Monday

A Beginner’s Guide to Stage Sound – Part 2: Setting up your PA

So now you’ve got all of your equipment and you’re ready to test it out. That’s right… test it out. Hook everything up and run through the numbers. Don’t tell me that you just wanted to get on stage and see how it sounds. It’s very important to thoroughly familiarize yourself with your equipment before it’s time to hit the stage. Let’s start with simply getting everything in place and hooking everything up.

Inputs

Once your mixer is in place, it’s time to run your inputs. I like to start with my vocal mics then progress across the channel list until everything is hooked up. At this point, nothing is powered on, and all faders and volume knobs are bottomed out. As you position your microphones, consider your stands. Make sure they are stable and non-intrusive. Make sure your cables are long enough to comfortably reach your mixing board (Or snake) and provide some slack for adjustments if needed. Make sure your stands won’t be tripped over or bumped into during those more enthusiastic ballads.
A good Vocal microphone is cardioid, which means that they have to be sung into directly. When positioning your microphone, consider what sound sources will be aiming at the front of that mic when the singer moves his/her head. Since monitors are generally placed on the floor in front of the singer, it is a good practice to angle the microphone so it aims almost directly away from the monitors. This will be a great step to preventing feedback. And remember never to position your FOH speakers behind your microphones. If you have a guitar cabinet behind you, try getting it, or your mic, positioned in a manner where the speakers don’t aim at the front of the mic. One of the best ways is to angle the mics upward slightly. If the mic almost pointing at your mouth from below it not only blocks less of your face to the audience, but provides better cardiod isolation from your back line.
 
Instrument mics are also cardioid, but often more so than vocal microphones. If you are placing a microphone in front of a speaker cabinet, place the mic as close to the protective screen as possible, and aim it directly at the sound cone of one of the speakers (Not the center of the speaker). Boom stands and short instrument microphone stands are helpful for this application, but be mindful of cable routing and positioning to avoid snags and trips. Isolation from other instruments is important, so if you have the drums next to you, pick the speaker furthest from the drums. If you’re right next to another cabinet or the bass amp, try to get some isolation (space) between their amps and your microphone if possible.

Equalization

Now that our inputs are in place, we need to push that signal out for processing and amplification. Run the outputs from the mixer to your 2 –channel equalizer. One mono channel will be dedicated to your Mains (FOH) and the other will go to your monitors. If you plan on using multiple monitor mixes, more equalizers should be considered. Equalizers can be your best tool for accenting or brightening certain frequency ranges while suppressing others. On your monitor channel, it will be a great tool for finding feedback frequencies and suppressing them.
Though equalization takes place later, during sound check, we will discuss it a bit here. Equalizing a signal can be a lengthy process, and sometimes needs to be repeated when you visit a venue with different acoustics. Taking notes of your EQ settings can save a lot of time. (Photos using your camera phone can be a great way to archive your settings for future visits.) My favorite method is the basic sweep method. Hook up an MP3 player and play some music that matches the tone and style you want to equalize. Start with all faders centered. Starting with your lowest frequency, slowly sweep the knob up and down while listening to the effect it has on your sound. Find the spot that sounds the best and move on to the next fader. After going all the way to the highest fader, work back through from High to Low.
 
To use an equalizer to eliminate feedback, you need to find your “invasive frequencies.” There are some specialized pieces of hardware available that specifically help to find and suppress invasive frequencies, but these can be expensive additions to your PA. There are also some great equalizers that will have a light on the fader to indicate which frequency is causing your feedback. But I’ve found that I can gain similar results with a simple, free iPhone application called “Feedback Detector.” Simply hold your phone near the speaker feeding back and it will tell you the approximate frequency of the intrusive signal. Lower the corresponding faders on your EQ and you will be suppressing the likelihood of feedback.
 
Fighting feedback sometimes means deliberately causing feedback. Whenever you are doing feedback testing, be extremely careful not to overdrive your FOH speakers or monitors. Keep someone at the faders so they can mute the channels if the feedback gets too hot. If you do this in a club, let the club owners know what you are going to be doing so they don’t get concerned. Fortunately, if you don’t change your equipment, you can take note of your dangerous frequencies and apply what you’ve learned at most similar venues. You don’t necessarily need to do this every time. Once you’re pleased with your equalization, the signal can be split and amplified.

