Alsa: Creating a virtual microphone

Question

How can I correctly change a .asoundrc file to realize a virtual microphone?

I have the follow .asoundrc file, but it doesn't work:

pcm.audinp {
        type file
        slave.pcm front
        file /dev/null
        infile /writable/home/user/virtualmic.pipe
        format "raw"
}

pcm.!default {
        type plug
        slave.pcm "audinp"
}