wish that would happen more often (hint, hint, hint…). Anyway, I was sent a message on The Small Venue Survivalist’s Facebook page, asking if I could render an opinion on why a bass guitar seemed to have a surprising amount of LF information in the main speakers. The mains were being used alongside a subwoofer, with the sub providing a crossover filter at 100 Hz. What could the issue be?

There are a few explanations that would seem reasonable, if one discounts “catastrophic” issues like the crossover filter simply failing to operate as advertised.

1. Crossover filters, especially those implemented in active electronics, have a tendency towards a relatively steep slope. Even so, they usually aren’t brick-wall implementations. Everything below the cutoff doesn’t simply disappear – rather, it’s attenuated at a certain rate. With a filter set to roll off everything “below 100 Hz,” the mid-highs are still being asked to do a fair bit of work at the crossover frequency. The general vicinity of 100 Hz is actually quite bassy (depending on who you ask, of course), so the mains might be perceived as doing more than they should when everything is quite normal.

2. If a sizeable pile of low-frequency energy has been dialed into the bass-guitar channel, or the bass-guitar’s pre-console tone, that big hill-o-bass won’t be tamped down by the crossover. It will be split up proportionately, but following on from the first point, the mid-highs will still be tasked with reproducing their allotted piece of that big LF mound. Consequently, a surprising amount of energy may be present in the tops.

3. I have a suspicion that plenty of modern, two-way boxes receive some degree of “hyping” of their low-end at the factory. This makes them sound more impressive, and the manufacturer can get away with it because of safety limiters placed post-EQ. (The limiter prevents the low-frequency amplifier from supplying more voltage than the woofer can handle, and there may even be a level-dependent high-pass filter in play.) A low-frequency boost that occurs after the crossover reduces the crossover’s apparent effectiveness. Sure, the signal leaving the crossover might be down 12 dB at 75 Hz, but a +6 dB shelving filter put in place by the manufacturer at 100 Hz “undoes” that filtering to only lose 6 dB. Once again, a potential situation develops where the mid-highs are being asked to reproduce more “boom” than you expected.

It is entirely possible that an apparent problem isn’t covered by the three possibilities above, but they should catch quite a few scenarios where everything is hooked up properly and configured correctly.


The Unterminated Line

If nothing’s connected and there’s still a lot of noise, you might want to call the repair shop.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

“I thought we fixed the noise on the drum-brain inputs?” I mused aloud, as one of the channels in question hummed like hymenoptera in flight. I had come in to help with another rehearsal for the band called SALT, and I was perplexed. We had previously chased down a bit of noise that was due to a ground loop; Getting everything connected to a common earthing conductor seemed to have helped.

Yet here we were, channel two stubbornly buzzing away.

Another change to the power distribution scheme didn’t help.

Then, I disconnected the cables from the drum-brain. Suddenly – the noise continued, unchanged. Curious. I pulled the connections at the mixer side. Abruptly, nothing happened. Or rather, the noise continued to happen. Oh, dear.


When chasing unwanted noise, disconnecting things is one of your most powerful tools. As you move along a signal chain, you can break the connection at successive places. When you open the circuit and the noise stops, you know that the supplier of your spurious signal is upstream of the break.

Disconnecting the cable to the mixer input should have resulted in relative silence. An unterminated line, that is, an input that is NOT connected to upstream electronics, should be very quiet in this day and age. If something unexplained is driving a console input hard enough to show up on an input meter, yanking out the patch should yield a big drop in the visible and audible level. When that didn’t happen, logic dictated an uncomfortable reality:

1) The problem was still audible, and sounded the same.

3) The input meter was unchanged, continuing to show electrical activity.

4) Muting the input stopped the noise.

5) The problem was, therefore, post the signal cable and pre the channel mute.

In a digital console, this strongly indicates that something to do with the analog input has suffered some sort of failure. Maybe the jack’s internals weren’t quite up to spec. Maybe a solder joint was just good enough to make it through Quality Control, but then let go after some time passed.

