|Dome tweeters are extremely popular and many of us depend on the manufacturers’ specifications when choosing one.. The Frequency Response plot is often the first data we look at as it shows how linear the tweeter’s response is.
Fig 1 is Peerless BC25TG15-04 response supplied by the manufacturer. It is flat, which makes it highly desirable. Unfortunately, there’s more to it than a manufacturer’s published measurement.
Fig 2 is the actual response of the tweeter when it is mounted onto a 13 liters box with a baffle width of 8.5″. Note that it is no longer flat. Why is it different? Well, manufacturers follow the industry standard, which stipulates that the driver must be mounted onto an IEC panel when doing measurements. And what is the IEC panel size? 135cm x 165cm. That is 4.4FT x 5.4FT.
Most of us would agree that we don’t have speakers with front baffle of 4FT x 5FT. Okay, so it is not flat when in a box. What’s the problem.
Well, the depression at 2.6kHz creates a lot of issues in the crossover region. The Red plot in Fig 3 is horrible when the tweeter is crossed at 2.2kHz at 24dB/oct. This is the effect of what is called Baffle Edge Diffraction. All dome tweeters suffer from this phenomenon. That’s why you see speakers with all sorts of shapes, from pyramids to deeply beveled front baffles. It all has to do with trying to minimize this baffle diffraction that dome tweeters suffer from.
Is there a way to get around this Baffle Edge Diffraction?
Yes, replace the dome tweeter with a horn.
The Black plot in Fig 4 is the Raw response of a Peerless XT25TG30-04 Dual Ring Radiator Tweeter mounted in a 13 liter box with a front baffle of 8.5″. The effects of baffle diffract can clearly be seen.
The Red plot is the same XT25TG30 with the face plate replaced by a Pellegrene waveguide. Problem solved. No baffle diffraction. Now, that is what I call a beautiful response.
If dome tweeters are so problematic, why are so many speakers still using them?
To a manufacturer, it’s easier to make money with a speaker sporting a dome tweeter than a horn because buyers have the perception that horns sound bad. To make matters worse, horns are more complicated to design and they are generally more expensive. Only a handful of speaker designers dare challenge this market mentality. Klipsch is one of them. That’s why vintage Klipsch speakers are still sort after.
September 6, 2017Articles