Back to School : What is Professional Audio Systems?
In short, a PA system—more formally, public address system, and also known as a sound reinforcement system—is an electronic amplification system used to get sound from the performer(s) to the audience. It's made up of several components, and while one system can vary greatly from the next, each one handles these same basic functions:
- Converting acoustic sound into electronic signals, most often with microphones or line inputs.
- Processing and mixing these electronic signals, using mixing boards and effects modules
- Amplifying signals with a power amp
- Delivering the sound through speakers
- Monitoring the performance with speakers or in-ear monitors
Different PA equipment will have different capabilities, features, and designs associated with each of these functions. Your specific needs will determine what you want out of each.
Live Sound Mixers
Electronic signals from microphones and instruments need to be balanced, processed, and mixed together before they can be amplified and routed to PA speakers and monitors. In your live music PA, this is where the mixer comes in.
Mixers range in size from simple 4-channel units to much larger consoles that have hundreds of channels. A channel is essentially a signal path. A mixer with a large channel count allows more things to be connected and routed through it.
Channels are usually designed to accept microphones and/or line-level devices such as amplifiers, preamps, or signal processors. (Microphones and instruments such as guitars and basses output electronic signals that have a much lower level than line-level devices.)
On a mixer, audio signals are assigned to separate channels, so a fundamental question to start narrowing down your mixer selection is “How many channels do I need?” Generally, you will want to make sure you have more channels than you think you will require. So it's important to take an inventory of what you will be amplifying and make sure you have sufficient inputs and outputs while allowing for future expansion of your PA system.
For instance, if you are plugging a standard five-piece rock or pop band into your PA, your first instinct might be to shoot for an 8-channel mixer. After all, that would allow for five instruments with room to spare. However, once you add up the inputs for everything you need for the group, you will find that your 8 channels actually won't give you enough inputs.
With a five-piece band, the live sound arrangement might look something like this: one mic for lead vocals and one for backup (two channels), one mic for the guitar amp, one for the bass amp, and at least one direct input for synthesizers (three channels). Then there's the drum kit, which will have its own miking considerations. With two overhead mics for cymbals, a mic on each tom, the snare, and the bass drum, you are looking at a minimum of seven mics for a fully miked kit.
This brings your total required inputs to 12 channels, so you’d want a model with a minimum of 16 channels giving you room to expand.
Analog Mixers versus Digital Mixers
Analog mixers are the mainstay of any audio system, and range in price and features. There are some diehard analog enthusiasts who will not move to a digital mixing board, as they believe the analog components sound superior to digital. If you are mixing a live band, you will want some additional signal processors to shape the sound of each instrument. Most analog mixing consoles will offer a built-in four-band parametric EQ, which helps balance the tonal sound and carves out space for each instrument in the mix. It is rare to find analog consoles with built-in dynamics available on every channel. Therefore, an all-analog setup will require several racks of gear to accommodate the additional signal processing, such as compression and gates for each channel.
Another aspect to consider is the use of wedge monitors or stage monitors. These are speakers that are typically on the floor and angled up toward the performers, offering a dedicated mix, which allows the musicians to hear themselves on stage. Feedback can become a problem, so the use of graphic EQs will be needed to remove the frequencies that are feeding back. Add in additional signal processors like multi-effects, delays, and reverbs and you can see the analog setup may sound better, but will cost more money with the additional signal processing, plus there are additional racks, cabling, troubleshooting, and maintenance involved.
Digital mixers have made some considerable advances in recent years regarding the quality of the sound, and pricing that is comparable to many moderately priced analog consoles. Digital mixers offer the best solution for any touring band, with a large channel count and each channel packing four-band EQ, compression, and gating. Additionally, each output features graphic EQ for ringing out monitors. Many mixers feature internal effects with up to eight insert slots for use with internal sends. You can still use your favorite outboard gear, but the digital platform reduces the amount of gear substantially. Another benefit of the digital mixer is the wireless control options. Many mixers offer iOS and Android control apps.
If the FOH position is in a less than desirable place, the engineer can move about the room to make informed adjustments based on the audience’s perspective. This also allows the engineer to tweak monitors from the stage, while standing next to the musicians. Many mixer platforms will allow multiple device setups in which band members may adjust their own mix in real time, allowing the FOH engineer to focus on the main mix. Other features now incorporated in the digital platform include spectral analysis and a real-time analyzer (RTA) for making adjustments to monitors or to the entire mix. However, I still recommend a dedicated speaker processor for tuning the sound system.