In any case, we didn’t have a problem we could fix directly. Luckily, we had some spare channels at the other end of the input count, so we moved the drum-brain connections there. The result was a pair of inputs that were free of the annoying hum, which was nice.

But if you looked at the meter for channel two, there it still was: A surprisingly large amount of input on an unterminated line.


THD Troubleshooting

I might have discovered something, or I might not.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

Over the last little while, I’ve done some shows where I could swear that something strange was going on. Under certain conditions, like with a loud, rich vocal that had nothing else around it, I was sure that I could hear something in FOH distort.

So, I tried soloing up the vocal channel in my phones. Clean as a whistle.

I soloed up the the main mix. That seemed okay.

Well – crap. That meant that the problem was somewhere after the console. Maybe it was the stagebox output, but that seemed unlikely. No…the most likely problem was with a loudspeaker’s drive electronics or transducers. The boxes weren’t being driven into their limiters, though. Maybe a voice coil was just a tiny bit out of true, and rubbing?

Yeesh.

Of course, the very best testing is done “In Situ.” You get exactly the same signal to go through exactly the same gear in exactly the same place. If you’re going to reproduce a problem, that’s your top-shelf bet. Unfortunately, that’s hard to do right in the middle of a show. It’s also hard to do after a show, when Priority One is “get out in a hurry so they can lock the facility behind you.”

Failing that – or, perhaps, in parallel with it – I’m becoming a stronger and stronger believer in objective testing: Experiments where we use sensory equipment other than our ears and brains. Don’t get me wrong! I think ears and brains are powerful tools. They sometimes miss things, however, and don’t natively handle observations in an analytical way. Translating something you hear onto a graph is difficult. Translating a graph into an imagined sonic event tends to be easier. (Sometimes. Maybe. I think.)

This is why I do things like measure the off-axis response of a cupped microphone.

In this case, though, a simple magnitude measurement wasn’t going to do the job. What I really needed was distortion-per-frequency. Room EQ Wizard will do that, so I fired up my software, plugged in my Turbos (one at a time), and ran some trials. I did a set of measurements at a lower volume, which I discarded in favor of traces captured at a higher SPL. If something was going to go wrong, I wanted to give it a fighting chance of going wrong.

Here’s what I got out of the software, which plotted the magnitude curve and the THD curve for each loudspeaker unit:

I expected to see at least one box exhibit a bit of misbehavior which would dramatically affect the graph, but that’s not what I got. What I can say is that the first measurement’s overall distortion curve is different, lacking the THD “dip” at 200 Hz that the other boxes exhibit, significantly more distortion in the “ultra-deep” LF range, and with the “hump” shifted downwards. (The three more similar boxes center that bump in distortion at 1.2 kHz. The odd one out seems to put the center at about 800 Hz.)

So, maybe the box that’s a little different is my culprit. That’s my strong suspicion, anyway.

Or maybe it’s just fine.

Hmmmmm…


The Effervescent Joy Of Meeting A Knowledgeable Outsider

Some of the best folks to find are those who know the craft, but aren’t invested in your workflow.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

Last week, I got to spend a few days with students from Broadview Entertainment Arts University. The Live Sound class needs some honest-to-goodness shows to work on, so Bruce (their actual professor) and myself worked out a bit of a mechanism: I put a couple of gigs together every quarter, BEAU provides the room, I bring the PA, and we spend three days getting our collective hands dirty with building the thing.

Last week was the first round. As usual, I spent too much time talking and we didn’t get as far as maybe we should have. I also made some hilarious blunders, because everything involved in putting on a live gig is a perishable skill, and I sometimes have sizable gaps between productions. (For several minutes, I couldn’t find the blasted aux-in remap selector for my X32, even though I was on the “Input” routing page and staring right at it. I also absent-mindedly walked off the drum riser while I was mid-sentence. You can’t make this stuff up, folks.)

