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Sound Advice

Controlling Feedback Onstage Using Phase To Your Advantage Part 2: The Interaction Between Speakers by Peter Janis

April 19th, 2010

When an acoustic guitar is used onstage, it is usually connected via a direct box that splits the signal to the onstage amplifier and the PA system. The PA then will split the signal again to drive wedge monitors and the main house sound system. When all of these loudspeakers are blasting at the same time, they interact. In fact, they mostly interact in the bass region where the longer, low frequency sound waves meet to either reinforce each other or cancel each other out. This effect is known as modal distortion. Recording studios commonly employ bass traps to reduce hot spots known as room modes. These are exaggerated depending on the room geometry or the room’s natural resonant frequency. And guess what … room modes, like gravity, exist everywhere including on a live sound stage.

Here’s what happens: You play a chord on the guitar and, depending on where you are standing, the sound waves from the wedge monitor and the PA system will either amplify each other if they are in phase or cancel each other out if they are out of phase. When they are in phase, the resulting amplitude at that particular frequency will increase or even double depending on where you are standing. If you find that a certain frequency is feeding back when you stand in front of your monitor, in all likelihood, you are experiencing two or more waves that are combining, causing a resonant feedback problem. There is absolutely no point trying to figure it all out by calculating the phenomena as this will occur based on a host of variables such as the PA system, the monitors, the size of the room, the room acoustics, and so on. But you can try reducing feedback by following this simple procedure.

First, start by eliminating unneeded bass frequencies by rolling off the low end below 100 Hz. This is the one fix that you should absolutely consider before doing anything, as low frequencies are the primary problem with resonant feedback. Bass below 300 Hz is considered to be omni-directional, meaning that it will be everywhere. By eliminating excessive low end, you make the task of controlling feedback easier. There is also another benefit – ever notice that it is way easier to get feedback from an electric guitar when the sound is distorted? Guess what. Like gravity and modal distortion, the same laws of physics apply everywhere. So, if your acoustic guitar is distorted, you will get more feedback. To eliminate distortion, make sure you use a high-quality direct box that is able to handle transients without choking. Since most of the sound energy is contained in the bass, when you roll off the low end, you are actually making it easier for the buffer or amplifier inside the DI box to work. Less distortion = less feedback.

Now that you have rolled off the bass, you are now ready to turn up your PA system and monitors. Start playing chords and let the guitar ring. Turn your system up until it begins to resonate. Now, take a step away from in front of your wedge monitor to see what happens. Now move sideways.

As you move around, the feedback character will change. This is because you are in the middle of a multitude of room modes. If the feedback is most active near the monitor, try moving the monitor electronically by reversing the electrical phase. Most professional DI boxes have a 180-degree polarity reverse switch to enable you to do this. What you are doing basically is causing the modal distortion to change. This can often move a phase-adding mode from where you are standing which can help reduce feedback.

Another possible fix is to “imply move” the wedge monitor away from where it is so that the physical relationship changes. If you have an instrument amp on stage, moving it back a few inches can also help. This will cause different frequencies to either amplify each other or cancel each other out depending on where you stand. Point being, we have yet to EQ the sound, but are dramatically shifting the way the natural sound will interact so that we minimize feedback naturally. Once you have maximized the output, you can then fine-tune your system using the EQ.

Peter Janis is the President of Radial Engineering, the PortCoquitlam, BC-based manufacturer of music and audio equipment. Visit www.radialeng.com for more information.

It’s All In The Ears by Laurence Currie

April 19th, 2010

While teaching as a guest speaker at Dalhousie University and at the community college in Halifax, I had so many students ask, “What setting do you use on that?” I would have to tell them every time, “I don’t have a setting. The setting is whatever my ears tell me it should be.”

To think that every single base-track has to be used through a particular type of compressor EQ and have this on it or that on it is a total misnomer. It’s on a case-by-case contingency. Anyone who’s thinking about becoming an engineer should either find someone who’s willing to tutor you, or, find a reputable place where you can learn a little bit about it. I originally learned the trade of sound engineering from a school that relied very heavily on technical knowledge. If you want to become a really good engineer, you have to know all of that stuff. Above and beyond that, it’s a lot of experience, a lot of trial and error. The most important tools you have are your ears. Using them is the main thing – and have a good head on your shoulders that houses those ears.

Laurence Currie is a professional sound engineer, and Co-Host of MasterTracks, currently airing on AUX.tv.

Controlling Feedback Onstage Using Phase To Your Advantage: Part 1 by Peter Janis

April 19th, 2010

Anyone who has played an acoustic instrument onstage knows that feedback can be a serious problem. What few realize is, there are solutions beyond radically altering the EQ. The following looks at the various problems and solutions at hand. For simplicity, we will discuss an acoustic guitar – but the same principles apply equally to a violin, mandolin, banjo, and contra-bass.

