Edit_Update: On checking again, the direct link to the Widex Pro site for the 2006 technical paper on vent size now works again. I had complained to Widex that the link was broken and their response, from a Widex website administrator, was just to ask how I came by the link in the first place(?!). I pointed out that the other versions of the document were available on the Internet, that the paper was cited in a review posted online in the NIH National Library of Medicine and if Widex was going to restrict access to such useful and informative information that would prejudice me against Widex in future hearing aid purchases. Don’t know if that dialog had anything to do with the link being restored but ReSound in allowing the general public to view the contents of their PRO site sets a standard to match. Hearing aids ReSound - Courses
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The link I cite above seems to no longer work (Widex locked the door to non-providers?). Here is a link that works as of this posting:
https://pdfs.semanticscholar.org/5a36/0e417ffe80faf248a7581d01aef168c631db.pdf
Edit_Update: A copy of the paper to download is also available in less colorful, less well laid out form from Hearing Review, the original publication source: Fitting Tips: How Do Vents Affect Hearing Aid Performance? | The Hearing Review
(Google Chrome warns that this site is not secure as it does not implement HTTPS: communication)