In Reply to: building a two station intercom posted by dave on 05/20/00 at 7:32 PM:
I sent an e-mail directly to you earlier today, but just in case, consider the following.
The topic of intercoms isn't trivial. I presently design intercoms for airborne and shipboard applications and there are complex problems involving sound pressure levels which depend on the operating environment, EMI suppression, ambient noise cancellation techniques and other factors.
For example, I suggest you start with the definition of sound pressure level (SPL) which is that:
Zero dB SPL = 20 Micronewtons / m² = 0.0002 Dynes / cm².
Thus, an Astrocom (take a look at their web page) microphone, which has its electrical output signal level specified at an SPL of 28 Dynes / cm² --> 20 * log (28 / 0.0002) = 103 dB SPL, is specified to work at a fairly loud sound level. Then reflect on the fact that a US military helicopter environment can exceed 120 dB SPL. Compare these sound levels to normal environments such as street corners, factories and subway tunnels.
Take a look at the noise cancelling microphones which Astrocom makes. There are actually two microphone elements connected in series, which cancel each other out vis-a-vis far field sound sources. They pick up the user's voice because the user's mouth is right close in and doesn't get picked up equally by the two elements because the user almost never is in the exact, single, critical position for which cancellation would occur.
Look up local speaker-audio cancellation circuits too. You'll see how a sample of an intercom station's speaker signal is fed into the microphone amplifier path to cancel acoustic feedback of the speaker's audio back to its origin.
John Dunn - President