code: private void deliverOnAirUpdate(Set<Integer> audibleChannels) {
          for (final OnAirListener listener : getOnAirListeners()) {
              try {
                  listener.channelsOnAir(audibleChannels);
              } catch (Throwable t) {
                  logger.warn("Problem delivering channels on-air update to listener", t);
              }
          }
      }
docs: Send a channels on-air update to all registered listeners.
  
  @param audibleChannels holds the device numbers of all channels that can currently be heard in the mixer output