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Q: How can I get sound from the stage all the way to the back of the audience?

When I do live sound, it's always a lot louder at the front of the auditorium than at the back. How can I get the sound all the way to the back of the room?

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Imagine an omnidirectional sound source in free space. Not outer space of course, just hanging high in the air.

Since the energy from the sound source has to cover a larger and larger area as it spreads out, the further away from the sound source you are, the quieter the sound you hear will be.

In fact, as you move further away from the source, the sound level will drop by 6 decibels for every doubling of distance.

For reasons that we won't go into here, this is called the 'inverse square law'.

In fact, ignoring the effects of any absorption, this is the fastest rate sound level can decay with distance.

What we want however is for sound to decay at a lesser rate with distance.

The way to do this is to use a focussed sound source.

We can focus light with a lens. Although it is theoretically possible to focus sound in a similar way, a method that is practical for real-world live sound use has not yet been found.

We therefore have to use a sound source that is intrinsically focussed.

The way do do this is amazingly simple in concept - just make the sound source larger!

The larger the sound source, the more focussed the sound will be and the less the sound will spread out. The less it spreads out, the less will be the difference in level from the front of the room to the back.

There is a refinement...

In a practical live sound scenario, you would normally want to spread the sound broadly to cover the full width of the audience. At the same time you would want to focus the sound vertically so that you were not spraying it pointlessly on the ceiling and floor.

You can do this by making the loudspeakers large vertically and narrow horizontally.

You can see this in so-called column loudspeakers, and in line-array loudspeaker systems.

Bear in mind however that sound is difficult to control and neither system works to perfection. Any benefit however is well worth the effort.  

By David Mellor Friday October 15, 2010
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