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How to Distributed Sound System for Political Demonstrations

Having an amplified sound system is a key component at marches, rallies, demonstrations, press conferences (and especially car caravans in the COVID-19 era). But too often events are under-resourced - where small speakers simply aren’t enough for large crowds to hear.

One solution: distributed sound systems - using an FM Radio transmitter to allow for additional speaker systems (or car stereos) to amplify the sound coming from a main stage to a much wider area. For large crowds of thousands of people - this can be a functional (and often better) alternative to hiring a large audio production company that can easily cost $5-10k.

Why do we want to ensure everyone can hear?

This system allows for multiple sound vehicles to play the same sound, so it becomes possible to convey information to much larger crowds. 
  • Making our politics and demands clear: Demonstrations are key moments with community leaders speaking to build political alignment, shared purpose, and motivation for our movements.  Rallies can often lose the crowd when people can’t hear.
  • Better Organizing: If we can’t communicate, we can’t organize, and we have all experienced how hard it can be to hear the demo when crowd sizes are 1000+
  • COVID Safety: People will crowd toward the sound system if they can’t hear, increasing COVID infection risk.
  • Hype: Sound keeps people’s energy up! It keeps people informed! It keeps us together.
  • Accessibility: People with disabilities are able to stay tuned in while in their cars. People who are hard-of-hearing will have a better chance of understanding what is going on.

The Sound System:
  • We can interface with any sound system used as a main stage (or sound truck).
  • Using the radio station (IE 88.3fm) and a radio transmission system with the main sound system, we transmit organizer’s speeches, chants, and playlist over a large area.
  • Every block or where original sound can’t be heard, additional speakers (or sound vehicles, or cars) can re-transmit the audio at maximum volume in real time
  • Our audio range is most clear when line of sight is possible between the main stage/transmitter and receiving vehicles. It’s ideal to park the main sound truck at an intersection.
  • For moving marches, vehicles at the end and in the middle of the march can blast out exactly what is being played/chanted/spoken at the front. Any car can tune in and help amplify the sound.
  • For more stationary demonstrations, we can strategically place sound vehicles on a grid so everyone can hear. (See diagram below)

Current Sound vehicle Rig:

Each sound vehilce needs 4 basic components, the vehicle, the speaker(s), the power souce, and the audio source (a FM radio)
  1. A Vehicle
    1. Pickup Trucks are ideal, as people can use the bed as a stage and there is a high capaicy for many speakers
    2. Toyota Priuses can be driven with the hatch back open, and have the low voltage (12-volt)  battery in the trunk, making it very easy to plug in  high-power speakers (see inverters below.) Pro-tip: Put a bungee cord hook in the latch to prevent “door ajar” beeping
    3. Cargo bikes are great as they can safely navigate dense crowds
  2. Speakers
    1. Wall Powered, or active PA speakers are very commonly owned by musicians and venues. In COVID, these are left unused. They are self contained sound systems, needing only a sound source and power to operate. We reccomend borrowing or renting these, as they can cost $500- $1500 each.  The QSC 12.2 is an example of model that is very loud.
    2. Battery powered speakers are generally not loud enough for this application, with some excpetions for high-end systems. Loudest, and pretty loud
    3. Passive speakers require a seperate amplifier and the loud professional models are easy to damage if used by unexperienced people. As such, they are hard to borrow. While they can be the loudest option, they should be operated by expereinced people.
    4. We have two cars set up with outdoor PA horns which are incredbly loud and carry voice very well. They also look great. They are lacking in bass response, and a little complicated to set up. Horns and special 70v ampliefier
    5. Boom boxes are a good way to keep a crowd engaged, but they are not loud enough to provide sound to more than 5 or so people in the loud demo environment.
  3. Power source
    1. Generator (not a good option unnless you have a truck or can attach to the outside of your vehicle)
    2. Inverters are devices that allow you to plug home appliances into your car battery. They are rated with a maximum wattage of how much power they can provide. Note that in order to use more than 120 watts, you need to plug the inverter directly into a car battery, and you cannot use the "cigarette lighter" outlet.
      1. Inverter connected to your car battery (leave engibe running, or you may need a jump)
      2. Inverter connected to a charged battery not part of your car
    3. Self-contained battery solutions, such as the Goal-zero yeti
    4. Battery powered speakers don't need a power source, but are generally not loud.
  4. A Radio
    1. Your Car Radio. Suprisingly, your car radio is best for this, as it has a very good antenna.

