A blind listening evaluation of classical guitar soundports was performed to ascertain whether players could hear the difference between an instrument with an open port and the same instrument with a closed port, all in a context that is representative of a player's instrument selection decision. The study was conducted to comply with the specifications detailed in standard ASTM E 2139-05, Standard Test Method for Same-Different Test from ASTM International (formerly called the American Society for Testing and Materials). Twenty four guitar player subjects were recruited to play a single guitar twice while blindfolded. In each of these two trials the open/closed port configuration of the guitar was selected at random. After playing the instrument in each port configuration the subject was asked to indicate whether there was or was not a difference heard between the two configurations. Results of this experiment indicate that the port open and port closed states are not perceivably different.
Mottola, R.M. "Blind Listening Evaluation of Classical Guitar Soundports" American Lutherie #96, 2008, p. 54.