For singles, duos, acoustic trios, or any group that plays in smaller clubs and coffeehouses, you'll probably want a PA that is compact, easy to carry, and can fit easily in your car with your other instruments. For you, a powered mixer is probably the best choice. Most of what you need - power amps, mixer, and often EQ and effects - are usually combined in one box. Plug in a mic and speakers and you're good to go!
PA Power Amplifiers
One of the most important questions when it comes to PA systems is “How much power do I need?” This is a consideration when purchasing a power amp for the system.The power amp's job is to boost the low-level signals coming from the mixer and broadcast them through the speakers. How much power it produces is measured in watts. And you want to make sure you've got enough wattage to fill the venue without compromising the sound quality.
Exactly how many watts you need hinges on a number of variables. The most obvious of these is the performance location (room size, indoor/outdoor, acoustics). However, there are additional factors that complicate the issue. For instance, there is the efficiency of the speakers (i.e., how much sound the speakers produce per watt of power). There also is the concept of headroom (how much power it takes to handle peaks without distorting) and the desired volume level of the music.
Using speakers with average sensitivity, a rock band playing in a medium-sized club will need around 1,500 watts total power at a minimum, whereas a pop or jazz group might need between 250-750 watts. For simple folk music in the same venue, that requirement can come down to as little as 60 watts. Keep in mind though that these power estimates are generalizations; difficult performance spaces and music with a lot of dynamics can require considerably more power. As we note below, factoring in plenty of headroom will help ensure great sound when you’re performing in a challenging environment.
It's important to buy an amp with plenty of power to drive your speakers plus enough headroom to prevent distortion. When shopping for speakers, you'll see that they have a power rating, measured in watts. As a general rule, you will probably want an amp with twice the wattage of your speaker's rated power handling to ensure a clean, undistorted signal gets to them. We will discuss this further when we cover PA speakers and their power requirements.
Keep in mind that a stereo power amp provides two channels, each able to drive its own speaker load. So if your amp provides 500 watts per channel, a pair of speakers rated for 250 watts would be a good fit. Note that the rated output for stereo power amps is usually given on a per-channel basis. A rating of “2x450W” indicates that the amp generates up to 450 watts into each of its stereo channels.
Once your mixer, signal processing gear, and power amp have shaped your audio signals, it's your speakers' job to turn those signals back into physical sound waves. Speakers reinterpret the signal by using the voltage from the amplifier to move their cones back and forth, producing the sound waves that reach the audience’s ears.
Maybe it goes without saying, but speakers play a critical role in delivering quality sound to an audience, and it’s an area where quality gear can make a real difference.
As is true for the power amp, the size of the venue you play will help you decide on the power handling (wattage) and size of the speakers will need. For example, smaller gigs, conferences, and lectures may require about 350-500 watts, while club bands, garage bands, and mobile DJs may need 500-1,000 watts, or even more, depending on the venues they perform in.
In choosing PA speakers, the key trade-offs to consider are portability versus performance. While full-range speaker cabinets that contain a woofer, mid-range driver, and tweeter are more portable and easier to set up, they typically won’t deliver the same performance as high-end speaker arrays.
What type of individual speakers are there?
Speakers are designed to reproduce specific frequency ranges. So-called “full range” speaker cabinets usually contain a woofer and tweeter and are referred to as a two-way system. Some PA speaker cabinets also include a mid-range driver and are referred to as three-way systems.
Your choice of speakers should be based on coverage requirements and the size of the venue. There are some things to consider regarding the shape of the room and how the speakers will interact with boundaries, such as the walls, the ceiling, and the floor.
You want to get the best speakers your budget will allow. Start by figuring out what you can afford and then determine what sounds best to you within that price range. Always listen to the speakers before buying, as not all of them are made equal. When choosing a speaker, you’ll want to consult the specification sheet, which should be readily available from most reputable manufacturers. The most important specs to know are the frequency response, SPL output, and dispersion. If you are using passive speakers, then you’ll need to know the wattage and impedance (ohms resistance).
A full-range speaker with a frequency response of 60 Hz to 18 kHz may be fine for many genres of music, such as country, folk, or folk-rock, where the kick drum and bass don’t need additional punch. For rock, metal, pop, hip hop, EDM, etc., you will want a subwoofer. A subwoofer extends the frequency response down to 45 Hz or lower and will allow the full-range speakers additional headroom and increased output.