Anyway.

We had a really solid group of students all around. One of the most solid students was Patrick. Patrick is a guy who’s coming at this whole live-sound thing with a background in telecom. Telecom, like audio for entertainment, is the sort of business where you have to manage and troubleshoot every possible species of signal-transfer problem imaginable. Telecom skills are also becoming increasingly relevant to audio because of our increased reliance on high-speed network infrastructure. When all your audio, control, and clock signaling gets jammed onto a Cat6, it’s important to have some sort of clue as to what’s going on. (I have just enough clues to make things work. Other people have many more clues.)

As the story ended up going, we had a problem with my digi-snake. We got everything plugged together, and…oh dear. The consoles were only seeing one stage box, instead of both cascaded together. I walked over to the deck and started puzzling through things. Did the cascade connection get partially yanked? No. Did the boxes simply need a reset? No. Had I crunched the cascade cable at some point? No. I was on the brink of declaring that we’d just have to muddle through with one box when Patrick got involved.

Had I tried running a signal directly to the second box? Well, actually I hadn’t, because I was used to thinking of the two boxes as a unit.

Click.

Oh, look! The second box illuminated its green light of digital-link happiness.

Had I tried plugging directly into the secondary connection on the first box? Well, actually I hadn’t.

Click.

No happy-light was to be found.

I considered all that very nifty, but still being invested in my way of doing things, I failed to immediately see the obvious. Patrick enlightened me.

“The B-jack on the top box is the problem. Just connect them in reverse order, and you’ll have both. You can always change them around in the rack later.”

Of course, he was exactly right, and he had saved the day. (I was really glad were working on the problem the night before the show, instead of with 30 minutes to spare.)

The point here is that Patrick’s skillset, while not directly related to what we were doing, was fully transferable. He didn’t know the exact system we were working on, but he had plenty of experience at troubleshooting data-interconnects in general. He also had a distinct advantage over me. He was looking at the problem with a set of totally fresh eyes. Not being locked into a particular set of assumptions about how the system was supposed to work as a whole, he could conceptualize the individual pieces as being modular rather than as a single, static, integrated solution. I was thinking inside the flightcase, while Patrick was thinking outside the flightcase about everything inside that same flightcase. There’s a difference.

The whole situation was the triumph of the knowledgeable outsider. A person with the skills to make your plan work, but who isn’t yet invested in your specific plan may be just what you need when the whole mess starts to act up. They might be able to take a piece of the whole, reconfigure it, and slot it back in while you’re still getting your mind turned around. It’s really quite impressive.


When The Control Surface Fails

You may have to reboot – or you might not want to.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

Back in “the day,” we got wind of an exciting development: Consoles now existed that had a measure of independence between the actual audio processing and the control system. If the controls – the “surface” – had a problem, you could restart the surface without interrupting your show. Neat!

Of course, only the big boys and girls had access to this. I still have in my possession a pair of digital consoles that do not allow that kind of behavior. When they were newly built, the asking price per each was $3000. Nowadays, you can swipe a card for $450 and get the DSP part of a digital console equation that’s noticeably better.

These new, mini-consoles are designed to connect to a tablet or computer via a network, presenting a virtual surface through the external device. The convenient and fast way to do this is over WiFi, and it’s great when it’s really working…but it’s not so great when something goes amiss. (To be brutally frank, it’s another case of “It takes a pretty darn spendy wireless unit to be as good as a $5 cable.”) The console keeps charging along, passing audio without a hitch. You, on the other hand, are sitting there, somewhat alarmed that your display is freezing and lagging like a Tenderfoot Boy Scout on his first cold-weather hike.

So, what do you do?

Well, first, I would urge you to remember that disrupting a show or event is the last thing you want to do. Second, you need to keep in mind that some control is better than no control at all. Third, having no control at a critical moment will disrupt the show. (You see, Simba, we are all connected in the great circle of…mic cables…no…loading in and out…no, that’s not it…)

Anyway.

