axd-CP4/A INTERCOM FEEDBACK ISSUE

Leo,
The feedback you hear is actually caused at the other end; it is sending back the mic's audio through the speaker, and you probably have this issue on both stations. There are a number of ways to resolve this:
1) Move the speaker away from the microphone - this means adding external speakers and may not be an option for your install since the walls are likely to be solid in the Netherlands (in the US they are usually hollow and allow for easier installations).
2) Change the system to a push-to-talk (half duplex) scenario. The mic becomes active only when the button is pushed while muting the speaker. Switch the speaker back on and kill the mic when the button is released. Do this for both sides - pretty easy to write in code and the Extron switcher should be fast enough to make this work well.
3) Add an electronic echo canceller in the audio path, like a ClearOne XAP or Polycom Vortex. This is a rather expensive option (at least $2000) but allows you to have full duplex conversations. You will need to feed the speaker and microphone audio signals through it so it can remove the speaker's audio from the mic signal. Do this for both directions.
3) You could try and equalize out the offending frequencies using a parametric equalizer, but this may or may not work, depending on the sound pressure caused by the speakers.

Leo,

If you used the push-to-talk feature that Wilbert described in his post you should be able to use the mic and speaker mute functions in the panel and not have to do anything different to the switcher.

Danny