ENIGMAcoustics Sopranino electrostatic supertweeter Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the EnigmAcoustics (Enigma from now on) Sopranino's frequency response in the farfield, and an Earthworks QTC-40 for the spatially averaged room responses. It was difficult to estimate the Sopranino's voltage sensitivity, as I usually calculate this with a 20kHz bandwidth and, as you will see, the Enigma supertweeter's output is mainly higher than that frequency. But the sensitivity below 20kHz appears to be 87dB/2.83V/m, with the switch reducing this by the specified 3dB.

Fig.1 shows the Sopranino's electrical impedance (solid traces) and electrical phase (dotted traces) with the 12dB/octave high-pass crossover filter set (from top to bottom, impedance; from bottom to top, phase) to 12, 10, and 8kHz. The impedance above 14kHz averages 2.5 ohms, the magnitude increasing below 12, 10, and 8kHz, respectively, and is not significantly affected by the 3dB sensitivity pad. The phase angle becomes very capacitive about an octave below each nominal crossover frequency, but that won't be an issue because the Sopranino's magnitude increases with decreasing frequency. However, with the 8kHz filter setting, there is a very high inductive phase angle between 7 and 12kHz, a region where the impedance magnitude drops to less than 2 ohms. This makes the Sopranino very difficult to drive at this setting, though it's fair to point out that a lot of music has limited energy in this region.


Fig.1 EnigmAcoustics Sopranino, electrical impedance (solid) and phase (dashed) with Gain set to "0" and crossover to (magnitude from top to bottom at 500Hz, phase from bottom to top at 1kHz): 12kHz, 10kHz, 8kHz (2 ohms/vertical div.).

The Sopranino won't be used on its own, of course, so I measured the impedance of the combination of the Enigma tweeter in parallel with the 1978 BBC LS3/5A that I used for some of my auditioning. The worst-case result, with the supertweeter's filter set to 8kHz, is shown in fig.2. The top solid trace shows the impedance of the LS3/5A alone, the lower solid trace the impedance of the combination. Although this version of the BBC's LS3/5A is a 16 ohm design, the Sopranino steadily decreases the impedance above 1kHz, reaching a minimum magnitude of just 1.33 ohms at 9kHz. Fortunately, the phase angle of the combination (bottom dotted trace below 100Hz, top dotted trace above 8kHz) is low in this region. Still, it reaches +47° at 30kHz and –64° at 3kHz, frequencies where the magnitude is still low, at 2 and 4 ohms, respectively. The phase angle is a little more benign with the Sopranino's filter set to 12kHz (not shown), but those wanting to try the Sopranino will need to be sure of using an amplifier than has no problem driving such low impedances.


Fig.2 Rogers BBC LS3/5A with EnigmAcoustics Sopranino set to 8kHz and "0," electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Turning to acoustic measurements, fig.3 shows the Sopranino's farfield frequency response averaged across a 30° horizontal window centered on the middle of the horn. I have added a vertical green line to this graph at 14.8kHz to indicate the HF cutoff of my hearing at normal listening levels. The filter was set to 10kHz for this measurement, and you can see that the supertweeter's output plateaus between 10 and 15kHz, then rapidly rises to reach a level 17dB higher at the 30kHz limit of this graph. Interestingly, this rise in response sort of matches the typical loss of hearing sensitivity with increasing frequency, at least up to 20kHz. Sort of. Setting the crossover to 8kHz increases the response below 10kHz by a couple of dB, while setting it to 12kHz reduces the level at 10kHz by 5dB (not shown).


Fig.3 EnigmAcoustics Sopranino, anechoic response on axis at 50" with crossover set to 10kHz, averaged across 30° horizontal window and corrected for microphone response.

The Sopranino's lateral dispersion with the crossover set to 10kHz is shown in fig.4 (actual responses shown). The supertweeter's output falls off rapidly to the sides. The vertical dispersion (not shown) is similar. Listeners need to arrange the Sopraninos' placements so that they sit with their ears on axis.


