I used these in my brother’s speakers, Speaker III.  I’ve always had a soft spot for 3/4″ dome drivers.  I used them in my first design, Speaker I, listened to them for years.  When I switched out to a pair of Polk RTi4’s with 1″ tweeters, one of my first impressions was the very top end lost some air, some imaging.  I could localize symbols to the speakers, which I didn’t recall with my own design.  I have always – and possibly erroneously – attributed that to smaller diameter tweeter.

This Seas 3/4″ uses an aluminum/magnesium alloy for the dome, fabric (Sonomex) surround, and ferro fluid for additional power handling.  Resonance is 1100 Hz, and sensitivity rated at 92 dB.  Testing was therefore done at -2 dBW.

Frequency response is very smooth, and quite extended.

Distortion isn’t as a good news story.  Performance here is generally average, with the good news that its primarily second order distortion.  I believe the distortion suggests that a 3 kHz or higher cross-over is the best choice, and as low as 2 kHz would likely work (my Speaker III crossover is 2.5 kHz).

The D27TG-05-06 is a conventional 1″ fabric dome tweeter with ferro fluid (pictured below on the left).  Resonance is a respectable 1 kHz with a Qts of 0.77, sensitivity is advertised at 92 dB.  Testing is done at -5 dBW (should have done at -2 or -3 dBW).

The frequency response is quite smooth, with a few dB of gradual roll-off on the top end.

What is even more impressive is the distortion performance – this unit is 6 years old, been sitting around in a box!  Distortion is at -50 dB from the fundamental through most of the band above 2 kHz.  The third harmonic only rears its head at 4-5 kHz.  This tweeter could easily cross over at 2 kHz at 2nd order, and maybe 1.8 kHz at 4th order if you wanted to push it.  That explains why this tweeter has had such a long life and is still available.

Dayton Audio produces a 1″ polyamide compression driver and several threaded horns.

The combination compression driver with horn is very efficient.  I estimate about 105 dB at 2.83 vRMS / 1 meter based on the measurements, which matches the manufacture’s specs.  Thus all testing is performed at -15 dBW.  The frequency response has a gentle roll-off of about 2 dB / octave until 12 kHz.  Then it drops off more quickly, and shows resonant behaviors.

The distortion curve is quite surprising.  It is completely dominated by second order harmonic distortion!  Reportedly second order harmonic distortion is the least offensive.  I’ll need to build something from this and find out how it sounds.  Recommended cross-over is 1.6 kHz, and that looks about right provided the slope is 2nd order or better.

This is a ceramic dome tweeter with a neodymium magnet.  I picked these up based on the manufacturer’s claims of low distortion, extended response (to 30 kHz), and the compact mounting dimensions.  A plastic mesh grid covers and protects the dome.  Its the center tweeter shown below.

Here’s the first unit frequency response

Very smooth, and extended to 700 Hz, but a combination null / peak at the top end.

The distortion measurement

A little higher than expected.  Other tweeters and systems that I will be showing have the harmonic distortion as low as 50 dB below the fundamental.  The distortion is rising below 2 kHz which limits the crossover frequency and slope to be 2.5 kHz at 3rd order.

Finally getting comfortable enough with the Room Equalization Wizard software to start some serious testing.  Another audiophile has set the bar high – Zaph Audio.  He has done large tests of tweeters, mid-ranges, and woofers.  One of his claims is harmonic distortion is a key discriminator in the sound of speakers and their drivers, and is an important factor in setting the cross-over frequency.  So my goal is to mimic his methodology.  Part of the Zaph Audio approach is consistency – testing all of the drivers using the same process.  So even if it turns out the process has some hidden defects, the ranking between drivers are likely to remain valid.

I started by building a large baffle with a replaceable 12″x12″ panel to which drivers can be mounted.  Flanking on the left/right are two of the GIK Acoustics panels borrowed from the living room.

The problem with this setup is the reflection from the ground.  The driver center is about 30″ above the ground, and the microphone is 18″ away.  Doing some math, you can show that the round trip travel time will be about 3.85 msec for the ground reflection compared to the main impulse response.  Here’s an example.  Notice the “blip” at 3.85 msec?

Why is the extra blip a problem (as well some other smaller, extra blips)?  It distorts the frequency response calculation.  Show below is the result with the full 10 msec of data:

Next, with the data limited to a 3 msec data record:

So how to get a longer, clean data record?  How about laying the board down so that the driver faces up?  The ceiling is 10′ high in the room.  There is a lot of equipment in the room which will generate reflections.

Here’s the resulting impulse response – clean for a full 10 msec!  (Alright, not perfectly clean, but significantly better.)

The resulting frequency response, compared with the upright measurement with a 10 msec data record.  The imperfections in the curve – likely the result of the finite test baffle – are sharper and more exaggerated with the reflection present.

For completeness, repeat the comparison, 10 msec data record on the floor vs. 3 msec data record upright with reflection.

What else needs to be controlled?  For the distortion measurements to be comparable, the input power should be set so the driver output is at a consistent value – I’ll use the same value of Zaph Audio of 90 dB at 1 meter.