PA System (an integration)

Several years ago I put together a simple Public Address (PA) system for FLARE. I and my team have used this system many, many times, including the last time we hosted a National Sport Launch in Alamogordo, NM. We haul it out whenever we host scout launches or other school events. If we are expecting more than 25 people we use the system.

Back in 2009, 2010 FLARE was working with member Dave Kovar to do a series of launch events for commemoration of the 40th Anniversary of the Apollo Program. A few of the events we staged included astronauts and flight directors from NASA and were attended by several thousand students and the general public. Planning meetings included educational program offices at our local university and one of my priorities was a suitable PA system to host these events.

I put together a proposal to build this system and requested modest funding to do so. The total costs were less than $300. The system included a wireless dual microphone system, amplifier, speakers, and two 9 ft. stands on which to elevate the speakers.

When I asked Laura, the program director, about funding the project she stated that her husband had looked over my proposal and said that it would not work because the amplifier would only work in a car. Rather than attempt to discuss the essential "car-ness" or "not car-ness" of a 12 vdc audio amplifier, I concluded that there would be no funding for the project. The meeting moved on to other subjects.

I went ahead and built the system out of my own pocket and later asked FLARE to recompense — which they did without hesitation. The system had worked flawlessly at several events already, and if it was good enough for Gene Kranz then it was pretty darn good.

This system was designed for outdoor use on a large field where the audience are all behind a stage area or flight line. It is optimized for voice frequencies.

Rocket launches have a clear set of mandatory guidelines (the Safety Rules) to make sure that everyone is aware that a rocket is about to launch. This is so that everyone is paying attention in case something goes wrong and a rocket is headed your way. Modern life and people being as they are, there is someone on a cell phone chatting or looking at social media, and someone else so busy prepping their rocket that they are not aware. This is where a PA system really helps. It is an important part of public safety, especially at a large launch.

It is not terrifically difficult to put together a capable system using off the shelf components. Knowing how to do this is a craft — as is knowing how to use them effectively. So this is a bit about that craft.

The Components

Everything needs to work off a 12v DC power source. Hauling inverters out to the field in neither practical nor efficient. 12v power amplifiers designed for automotive or marine use are not that hard to come by (as least not until recently, more on this later). From easiest to hardest to acquire, you need:

  • A 12v DC power source capable of 2 or 3 Amps continuous supply

  • 16 gauge lamp cords 20-30 feet long to connect amplifier to speakers

  • Horn speakers, because they are optimized for voice frequency broadcast

  • Microphone(s) and preamp to feed the sound into the amplifier

  • Speaker stands 8 feet or taller, to get those speakers above head level

  • An audio amplifier, mono or 2-channel, of 50-200 watts

For a power source I really love automotive jump starter battery packs, which are Lithium Ion batteries that are so easy to charge and use. These things vary in quality and there are a few tricks to using them. You can get a decent one for $60-80. I use these for launch controllers and PA system.

Horn speakers, looking to eBay, are not too dear. I like the ones offered by Pyle. I have used these for many years and can say that they are tough and effective. Those rated for 50 watts are sufficient. The 6 inch rectangular horns are excellent. The only precaution is that it is possible to break one. Do not toss them around or drop them, as the internal speaker element can snap off and the speaker will no longer work.

My choice for the Microphone/preamp system is one that provides wireless headsets or lapel mic's for hands-free operation. It is surprising how cheap these are now (God bless the Chinese). You can find really nice systems for under $50 on eBay. The base station runs on 12vdc and provides volume controls and preamp. You can spend less, but after using these you will not regret the choice.

Speaker stands are the most expensive component. Especially those providing 8ft or taller extension. You will spend more than $100 on these but they are essential to the operation of your system. You must get those speakers above the level of vehicles, structures, and people in order to get clear sound out to everyone. Professional quality speaker stands will not blow down in the wind.