Before we get too far ahead of ourselves, we need to first identify the problems. There are basically two types of feedback that occur onstage: high-frequency whistles and1low-frequency resonance. Both are caused by the sound emanating from the loudspeaker being so loud that it overtakes the instrument by feeding itself back through the pickup, causing the sound to feed back into the PA system forming an audio loop. This endless cycle is called a feedback loop, or feedback for short.

High-frequency feedback is often caused by sound from a wedge monitor going directly into the instrument’s microphone. This can also occur with piezo-type transducers. The usual fixes for high-frequency feedback are to turn down the volume, reposition the microphone, or employ some form of equalization to eliminate the problem frequency.

Low frequency feedback occurs when bass energy from the speaker system causes the instrument to vibrate. This is also known as resonant feedback. The sound system causes the soundboard (top of the guitar) to vibrate in sympathy with a particularly loud bass frequency. The vibration is picked up by the instrument pickup and recycles itself as feedback. Some musicians will seal the sound hole using a rubber plug. This can reduce feedback, but also degrades the sound quality of the instrument.

The Down Side To Using EQ To Solve Feedback Problems:
The most common approach to eliminating feedback is to use some sort of notch filter to find the offending frequency and remove it with a narrow band EQ. The problem with this approach is it winds up as a catch 22. For instance, if you reduce midrange to eliminate feedback, you are actually removing the “meat” or most important part of the sound out of the monitors. To make up for the loss in the midrange, folks invariably increase the stage volume and guess what … more feedback.

To make matters worse, some will introduce a form of automatic feedback filtering system to “magically” solve the problem. These devices introduce a series of very narrow filters that rapidly move around to squash feedback as soon as it occurs. The resulting sound is best described as comb-filtered, an effect that studios spend thousands trying to eliminate!

Does this mean that using an equalizer is bad? Of course not. The number one rule with EQ is and will always be less is best. In some situations, the only option you may have will be to introduce some radical EQ curves into your monitors, but when you do so, keep in mind that you are moving further and further away from the natural sound of the instrument instead of actually solving the problem.

Peter Janis is the President of Radial Engineering, the Port Coquitlam, BC-based manufacturer of music and audio equipment. Visit www.radialeng.com for more information.

Choosing a USB Audio Interface by Alec Watson

April 19th, 2010

Computer recording keeps getting easier and more accessible.

Just a few short years ago, in order to do any “real” recording, one needed some kind of expensive internal controller card (and the guts to break open their computer to install it), a digital converter, and some good outboard microphone preamps; and we’re not even touching on the gear necessary to monitor your music. Today, there are so many choices for getting pretty darned good audio into your computer (at a good price) that it has once again become a little confusing when it comes to making the right choice. In fact, I was “Ebaying” last night and found a “Professional Engineer” who is willing to sell you his thoughts on purchasing the “right” USB audio interface. Not that I want to go denying dude his Ebay income, but as a little gift from CM to you, save your $16 US, (you can apply it to your new interface) here is what you need to know…

USB or Firewire?
I am almost certain I am going to get some hate mail from some better informed tech guy as to why I am wrong, but the honest truth is – it doesn’t really matter. That said there are a few considerations. No, USB and Firewire aren’t going to sound any different, but there may be some usage differences. If you have a computer that has all sorts of USB peripherals plugged in – printers, hard drives, card readers, USB Coffee Maker … and you have a Firewire port sitting empty, then it would probably be wise to go with a Firewire audio interface; you will never receive the dreaded “USB device not recognized” message AND you are likely to be able to achieve lower latencies due to less bus traffic … if that sounds like a bunch of techno crap, apart from the fact that it is (techno crap), rest assured I will explain it later so that you too can impress your friends!

On the flip side, I would tend to go with a USB interface if I was using it with my laptop. Yes, my laptop does have a Firewire port, but it also has six USB ports. A lot of the USB interfaces run off the power supplied by the USB port and as I don’t have a lot of USB peripherals plugged into my computer and I don’t want to carry a wall wart (power adapter) around with me, the USB interface is likely the more robust choice when it comes to powering external devices from my laptop.