Sound Pressure Level to Decibels Distance
Dispersion is the way the sound is projected horizontally and vertically from the speaker. This is incredibly useful for determining the placement of speakers, as you can direct the sound away from boundaries, such as walls and ceilings. For instance, a speaker with a 60-degree horizontal dispersion might work well for a narrow room, while adding an additional speaker could increase the dispersion to 120. The goal is to offer coverage to the entire audience, while directing the sound off the walls. Many speakers are designed to couple by utilizing a trapezoidal enclosure, versus a square or rectangular enclosure. The trapezoidal design allows for easy placement of the speakers, as they can be placed together in tight-knit group or array, which allows for coupling with reduced interference between speakers.
Active Speaker vs. passive Speaker
There are pros and cons to both active and passive speaker designs. Active speakers are the easiest to deploy with built-in amplifiers that are matched to the speaker components (woofers, mid-range, and tweeters—typically compression drivers). They also feature crossovers, which isolate and route frequency ranges to each component, and built-in limiters for protecting the drivers. A three-way active speaker will have two or more built-in crossovers, which isolate the high, mid, and low frequencies. The advantage of active speakers is the ease of setup and operation. They only require a line level input and you won’t have to use separate amplifiers to power them.
Passive speakers require amplification, speaker cables, and may require an outboard crossover and other signal processing. Some passive speakers will utilize an internal crossover network, which functions much like the active speakers. Other speakers are designed to be bi-amped or tri-amped, which can be a benefit, as this allows greater control over the speaker components, but also requires a separate amplifier for each component of the speaker. If you decide to go with a passive speaker design, you’ll need to look at the specification sheet provided by the manufacturer to determine the correct amplifier(s).
The input range of a speaker is typically given in continuous, program, and peak wattage measurements. You will most likely see the continuous output and either program or peak. The general rule is a doubling of the continuous results in program, while doubling the program will give the peak performance. For instance, a speaker with a continuous input of 1400 watts will offer a program of 2800 watts and a peak of 5600 watts. The larger the amplifier, the more headroom will be available. Do you really need to match 5600 watts to this speaker? Most professionals will say no. A good formula is to aim for 1.5 x the continuous input. A 1400 watt input x 1.5 is 2100 watts and should be the bare minimum for this speaker. A safer bet is to match the speaker to the program output of 2800 watts.
Another consideration is the impedance or ohms resistance for the speaker. You will need to consult the amplifier specifications to determine how much power an amplifier is able to produce at a given impedance. Most manufacturers will boast the highest output of both channels at the lowest resistance. When matching your amplifier to your speaker, it’s important to consider the ohms rating and wattage. For instance, an amplifier that is rated at 4000 watts (2000 watts per channel) at 2 ohms will realistically deliver 1400 watts at 4 ohms and 850 watts at 8 ohms. We could certainly use this amp with our 1400-watt speaker, but at its continuous output rate, it doesn’t leave much headroom. Without headroom, it is entirely possible we could drive the amplifier into clipping and potentially damage the speakers.
Some amplifier manufacturers will indicate power draws as 1/8 power, 1/3 power, and full power. 1/8 power delivers the amplified signal below the built-in clip limiters, while 1/3 power will have the clip limiters occasionally flashing. Full power will have the limiters in constant activity. When engaging the clip limiters, you are actually rounding off the audio signal to prevent distortion, but the signal of the audio will be compromised. I prefer to run the amplifiers at 1/8 power, which will give plenty of headroom without squaring off the waveforms. You may also use a higher-rated amplifier at 1/8 power without fear of damaging your speakers. Remember, the quickest way to blow a speaker is to underpower it.
Stage Snakes and Stage Boxes
A stage box or multi-channel snake is highly beneficial for reducing clutter on the stage. Some larger stage setups use a splitter that splits the signal from all the sound sources on stage between FOH and monitors. Most mid-level bands typically don’t have a dedicated monitor engineer, so the FOH engineer will perform both main mixing and monitor duties. With an analog setup, you’d be working with a 16- to 24-channel audio snake with a cable run of 100+ feet. A drum kit may have 8 to 12 microphones set up to capture the sound, so a dedicated sub-snake allows for shorter mic-cable runs and a much cleaner stage setup.
In order for musicians to be heard, microphones are used to capture vocals, guitar amplifiers, and drums. The mainstay of live music is the use of dynamic microphones. There are many microphone manufacturers, but the favorite of most clubs is still the Shure SM58 for vocals and SM57 for instruments. They have proven their value over time by sounding good and being incredibly rugged. They can literally take a beating and still function. If there is the budget and desire for wireless microphones, I personally recommend the Shure QLX-D series digital microphones. They offer clean, clear sound without any artifacts, and with a simple setup.
