Single spacing is used within paragraphs, double spacing between paragraphs.

Try your double low-tone whistle with the same temporal spacing and the same notes, and you will see the text "two low" printed to the screen. If you want to see the functioning of the cmdWhistle.pl

By varying the groove spacing, the diameter of the single or double diameter hole, and the rear air cavity depth and content, one can obtain absorption over a wide variety of frequencies.