Multi-robot collaboration is essential for advanced manufacturing environments, especially in smart factories where seamless, real-time communication between robots enhances efficiency and adaptability. Conventional communication methods, such as WiFi and Bluetooth, face limitations in multi-robot scenarios without network due to infrastructure dependency. To address these challenges, we developed a novel, dynamic inaudible frequency-shifting communication method for multi-robot communication without relying on network infrastructure. By utilizing inaudible frequencies in the 18 to 22 kHz range, the proposed method enables robust, interference-resistant communication suitable for uncontrolled environments. The proposed method was tested with heterogenous robots in one-to-one and one-to-multiple communication scenarios, consistently achieving high accuracy above 97.5 % at distances up to four meters. Minimal reductions (up to 0.35 %) in accuracy in one-to-multiple communication highlight the robustness of the method in handling simultaneous commands across multiple robots, effectively supporting multi-command scenarios and providing reliable communication even in dynamic, uncontrolled environments. These results demonstrate the potential of the proposed method for enabling decentralized, scalable, real-time robotic interactions within complex manufacturing settings.