# How to Locate the Speakers in a Public Address System?

Designing Public address and Mass Evacuation systems is a very critical task. The designer should not only adhere to the approved code, but also make sure the notification messages are clear to the listeners’ ears. Understanding the sound pressure level (SPL) is key to a successful design.

#### What is SPL?

Sound Pressure Level (SPL) measures the difference between the pressure of the sound created by the source (e.g. speaker) and the ambient pressure at a specific location. SPL is measured in decibels (dB) and can indicate the loudness of the sound at a certain point in space.

#### Distance and Power

SPL is affected by distance and power. The further you move from the source the lower the SPL you get. The more power you inject in the source, the higher the SPL you get. As a rule of thumb, SPL drops by 6B when you double the distance, and increases by 3dB when you double the power.

#### Edge-to-Edge Configuration

Edge-to-edge configuration of the speakers is widely used (as shown in the image above). This configuration prevents overlaps and proves to be the most cost-effective method. The space between the speakers is determined such as the speakers meet at a point where the SPL drops 6dB below its on-axis reading. This reading is measured at a height of 1.5m (5ft) from the ground, where NFPA requires the sound level to be measured.

#### Formula

To calculate the distance between the speakers in an edge-to-edge configuration, you can use the formula below:

S= 2 x (H-L) x tan(D/2)

Where: S= Spacing between speakers | H= Height of the speaker from the floor | L= Listener’s level (usually 1.5m) | D= Dispersion Angle of the Speaker

### 1 Comment

1. Mohamed meeran

Dear Matbouli,

Could you clarify the above given formula is right.
So,
S=2*(3mtr-1.5mtr)*tan(60°/2)
S= 2*1.5*tan(30°)
S= 2*1.5*0.5773
S= 1.7319m

I hope that this is not correct one. .