Zaph|Audio

Tweeter Off Axis Response

Jan 22, 2006 - Initial Publication
Feb 19, 2006 - Minor updates to address recent questions
May 15, 2008 - Added Hi-Vi T20 and Seas DXT
June 4, 2009 - added Tangband 75-1558SE

Question: How do various types and sizes of tweeters vary with regards to off axis response?

Top end rolloffs for tweeters of a given size and type are generally similar. Of course, extension and response shape can be all over the map but rest assured you can make a good estimate of how much it will roll off based on it's design. Here I have gathered samples of various types and sizes so you can see how they compare. All curves are infinite baffle, taken at 0, 15 and 30 degrees off vertical axis. After each image, I will specify the make and model of tweeter, it's size, and describe what makes it different. Following all that you will find a discussion of what to make of all this.

Vifa DX19 3/4" wide surround

Vifa XT19 3/4" ring radiator

Dayton ND20TB 3/4 dome with mild horn loading

Hi-Vi 3/4 dome with no horn loading

Seas 27TBFCG 1" dome with phase shield

Seas 27TBCDGB-DXT with unique waveguide design

Vifa D26NC55 1" standard dome

Vifa XT25 1" large ring radiator

B&G Neo3 PDR 1.5" x 3" medium planar - Note: vertical off axis shown. Horizontal is roughly equivalent to a 1" dome.

Fountek NeoCD2.0 10mm x 5" tall ribbon - Note: vertical off axis shown. Horizontal is roughly equivalent to a 3/4" dome.

Tangband 75-1558SE, 3" dome mid - Note: 45 & 60 deg also shown.

Hi-Vi B3S, 3" full range driver

Notes: all of the 3/4" models have very little off axis droop. You can see the differences between a full dome, phase plug and 3/8" horn loading on the Dayton model. The Seas 27TBFCG has a clear plastic phase shield under the hexagrid, which vastly improves off axis response for a 1" dome. Obviously, when you listen to a tall ribbon, you might want to sit down and not move. The 3" full range is thrown into this comparison for good measure.

The importance of top octave response is highly overrated. It has absolutely nothing to do with sound quality, and everything to do with flexibility of placement. Simply, all it means is that you might want to toe larger drivers inwards. I say "might" because the usefulness of a response that's flat out to 20kHz is also overrated. While a peak at 10kHz is easily heard, (tape hiss anyone?) a few dB down at 10kHz is barely audible, and in some cases preferable. At the very top end, 15dB down at 20kHz is barely audible if at all. This is probably why many people don't mind tweeters like the Dayton RS28 or the many of the Morels, such as the MDT20. Dips in response at the extremes of audibility simply aren't very audible, even for those who still have clinically proven hearing out to 20kHz. (I'm good to about 17kHz in my old age)

System power response at and below the crossover is much more important than top octave off axis response. This is something to keep in mind with 3/4" tweeters that require substantially higher crossover points. Horizontal response may not be so bad, with only a mild null to deal with based on the woofer's diameter induced rolloff, but the vertical off axis response can have deep nulls that set in at a shallow angle if the crossover point is relatively high. What's not obvious to many folks is that vertical off axis nulls are still audible on axis. That's pretty much what power response is all about, and why it exists as a valuable measurement.

The most important criteria in selecting a tweeter is the harmonic distortion on the low end near the crossover point. This is the most audible defining characteristic of a tweeter, more so than top octave frequency response. System power response, while critical at the crossover point, becomes a non-issue higher in frequency. This is simply because the room plays less of a roll in the reflection of high frequencies back to the listening position. There is a lot of high frequency absorbtion in typical room construction, and even painted drywall almost completely absorbs a 15kHz tone. So, as I mentioned above, if you can toe in a system using a larger tweeter, all is well.

If this article sounds a little biased against 3/4" dome tweeters, that's because it is. In most cases, I see absolutely no reason to use a 3/4" tweeter, particularly with larger woofers. The only 3/4" tweeters I've used on this web site were selected based on cost and the fact that the mid/woofer used with it was small and had a very extended response. Maybe someday someone will create a 3/4" dome that can hang with the 1" boys at the low end, but for now it doesn't exist.

Page done by John "Zaph" Krutke © 2006
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