Spread spectrum works by "multiplexing" multiple signals by sending them along different frequencies within the allotted frequency range for a wireless intercom. Or in other words it splits the spectrum into tiny slices of time.
Hollywood actress Hedy Lamarr and musician George Antheil filed for a patent on a spread spectrum communications system in 1942, though real-world deployment occurred later. A spread spectrum system inverts the static model of transmitting with high power on a narrow channel.
Using low power and spreading the signal across a range of frequencies, it's possible to carry more transmissions simultaneously. The basic notion is that if the transmission is broken into pieces, each of which is tagged with a code, a receiver that knows the code can reconstruct the message. The wider the spreading, the more space there is in between the coded packets to send other signals at the same time (see Figure 7).
Taking spread spectrum to its logical conclusion, if the signal is spread wide enough, the power density can be so low that the signal becomes effectively invisible to other systems in the same bands.
Radio frequencies are never totally empty of noise. Radiation-emitting devices such as hair dryers and microwave ovens, as well as cosmic background radiation, create a "noise floor" that all systems must contend with. Static systems do so by using high-enough power that it's easy to distinguish the high-power signal from the low-level noise.
With enough smarts, a spread spectrum system can transmit and receive very faint signals without ever raising above the noise-floor threshold.
For a wireless intercom, spread spectrum has several benefits. One is that it uses lower power so battery life is extended. Two is that it eliminates the noise that wireless intercoms with other technologies would experience. And three is that it makes it much more difficult for other devices to hear a conversation since the conversation is broken into tiny fragments over a wide range of frequencies.