Basics of Distributed Audio

Posted by Rick Mathewson

12/3/15 8:02 AM

What is distributed audio?

A distributed audio system simply delivers audio to many speakers rather than to just a few. Examples would be hallways in a school, supermarket paging, health club background music, etc. These systems are commonly referred to as “constant power systems” or “70 volt systems” — even though 100V and 25V systems are distributed audio systems too.

Why 70V speakers instead of 8-ohm speakers?

The typical 8-ohm speakers you might use in a home stereo system or a classroom sound system wouldn't work well as for a distributed audio system. The main reasons are that we're trying to amplify a lot more speakers and, critically, they're much, much farther away from the amplifier than they are in your living room.

To get around this, distributed audio systems ordinarily use 8-ohm speakers with an important modification:  a transformer that takes a 70V input and steps it down to the lower voltages needed by the 8-ohm speaker. To understand why this would make sense, we need to remember Ohm's Law, which states:

P=VI (power = voltage x current)

Mathematically, you can deliver the same power with many different combinations of voltage and current, but practically speaking, it matters a lot which you choose. To deliver the power we need for the distributed audio system, it's much more energy-efficient to have a higher voltage and lower current, because a high current can generate a lot of resistive heat (and therefore a waste of energy/loss of power). This is why the power lines coming from your nearest power plant start out at extremely high voltage (hundreds of thousands of volts) and get dropped down in stages via transformers until we receive 120V (in North America) at the house. The power company wants to lose as little energy as possible during the trip from plant to your house, so they keep the current low to avoid having their dissipate as heat on the lines. (They'd rather sell that power to you instead, of course.) Because the current is low, they have to have a very high line voltage to compensate, with the one drawback that it's extremely lethal (thankfully not the case with distributed audio)!

The reasoning behind power line design also applies to distributed audio. We want to be able to “push” the electrical audio signal over long distances and to many speakers without losing a lot of energy. So, we use a 70V transmission standard and then “step down” the voltage with small transformers to the level that each 8ohm speaker can handle. The trade-off is that all those transformers add cost, and they tend to degrade the sound quality (acceptable for a supermarket or paging, but not the way you'd design a home entertainment system).

Explain transformer “tapping” please!

In our distributed audio system, we want audio levels to be constant and to cover as much area as necessary. To do this we need to determine the power required from the transformer. For school use, we typically see transformers with taps (settings) from 0.5 Watts to 10 Watts for areas such as hallways, cafeterias, and common areas. A tap is colored wire, a label, or a switch. This is referred to as the primary side of the transformer. Which tap you use is determined by how much area needs to be covered and how loud the audio should be. In a typical hallway there are many speakers and a narrow corridor, so the tap is typically set to a relatively low wattage (but each application can certainly differ).

Here is an example of a 70V transformer by itself showing its various taps, and then mounted to an 8ohm speaker:

A 70V transformer alone and mounted to an 8ohm speaker

How are the speakers wired?

Distributed audio speakers are daisy-chained from the source, and the selected taps are a continuous electrical path of speaker cabling. Note that the most efficient wiring would allow for multiple paths so that a single break in a wire wouldn't cause all of the speakers to lose audio.

Wiring example for distributed audio speakers

What type of amplifier is best suited for this application?

There are amplifiers specifically designed to drive distributed audio speakers. These are commonly referred to as “paging amps” or “70V” amps. Most of these amplifiers will allow you to drive 25V/70V/100V speakers. You should not drive 25V/70V100V speakers from an amplifier that is to be used in a classroom — these amplifiers are meant to drive a handful of 8ohm speakers only. Note that while you can drive more than four speakers from some classroom audio amplifiers, you should still be careful to consider wiring schemes, audio balancing, amplifier overheating and volume levels.

How many speakers can I connect to a single distributed audio amp?

This depends on the power available from the amplifier (measured in Watts) and how the speaker transformers are tapped. For example, if you have 20 speakers tapped at 5W you would need at least a 100W amplifier. But, it is always a good idea to give yourself at least 20% overhead. For this example you would need 5Wx20 speakers = 100W+20W overhead, so a 120W amplifier would be recommended. Keep in mind that amplifiers typically go up in 50W (or more) increments. So a 150W or 200W paging amplifier would be chosen for this example.

What if I need multiple zones?

In paging and intercom systems, it's normal to want “common areas” outside of the classrooms to be treated as different zones for communication. Let’s say that you have four of these: hallways, bus loading area, cafeteria, and administrative offices. Each of these areas would need to have its own set of 70V speakers connected together and wired back to the 70V amplifier. Therefore, you would need either four separate amplifiers or a single amplifier that has four channels.

Your decision about which type of amplifier to use will probably be influenced by these speaker wiring requirements and amplifier placement. It may make sense to run all speaker wires back to one location, or it might be more efficient to stragecially place the amplifiers in various locations to minimize cable runs.

How does distributed audio work with a Frontrow Conductor system?

You can use different FrontRow components to design the right solution for each area of your campus. For example, it makes sense for your classrooms to have hardware that combines local audio (mics and media) with paging, bells, and intercom — for this, you'd use something like FrontRow ezRoom or Juno Connect. For speakers in a hallway chain, however, that kind of hardware doesn't make economic sense, nor is it even necessary to have each speaker uniquely addressable. The solution is to add a Frontrow CM800 audio decoder/controller as the paging amplifier source so you get the benefit of an IP-based solution with the cost-effectiveness of the analog distributed audio system.

Here's how you would use a Frontrow CM800 to push a Conductor audio-over-IP stream out over an analog distributed audio system:

Distributed-Audio-Tips-Conductor-Hallway-Paging-Example.png

Any questions, just let us know!

Topics: Integrated communications, Networked PA, bells and intercom, audio, Conductor