The point is that if you reboot your surface, or the WiFi module that communicates with it, you are no longer a “pilot in command.” Instead, you’re a pilot strapped to a jet that is going to do whatever it was last told to do, come hell or high water. That might be a good thing; A right thing. It might also be the wrong thing, or a thing that’s so horrifically bad that you want to hide your eyes and run for an exit. In whatever state you are, you are going to be stuck until the surface or network is back up. How long will that take? A few seconds? A minute? Several minutes?

You may not be able to be sure.

If the problem is degrading your control, but not completely preventing it, keep what control you have. Only reboot if you actually lose control, and that’s what you need to do to return to the driver’s seat. If it looks like you’ll soon be forced to let the system drive itself for a bit, try to use what influence you have left to make your mix stable and accommodating of coming changes. Open all channels that might need to be un-muted in the next while, and pull your output masters down a bit to guard against feedback.

Otherwise, just let the situation ride. Things might be clumsy and disconcerting, but you’ll be able to get through.

And have an alternative control connection available if at all possible. Like something that uses a $5 cable.


EQ: Separating The Problems

You have to know what you’re solving if you want to solve a problem effectively.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

Want to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

There’s a private area on Facebook for “musicpreneurs” to hang out in. I’ve been trying to get more involved, so I’ve asked people to pose their sonic quandaries to me. One person was asking how to set up their system so as to get a certain, desired tone from their instrument.

I won’t rehash the whole answer here, but I will definitely tell you the key to my answer: Separate your problems. Figure out which “domain” a particular issue resides in, and then work within that area to find a solution.

That’s a statement that you can definitely generalize, but the particular discussion was mostly in the context of equalization. Equalization of live-audio signal chains seems to invite unfocused flailing at least as much as anything else. Somebody gets into a jam (not a jam session, but rather a difficult situation), and they start tweaking every tonal control they can get their hands on. Several minutes later, they’ve solved and unsolved several different problems, and might be happy with some part of their fix. Of course, they may have broken something else in the process.

If you’re like me, you’d prefer not to do that.

Not doing that involves being very clear about where your problem actually is.


Lots of people use the “wrong” EQ to address a perceived shortcoming with their sound. I think I’ve mentioned before that a place to find this kind of approach is with vocal processors. I’ve encountered more than person who, as far as I could tell, was trying to fix a PA system through the processing of an individual channel. That is, at a regular gig or rehearsal, they were faced with a system that exhibited poor tonality. For instance, for whatever reason, they might have felt that the PA lacked in high-end crispness.

So, they reach down to their processor, and throw a truckload of high-frequency boost onto their voice. Problem solved!

Except they just solved the problem everywhere, even if the problem doesn’t exist everywhere. They plug that vocal processor into a rig which has been nicely tuned, and now their voice is a raspy, irritating, sand-paper-esque noise that’s constantly on the verge of hard feedback.

They used channel-specific processing to manage a system-level problem, and the result was a channel that only works with one system – or a system with one channel that sounds right, while everything else is still a mess. They found a fix, but the fix was in the wrong domain.

The converse case of this is also common. An engineer gets into a bind when listening to a channel or two, and reaches for the EQ across the main speakers. Well, no problem…except that any new solution has now been applied to EVERYTHING running through the mains. That might be helpful, or it might mean that a whole new hole has just been dug. If the PA is well-tuned, then the problem isn’t the PA. Rather, the thing to solve is specific to a channel or group of channels, and should be addressed there if possible.

If you find yourself gunning the bottom end on every channel of your console, you’d be better served by changing the main EQ instead. If everything sounds fine except for one channel, leave the main processing alone and build a fix specific to your problem-child.

Obviously, there are “heat of the moment” situations where you just have to grab-n-go. At the same time, taking a minute to figure out which bridge actually has the troll living under it is a big help. Find the actual offender, correct that offender, leave everything else alone, and get better results overall.


