The Tweeters

The selection of high-quality, sheilded tweeters in March of 1998 was darn sparce. Only one unit from each Seas, Focal and Morel. Vifa was the only player offering 3 choices, the D27ASG-05 (aluminum dome), the D26SG-05 (textile dome), and the D27SG-05 (silk dome). Unfortunately, none of Vifa's shielded tweeters include Vifa's "tinsel leads." The "tinsel leads" must be more flexible, more resilant to long tweeter dome excursions. All of the non-shielded tweeters that carry this feature rate 100 watts of power handling with a 12 dB/octave crossover at 3 kHz. Similar tweeters without these special leads are also rated at 100 watts, but with a 12 dB/octave crossover at 4 kHz. (It is also worth noting that all 3 shielded tweeters have lower power handling ratings than similar, non-shielded units. I can only speculate the cause; higher expected tweeter power levels in a home theatre environment?) The aluminum and textile dome units have resonant frequencies about 1.5 Khz, so the choice Vifa tweeter was the silk dome unit with a resonance of 1 kHz.

So far I have been able to evaluate the Vifa, Focal, and Morel units. At some point I may check out the Seas unit. It is rather expensive at $64, but looks to be a top-of-the-line tweeter with every bell and wistle that Seas has to offer (even silver voice-coil wire). My final selection is the Vifa, with the Morel being held for further testing (modification of the back chamber for lower resonance).

Model	Vifa D27SG-05	Focal TC90KB	Morel MDT-30-S
Pic
Material	Silk	Kevlar	Textile
Faceplate	Plastic	Plastic	Metal
Ferrofluid	Yes	No	Yes [?]
Resonance Frequency (measured)	1000 Hz (1090 Hz)	950 Hz	700 Hz (837 Hz)
Moving Mass	0.3 g	?	0.44 g
BL product	2.7 NA	?	3.5 NA [?]
Qts (measured)	? (0.86)	0.90	0.57 [?] (0.64)
Qes (measured)	? (1.24)	1.21	? (1.02)
Qms (measured)	? (2.83)	3.47	? (1.71)
Re	4.6	5.9	5.2 [?]
Efficiency	92 dB	92 dB	90 dB
Power Handling	80W	75W	200W
Recomended Crossover	12dB/Oct @ 4k	12dB/Oct min	?
VC Height	1.6 mm	2.2 mm	2.7 mm [?]
Gap Height	2.0 mm	3 mm	2.5 mm [?]
Price	$22.95	$59	$60

Notice the white stuff around the edges of the Focal tweeter. It is from the styrofoam packaging where the surround was touching during transportation. I could find nothing to remove the styrofoam contamination without also harming the foam surround. Needless to say, I was pretty disappointed.

Testing Methodology

Vifa	Focal	Vifa/Morel Baffle

Shown above is the rather crude testing methodology that I began with. The white board in the first two pictures is a 40" x 60" foam board that can be found at any office supply store. It is about 0.2" thick, so not particularly strong. I glued an additional piece of foam board on the backside to stiffen up the area around where I mounted the drivers. The last photo shows a double layer, 1.5" thick "skinny" test baffle that I built, the stand for it, and two more 40" x 60" foam boards. The two white circles on the test baffle cover up the mounting holes for the Vifa and Morel tweeters. All the tweeters were flush mounted for the tests, as was the mid-range.

The frequency and phase responses shown below have to be taken with a grain of salt. Later testing showed that the foam board is at least partially acoustically transparent in the mid-range frequencies, and possibly has a mid-range resonance as well (see The Tweeters, Part 2). By the time I made this determination, I had already returned the Focal tweeter, so I have not been able to verify which response artifacts are "real" and which are "foam board." However, discussions with fellow speaker builders verified the Focal's severe resonance problem above 10 Khz as real. Between that, the unit's high price, and the poor arival condition, I incurred no emotional loss returning the Focals.

Distortion measurements were all made at a distance of less than an inch to maximize the number of valid data bits. Since the card's sample rate is just over 40 Khz, 3rd order harmonic distortion measurements above 7 Khz were not possible, nor 2nd order harmonic distortions above 10 Khz.

Frequency response and phase plots.

Frequency waterfall of the above. There is some activity associated with the response dip around 7kHz. This indicates some type of possible resonance behavior. As mentioned in the intro, this could be an artifact of the foam board.

THD plot at a signal level of -19.5 dB.

Impedance plot with a signal level of -39.0 dB measured while on prototype cabinet 1. This is very smooth and does not indicate any resonance behavior as measured while on the foam board. This smoothness is possibly the result of ferrofluid's mechanical damping.

Frequency response and phase plots. Notice the dip at 2kHz, and the crazyness above 10 Khz. The dip at 2 kHz is present in Focal's own response data, as is the wierdness above 10 kHz.

Frequency waterfall of the above. Wow, look at the activity above 10Khz! This is pretty bad ringing.

THD plot at a signal level of -19.5 dB. Notice the Focal unit's THD is evenly distributed between the 2nd and 3rd harmonics (plot 2 of 2). Both the Vifa and Morel units' distortions are dominated by 2nd order distortion products. This characteristic is maintained at lower signal levels.

Sorry, no impedance plot for the Focal. However, other data I recorded shows impedance anomylies at 2.5 kHz, 5 kHz, and several above 10 kHz.

Morel MDT-30-S

Frequency response and phase plots. Notice the small dips at 4.4 kHz and 12 Khz.

Frequency waterfall of the above. The peaks/dips seem fairly well damped. However, the z-axis is only 0 to -20 dB whereas on the other drivers the range is 0 to -25 dB. :(

THD plot at a signal level of -19.5 dB.

Impedance plot at a signal level of -39.0 dB. Note the anomyly around 12-15 kHz. There is also an anomyly around 5 kHz, but it is much more subtle. This indicates a mechanical resonance in the system which can store energy. At higher test signal levels the anomylies are more pronounced.