This advice comes from Dragon Newsletter (I have a Samson C01 U which is also a USB mic):

Select a USB Digital Microphone: The built-in array microphones are generally poor performing in most machines, including netbooks. Although this type of microphone may work fine for applications such as webcam conversations, the poor quality of the microphone will make it difficult for Dragon to differentiate words within your dictation.

Using an external analog headset is an option, but a better choice is to select a USB digital microphone since this takes the audio circuitry of the netbook out of the equation and will deliver the best accuracy.

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