Fig.4 EnigmAcoustics Sopranino, lateral response family at 50" with crossover set to 10kHz, from back to front: responses 90–5° off axis, on-axis response, responses 5–90° off axis.

I performed my usual spatially averaged in-room response measurements with the Sopraninos sitting atop a pair of Joseph Audio Perspective speakers. The blue trace in fig.5 shows the averaged response of the Josephs at my listening position; the red trace shows what happened when I added the Sopraninos set to 8kHz and 0dB gain. Again, the vertical green line shows the 14.8kHz cutoff of my hearing sensitivity. There is 1–2dB greater output between 7 and 15kHz with the supertweeters, which I could hear, and a major boost in response between 15 and 30kHz, which I couldn't hear. (I plotted these responses to 40kHz rather than my usual 30kHz.)


Fig.5 Joseph Audio Perspective, spatially averaged, 1/6-octave response in JA's listening room (blue), and with EnigmAcoustics Sopranino set to 8kHz and "0" (red).

Turning to the time domain, the Sopranino's step response (fig.6) indicates that it is connected in positive acoustic polarity. Whether or not its output adds to or subtracts from that of the main speaker's tweeter can be adjusted by inverting the Sopranino's polarity. As anticipated, the supertweeter's cumulative spectral-decay plot (fig.7) is very clean.


Fig.6 EnigmAcoustics Sopranino, step response on tweeter axis at 50" with crossover set to 10kHz (2ms time window, 30kHz bandwidth).


Fig.7 EnigmAcoustics Sopranino, cumulative spectral-decay plot on tweeter axis at 50" with crossover set to 10kHz (0.15ms risetime).

As I mentioned in my "Listening" comments, as subtle as I felt the effect of the Sopranino sometimes was, toward the end of the review period it suddenly got more subtle. I redid the in-room measurements, this time using my BBC LS3/5As. Fig.8 shows the response at the positions of my ears of the right (blue) and left (red) channels. It appears that the right Sopranino was no longer outputting energy above 15kHz, though it was still operating below that frequency. Perhaps, when they receive the review samples, EnigmAcoustics can examine what went wrong.—John Atkinson


Fig.8 Rogers BBC LS3/5A with EnigmAcoustics Sopranino set to 8kHz and "0," 1/6-octave response of right (blue) and left (red) speakers at the listening seat.

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remlab's picture

..around an inch long. The distance of the super tweeter's center looks to be several inches from the main tweeter's center. Relatively speaking, it would be like placing a subwoofer a hundred feet from the mains(Kind of). It seems like the potential for destructive interference due to timing variations would be pretty bad. For this reason, off axis measurements/w main speaker would have been interesting.
Another thing, regarding show conditions, is that if you are not sitting perfectly on axis with these supertweeters(90% of show listeners usually aren't), what you are hearing from it is probably just your imagination, or destructive interference at the crossover point.

dalethorn's picture

"There was a sense of loss every time I disconnected the supertweeters, the imaging losing some of that addictive palpability."

That's the key I think - that direct comparisons don't reflect the full difference. I've learned that inserting a component that improves the sound in this subtle way doesn't make quite the impression that removing said component does, once the listener has accomodated to it.

hnipen's picture

John, I do have asthma too, suddenly I realize how this may bee a good thing, thx for info John :D

Catch22's picture

If I start smoking, can I get asthma too?

remlab's picture

So there is no way for me to stay in the sweet spot long enough to hear any benefits anyway..

hnipen's picture

@Catch22, Asthma is allergy related and has to do with a narrowing of the airways, due to these allergic reactions, smoking can reduce your lung functions and make other problems with your airways, but it can't give you asthma.... as far as I know (I'm not a doctor, though)

When I was younger I also had asthma when exercising, but as far as I know, it's always allergy related, but potentially being more disclosed when you exercise, but still it's allergy that's always the core reason, I believe

remlab's picture

..catch22 and I were just trying to be funny(out of insane jealousy, of course).

hnipen's picture

Geir Tømmervik, master chief of Oslo Hi-Fi center has a supertweeter that goes to 200KHz for his Audio Physic Medea (well, the medea seemingly has been replaced with Kef Muon) .... He wholeheartedly claims that everything gets better when he employs the supertweeters, even the bass!

