A Musician's Guide to the PA System
As musicians, we use a PA system for the same reason we make music: we want to be heard. And whether it’s for 25 intimate friends or 2,500 adoring fans, the PA system lies at the heart of being able to share your musical talents. So if you’re looking to buy your first PA system, expand an existing system, upgrade to a new level, or just gain some knowledge and insight, give this guide a quick read and we’ll walk you through all the basics.
To begin with, the initials in "P.A. System" stand for Public Address system—a somewhat catch-all phrase for all types of sound reinforcement equipment used in all sorts of situations; this includes everything from a school pep rally or a yoga class to an acoustic coffeehouse or a professional touring band—and beyond. There are PA systems ideal for permanent installation at the venue, and systems designed for the ultimate in portability; there are systems engineered to deliver superior clarity, and systems created for maximum sound and coverage. But no matter the size or application, all PA systems do the same thing: combine a variety of audio sources, enhance the sound, and play this sound back at comfortable listening level through a series of loudspeakers. Sounds simple, right? Well, in a way it is. But as they say: the devil is in the details.
Learning the Components
Probably the best way to understand the details of a PA system is to explore each of the components separately, learn what each one can do, and appreciate how they all come together. From one end to the other, the "Signal Chain" starts at the mixer inputs, and ends when the sound comes out of the speakers. Here are the components in a typical PA system:
- Effects & Signal Processing
- Crossover circuitry
- Stands and Mounts
Now don’t get alarmed—the idea is not to build a huge shopping list. The fact is, nowadays many—or all—of these individual components can be combined in a single component. There are powered mixers that combine the mixer and amplifier; active loudspeakers that combine the amplifier, crossover and speaker in a single box, and many mixers have reverb and signal processing built right in. That said; let’s have a closer look at each component.
Just as the name suggests, here is where all of the audio sources that will be amplified by the PA system are connected, and then mixed together to balance the volume and tone of each input. Normally, vocal microphones are the most important signal source. Acoustic instruments such as sax, flute, violin, and even acoustic guitars are next—using either more microphones or the instruments’ built-in pickups. Often, the electric guitar amplifier and the drums have enough stage volume so that they do not need the additional boost of a PA system. However, if you are trying to create a live recording using the mixer—or are playing in a larger venue—you will need to mic' up the drums and the stage amplifiers of the other instruments. Many guitar, bass, and instrument amplifiers feature a line output that can plug directly into the PA system. An additional tool often used for bass, keyboards and other instruments is a direct box, which splits the signal, allowing the player to use his stage amp as a monitor, and also sends the sound to the PA system.
On the mixer, each input will generally have the same set of controls, duplicated in what is called a "Channel Strip."
The Gain Control (Input Level) is used to set the level coming from the input source as high as possible WITHOUT "clipping" or allowing the sound to distort. Although this control will affect the level, it should not be thought of as a volume control.
EQ (TONE CONTROLS)
Using these controls (when offered) allow for the shaping of the tone of each input source individually.
Many mixers have some type of built-in effects—often reverb and/or delay. The individual Effect knob allows the effect to be applied to each input in varying amounts. If the effects are not built in, there is usually a series of inputs and outputs to accommodate an external effect processor; in this case, these knobs will create a separate mix of all of the inputs that will feed into the external processor.
Monitor speakers—either on the floor or in the ear—allow the musicians on stage to accurately hear themselves as they play. Once again, the on-stage amplifiers are often all that is needed for the instruments to be heard during performance. Vocals are a different issue, and the monitor speakers are needed to allow the performers to hear their own vocals. The monitor level knobs allow the monitor speakers to receive and reproduce a separate monitor mix.
The Pan control determines the placement of the input sound in the Left – Right stereo field. In practical use, it may be best to use the pan control sparingly. Creating a stereo mix is great when recording a CD; in a live situation, panning an instrument or microphone far to the left may inadvertently cause the sound to be unheard by listeners on the right side of the venue.
Often a fader, sometimes a knob, this control is used to set the volume level of a specific audio input in relation to the other audio inputs.
In addition to these individual channel controls, the mixer will feature a number of Master Controls—Master Level, Monitor Master Level, Effects Master Level, etc. Unlike the EQ and Tone controls available on each individual channel, the Master EQ (often a graphic EQ) is primarily used to control the overall sound of the PA system in a particular venue.
Finally, there are some other very useful features available on select mixers. An "AUX" auxiliary input—occasionally a Bluetooth input—allow the band or the venue to play MP3s or CDs or another audio player between sets or for any other use.
