Location awareness is an important service in ubiquitous computing environments. This paper presents Lightning Protocol, a hard real-time, fast, and lightweight protocol to elect the sensor closest to an impulsive sound source, for the purpose of either proximity-based localization or leader election for sensor collaboration. This protocol utilizes the fact that electromagnetic wave propagates much faster than acoustic wave to efficiently reduce the number of contending sensors in the election process. With only simple RF bursts, most basic comparison operations, no need of clock synchronization, and a memory footprint as small as 5330 bytes of ROM and 187 bytes of RAM, the protocol incurs $O(1)$ transmissions irrespective of the sensor density and guarantees hard real-time ($O(1)$) time delay in localization. Experiment results using UC Berkeley Motes in a common office environment demonstrate that the time delay for Lightning Protocol is in the order of milliseconds. The simplicity of the protocol reduces memory cost, computation complexity, and programming difficulty, making it especially desirable for low-end wireless sensors.