Public address system problem

[gonelli]

I was curious as to how a public address (PA) system that operates across a whole school is able to produce signals that allow announcements to be just as loud near the amplifier as they are at the furthest end of the school.

 

I was thinking about my highschool where announcements are made in the main office and will be heard over the whole school even to speakers that are about 200m away. The speakers are all connected in parallel with copper wire, so I was thinking that a lot of the signal strength would be lost as it travels along such a long distance of wire.

 

Any ideas on how they do it?

[YT2095]

a dropping resistor of differing values in each box would be the logical method.

[gonelli]

That does sound like the easiest way to achieve an even sound distribution.

 

I suppose the resistor would be as simple as a volume control dial that would be a little harder to change than just turning a nob.

[big314mp]

PA systems also run at higher voltages than standard audio, so you should have lower resistive losses for a given amount of power.

[John Cuthber]

Long ago when I was at school I noticed that there were transformers in the boxes with the speakers in. I later learned that these meant that the signal could be distributed at a higher voltage (typically about 100V) and lower current. Since the current is quite low the power loss due to the resistance of the cables isn't a problem. Searching for "100 volt line transformer" will get lots of examples.