Force detection of all words, even acronyms

Question

I need to recognize phrases like "CN Detector Yale". I only want a successful recognition when ALL elements of the phrase are spoken, including the acronym. However, I am finding that I am getting successful recognitions in cases where only "Detector Yale" is spoken.

Here is what I've been using:

SpeechRecognitionEngine recognizer;
GrammarBuilder gb = new GrammarBuilder();
gb.Append("CN Detector Yale");
recognizer.LoadGrammar(new Grammar(gb));
recognizer.SetInputToDefaultAudioDevice();
recognizer.RecognizeAsync(RecognizeMode.Multiple);

How can I force it to require all parts of the phrase for a successful match?