Crossovers

Crossovers take the single FOH sound signal and split it up into multiple signals. The two popular uses are called Bi-amp and Tri-amp setups. Bi-Amp setups will split the signal into a low signal and a Mid/High signal. Tri-Amp setups split it into a Low, mid, and high range. I find that many decent FOH speakers include a basic internal crossover that will split your Mid and high signals for you, so in many cases, Bi-Amping is sufficient. You can even get away with a single signal for some venue, but I am a big fan of subs, so I will advocate the added expense.
 
Crossovers will have dials which allow you to manipulate where in the frequency spectrum the split will occur, and how much of each frequency will be able to fade into or overlap into the other channel. This is another place where experimentation will be helpful. Find a cutoff frequency for your low signal (I set my PA around 20-30 Hz) and simply see how your subs perform. If you think your subs can handle more, dial it up a bit, or if you just want hard hitting lows, dial it back. I find 20-30 Hz to be a nice compromise. This is a setting that you will likely only need to do once over the life of the PA. Once you’ve found the “sweet spot,” you shouldn’t need to adjust it again, barring a major change in your band’s sound. Now your signal is ready to be amplified.

Amplifiers

Amplifiers will boost your signal so that it can drive your speakers. Some amplifiers are built right into the speakers creating “Powered” or “Active” speakers. Other amplifiers are separate from the speakers, making the speakers “Passive”. Active speakers are great for novices because the internal amplifiers are perfectly rated for the speakers they are attached to. The downside is that you will need to plug each speaker into its own power source and they also tend to be more expensive than passive speakers.
 
Amplifiers will have an output power and an impedance rating for each amplifier channel that should be carefully noted. It is very important that your amplifier is compatible with your speakers. A mismatch could result in an overdrive or frequent clipping, both of which can cause permanent damage to your equipment. There are numerous arguments about how to properly match power rating to the speakers peak and optimal RMS. I’m going to take a stab at explaining it.

Pairing Speakers and Amplifiers

Amps are rated for output power (Watts) at a certain impedance (Ohms). Simply stating that you have a 1000 watt amp may not be accurate, since the “power” will not always necessarily be consistent. As the impedance (number of Ohms)of a line changes, the power to each speaker will change with it. For example, a fairly popular amplifier, the Peavey CS-2000, delivers 1075 watts (rms) per channel @ 2 ohms, 750 watts @ 4 ohms, and 495 watts @ 8 ohms.
 
Speakers have an impedance rating in ohms, and an optimal (program) and peak power rating. The Peavey PR-15 is an 8 ohm speaker that handles 400 watts program and 800 watts of peak power. When speakers are “daisy chained” off of a single amplifier channel, the line impedance changes. How it changes depends on if the speakers are in series or parallel. If you have 2 speakers in a parallel chain (I would never advise chaining more than 2 for common applications), then your impedance is divided by 2. If they are in series, the impedances are added together. Peavey PR-15’s are connected in parallel (this information is usually labeled on the connector plate.), so the line impedance will now be 4 ohms.
 
As we can see above, the CS-2000 amplifier provides 750 watts at 4 ohms. This means that 750 watts will need to be divided amongst all of the speakers in that chain. In this case, each speaker will get 375 Watts of peak power. In this case, it falls under our program power, and not even close to our peak power. Though I’ve seen this application work, it is not healthy for the equipment. I would recommend getting a slightly more powerful amplifier. The pros say that the Amplifier should be able provide roughly 2/3 (66%) peak power to each speaker. If we have a pair of PR-15’s, we will be looking for an amplifier that can provide 66% of 800 watts, or approximately 528 Watts at 8 Ohms. Since we are hoping to use them in parallel, then we need double that, or 1056 Watts at 4 Ohms.
At first glance, it looked like the Peavey CS-2000 was a good pairing for our PR-15, but after we calculated what the output power would be, it fell short. Now, if we decided to only hook up one speaker per channel, the impedance would be 8 Ohms. So each speaker would receive 1075 Watts of peak power. This is higher than the speaker’s peak power rating. Fortunately, the CS-2000 has a dial to reduce the peak power output. A user could simply dial the knob back to the center mark, and each channel would receive half of that 1075 watts (approximately 537 Watts). This will put the peak output power almost perfectly at our 66% range.
 
What have we learned? Do your homework before deciding how to connect your speakers to your amplifier. Matching power isn’t always as straight forward as it looks.

Note about wiring – Always keep your load above 2 ohms and below 16 Ohms .Most amplifiers are not designed to handle anything more than that.
 
~Now that we are hooked up and ready to go, In the next article we will discuss performing a sound check.~

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