Selecting and setting up a PA system for a live performance can be an overwhelming process. There are the microphones on stage, the mixing console at front of house, the power amps and the speakers. Everything needs to be specifically calibrated otherwise you run the risk of damaging the equipment, or worse, having to cancel the show and refund everyone’s money.
With all of that being said, selecting the proper PA is actually much simpler than it seems, and deciding what is right for you starts with just a few questions. Whether you’re in a band looking to supply your own system for shows, an engineer looking to rent and operate PA systems for events, or even a venue manager looking to upgrade your rig, continue reading below to discover how to choose a PA and set it up.
Selecting the Right PA
Let’s start with the basics. There are three components to every PA system:
- An input transducer (microphones or line-level output from an instrument)
- Amplifiers (both preamps in the console and power amps in the speakers)
- An output transducer (speakers or headphones)
That can range from simple DJ setup with two turntables and a microphone, to a full blown stadium spectacle with line arrays and time-aligned fill speakers.
To select the perfect PA for your event, you need to know a few details about the gig. You don’t want to show up to a coffee house with a semi truck full of line array speakers, or worse, show up to the arena gig without one.
Obviously, larger spaces require more powerful PA systems to fill the venue with sound, but to determine exactly how powerful, you need to know four things.
1. How far is it from the speakers to the back of the room?
The further you are from the speakers, the quieter signal becomes. In smaller venues this likely won’t be an issue, as the level difference from the stage to the back of the room will barely be noticeable. But, as you get into larger venues, the level decrease becomes more apparent. While every venue is different, here’s are some approximate dimensions to get you in the ballpark:
- Coffee shop, restaurant, bar: 5~10 m (16~32 ft)
- Club, auditorium, house of worship, small outdoor festival: 10~30 m (32~96 ft)
- Concert hall, stadium, arena, large outdoor festival: 30 m+ (96 ft+)
Once you know the distance from the loudspeaker to the farthest listener, you can determine exactly how much signal loss you’re going to incur at the back of the venue.
Every time you double your distance from the speakers, the SPL will decrease by 6dB. For example, if you’re standing 1 meter (~3ft) away from a speaker pumping out 110dB SPL the signal will decrease as follows:
- 1m (~3 ft): 110 dB SPL
- 2m (~6.5 ft): 104 dB SPL
- 4m (~13 ft): 98 dB SPL
- 8m (~26 ft): 92 dB SPL
- 16m (~52 ft): 86 dB SPL
- 32m (~105 ft): 80 dB SPL
- 64m (~210 ft): 74 dB SPL
- 128 (~420 ft): 68 dB SPL
So, even though they may be plenty loud in the front row, those in the back can easily talk over the music. That’s when fill speakers come into play, which are additional speakers strategically placed to keep the SPL level consistent throughout the venue.
2. How loud does it need to be?
The general rule of thumb here is the heavier the music, the louder it should be. Genres like folk, jazz and classical typically range from 80-95dB SPL, while genres like pop, hip-hop and rock are typically around 95-110 dB SPL.
After determining the desired volume (be sure to check local noise ordinances!), you can select appropriate speakers for the gig. Make sure you find something with a maximum SPL capability high enough to meet your needs!
*Author’s Note: Because loudspeakers are tested in anechoic chambers, the technical specs don’t account for room reflections. If you’re mixing indoors you’ll typically see a 6dB SPL increase throughout the venue, which can be really handy when trying to squeeze a little more volume out on an undersized PA.
3. What's the sensitivity rating of your speakers?
All speakers have a listed sensitivity rating, which essentially measures how efficiently a speaker converts wattage from power amps into SPL.
Smaller PA speakers have lower sensitivity, meaning they operate less efficiently than larger PA speakers. Speaker sensitivity is calculated by sending a speaker 1 watt of power and measuring the SPL from 1 meter away. Professional PA speakers can range anywhere from 95 to 110 dB SPL/Watt/meter, meaning a single watt of power can produce anywhere from 95 to 100 dB SPL, depending on the speaker.
Now that you know the technical specs on your PA speakers, there’s only one more question before selecting the right power amps for the gig.
4. How much headroom do you need on your power amps?
Live music can have transient peaks as high as 25 dB above the average level, which can cause power amps to peak and create distortion. There are two options for dealing with this. 1. Use a limiter on your master output and prevent signals from peaking above a certain point. 2. Provide enough headroom on your power amps that the peaks won’t cause clipping.
That’s much easier said than done. To add 3 dB of headroom you need to double the amount of power you’re supplying to the speakers. That means if a 1000 watt amp allows you 3 dB of headroom, you need a 2000 watt amp for 6 dB of headroom.
While more is almost always better, you can get by with as little as 3-6dB of headroom in most cases. However, it would still be wise to include a limiter just to be safe.
Now that you’ve answered all four questions about the gig, it’s time to do the math and determine exactly how much power you’ll need to fill the venue. But hey, you’re an audio engineer, not a mathematician, just use a calculator!
As you can see, power requirements can quickly skyrocket. Here’s a quick reference guide for common shows:
- Coffee shop, restaurant, bar: ~250 W
- Small/medium club, auditorium, house of worship, outdoor festival: ~250-1500 W
- Medium/large club, auditorium, house of worship, outdoor festival: ~1500-3000 W
- Large concert hall, outdoor festival: 4000-15,000 W
- Stadium, arena: 15,000 W+
Author’s Note: Some major production companies tour with as much as 400,000 watts of power!
Pairing Speakers + Power Amps
Most permanent or large installations use passive PA systems, meaning the speakers require external power amps. This allows you to fine-tune the system and dial in the perfect amount of power for every venue. To find the right power amps for your system, check the tech specs of your speakers for the following stats:
- Nominal Impedance
- Continuous/RMS/Program Power Capacity
Impedance is a form of electrical resistance, measured in Ohms. Most speakers are designed to operate at 4, 8, or 16 Ohms—as are most power amps. This information can be found in the tech specs.
It’s important that you match the impedance rating of your speakers to the impedance rating of your power amps. If your speakers' impedance is too low for the amp, the amp will send more power than the speaker can handle, and can damage your equipment. If your speakers' impedance is too high for the amp, it won’t send enough power to the speaker, and the system will sound weak.
Power Capacity is essentially a measurement of how much wattage a speaker can continuously handle. This number should not be confused with Peak Power Capacity, which is a measurement of the most power a speaker can handle in a given instant. If you try to continuously supply a speaker with the peak power wattage, it will wear out very quickly.
As a general rule of thumb, you want an amp that can supply 2-4 times the continuous power capacity, which allows for 3-6dB of headroom for peaks. This should be sufficient headroom for any genre as long as you have a limiter on your master output. If that’s not possible, you may want to supply more power to account for larger signal peaks.
Setting Up a PA System
After selecting the speakers and power amps for the gig, it’s time to put it all together! Although every PA system is different depending the particular equipment being used, here’s a basic overview of PA signal flow:
- Instruments and microphones connect to the stage box/snake on stage.
- Signals are split and identical copies are sent to front of house and monitor world.
- Signals are processed on the console, and sent out the console outputs.
- Each output is processed by an EQ, and in the case of front of house, most likely a limiter.
- Depending on the system, signals may be run through a speaker distribution system to disperse the main left and right outputs to a variety of speakers.
- After distribution, signals are routed to their respective power amps.
- Power amps are connected to their respective speakers.
Now that the PA is set up, all that’s left to do is test the system, tune the room, and soundcheck the band before showtime!