I Think My Spaceship Knows Which Way To Go

A superbly talented and highly rehearsed band roars back from the brink of disaster.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

mttmWant to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

It’s funny how we’re often separated by a common language.

If you’re a regular reader, you are certainly aware by now of how much I emphasize logistics and preparation. On deck to handle the sound reinforcement for a Major Tom & The Moonboys appearance at the Sugarhouse Farmer’s Market, I was feeling pretty confident. We’d managed to talk the event folks into an extra hour for setup, and we were on track to make good use of the time. The van o’ audio was 75% unloaded into Fairmont Park’s west pavilion. We were cookin’.

We then got the news that we were in the wrong place.

You see, when the Farmer’s Market folks said “the west pavilion,” what they meant was, “the pavilion on the west side of the Farmer’s Market.” Unfortunately, the additional qualification wasn’t what we heard. What we heard was “THE west pavilion.”

So that’s where we were.

And being both on time and industrious actually worked against us.

We could easily pull our vehicles right up to where we were really supposed to be, but the van was almost empty. To make matters worse, it would take just as long to walk everything back to the van as it would to walk it over to the correct location. By the time it was all done, our lead time had evaporated. The situation was now “throw and go” with a band that is decidedly NOT meant to be “throw and go.”


Some weeks earlier, my cautionary inner voice had said, “You know, Danny, you probably don’t want to be dialing up monitor world from scratch on this gig.” As such, I had gone out to a rehearsal and built a preset monitor solution. This did indeed turn out to be a Very Good Idea™ in the end, but at first it tripped us up. With the stage not necessarily being patched in a house-left-to-house-right order, but rather jumping around a bit, it wasn’t possible to set out a simple “patch logic” and have other folks go to town. I couldn’t walk out to FOH and work on that setup while the stage was getting taken care of. Every task had to be done in series, with me directing traffic in detail.

And, of course, my danged CAT6 cables for the stagebox connections got tangled in the box. It’s amazing how even nice coils will find a way to glom onto each other. With the help of Layne, the percussionist, the two of us managed to sort out 200 feet of pissed-off, solid-wire data cable in decent time – but we were still late, and nowhere near where we needed to be.

We were a little over halfway patched overall when FOH control finally came together. Or sort of did. There’s a special kind of horrified panic that audio humans experience when something that, by all measures should be working…flat-out fails to work. We had this whole plan for a grunge-a-delic break music solution involving David Bowie instrumentals coming through a mic’ed boombox. The boombox was working, and the mic was working, so why wasn’t anything coming through the FOH PA when I pushed the fader up? Even worse, why was the channel routed to the main bus, but the main bus meter showed no signal?

I was racking my brain.

I checked all the global routing I could think of, with my half-panicked brain going mushy with reinterpreting the odd machinations required to string two X32 consoles together in a daisy chain. Had I reset something by accident? How was that even possible?

Finally, the lead videographer made the simple suggestion: Just restart the software. Of course this had not occurred to me, because a problem of this nature could not possibly occur without active misconfiguration, right? Well, at that point I was ready to try anything. Ten seconds later, FOH was in business.

So, if you didn’t know, it is indeed possible for X32-Edit to connect to a console and “see” meter activity, yet not successfully send control data.


At this point we were over half an hour past our scheduled downbeat. Michael, the guitar player, said what I was very definitely starting to think. “Let’s just plug in the keyboards and bass and go for it.” So, with neither guitar in the PA, nor drums, and the FOH subwoofers metaphorically thrown under the bus, we went for it.

Our luck changed immediately.

The band dove in, and our prep work started to pay off. I had also prebuilt some of FOH, which meant that I could just grab faders and basically have something usable come out of the system. What was coming out of the system was the music provided by seasoned pros with hours upon hours of rehearsal. I think it’s quite fitting that a Bowie tribute act would embody the line “I think my spaceship knows which way to go.” From everything I could perceive, the audience was IN LOVE.

