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Why supertweet? (Harbeth SHL5+XD)

Posted by Brian H P (A) on August 14, 2023 at 12:17:05

This Harbeth model gets a very nice write-up from HR in the new S'phile. It looks in many ways to be a solid implementation of classic BBC design principles, but I'm puzzled by its use of a 22mm supertweeter crossed in above 10kHz. I think this feature is based on another decades-old BBC model, but this is the ONLY current Harbeth model that has such a thing.

My question is, why bother? Because the center-to-center distance from the main 26mm tweeter is necessarily greater than one wavelength at this frequency, and because of additional phase error in the crossover, the measured response above 10kHz is a nasty-looking mess, both on and off axis. It hurts my ears just to look at it!

It's not like it's necessary to resolve the top octave, either. SEAS makes (and Harbeth uses) a number of 26mm tweeters that extend flat well beyond 20kHz. With the high, 3.5kHz crossover from the woofer to the main tweeter, an off-axis dip followed by a flare is already inevitable, but may be an acceptable tradeoff for keeping all vocal fundamental frequencies coming from the same driver. This may result in greater coherency.

But I can make no such case for the use of the supertweeter. Any hypotheses on why Harbeth chose this design approach, and what advantages it might offer?

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