      1. Aux Out adapter - installed behind your stereo (note many cars have aux in, but aux out is only present on after market audio systems)
      2. Taping a microphone to your car speaker
    2. A portable radio - this is great for bikes, but the reception and range will be less than with a car stereo

Current Transmit Rig:

We are using the following equipment. Publicly I am pretending the transmitter is bluetooth and playing dumb if cops ask.
  1. The Transmitter
    1. This takes audio signals and transmits them over FM through a separate antenna
      1. Low power Transmitter 7W Ebay model ~$100 (enough for small demos)
      2. Higher Power Transmitter 30W Ebay model (Actually 25W) This one seems identical  
  2. The Antenna
    1. This actually emits the signal.
      1. Magenetic Compact antenna
      2. Antenna and magnetic base and adapter ~$50(Ham radio outlet is open for pickup orders) Antenna must be cut to match frequency, so I am keeping to 88.3FM so I can use it at full length
    2. Large vertical antenna
      1. if you have the height available there is a Better longer more directional antenna  Spec Sheet (doubles horizontal power (increasing range by 50%) but is 8 feet tall)
    3. Directional antenna
      1. If you are going from a high point and want to cover a specific area, you can use a much better Yagi directional   Spec Sheet (hilltop transmission, emits a cone of conventrated RF)
  3. Wired microphones are recommended as the FM seems to interfere with range and quality. Foam windscreens are recommended so it can withstand frequent sanitizing with harsh alcohol spray.
  4. A mixer is optional, but highly recommended.
    1. Features it should have:
    2. “VU” sound level meters which are key so you can broadcast at the right volume. Makes it easy to switch between music, live speeches, and folks calling in over zoom or Jitsi.
    3. (If doing a digital stream also) USB interface allows easy hookup to phone(ios adapter, android adapter) or a computer to get clean audio out over digital streams (youtube live, facebook live)
    4. Reccomendation: Peavy PV6BT it has built-in compression which vastly reduces how often you need to make adjustments for microphone users of various voliumes, compression helps keep the FM broadcast from blowing out(over modulating) and being illegible.
  5. Cables: Stereo Male RCA to 1/8” (3.5mm) AUX male, iphone dongle, misc others, phone charger
  6. A PA can be hooked up to the mixer (consider using phones, not main out, as you need the VU level meter for your FM) or it can just be plugged into a portable FM radio.
  7. FM headphones to constantly check your transmission for distortion and that it is actually working

Improvements planned:
  1. Collaborate with a local radio station and do this legit. It will have much better range and maybe some folks will listen in!
  2. Hill-top transmission go much further than city-to-city transmission
  3. ios internet radio simultaneous (20 sec delay) broadcast, worked great! Mixlr or www.Radio.co used with pocket streamer USB mixer hooked up to iphone with “USB camera adapter”

Set Up Instructions:
  1. NEVER power transmitter without antenna connected. Will instantly damage or destroy the transmitter.
  2. Attach antenna to roof of car, the closer to the center the better. See diagram below. If using a mast antenna, read included instructions.
  3. Use a radio to find a vacant station. Ideally one with plenty of space on both sides. In the east bay, we used 88.3 with good results.. Here is a tool to get you started, but it is not accurate
  4. Mix your stereo audio signals to a standard level, ideally the mixer has a level meter
  5. Tune a radio to the station you have selected
  6. If applicable, tune your antenna to the right length. (See manual)
  7. With the antenna attached, Power on the transmitter. Select the station with the + - buttons.
  8. Adjust the audio level of the transmitter slowly, starting from the bottom. 9 o’clock is probably about right, turning it too high will distort the signal
  9. With your radio, switch between a big radio station and our lil one with presets. Adjust the level knob until our audio sounds about as loud as theirs.
  10. That’s it! You get about 2 blocks in both directions line-of-sight. Once you turn a corner it is kinda all static.

  1. Don’t swear, the FCC apparently cares about this a lot. Don’t say your name.
  2. Tell people on the outside of the action it is bluetooth. Be vague and play dumb.

  1. Distributed Dance Party boom box parties, I have contacts in SF with lots of boomboxes.
  2. Local Shops
    1. HAM Radio Outlet (Open during COVID)
    2. Best Buy is open also.
    3. B and H is shipping quicky from NY
    4. RF Emporium

  1. Can a repeater be built simply? No, apparently impossible
  2. Can we use two transmitters together for more power? No  apparently impossible
  3. How important is antenna type? IE 1/4 wave antenna vs 5/8 wave antenna? 5/8 gives you ~double signal in the horizontal direction!  

Fresnel zone around line of sight. Better to be higher and further