Signal Processing (Effects)
Mixers may have some built-in effects, but if you really want to make your sound stand out, additional outboard processing gear can help add drama and sparkle to your sound. Although they're not essential in all situations, signal processing and effects such as compression, limiting, reverb, and delay can add sonic interest to your sound that make them well worth the cost.
Here are some basic effects and signal processing tools that can enhance your PA system:
Compression and limiting
A compressor as the name suggests compresses the overall dynamics of the audio signal limiting the amount of variation between the loudest and softest sounds.It smooths your sound and protects gear by helping to avoid damage caused by clipping—a speaker-destroying phenomenon resulting from overdriving the amplifier into distortion. Well designed compressors not only prevent signal distortion, but add pleasing sustain to your sound.
A similar tool, the limiter keeps your speakers and ears from getting blown out by limiting the peaks in the music. A limiter allows compression to occur only above a set threshold, and the compression ratio can be very high. This prevents clipping, distortion, and other related problems.
Reverb and delay
Reverb (from reverberation) is an effect that makes sounds richer, as if they are reflecting off surfaces. Reverb adds depth and dimension—that’s why your voice sounds better when you sing in the shower. The hard, reflecting surfaces add “space” and drama to your vocal performance. The adjustable parameters on a reverb unit allow you to control these reflections to simulate various acoustic environments.
Delay is one or more echoes that have a more distinctly audible space between the initiation of the original sound and its reflection. There is a more distinct repetition of the original sound (partial words, musical phrases, etc.) and an eventual diminishing of volume over time. Delay is the effect you experience when you shout into a large canyon: your voice bounces back to you in diminishing waves.
Most mixers include an EQ section. The most simple mixer equalizer controls resemble those found on consumer electronics and typically raise or lower the low, mid-range, and high frequencies. More advanced mixers offer more fine tuning of frequencies as discussed below.
The important role of an equalizer is to balance out your sounds by allowing you to boost or cut highs, lows, and mid-range frequencies to improve your overall sound. The equalizer modifies the signal's frequency response by increasing (boosting) or decreasing (attenuating) selected ranges of frequencies. Equalization also can help fight feedback by letting you pinpoint and reduce the volume of frequencies where it occurs.
The built-in equalization circuits in your mixer may be adequate for fine-tuning your overall sound. But for the most demanding sound applications, a standalone equalizer will likely do a better job.
Types of equalizers
There are three types of equalizers commonly used with PA systems.
- Parametric EQ:
Semi-parametric/quasi-parametric EQ: These function in the same way as a parametric EQ, however the manufacturer sets the bandwidth—it's not user-adjustable. These units trade off somewhat easier operation with reduced functionality.
- Graphic EQ
Graphic equalizers provide more flexibility and control than simple two- or three-band tone controls, and they're quite easy to use. A graphic equalizer is a set of band-pass filters that divide the audio spectrum into 30 or more bands allowing you to control the amount of boost or cut in much narrower frequency ranges. Usually controlled with sliders, the effect of the equalizer is graphically depicted by the positions of the sliders—that's why it's called a “graphic” equalizer. The ease of use offered by graphic equalizers is largely due to the ability to easily visualize how equalization is affecting the overall audio signal by simply looking at the faders.
Graphic EQs are great for sound reinforcement and “tuning” rooms—adjusting the contour of your sound to match the environment. For example, if you’re performing in a space that produces a lot of boominess, you can cut the lower frequencies that are contributing to the problem. Graphic equalizers allow you to produce a consistent sound at every venue.
Many bands prefer to forgo the use of stage monitors and opt for in-ear-monitors (IEM). I’ve used the entry-level PSM300 Shure Personal Monitoring System for years and have had excellent results. With a digital mixer, the setup and operation is even simpler, resulting in very happy musicians who are able to set their own monitor mix—and without excessive stage volume.
As you can see, there are many directions one can choose when setting up a sound system for your band or event: analog mixers versus digital mixers; passive speakers and subs versus active designs. Each has its pros and cons. The most important thing is to use your ears when making decisions. Always listen to speakers before purchasing and, if possible, demo speakers and subs together, especially if you are using different brands. I can’t recommend enough the importance of having a dedicated speaker processor for any system, regardless of size or budget.
- South East Asia Musician