Question
Regarding polar plots, can you please tell me the best application for each of them?
Answer
The intent of a directional microphone is to provide greater amplification for sounds arriving from the front and reduce (attenuate) sounds coming from the side or rear. The graphical display of the output (or lack thereof) from different directions is referred to as a polar plot. Polar plots will vary by both microphone design and by frequency. There are two ways to change the polar pattern and, thus, the area of greatest attenuation in a directional microphone. The manufacturer can either change the external port spacing of the microphone(s) or the internal time delay. With digital instruments, the latter method can be used to allow for varying or adaptive polar patterns. So, within the same digital instrument it is possible to have several different polar patterns, including bi-directional, cardioid, hypercardioid, and supercardioid (see figure below).
What polar pattern works best is dependent on the listening environment. For example, if a hearing aid wearer is listening to speech from the front and the noise is directly behind them, then the cardioid pattern would provide the best audibility because the most attenuation occurs directly behind the listener (180 degrees). If the noise were coming directly from the side (90 and 270 degrees), then a bi-directional pattern would provide the best results because this polar plot attenuates best on the sides. Unfortunately, most real-world listening situations have noise arriving from all angles, or noise that continuously moves around to different angles. In situations such as this, the hypercardioid pattern has been used by many manufacturers to provide good performance for the listener. 
BIO
Angela Pool has been an Educational Specialist and trainer with Siemens Hearing Instruments for the last four years. Prior to joining Siemens she worked as a clinical audiologist at Mayo Clinic and the University of Florida Pediatric Hearing Program. Ms. Pool received her Masters degree in Audiology from the University of South Alabama.
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