Interesting fact from article about the Rega RS line.
Quote:
Although the new model has undergone other subtle changes, the RS1 differs from the R1 in two key areas: its gone up by £100; and it now incorporates Rega's radical new tweeter.
Radiation control
Coping with the rear radiation generated behind a tweeter's dome diaphragm has been an issue with speaker designers for years. Many drill a hole through the polepiece to avoid pressure build-up, but Rega has gone much further by drilling most of the polepiece out and leaving just a thin cylinder of metal instead of the usual thick rod.
By using powerful magnets, Rega's 19mm tweeter now has a 16mm-diameter hole through its polepiece, so the fabric dome effectively operates in free air and the rearward radiation passes through the motor into a generous cavity, shaped to avoid reflections.