The songs were being beautifully played by people who adored the material, and the whole thing was basically balanced – guitars and drums in the PA or not – because the players know how to be a band without a sound operator taking everything apart and putting it back together.

Exactly zero people complained about the lack of subwoofer material. (I eventually got the guitars into the system. I never finished patching in the subs.)

Kids were dancing.

The folks down front were smiling.

People were offering compliments on the sound.

As I’m sure happens almost every night all over the world, a supremely rehearsed and professional band had salvaged a bad situation so completely that the problems leading up to the music were essentially forgotten.

Boy, what a ride.


A Guided Tour Of Feedback

It’s all about the total gain from the microphone’s reference point.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

smoothed-monitorsWant to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

This site is mostly about live audio, and as such, I talk about feedback a lot. I’m used to the idea that everybody here has a pretty good idea of what it is.

But, every so often, I’ll do a consulting gig and be reminded that feedback can be a mysterious and unknown force. So, for those of you who are totally flummoxed by feedback monsters, this article exists for your specific benefit.

All Locations Harbor Dragons

The first thing to say is this: Any PA system with real mics on open channels, and in a real room, is experiencing feedback all the time. Always.

Feedback is not a phenomenon which appears and disappears. It may or may not be a problem at any particular moment in time. You may or may not be able to hear anything like it at a given instant. Even so, any PA system that is doing anything with a microphone is guaranteed to be in a feedback loop.

What matters, then, is the behavior of the signal running through that loop. If the signal is decaying into the noise floor before you can notice it, then you DO have feedback, but you DON’T have a feedback problem. If the signal is dropping slowly enough for you to notice some lingering effects, you are beginning to have a problem. If the signal through the feedback loop isn’t dropping at all, then you are definitely having a problem, and if the looped signal level is growing, you have a big problem that is only getting bigger.

Ouroboros

If every PA system is a dragon consuming its own tail – an ouroboros – then how does that self-consuming action take place?

It works like this:

1) A sound is made in the room.
2) At least one microphone converts that sound into electricity.
3) The electricity is passed through a signal chain.
4) At the end of the chain is the microphone’s counterpart, which is a loudspeaker.
5) The loudspeaker converts the signal into a sound in the room.
6) The sound in the room travels through direct and indirect paths to the same microphone(s) as above.
7) The new sound in the room, which is a reproduction of the original event, is converted into electricity.

The loop continues forever, or until the loop is broken in some way. The PA system continually plays a copy of a copy of a copy (etc) of the original sound.

How Much Is The Dragon Being Fed?

What ultimately determines whether or not your feedback dragon is manageable or not is the apparent gain from the microphone’s reference point.

Notice that I did NOT simply say “the gain applied to the microphone.”

The gain applied to the microphone certainly has a direct and immediate influence on the apparent gain from the mic’s frame of reference. If all other variables are held constant, then greater applied gain will reliably move you closer toward an audible feedback issue. Even so, the applied gain is not the final predictor of ringing, howling, screeching, or any other unkind noise.

What really matters is the apparent gain at the capsule(s).


Gain in “absolute” terms is a signal multiplier. A gain of 1, which may be referred to as “unity,” is when the signal level coming out of a system (or system part) is equal in level to the signal going in. A signal level X 1 is the same signal level. A gain of less than 1 (but more than zero) means that signal level drops across the in/ out junction, and a gain of greater than 1 indicates an increase in signal strength.

A gain multiplier of zero means a broken audio circuit. Gain multipliers of less than zero are inverted polarity, with the absolute value relative to 1 being what determines if the signal is of greater or lesser intensity.

Of course, audio humans are more used to gain expressed in decibels. A gain multiplier of 1 is 0 dB, where the input signal (the reference) is equal to the output. Gain multipliers greater than 1 have positive decibel values, and negative dB values are assigned to multipliers less than 1. “Negative infinity” gain is a multiplier of 0.


