The depreciation of sound vs distance assumes that measurements are taken in still air ‘open field’ conditions, and that the sound will not be interrupted by buildings, hills or other objects.
The sound level emitted by a sounder is usually measured in dB at 1 metre. As the distance from the sounder increases the audible level of the sounder reduces.
The rate of reduction, normally known as attenuation, is approximately 6 dB as the distance doubles.
e.g. For a sounder with an output of 100 dB at 1 metre, the audible level will be 94 dB at 2 metres, 88 dB at 4 metres, and 82 dB at 8 metres.
From the following table you can calculate the dB level required from a sounder to achieve a given alarm sound level at a known distance from the sounder. It can be seen on the table that to achieve 84 dB at 128 metres distance, a sounder with an output of 126 dB must be used.