Effects & Signal Processing
As we mentioned in the Mixer section, many mixers have built-in effects such as reverb, providing a certain sense of depth to the overall sound. If not, most mixers are set up to allow an external reverb or delay unit to integrated into the PA system, often with individual effect "sends" on each mixer input. And of course, you can always use an "in-line" effect on an individual source before plugging it into the mixer.
Raw Power. The amplifier takes the electronic signal coming from the mixer and increases the signal to a level high enough to drive the loudspeakers. Power is not the ultimate determining factor in how loud your PA system will be. Speaker efficiency, dispersion patterns, and other elements will also influence the final perceived volume. But in the end, clarity—not volume—should be the goal of a well-implemented PA system.
Loudspeakers & Crossovers
At the front end of a PA system, the diaphragm in a microphone converts mechanical energy (vibrations in the air) into an electrical signal that is sent to the mixer. At the other end, the loudspeakers convert this electrical energy back into mechanical energy by moving some air!
As we mentioned earlier, there are both active (powered) speakers—those with an amplifier built in—and passive (un-powered/non-powered) speakers. Larger speakers are generally able to reproduce lower frequency (bass) sounds more efficiently—but not always. Bass porting and other cabinet enhancements can allow smaller drivers to operate in the lower ranges with extra clarity. And of course, subwoofers can be added to most systems if extra low-end response is needed. With the upper frequencies, there are many different types of drivers—compressions drivers, radial horns, and multiple smaller speakers in a linear row (line arrays)—all with different advantages for providing clarity, coverage, and portability.
Unlike guitar amplifiers, which often use multiple full-range speakers of the same size, PA and sound reinforcement speakers cabinets usually contain separate bass and treble transducers—woofers and tweeters. Inside most speaker cabinets is a crossover; this electronic component sends the right frequencies to the right speakers. Below a certain frequency, the sound is sent to the woofer. Above that frequency, the sound "crosses over" to the tweeter elements. In many large touring sound systems and permanent installations, the crossover may be an external component. In this case, separate external amplifiers are used for the woofers and tweeters.
Stands and Mounts
Don’t overlook this important element of a great-sounding PA system. Speaker placement can influence the clarity, tone, and perceived volume of your sound. Treble sound is very directional, and is easily absorbed by people, carpet, chair cushions, etc. Bass sound, on the other hand, is not very directional, and the longer wavelengths are more easily reflected. By placing the loudspeakers on stands, you can move the speakers forward away from walls and corners to improve the projection and increase the size of the effective listening area. Raising the speakers will also lower the bass response of the speakers, which is ideal for increasing vocal clarity. Another way to provide projection and clarity is to use a "column" of smaller drivers—such as in the design of certain Bose systems. When speakers are placed directly on the floor, the bass frequencies are reflected off the floor—or walls and corners, if close enough—which can muddy the sound in certain situations. When adding a subwoofer to your raised-speaker PA system, placing the sub on the floor or in a corner can actually increase the bass response. Mounts are available for permanent installation and provide the same benefit as stands.
Tech Tip #1
Proper Gain Staging to Reduce Noise
Noise, to some degree, exists in every electronic circuit or product and is often referred to as the "noise floor." In addition, noise can be added in the form of distortion by "clipping" or "overdriving" the inputs of each gain stage in a PA system. The trick is to raise the signal level as high possible away from the noise floor at each gain stage, without clipping the input signal or adding distortion.
Tech Tip #2
Active or Passive Loudspeakers
Active loudspeakers are often bi-amped; using a separate amp for the woofers and the tweeters. One advantage here is that the crossover circuit is placed before the power amp, which allows the crossover to operation to be quieter and more efficient. One thing to keep in mind is that active loudspeaker cabinets require not only an audio connection, but will also need an AC power connection. This can mean extra wiring, and may therefore limit the placement of the speakers.
Tech Tip #3
Understanding Ohms and Power
Adding passive speakers to your PA system is possible, as long as you mind the Ohms (Ω) and power ratings of your amplifier or powered mixer. Named for early electric pioneer Georg Simon Ohm, the Ohm rating of a speaker cabinet is a measure of its resistance to the electric current carrying the signal. It may seem counter-intuitive, but when adding one 8 Ohm cabinet to the original 8 Ohm cabinet, the result is roughly a 4 Ohm load, not 16. If the outputs of your amplifier or amplified mixed cannot handle a 4 Ohm load, the amplifier will operate in an unsafe range and may overheat—or worse. Check the specifications or contact the manufacturer to be sure. Active speakers often a pass-through output, so the input signal can be sent along to additional active loudspeakers, without worrying about impedance loads, etc.