External Microphone input for MacBook Air 2012

External Microphone input for MacBook Air 2012

I am trying to get an external microphone (this Sennheiser ME3) into the
TRRS jack of my 2012 MacBook Air, through a headset splitter such as here.
(this is so that I can use speech recognition software -- the Sennheiser
is especially good for speech recognition)
The first check I do is on the headphones: I go to system preferences ->
audio -> output, and notice that inserting the splitter registers as
"headphones/headphone port" whereas removing it registers as "internal
speakers/built-in". so that is behaving correctly. I also note that
actually plugging and unplugging headphones into the other end of the
splitter doesn't register.
I also check that audio output works correctly TRRS -> splitter -> my
headphones
However, audio INPUT does not behave correctly. system preferences ->
audio -> input does not change when I plug and unplug the splitter (which
has the Sennheiser ME3 attached). it states "internal microphone/built-in"
regardless.
I've also done a simple experiment to confirm that the MacBook is indeed
receiving audio from the built-in microphone. i.e. It is completely
ignoring the fact that the Sennheiser is attached. I did this by launching
QuickTime and creating audio recording, then putting on the Sennheiser and
counting "123" with my face next to the screen then "456" with my head as
far from the screen as the cable permits. The first recording comes out
much louder, so it is indeed using the internal microphone.
How can I get to take input from the Sennheiser?
Here are a couple of related questions:
MacBook Pro (mid-2012) Microphone Input
MacBook Air audio input/output, mic/headset
PS If all else fails I will try another splitter: here