The apparent gain as referenced by the pertinent microphone(s) is what can also be referred to as “loop gain.” The more the reproduced sonic event “gets back into” the mic, the higher that loop gain appears to be. The loop gain is applied at every iteration through the loop, which each iteration taking some amount of time to occur. If the time for a sonic event to be reproduced and arrive back at the capsule is short, then feedback will build aggressively when the loop gain is positive, but also drop quickly when the loop gain is negative.

Loop gain, as you might expect, increases with greater electronic gain. It also increases as a mic’s polar pattern becomes wider, because the mic has greater sensitivity at any given arrival angle. Closer proximity to a source of reproduced sound also increases apparent gain, due to the apparent intensity of a sound source being higher at shorter distances. Greater room reflectivity is another source of higher loop gain; More of the reproduced sound is being redirected towards the capsule. Lastly, a frequency in phase with itself through the loop will have greater apparent gain than if it’s out of phase.

This is why it’s much, much harder to run monitor world in a small, “live” space than in a large, nicely damped space – or outside. It’s also why a large, reflective object (like a guitar) can suddenly put a system into feedback when all the angles become just right. The sound coming from the monitor hits the guitar, and then gets bounced directly into the most sensitive part of the mic’s polar pattern.

Dragon Taming

With all that on the table, then, how do you get control over such a wild beast?

Obviously, reducing the system’s drive level will help. Pulling the preamp or send level down until the loop gain becomes negative is very effective – and this is a big reason for bands to work WITH each other. Bands that avoid being “too loud for themselves” have fewer incidences of channels being run “hot.” Increasing the distance from the main PA to the microphones is also a good idea (within reason and practicality), as is an overall setup where the low-sensitivity areas of microphone polar patterns are pointed at any and all loudspeakers. In that same vein, using mics with tighter polar patterns can offer a major advantage, as long as the musicians can use those mics effectively. Adding heavy drape to a reflective room may be an option in some cases.

Of course, when all of that’s been done and you still need more level than your feedback monster will let you have, it’s probably time to break out the EQ.

Equalization can be effective with many feedback situations, due to loop gain commonly being notably NOT equal at all frequencies. In almost any situation that you will encounter in real-life, one frequency will end up having the highest loop gain at any particular moment. That frequency, then, will be the one that “rings.”

The utility of EQ is that you can reduce a system’s electronic gain in a selected bandwidth. Preamp levels, fader levels, and send levels are all full-bandwidth controls – but if only a small part of the audible spectrum is responsible for your troubles, it’s much better to address that problem specifically. Equalizers offering smaller bandwidths allow you to make cuts in problem areas without wrecking everything else. At the same time, very narrow filters can be hard to place effectively, and a change in phase over time can push a feedback frequency out of the filter’s effective area.

EQ as a feedback management device – like everything else – is an exercise in tradeoffs. You might be able to pull off some real “magic” in terms of system stability at high gain, but the mics might sound terrible afterwards. You can easily end up applying so many filters that reducing a full-bandwidth control’s level would do basically the same thing.

In general, doing as much as possible to tame your feedback dragon before the EQ gets involved is a very good idea. You can then use equalization to tamp down a couple of problem spots, and be ready to go.


An Adventure In A Peavey

Sometimes, a broken thing is less broken than you might think.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

PeaveyWant to use this image for something else? Great! Click it for the link to a high-res or resolution-independent version.

Remember that road gig I was talking about a few weeks ago? Well, during our setup for the first day, I noticed something. One of my monitors had a cooling fan that wasn’t being much of a fan. It was just sitting there, not moving any air molecules.

This was disconcerting to me, but there wasn’t much I could do about it. We ran the show with some signal going through it, and it apparently did not “thermal,” so – good on ya, PVxP!

Of course, even with the box having behaved itself, I couldn’t just leave it be. I wanted to know what was wrong, and if it could be fixed. So, I gathered up a few tools and went on a little adventure.


