Free space is only the starting point
Line-of-sight free-space calculations are useful references, but professional audio surveillance systems commonly operate around buildings, vehicles and other structures. In those environments, absorption, reflection, diffraction and scattering all modify the signal.
Frequency influences penetration and antenna size
Lower frequencies often experience lower free-space loss for the same antenna gains and can diffract differently around obstacles, but they also require physically larger antennas. Higher frequencies permit smaller resonant antennas but may experience greater attenuation through some building materials. The best operating band is therefore a system trade-off.
Walls are not all equal
Plasterboard, timber, brick, reinforced concrete, foil-backed insulation, metallised glass and steel structures can have very different RF effects. A range claim measured through one internal wall says little about performance through a reinforced service core or several floors.
Multipath can help and hurt
Reflections create multiple versions of the same signal arriving with different delays and phases. At some locations these combine constructively and at others they partially cancel. Moving the transmitter or receiver by a small distance can therefore change signal level substantially.
People and furniture are part of the environment
Human bodies contain water and can absorb RF energy, while metal furniture and equipment produce reflections. A room with people present may behave differently from the same room during an empty bench test.
Why site testing matters
For professional applications, representative testing in the intended environment provides more useful information than a single idealised range number. Receiver placement, antenna orientation and local interference should all be considered when evaluating system margin.