It is often called "phase shield". But what it actually does is mainly to dampen on-axis radiation spl to get smoother directivity - an old trick from seventies and it really works.
Vifa has very popular tweeters with the shield and also SEAS 27TBFC/G has a plastic tape shield under the grill
Vifa has very popular tweeters with the shield and also SEAS 27TBFC/G has a plastic tape shield under the grill
The Seas phase shield actually deteriorates the directivity, if you take frequencies up to 20 kHz into account. It beams the upper octave forward, preventing the frequency response from drooping on on axis measurements. Off axis response suffers above 8 kHz and is not smooth: as you move off axis there is peak at ~13 kHz followed by a dip at ~18 kHz.
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