Latency – What The Heck Is It And Why Do I Care?
Between the manufacturers of the USB audio interfaces there is a lot of hype about latency. Latency, in practical terms, is the delay that occurs between the moment your audio enters the interface, travels to the CPU (the main processing chip in your computer), is processed (effects and/or EQ that are applied to your audio), and then returns to your USB audio interface to be played by your speakers or headphones. Some USB interfaces have lower latencies than others; for me, however, any latency is too much! I prefer to “direct monitor”; most interfaces achieve zero latency times through this process. Direct monitoring really means that the USB interface is really splitting the audio into two paths, one path goes to your computer, the other goes directly to your headphones; the result is zero latency. The drawback is that you won’t be able to hear your vocal or guitar etc. with any of the cool effects that your computer can apply to them. For me, I would rather hear my voice dry than gooped up with effects and late.

Alec Watson is a Producer/Engineer that lives in Reno-hell Vancouver Island. He can be contacted at alec@alecwatson.com.

Something New Or The Same Old Thing? by Simon King

February 19th, 2010

Being a recording engineer these days can be boring. I think the era of affordable,
disruptive technologies has blunted peoples’ artistic and creative energies. In addition, the user is overwhelmed by so many choices. Our industry is awash in poorly-designed gear, with terrible user-interfaces. As a result, an inordinate amount of time
is wasted on “learning curves” as opposed to making music. Even worse, the gear in question may be destined for obsolescence.

Most professional studio engineers are schooled in the “tried and tested” industry
standards, but the convenience of recording and mixing inside the box is leading to terminal brain laziness. Consequently, the music charts are littered with recordings that have been tooled, based, looped, cut, pasted, and edited to death. The aural art of music recording has been replaced with mouse-driven video games.

Anyhow, in my neck of the woods, creativity, artistry, and innovation still rule. Here are some of my “mad” studio experiments:

Take 0
First, I thoroughly revised everything I knew about mono and stereo mic techniques. I then stepped out of my comfort zone, and embraced more exotic techniques like Ambisonics (Soundfield), the Decca Tree, the Fukada Tree, etc. (thanks DPA). It became clear to me that the overused standbys are not enough anymore.

Take 1 – Drums
Instead of the usual set-up, I have successfully deployed Crown GLM-200s on the kit, and Crown SASS models stuck on walls to capture room ambience. With artistic liberty, I have added MIDI triggers to the drums, and recorded the data to a sequencer. Under MPTE/MTC control, drum machines like the TR-909 and Korg ER-1 can be added to the acoustic kit with great results. Sometimes, an electronic kit with some insane “treatment” is what Dr. Drums ordered.

Take 2 – Acoustic Guitars
I usually position the stereo condensers at the sweet spots when tracking. If the guitar is pickup-equipped, the line output goes into a Roland JC-120 Chorus amp for further recording and sonic manipulation.

Take 3 – Electric Guitars
I normally make modeled amps (Line-6, Vox, Roland) part of the picture. Distorted digital amps sound horrible, but usable. For more fun and sonic assault, the vintage Roland GR-100 guitar synth (and G-series guitar) is hauled into the fray. With its hex fuzz/distortion, chorus, and filter (read HUGE wah), this old-timer loves to hang with Marshalls and Boogies; furthermore, the Supa-Fuzz and Supa-Wah pedals by Marshall may be added for more mayhem.

Take 4 – Electric Bass
First, I try to convince the bassist to think like a musician, and then plead with him to try the 6-string bass. Tuned to B-E-A-D-F#-B, the 6-stringer is a powerful musical tool. Now, our newly-forged bassist suddenly becomes creative; he’s thinking about how he can use the studio’s Boss GT-10B, Roland V-Bass, and the Moog Taurus Bass pedal.

Take 5 – Vocals
OMNIS are cool vocal microphones; however, I am always eager to see what DPA, Sanken, Schoeps, and Pearl can bring to the table. I might eventually settle on the Fostex M88-RP or the Milab VIP-50.

As Joe Meek once said… “If it sounds right, then it is right.”

Simon King is a producer/engineer based in Edmonton. He has recorded Moe Berg, Neo-A4, and other Alberta-based bands. He currently works as a composer in his private studio, Leo Project-Techworks.

Some Tips For Working Efficiently With The House Tech by Owen Whitehead

February 19th, 2010

  • First thing: always remember to be respectful. The house tech has seen all the things that have gone right and wrong in that specific venue. A five-minute conversation can save you hours of aggravation and also brings everyone on your side. Now, you still may want to do things your own way, but giving the five minutes
    brings everyone to your team.
  • Be direct with your demands, questions, and comments, though keep in mind that barking orders usually yields shotty work.
  • Above all, treat them as you’d like to be treated (as is usually the case). I have been on both sides of the situation and the best day is the day where everyone
    is enjoying himself or herself. It doesn’t matter how hard the work is – so long as you’re happy doing it.

Owen Whitehead is Production Manager for Mississauga, ON’s Metalworks Production Group. Visit www.metalworksproductiongroup.com for more information.


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