Finally, the Amplifier. Back in 2010 when I put this together there was a wider selection of auto after-market amplifiers. These days new cars are delivered with pretty fine sound systems, so this market is shrinking. Last time I focused on Marine class amplifiers designed for use in wet environments. This is still where I am looking, but the choices seem a lot more limited, and they are more expensive. I could spend a whole page describing the hazards of shopping for amplifiers, especially on eBay. You want an RMS Power rating of 50 watts minimum.

Nearly everything you see on eBay under $60 is actually providing a LOT less than this, no matter what the description says. There are little amps that say they are 200 Watts. When you get a look as the manual they are 15-18 Watts RMS. Not good enough! Did you ever hear that you are more likely to cut yourself with a dull knife than with a sharp one? Same principle here. You are far more likely to burn out a speaker with an underrated amplifier than you are with an amp that is rated to deliver more power than you speakers are rated for. This is because your speakers will not sound loud enough and you will turn the amp all the way up until is is pumping out distortion.

The other caution is subwoofer amplifiers. Many of the better rated amps that seem more affordable are optimised for base frequency amplification. Many have low-pass filters on the input which means that your voice frequencies will be filtered out. These will not work at all. However, they are all the rage in this market right now because young men love to make lots of noise. Most single channel (mono) amplifiers are made specifically for this function.

So I am again researching marine amplifiers and I am finding them to be in short supply. Since I am trying to produce systems for resale, I need to make sure that the product I design around will still be for sale when it comes time to produce the next one. This is not your problem. 'Just saying.

I have an eye on a Pyle 2-channel marine audio amplifier PLMRMP2A. I have experience with Pyle products and I trust them. These are currently priced around $60 making them the second most expensive component in your PA System. But they appear to be sturdy, water resistant, and seem to be made for integration into a system.

Range Layout

Understanding how to lay out a PA System is important. Some of this is not obvious if you have never heard about it before. Your audience is behind the flight line. This is where the sound needs to be directed.

The speakers need to be positioned well behind the microphones or feedback will send a shrill squeal out to your audience. Feedback happens when your microphones pick up the sound from your speakers and feed it back through again. Most microphone systems are fairly resistant to this, but none can prevent it entirely so take care to observe this rule. Also, don't forget to mute or turn off your microphone if you venture behind the flight line. You will remember what you just read should you ever forget this. Trust me, I've done it.

The speakers need to be 40 or more feet apart to maximize the range of your soundstage, and they need to be 8 feet or higher up in the air to clear any obstructions.

The systems I build have red and black speaker terminal clips so that you can hook up the speakers with the correct polarity. The lamp cords are marked for polarity; if you look closely you will see that one conductor is ribbed and the other is smooth. Call the ribbed one positive (red) and the smooth one negative (black). Honor this as you connect the wires to the amplifier and speakers. If you do not, the sound from the speakers will cancel each other out down range. This is because, out of phase, one speaker is pushing while the other is pulling. This is called destructive interference.

Another important factor is the impedance of the speakers. Most of the horn speakers I see are rated at 8 ohms. Most of the amplifiers I will drive 4 ohm or 8 ohm speakers — but will supply higher wattage to 4 ohm speakers. I most often connect both speakers in parallel to one channel, carefully observing the polarity on the connections. If I really needed to drive a large launch range, I would up my system to four speakers (even if they are still mounted on two towers) and wire the two speakers on each side in parallel.

If you know the basics of setting up a component sound system it is not difficult to put together a very effective and durable PA System for your launch range. Connectors for power and speakers, a power switch, and audio connectors and speaker cords are easy to buy on eBay and/or your local hardware store. The base I fashioned to put it all together was a large Polypropylene cutting board cut into suitable panels. Do not obstruct the heat sink on the amplifier. It is just fine to drive both amplifier and wireless microphone system from a single power source.

At the 2010 NSL I ran this PA System for 9 hours a day on a 2000 mAH battery. I had a spare ready the whole time and never needed it.

There is no question that a loud, clear PA System is a real asset at any launch. When we use one at our launches it gives our guests an unmistakable sense that we are professionals and we know what we are doing. It helps everyone to be aware of activities and cautious of rockets in flight. A PA System is an excellent investment in your club.