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.
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.
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