The same goes for Sound Pressure Level (SPL) - see the previous tutorial blog: remember that we measure SPL as indication of the power relation. As a calculation: sound pressure (p) relates to the square root of sound intensity (I), but by using the factor 20 we get the same dB value for the field property SPL as with the power property SIL.
Now it becomes clear that, for any distance from the speaker, if we double the distance we get an SIL and SPL decay of 20*log( 2r / r ) = 20*log(2)... which is approximately 6dB.
Now we know this, we can address the most important challenge in designing ‘Public Address’ sound reinforcement systems: to make sure everybody in the audience hears roughly the same sound level.
First, we need to set an acceptable difference of SPL between the listeners who are closest to and furthest from the loudspeaker. Let’s use an arbitrary difference of 6dB. If we then place the loudspeaker at ear height at one metre from the closest listener (sitting in the centre of the first row), the furthest listener (at the centre of the last row) can be only one metre further away: two times 6dB equals two times the distance. For a very small pub this one metre ‘spread’ would serve just a few listeners; for larger audiences we need to think of something else.
The solution is to increase the distance to the closest listener and decrease the distance ratio to the furthest listener. The constraint is the stage depth - to prevent feedback, the speakers have to be in front of the vocalists, and there’s always a limit to how far we can keep the first row of listeners away from the stage edge. Mounting the speakers on speaker stands to place them higher - or, in installed systems, simply suspending the speakers from the ceiling - helps a great deal. This increases the distance to the closest listener, and at the same time lowers the distance ratio of the closest and furthest listener. In the example of the very small pub with a ceiling height of three metres, the spread increases from one metre to three metres - already suitable for live music in a small pub.
For larger audiences, we may need to apply a distributed system and add delayed speakers or use speaker systems with different energy distribution properties, such as column speakers or line arrays. These are topics for future episodes of this micro tutorial blog.
For now, understanding the 6dB per double distance challenge gives us a useful design tool to cope with design challenges for small to medium size venues - which form the majority of uses for sound reinforcement systems.