Gunky Fan

This monitor had spent its previous life at Fats Grill. During that life, the fan had ingested a fair amount of “old Salt Lake City basement gunk,” which is a particular kind of evil dust that’s nearly impossible to clean up, and adept at invading everything. I imagine that every city has its own version of this stuff. At the outset, I was pretty sure that the poor fan had gotten all bound up with this goop, and might have burned itself out.

But, to be sure, I would have to get the back panel off the speaker and do some rooting around.

IMPORTANT: Poking, prodding, digging, tweaking, yanking, or otherwise messing with the internals of gear that runs on “wall” power can injure or kill you. I am NOT responsible if you attempt activities like this and end up surprised, hurt, or dead.

Internal Connection

The amplifier module connects to the rest of the speaker through this little bit of fun. The four-conductor part seems to be what mates with the drivers (the clues being labeling like HPF + and HPF -). As far as I can tell, the little two-wire connector is to do nothing more than light up the LED in the front of the box.

Internal Connection Closeup

Peavey just couldn’t resist gluing that LED connection shut. Getting it out meant pulling the entire contact assembly off the circuit-board pins. Geeze…

Connection Tub

An interesting design choice with these Peavey PVxP speakers is the “big tub of nothing” that the amplifier module screws into. All that’s in there is the little tiny PCB that acts as the bridge between the backplate electronics and the rest of the loudspeaker. I can guess that it makes sense from a modularity standpoint, as all the negative space seems to be able to accommodate either an active electronics unit or a passive crossover setup with no fuss.

Woofer And Wire

Nobody in here but us drivers!

And some wire, and an LED, of course.

Fan Connector

After extricating the amplifier module fully from the enclosure, I was able to get a look at where the fan got power. My first step in troubleshooting was to get the fan off the main chassis, in the hopes that I could figure out why it was no longer inclined to spin.

Spinning Fan

Much to my surprise, applying power to the amp module caused the fan to run like nothing was wrong at all. The hub was turning as smooth as glass, with no noise of rubbing or anything else being amiss. Well, that was surprising – but in a nice way. I had envisioned having to find a replacement fan, and maybe do some wire splicing to get back to full operation. It didn’t seem that I would have to do any of that now.

33 Volts

Since I had gone to the trouble of getting the major assemblies apart, I did want to satisfy my curiosity as to how much voltage was used to drive the fan. (If the fan would have refused to turn at all, my next step would have been to determine if it was getting any power.) Using my meter to take a reading across the pins got me about 33 VDC. Apparently, Peavey runs the fan a bit “hot,” because…

24V Fan

…the actual fan is a 24 VDC model. (Then again, the power supply tap could be a little “off.”)

Internal LEDs

Side note: When you shut down the amplifier module, these LEDs stay lit for a while. I imagine that could be code for, “The big capacitors on this thing still have PLENTY of charge in ’em, pal, so don’t touch anything right now!”

Reassembled

I screwed the fan back onto the amp housing, and THAT’S where things got interesting. I applied power, and…nothing. The fan was at a dead stop again. I wondered if there was something about the fan’s orientation that was giving it trouble. I got out my screwdriver, and started loosening the fan from the mounting holes. Suddenly, with a bit of a grating scrape, the fan sprung to life again! I ran a couple of the screws back in a turn, and the blades ground to a halt.

The problem the whole time was that the chassis had been pressed too tightly against the fan hub. As I said before, the apparent behavior of PVxP fans is to pull air into the enclosure. In the case of these fans, that means that their integrated “cage” faces in, instead of out. As such, the outer plate can pretty easily be brought into contact with the unprotected side of the fan hub, and that can stop things pretty efficiently.

I backed a couple of screws out just a touch, and what do you know – I had a working fan again.

With the amplifier re-mounted to the box, I ran some music through the enclosure. Everything seemed fine, and that made me a happy audio-human.