Geir is a person that always proves what he says, so I don't believe this to be rubbish.....

remlab's picture

Would sound even better!

Rick Tomaszewicz's picture

I wonder how much of such improvements is the result of things like the McGurk Effect, wherein what you see affects what you hear. Could a double blind test help in determining actual improvements?

See the following for an explanation.


John Atkinson's picture
Rick Tomaszewicz wrote:
I wonder how much of such improvements is the result of things like the McGurk Effect, wherein what you see affects what you hear.

Always a possibility with even the most experienced listeners. But then how would you explain that I detected that one of the supertweeters had stopped working properly, something I confirmed by measurement?

Rick Tomaszewicz wrote:
Could a double blind test help in determining actual improvements?

As I wrote in 1997, following my experience as a listener in many double-blind tests organized by others - see www.stereophile.com/asweseeit/407awsi/index.html - "double-blind comparative listening tests [are] the last refuge of the agenda-driven scoundrel."

John Atkinson
Editor, Stereophile

Rick Tomaszewicz's picture

as Remlab suggests, your recognition of the failed super tweeter was related to the sudden absence of additive or subtractive effects within the audible range of that channel.

As an avid reader of Stereophile and its sister publications, I'm familiar with the ongoing debate concerning double blind testing. Both points of view have been eloquently argued. Mikey F. has also commented on the need for educating one's ears prior to making judgements. I note Philips provides an online tutorial to help in this regard.


I acknowledge that you and your writers likely have among the best educated ears around. However, I do not understand why audiophiles should be exempt from the same rigour most other technical fields subject claims to; namely, double blind testing. It's for this reason I admire Mikey's blind listening polls to compare gear. (I don't think one has to have an agenda or be a scoundrel in order to seek such objectivity.)

I've suggested before that an interesting annual feature could be for you and your writers to blind listen to gear in differing price ranges, and rank all in absolute terms and then rank based on cost ratio. For the 99.9% of us, that last category would be most valued. Because, in the end, it should be about the music. And the closer we can get to the artist at the least cost, the better. (The wealthy can buy gear to impress friends, but perhaps some of them would value exposing "emperors without clothes".)

Rick Tomaszewicz's picture

I forgot to express thanks for the restraint of your conditional commendation. Such measured and sober assessment is rare among reviewers of glamorous high end gear.

Also, from a marketing POV, was it wise for the creator of such expensive gear, at the edge of perceptive benefit, to include the word "Enigma" in their name?

dalethorn's picture

I explored the Philips Golden Ears test/challenge, and found that while it's interesting, the cancellation** factors between their proposed 'colorations' and colorations of the devices you're listening with can lead to dubious conclusions. While I believe that the subtractive effect of switching from the 'better' component to the lesser component, and hearing the loss of resolution that incurs usually reveals an important difference, it isn't always that simple. Sometimes you just have to spend a lot of time trying different things.

**i.e., the source has an emphasis at 'x' khz, while the speaker has an equivalent recess at 'x' khz.

remlab's picture

..just be from destructive and constructive interference at the relatively low crossover point? If it is simply meant to extend the response of a loudspeaker, wouldn't it make more sense for it to to start at 20khz if the mains tweeter is more than capable of getting there itself? I'll bet that you would have a much more difficult time guessing wether it was in or out of the system in that case.

JRT's picture

If the loudspeaker performance is so poor that this bandaid fix may improve that performance, I would suggest getting better loudspeakers.

The MSRP of these supertweeters is $3690.

For $1499 from KEF-Direct, you can get a pair of KEF LS-50 satellite stand mount monitors.

The little KEFs need a subwoofer system for full range response. Some might argue that a subwoofer system is a bandaid fix, but I would counter that using sepatately located sources below the Schroeder frequency can provide a better solution to smoothing Eigentones in the modal response of the listening space. That woud be a better designed solution, not a bandaid fix.